

Ci a ion: Monko a, K.; Vasina, M.;
Monka, P.P.; Vanca, J.; Kozak, D.
E ec o 3D-P in ed PLA S uc u e on
Sound Re lec ion P ope ies. Polyme s
2022,14, 413. h ps://doi.o g/
10.3390/polym14030413
Academic Edi o : Houwen
Ma hew Pan
Recei ed: 21 Decembe 2021
Accep ed: 18 Janua y 2022
Published: 20 Janua y 2022
Publishe ’s No e: MDPI s ays neu al
wi h ega d o ju isdic ional claims in
published maps and ins i u ional a il-
ia ions.
Copy igh : © 2022 by he au ho s.
Licensee MDPI, Basel, Swi ze land.
This a icle is an open access a icle
dis ibu ed unde he e ms and
condi ions o he C ea i e Commons
A ibu ion (CC BY) license (h ps://
c ea i ecommons.o g/licenses/by/
4.0/).
polyme s
A icle
E ec o 3D-P in ed PLA S uc u e on Sound
Re lec ion P ope ies
Ka a ina Monko a 1,2,* , Ma in Vasina 2,3,* , Pe e Pa ol Monka 1,2, Jan Vanca 1and D ažan Kozak 4
1Facul y o Manu ac u ing Technologies, Technical Uni e si y in Kosice, 080 01 P eso , Slo akia;
pe e [email p o ec ed] (P.P.M.); [email p o ec ed] (J.V.)
2Facul y o Technology, Tomas Ba a Uni e si y in Zlin, Nam. T.G. Masa yka 275, 760 01 Zlin, Czech Republic
3Facul y o Mechanical Enginee ing, VŠB-Technical Uni e si y o Os a a, 17. lis opadu 15/2172,
708 33 Os a a-Po uba, Czech Republic
4Mechanical Enginee ing Facul y, Uni e si y o Sla onski B od, T g I ane B lic-Mazu anic 2,
HR-35000 Sla onski B od, C oa ia; [email p o ec ed]
*Co espondence: [email p o ec ed] (K.M.); [email p o ec ed] (M.V.); Tel.: +421-55-602-6370 (K.M.);
+420-57-603-5112 (M.V.)
Abs ac :
3D p in ing echnique is cu en ly one o he p omising eme ging echnologies. I is used
in many a eas o human ac i i y, including acous ic applica ions. This pape ocuses on s udying he
sound e lec ion beha io o ou di e en ypes o 3D-p in ed open-po ous polylac ic acid (PLA)
ma e ial s uc u es, namely ca esian, oc agonal, homboid and s a li s uc u es. Sound e lec ion
p ope ies we e e alua ed by means o he no mal incidence sound e lec ion coe icien based on he
ans e unc ion me hod using an acous ic impedance ube. In his s udy, a ious ac o s a ec ing he
sound e lec ion pe o mance o he in es iga ed PLA samples we e e alua ed. I can be concluded
ha he sound e lec ion beha io o he es ed PLA specimens was s ongly a ec ed by di e en
ac o s. I was in luenced, no only by he ype o 3D-p in ed open-po ous ma e ial s uc u e, bu
also by he exci a ion equency, he o al olume po osi y, he specimen hickness, and he ai gap
size behind he es ed specimen inside he acous ic impedance ube.
Keywo ds:
polylac ic acid; sound e lec ion; exci a ion equency; po osi y; 3D p in ing echnique;
hickness; ai gap
1. In oduc ion
A chi ec s ha e o en ocused on designing beau i ul buildings in ended o musical
pe o mances, especially conce halls, ope as, chu ches, mosques, o hea es. Acous ic
design wi h good sound dis ibu ion is c ucial in he in e nal spaces o such buildings [
1
–
3
].
In con as o conce halls, in which he bes possible sound acous ics a e needed, in he
case o main oads and o he places, whe e he e is high noise pollu ion, he aim is o abso b
o e ec i ely e lec noise, hus p e en ing he u he p opaga ion o sound wa es [
4
].
Howe e , o he ooms which a e used e e y day, such as class ooms, audi o iums, o ices,
o p oduc ion halls, also equi e well-p ojec ed sound p ope ies. Poo acous ic design
o such wo kspaces can cause discom o , dizziness, and cons an exposu e o excessi e
unwan ed sounds ha a ec physical and men al heal h. The e o e, i is c ucial o human
heal h and sa e y o use sui able sound-abso bing ma e ials in speci ic ooms, o ices,
manu ac u ing halls, ehicles, o ai c a .
Noise and sound ha e he same speci ica ion bu a e pe cei ed di e en ly. The speed
o sound can a y due o he di e ence in p essu e be ween he media h ough which he
sound p opaga es [
5
]. Sound can be conside ed as he p opaga ion o a diso de mainly
in liquid o solid phases. The wa e mo ion is igge ed when he elemen mo es he
nea es ai pa icle, which hen causes a p essu e di e ence in he medium in which he
Polyme s 2022,14, 413. h ps://doi.o g/10.3390/polym14030413 h ps://www.mdpi.com/jou nal/polyme s
Polyme s 2022,14, 413 2 o 17
wa e mo es [
6
]. Noise is pe cei ed as a nuisance by people, and noise pollu ion in he
en i onmen can also cause discom o [7].
Sound p opaga es h ough he ai o o he media as a longi udinal wa e, in which he
mechanical ib a ions un in he di ec ion o wa e p opaga ion. The p ope y o wa es
and sound associa ed wi h he echo phenomenon is known as e lec ion [7–10].
The sound wa e changes du ing he passage h ough space when i mee s an obs acle.
The wa e may a y due o e lec ion om he obs acle, di ac ion a ound he obs acle,
and ansmission accompanied by a e ac ion o he obs acle and o he space. One pa o
he soundwa e passes h ough he bounda y o space. The second pa is e lec ed. The
e lec ion i sel and i s in ensi y depend on he simila i y o he spaces [11].
The sound ene gy e lec ed om a bounda y/obs acle can be di ided in o specula
o di use. The o me is when he e lec ion appea s a he same angle a which i s ikes
he su ace, like he e lec ion o ligh om a mi o . In con as , di use e lec ion occu s
when he sound ene gy is sca e ed in non-specula di ec ions [
4
]. The di ec ion o wa e
p opaga ion is pe pendicula o he on o med by all he Huygens’ wa ele s. In e ms
o shape, sui able e lec o s a e used o di e en pu poses o e ec s. Fo example, a
pa abolic e lec o ocuses a pa allel sound wa e o a speci ic poin , making i easie o hea
a eeble sound. Such e lec o s a e used in pa abolic mic ophones o pick up sound om
a dis an sou ce o selec he loca ion whe e he sound is obse ed and hen ocus i on
he mic ophone. On he o he hand, an ellip ical shape can ocus sound om one poin o
ano he [8,12].
The e lec ion (abso p ion) o sound wa es is a ec ed by se e al pa ame e s, such as
su ace shapes, obs acles and hei spa ial dis ibu ion, hei dis ance and a angemen [
13
].
In addi ion o hese, one o he mos impo an ac o s ha has a signi ican e ec on he
p ope p opaga ion o sound is he ma e ial, including i s po osi y, densi y, hickness, and
he angle o sound incidence, su ace shape, exci a ion equency o he acous ic wa e
and he ype o in e nal s uc u e o he ma e ial [
1
,
6
]. Zulki li [
14
] and Lee [
15
] epo ed
ha he acous ic abso p ion o mul ilaye ma e ials is be e wi h pe o a ed pla es backed
wi h ai spaces. In mos cases, sound-abso bing ma e ials a e measu ed in a e e be a ion
chambe . In a s udy by Cucha e o e al. [
16
], he au ho s showed how angle-dependen
abso p ion coe icien s could be measu ed. The esul s we e in good ag eemen wi h sound
abso p ion coe icien s and con i med ha ma e ials ha e di e en a enua ion beha io o
sound wa es coming om di e en di ec ions.
In ecen yea s, wi h he de elopmen o manu ac u ing echnology, po ous ma e ials
ha e p esen ed a challenge o many esea che s o s udy hei sound abso p ion o e lec ion
p ope ies. The p ocess o e lec ing acous ic wa es in po ous ma e ials in he case o
oblique impac o acous ic wa es was s udied by Gimal dino e al. [
17
] The esul s showed,
ha wi h inc easing angle o impac , ansmission dec eases. Acco ding o [
18
], ligh weigh
ma e ials, such as oam o ab ic, a e o en oo po ous o e lec sound passing h ough, and
hei ene gy is con e ed in o hea wi h a educ ion in magni ude. This app oach is o en
used in cinemas and eco ding s udios o educe he oom’s e e be a ion ime. While
e ec i e in e nally, ligh weigh ma e ials a e no sui able ex e nally, so conc e e is s ill
p e e ed [
18
]. A balance be ween speed and densi y o sound and i s e ec on he acous ic
p ope ies o a po ous ma e ial was s udied by Kuczma ski e al. [
19
], while he e ec o
o uosi y, po osi y, and low esis i i y on e lec ion and ansmission o acous ic wa es in
po ous ma e ials was he opic o [
20
]. Sun e al. [
21
] demons a ed ha he main eason
o he di e ence in acous ic p ope ies o ma e ials p oduced ia selec i e lase sin e ing
is he di e ence in he uni s uc u es. The in es iga ion con i med ha ansmission loss
has be e esul s wi h mixed s uc u es han using indi idual componen s, and can be
imp o ed wi h pe iodically a anged sca e e s inside po ous ma e ials.
To s udy he na u al beha io o he uni o m in ini e laye s o a po ous ma e ial
was he aim o [
22
], which conside ed he ela ion be ween p essu e and eloci y. The
esul s o his s udy enabled e alua ion o he in insic p ope ies o he ma e ial based
on plane wa e acous ic su ace impedance. A subsequen comp ehensi e li e a u e e-
Polyme s 2022,14, 413 3 o 17
iew, wi h de ailed desc ip ion o he p edic ion o acous ic abso p ion beha io , a ious
echniques o enhancing sound abso p ion and he applica ion o Wilson’s elaxa ion-
ma ched equi alen luid model, was p o ided by O a u [
23
]. The compound impedance
and sound abso p ion in a po ous ma e ial in he i s esonance ca i y we e examined
by Fan [
24
]. The esul s showed ha an acous ic boa d, consis ing o aluminum oam as
he inne s uc u e and mic o-po ous boa d as he ou e s uc u e, wi h polyes e be ween
hem, showed good sound abso p ion pe o mance in he low- equency zone. Simila ly,
Talebi oo i e al. [
25
] in es iga ed double-walled po oelas ic composi e shells o achie e
di use sound ansmission in he high- equency ange. A posi i e e ec was obse ed in
op imizing ib oacous ic i ness.
A summa y o ab ica ion, e olu ion and p edic ion models o sound abso p ion in
po ous ma e ials is desc ibed in he wo k o Cao e al. [
26
], while he wo k o Chen [
27
]
demons a ed he ela ion be ween geome ical pa ame e s and sound abso p ion based on
he es ablishmen o s a ic low esis i i y and de i a ion o he ac al acous ic model. The
ini e analysis me hod was applied o assess sound insula ion pe o mance in a sandwich
pla e wi h la ice co e, he esul s showing ha a sandwich pla e wi h a la ice co e could
ou pe o m adi ional la ice co es [28].
The p esen s udy ep esen s an o iginal con ibu ion o he esea ch o sound p ope -
ies o addi i ely p oduced ma e ials wi h egula ly dis ibu ed po ous s uc u es. I sough
o in es iga e he sound e lec ion beha io o ou di e en ypes o 3D-p in ed open-
po ous polylac ic acid (PLA) ma e ial s uc u es, namely ca esian, oc agonal, homboid
and s a li s uc u es, conside ing a ious ac o s a ec ing he sound e lec ion pe o mance
o he in es iga ed PLA samples. The sound e lec ion p ope ies we e e alua ed by means
o he no mal incidence sound e lec ion coe icien based on he ans e unc ion me hod
using an acous ic impedance ube.
I can be seen om he o e iew abo e ha he concep o using ligh weigh ma e ials
in echnical p ac ice is no en i ely new. Po ous ma e ials gi e he p oduc a speci ic combi-
na ion o p ope ies, especially in e ms o ligh ness (weigh educ ion), while main aining
su icien igidi y, s eng h, and esis ance o load/s ess. Thei ad an ages include, no
only hei mechanical p ope ies, bu also o he physical cha ac e is ics, such as acous ic,
he mal, o iscous p ope ies. Reduced ma e ial consump ion will also educe he cos o
p oducing he componen and ecycling i , making po ous ma e ials mo e en i onmen ally
iendly. Cu en ly, he p oduc ion o cellula ma e ials is possible mainly due o he apid
de elopmen o addi i e echnologies. Wi h he help o 3D p in ing echnique, i is possible
o p oduce e en complex objec s ha canno be p oduced by o he (classical) me hods. All
po ous ma e ials a e s ongly dependen on hei h ee-dimensional (3D) mic os uc u es,
including he po osi y, po e sizes, shapes, and connec i i y o he po ous space [
29
,
30
]. In
e ms o he di ision in o po ous s uc u es, egula and i egula , he egula a angemen
o cellula s uc u es allows be e con ol o p edic ion o hei p ope ies, so he au ho s
decided o add ess his ype o s uc u e in he sound e lec i i y esea ch.
To he bes o he au ho ’s knowledge, no ele an s udies ha e ye been published
which examine he sound e lec ion o he abo e ypes o s uc u es made o PLA plas ic
ma e ial, conside ing he in luence o se e al ac o s, such as hei s uc u al ype, exci a ion
equency, he o al olume po osi y, he specimen hickness, and he ai gap size behind
he es ed specimen inside he acous ic impedance ube.
2. Ma e ials and Me hods
2.1. Cha ac e is ics o Samples
Ai low esis i i y is a physical pa ame e cha ac e is ic o po ous and ib ous ma-
e ials, which quan i ies, pe uni leng h, he abili y o oppose esis ance o he mo ion
o ai pa icles inside a ma e ial. As is well-known, his p ope y is impo an o e alu-
a e he acous ic beha io o hese ma e ials in a ious ields o hei applica ion, such as
au omo i e noise mi iga ion, a chi ec u al acous ics, and building acous ics [31,32].
Polyme s 2022,14, 413 4 o 17
The samples we e p oduced ia an addi i e app oach om polylac ic acid o polylac-
ide (PLA) ma e ial (P usa Polyme s, P ague, Czech Republic), which is a he moplas ic
made om ege able s a ch. The p ocessing o his ma e ial was ca ied ou a a empe -
a u e o app oxima ely (180–220)
◦
C. The manu ac u e s a es in he ma e ial shee he
ollowing p ope ies o he PLA ilamen used in he expe imen s o his esea ch: peak
mel empe a u e (145–160)
◦
C, glass ansi ion empe a u e (55–60)
◦
C, speci ic weigh
1.24 g cm
−3
, mois u e abso p ion 0.3% a 28
◦
C and yield s eng h o he ilamen 57.4 MPa.
A e hea ing his ma e ial, i is possible o smell a swee scen . PLA ma e ial is
esis an o de o ma ion, non- oxic, b i le, and less hea esis an han ABS ma e ial. The
ad an age o his ma e ial is ha i is no ha m ul o heal h and is biodeg adable. Ano he
ad an age o his ma e ial is ha i canno be easily o n and can be used wi h p in e s
ha do no ha e a hea ed p in ing bed. Compa ed o o he ma e ials, PLA cooling akes
longe , and e icien cooling can be ensu ed by ans. Componen s made o his ma e ial
can be inely machined, e.g., by g inding. P oduc s ex uded om his plas ic should no
be exposed o di ec sunligh . The p oduc s a e no sui able o high loads. In si ua ions
o excessi e humidi y, hese p oduc s abso b wa e , which also educes hei quali y. PLA
ma e ial is mos o en used o p oduce ood aids, p o o ypes, oys, design models, e c.
Howe e , i can be used o ad an age in sui able echnical applica ions in enginee ing and
cons uc ion, and he au omo i e o ae ospace indus ies [33,34].
Fou di e en ypes o 3D-p in ed open-po ous s uc u es, namely, ca esian, oc ago-
nal, homboid and s a li s uc u es, we e selec ed o in es iga e sound e lec ion p ope ies
wi hin he p esen ed esea ch. The main eason o selec ing he la ice s uc u es e e ed
o abo e was he comple ely di e en ype o egula ly epea ing cells o ming he s uc u e,
as well as he na u e o hei dis ibu ion in he c oss-sec ional a ea ( o a ed a ound he
cylind ical axis o slid in ec angula axes). Vi ual models o he samples we e gene -
a ed in so wa e PTC C eo 7 (Pa ame ic Technology Co po a ion Inc., Bos on, MA, USA).
The samples we e cylind ical wi h a diame e o 29 mm so ha hei ou e shape and
dimensions ma ched he impedance ube o es ing; he s uc u e i sel had a diame e
o 25 mm, and he ou e shell o con inuous ma e ial had a hickness o 2 mm. Fo each
ype o s uc u e (ca esian, oc agonal, homboid and s a li ), samples we e p oduced in
h ee di e en olume po osi ies P(30%, 43% and 56%) and in h ee di e en hicknesses
(10 mm, 20 mm and 30 mm), making in o al 36 pieces.
The o al olume po osi y o a po ous ma e ial is one o he essen ial cha ac e is ics o
a s uc u e (apa om i s geome y) and is de ined by Equa ion (1) [35]:
P(%)=Va
VT
×100 (1)
whe e Vais he ai olume in body ca i ies, and VTis he o al body olume. The olume
po osi y o he specimens was egula ed by he hickness o he s u in a la ice s uc u e
and by he cell size.
I he appa en densi y o he samples is known (in ou case i was 0.5456; 0.7068 and
0.868 gcm−3), he po osi y can be exp essed by means o Equa ion (2)
P(%)=1−ρA
ρ×100 (2)
whe e ρAis he appa en densi y and ρis he densi y o PLA ma e ial (1.24 gcm−3).
A summa y o he basic cha ac e is ics o la ice s uc u es made o in es iga e he
sound e lec ion p ope ies is gi en in Table 1, whe e x/y/z a e he dimensions o he base
cell, whe e he o ien a ion “z” is he di ec ion o sample building and i is pe pendicula o
he posi ion o he pla o m “x,y”.
Polyme s 2022,14, 413 5 o 17
Table 1.
Basic cha ac e is ics o la ice s uc u es designed o in es iga e he sound e lec ion p ope ies.
S uc u e
Type Label Volume Po osi y
(%)
S uc u e
View
S u Diame e
(mm)
Cell Size
x/y/z
(mm)
Ca esian
56
Polyme s 2022, 14, x FOR PEER REVIEW 5 o 18
Table 1. Basic cha ac e is ics o la ice s uc u es designed o in es iga e he sound e lec ion
p ope ies.
S uc u e Type Label
Volume
Po osi y
(%)
S uc u e View
S u
Diame e
(mm)
Cell Size
x/y/z
(mm)
Ca esian
56
1 5/5/5
C 43 1.4 5/5/5
30 1.8 5/5/5
Oc agonal
56
1 6/7/5
O 43 1.4 6/7/5
30 1.7 5.5/7/5
Rhomboid
56
1 5.5/7/7
R 43 1.35 5/7/7
30 1.7 5/7/7
S a li
56
1 8/9/5
S 43 1.4 7.5/9/5
30 1.8 8/9/5
The abili y o p oduce such complex s uc u es in he co e o an objec (e.g., a u u e
sound wall wi h he necessa y sound e lec ion p ope ies) is made possible oday by
echnologies which wo k on he addi i e p inciple. The p oposed es specimens we e
p in ed on a P usa i3 Mk2 3D p in e (P usa Resea ch a.s., P ague, Czech Republic), using
used deposi ion modeling (FDM) echnology o 3D p in ing. This echnology is one o
he mos widesp ead addi i e p oduc ion echnologies and is included in he g oup o
me hods used o p oduce solid p o o ypes based on he p inciple o ex usion. As wi h
o he echnologies, including FDM, he equi ed body is c ea ed by deposi ing hin laye s
o semi-liquid ma e ial. The ma e ial is wound in he o m o a ilamen on a spool om
whe e i is led o a nozzle loca ed in he applica ion head. The mo emen o he nozzle is
gi en by a 3D model o he desi ed body, which is sliced in o hin laye s in he so wa e
be o e he 3D p in ing i sel . I is “con olled” by he con ou s o he body in he gi en
laye and he chosen s a egy o illing con inuous su aces wi h he ma e ial. A e ap-
plica ion, he ma e ial adhe es o he p e ious laye and g adually cools. The building
pla o m, on which he body is buil , d ops lowe wi h inc easing laye hickness and he
applica ion p ocess is epea ed un il he componen is comple ed. The quali y and
s eng h o FDM componen s essen ially depend on he p ocess pa ame e s [36,37]. The
ma e ial is wound on a spool a ached o he ex ude wi h i s mo o , which pushes he
h ead h ough he ho end. The ho end main ains a cons an empe a u e o mel he
ilamen e y quickly in o a iscous liquid which is ex uded h ough a small nozzle,
i s ly on o he pla o m, and hen in subsequen laye s, con inuing o apply he ma e ial
un il he en i e p o o ype is o med.
The 3D p in ing s a egy, as well as he nozzle and pla o m mo emen , we e con-
olled by a compu e on which P usaSlice so wa e o he same manu ac u e as he 3D
p in e machine (P usa Resea ch a.s., P ague, Czech Republic) was ins alled.
The echnical and echnological pa ame e s used in 3D p in ing we e selec ed ac-
co ding o he equipmen manu ac u e ’s ecommenda ions o PLA ma e ial. Howe e ,
a ange o empe a u es and speeds could be used. A nozzle empe a u e o 220 °C and a
p in speed o 40 mms−1 o he whole sample we e applied o p oduce a p elimina y
sample. In his case, he 3D p in e s inging e ec appea ed as seen in Figu e 1.
1 5/5/5
C 43 1.4 5/5/5
30 1.8 5/5/5
Oc agonal
56
Polyme s 2022, 14, x FOR PEER REVIEW 5 o 18
Table 1. Basic cha ac e is ics o la ice s uc u es designed o in es iga e he sound e lec ion
p ope ies.
S uc u e Type Label
Volume
Po osi y
(%)
S uc u e View
S u
Diame e
(mm)
Cell Size
x/y/z
(mm)
Ca esian
56
1 5/5/5
C 43 1.4 5/5/5
30 1.8 5/5/5
Oc agonal
56
1 6/7/5
O 43 1.4 6/7/5
30 1.7 5.5/7/5
Rhomboid
56
1 5.5/7/7
R 43 1.35 5/7/7
30 1.7 5/7/7
S a li
56
1 8/9/5
S 43 1.4 7.5/9/5
30 1.8 8/9/5
The abili y o p oduce such complex s uc u es in he co e o an objec (e.g., a u u e
sound wall wi h he necessa y sound e lec ion p ope ies) is made possible oday by
echnologies which wo k on he addi i e p inciple. The p oposed es specimens we e
p in ed on a P usa i3 Mk2 3D p in e (P usa Resea ch a.s., P ague, Czech Republic), using
used deposi ion modeling (FDM) echnology o 3D p in ing. This echnology is one o
he mos widesp ead addi i e p oduc ion echnologies and is included in he g oup o
me hods used o p oduce solid p o o ypes based on he p inciple o ex usion. As wi h
o he echnologies, including FDM, he equi ed body is c ea ed by deposi ing hin laye s
o semi-liquid ma e ial. The ma e ial is wound in he o m o a ilamen on a spool om
whe e i is led o a nozzle loca ed in he applica ion head. The mo emen o he nozzle is
gi en by a 3D model o he desi ed body, which is sliced in o hin laye s in he so wa e
be o e he 3D p in ing i sel . I is “con olled” by he con ou s o he body in he gi en
laye and he chosen s a egy o illing con inuous su aces wi h he ma e ial. A e ap-
plica ion, he ma e ial adhe es o he p e ious laye and g adually cools. The building
pla o m, on which he body is buil , d ops lowe wi h inc easing laye hickness and he
applica ion p ocess is epea ed un il he componen is comple ed. The quali y and
s eng h o FDM componen s essen ially depend on he p ocess pa ame e s [36,37]. The
ma e ial is wound on a spool a ached o he ex ude wi h i s mo o , which pushes he
h ead h ough he ho end. The ho end main ains a cons an empe a u e o mel he
ilamen e y quickly in o a iscous liquid which is ex uded h ough a small nozzle,
i s ly on o he pla o m, and hen in subsequen laye s, con inuing o apply he ma e ial
un il he en i e p o o ype is o med.
The 3D p in ing s a egy, as well as he nozzle and pla o m mo emen , we e con-
olled by a compu e on which P usaSlice so wa e o he same manu ac u e as he 3D
p in e machine (P usa Resea ch a.s., P ague, Czech Republic) was ins alled.
The echnical and echnological pa ame e s used in 3D p in ing we e selec ed ac-
co ding o he equipmen manu ac u e ’s ecommenda ions o PLA ma e ial. Howe e ,
a ange o empe a u es and speeds could be used. A nozzle empe a u e o 220 °C and a
p in speed o 40 mms−1 o he whole sample we e applied o p oduce a p elimina y
sample. In his case, he 3D p in e s inging e ec appea ed as seen in Figu e 1.
1 6/7/5
O 43 1.4 6/7/5
30 1.7 5.5/7/5
Rhomboid
56
Polyme s 2022, 14, x FOR PEER REVIEW 5 o 18
Table 1. Basic cha ac e is ics o la ice s uc u es designed o in es iga e he sound e lec ion
p ope ies.
S uc u e Type Label
Volume
Po osi y
(%)
S uc u e View
S u
Diame e
(mm)
Cell Size
x/y/z
(mm)
Ca esian
56
1 5/5/5
C 43 1.4 5/5/5
30 1.8 5/5/5
Oc agonal
56
1 6/7/5
O 43 1.4 6/7/5
30 1.7 5.5/7/5
Rhomboid
56
1 5.5/7/7
R 43 1.35 5/7/7
30 1.7 5/7/7
S a li
56
1 8/9/5
S 43 1.4 7.5/9/5
30 1.8 8/9/5
The abili y o p oduce such complex s uc u es in he co e o an objec (e.g., a u u e
sound wall wi h he necessa y sound e lec ion p ope ies) is made possible oday by
echnologies which wo k on he addi i e p inciple. The p oposed es specimens we e
p in ed on a P usa i3 Mk2 3D p in e (P usa Resea ch a.s., P ague, Czech Republic), using
used deposi ion modeling (FDM) echnology o 3D p in ing. This echnology is one o
he mos widesp ead addi i e p oduc ion echnologies and is included in he g oup o
me hods used o p oduce solid p o o ypes based on he p inciple o ex usion. As wi h
o he echnologies, including FDM, he equi ed body is c ea ed by deposi ing hin laye s
o semi-liquid ma e ial. The ma e ial is wound in he o m o a ilamen on a spool om
whe e i is led o a nozzle loca ed in he applica ion head. The mo emen o he nozzle is
gi en by a 3D model o he desi ed body, which is sliced in o hin laye s in he so wa e
be o e he 3D p in ing i sel . I is “con olled” by he con ou s o he body in he gi en
laye and he chosen s a egy o illing con inuous su aces wi h he ma e ial. A e ap-
plica ion, he ma e ial adhe es o he p e ious laye and g adually cools. The building
pla o m, on which he body is buil , d ops lowe wi h inc easing laye hickness and he
applica ion p ocess is epea ed un il he componen is comple ed. The quali y and
s eng h o FDM componen s essen ially depend on he p ocess pa ame e s [36,37]. The
ma e ial is wound on a spool a ached o he ex ude wi h i s mo o , which pushes he
h ead h ough he ho end. The ho end main ains a cons an empe a u e o mel he
ilamen e y quickly in o a iscous liquid which is ex uded h ough a small nozzle,
i s ly on o he pla o m, and hen in subsequen laye s, con inuing o apply he ma e ial
un il he en i e p o o ype is o med.
The 3D p in ing s a egy, as well as he nozzle and pla o m mo emen , we e con-
olled by a compu e on which P usaSlice so wa e o he same manu ac u e as he 3D
p in e machine (P usa Resea ch a.s., P ague, Czech Republic) was ins alled.
The echnical and echnological pa ame e s used in 3D p in ing we e selec ed ac-
co ding o he equipmen manu ac u e ’s ecommenda ions o PLA ma e ial. Howe e ,
a ange o empe a u es and speeds could be used. A nozzle empe a u e o 220 °C and a
p in speed o 40 mms−1 o he whole sample we e applied o p oduce a p elimina y
sample. In his case, he 3D p in e s inging e ec appea ed as seen in Figu e 1.
1 5.5/7/7
R 43 1.35 5/7/7
30 1.7 5/7/7
S a li
56
Polyme s 2022, 14, x FOR PEER REVIEW 5 o 18
Table 1. Basic cha ac e is ics o la ice s uc u es designed o in es iga e he sound e lec ion
p ope ies.
S uc u e Type Label
Volume
Po osi y
(%)
S uc u e View
S u
Diame e
(mm)
Cell Size
x/y/z
(mm)
Ca esian
56
1 5/5/5
C 43 1.4 5/5/5
30 1.8 5/5/5
Oc agonal
56
1 6/7/5
O 43 1.4 6/7/5
30 1.7 5.5/7/5
Rhomboid
56
1 5.5/7/7
R 43 1.35 5/7/7
30 1.7 5/7/7
S a li
56
1 8/9/5
S 43 1.4 7.5/9/5
30 1.8 8/9/5
The abili y o p oduce such complex s uc u es in he co e o an objec (e.g., a u u e
sound wall wi h he necessa y sound e lec ion p ope ies) is made possible oday by
echnologies which wo k on he addi i e p inciple. The p oposed es specimens we e
p in ed on a P usa i3 Mk2 3D p in e (P usa Resea ch a.s., P ague, Czech Republic), using
used deposi ion modeling (FDM) echnology o 3D p in ing. This echnology is one o
he mos widesp ead addi i e p oduc ion echnologies and is included in he g oup o
me hods used o p oduce solid p o o ypes based on he p inciple o ex usion. As wi h
o he echnologies, including FDM, he equi ed body is c ea ed by deposi ing hin laye s
o semi-liquid ma e ial. The ma e ial is wound in he o m o a ilamen on a spool om
whe e i is led o a nozzle loca ed in he applica ion head. The mo emen o he nozzle is
gi en by a 3D model o he desi ed body, which is sliced in o hin laye s in he so wa e
be o e he 3D p in ing i sel . I is “con olled” by he con ou s o he body in he gi en
laye and he chosen s a egy o illing con inuous su aces wi h he ma e ial. A e ap-
plica ion, he ma e ial adhe es o he p e ious laye and g adually cools. The building
pla o m, on which he body is buil , d ops lowe wi h inc easing laye hickness and he
applica ion p ocess is epea ed un il he componen is comple ed. The quali y and
s eng h o FDM componen s essen ially depend on he p ocess pa ame e s [36,37]. The
ma e ial is wound on a spool a ached o he ex ude wi h i s mo o , which pushes he
h ead h ough he ho end. The ho end main ains a cons an empe a u e o mel he
ilamen e y quickly in o a iscous liquid which is ex uded h ough a small nozzle,
i s ly on o he pla o m, and hen in subsequen laye s, con inuing o apply he ma e ial
un il he en i e p o o ype is o med.
The 3D p in ing s a egy, as well as he nozzle and pla o m mo emen , we e con-
olled by a compu e on which P usaSlice so wa e o he same manu ac u e as he 3D
p in e machine (P usa Resea ch a.s., P ague, Czech Republic) was ins alled.
The echnical and echnological pa ame e s used in 3D p in ing we e selec ed ac-
co ding o he equipmen manu ac u e ’s ecommenda ions o PLA ma e ial. Howe e ,
a ange o empe a u es and speeds could be used. A nozzle empe a u e o 220 °C and a
p in speed o 40 mms−1 o he whole sample we e applied o p oduce a p elimina y
sample. In his case, he 3D p in e s inging e ec appea ed as seen in Figu e 1.
1 8/9/5
S 43 1.4 7.5/9/5
30 1.8 8/9/5
The abili y o p oduce such complex s uc u es in he co e o an objec (e.g., a u u e
sound wall wi h he necessa y sound e lec ion p ope ies) is made possible oday by
echnologies which wo k on he addi i e p inciple. The p oposed es specimens we e
p in ed on a P usa i3 Mk2 3D p in e (P usa Resea ch a.s., P ague, Czech Republic), using
used deposi ion modeling (FDM) echnology o 3D p in ing. This echnology is one o
he mos widesp ead addi i e p oduc ion echnologies and is included in he g oup o
me hods used o p oduce solid p o o ypes based on he p inciple o ex usion. As wi h
o he echnologies, including FDM, he equi ed body is c ea ed by deposi ing hin laye s
o semi-liquid ma e ial. The ma e ial is wound in he o m o a ilamen on a spool om
whe e i is led o a nozzle loca ed in he applica ion head. The mo emen o he nozzle is
gi en by a 3D model o he desi ed body, which is sliced in o hin laye s in he so wa e
be o e he 3D p in ing i sel . I is “con olled” by he con ou s o he body in he gi en laye
and he chosen s a egy o illing con inuous su aces wi h he ma e ial. A e applica ion,
he ma e ial adhe es o he p e ious laye and g adually cools. The building pla o m, on
which he body is buil , d ops lowe wi h inc easing laye hickness and he applica ion
p ocess is epea ed un il he componen is comple ed. The quali y and s eng h o FDM
componen s essen ially depend on he p ocess pa ame e s [
36
,
37
]. The ma e ial is wound
on a spool a ached o he ex ude wi h i s mo o , which pushes he h ead h ough he ho
end. The ho end main ains a cons an empe a u e o mel he ilamen e y quickly in o a
iscous liquid which is ex uded h ough a small nozzle, i s ly on o he pla o m, and hen
in subsequen laye s, con inuing o apply he ma e ial un il he en i e p o o ype is o med.
The 3D p in ing s a egy, as well as he nozzle and pla o m mo emen , we e con-
olled by a compu e on which P usaSlice so wa e o he same manu ac u e as he 3D
p in e machine (P usa Resea ch a.s., P ague, Czech Republic) was ins alled.
The echnical and echnological pa ame e s used in 3D p in ing we e selec ed acco d-
ing o he equipmen manu ac u e ’s ecommenda ions o PLA ma e ial. Howe e , a
ange o empe a u es and speeds could be used. A nozzle empe a u e o 220
◦
C and
a p in speed o 40 mms
−1
o he whole sample we e applied o p oduce a p elimina y
sample. In his case, he 3D p in e s inging e ec appea ed as seen in Figu e 1.
A e se e al ials o adjus he se ings, he ollowing 3D p in ing pa ame e s we e
inally selec ed: nozzle empe a u e 215
◦
C o he i s laye and 210
◦
C o he o he s; bed
empe a u e (building pla o m) 60
◦
C, p in ing speed on he pe iphe al ci cui s 30 mm/s
and p in ing speed o he in e nal s uc u e 20 mms
−1
. A 1.7 mm diame e ilamen was
used o 3D p in ing o he samples, which was applied in laye hickness o 0.254 mm

Polyme s 2022,14, 413 6 o 17
using a 0.4 mm diame e nozzle. The samples we e posi ioned so ha hei cylind ical axes
we e pe pendicula o he building pla o m. They we e buil wi hou suppo s because
he la ice s uc u es we e sel -suppo ing. The quali y o he samples was checked isually,
and no c acks o hin s ands o plas ic (as he esul o 3D p in e s inging) we e obse ed.
Polyme s 2022, 14, x FOR PEER REVIEW 6 o 18
Figu e 1. The 3D p in e s inging e ec appea ance o a p elimina y sample.
A e se e al ials o adjus he se ings, he ollowing 3D p in ing pa ame e s we e
inally selec ed: nozzle empe a u e 215 °C o he i s laye and 210 °C o he o he s;
bed empe a u e (building pla o m) 60 °C, p in ing speed on he pe iphe al ci cui s 30
mm/s and p in ing speed o he in e nal s uc u e 20 mms
−1
. A 1.7 mm diame e ilamen
was used o 3D p in ing o he samples, which was applied in laye hickness o 0.254
mm using a 0.4 mm diame e nozzle. The samples we e posi ioned so ha hei cylin-
d ical axes we e pe pendicula o he building pla o m. They we e buil wi hou sup-
po s because he la ice s uc u es we e sel -suppo ing. The quali y o he samples was
checked isually, and no c acks o hin s ands o plas ic (as he esul o 3D p in e
s inging) we e obse ed.
An example o he samples p oduced is shown in Figu e 2. In Figu e 2a, samples o
hickness = 30 mm wi h di e en ypes o s uc u e a e shown. In Figu e 2b homboid
samples wi h di e en olume po osi ies a e shown, and in Figu e 2c a se o samples
wi h oc agonal s uc u e is p esen ed.
Figu e 2. Examples o samples p oduced. (a) samples o hickness = 30 mm and olume po osi y P
= 43% wi h di e en ypes o s uc u e, (b) homboid samples wi h di e en olume po osi ies and
(c) a se o samples wi h oc agonal s uc u e. (No e: he numbe s on he sample labels in he igu e
indica e he o al olume po osi y in %).
2.2. Measu emen Me hodology
2.2.1. Sound Re lec ion Coe icien
When sound p opaga es om a sound sou ce o a ma e ial’s su ace, he inciden
sound ene gy E
I
is ei he e lec ed o abso bed by his ma e ial [38,39]. The sound e lec-
ion coe icien β is he measu e o how much sound is e lec ed om a ma e ial [40]. I
can be exp essed as ollows [41–44]:
𝛽=𝐸
𝐸=𝐸−𝐸

𝐸=1−𝛼 (3)
whe e E
R
is he e lec ed sound ene gy, E
A
is he abso bed sound ene gy, and α is he
sound abso p ion coe icien .
Figu e 1. The 3D p in e s inging e ec appea ance o a p elimina y sample.
An example o he samples p oduced is shown in Figu e 2. In Figu e 2a, samples o
hickness = 30 mm wi h di e en ypes o s uc u e a e shown. In Figu e 2b homboid
samples wi h di e en olume po osi ies a e shown, and in Figu e 2c a se o samples wi h
oc agonal s uc u e is p esen ed.
Polyme s 2022, 14, x FOR PEER REVIEW 6 o 18
Figu e 1. The 3D p in e s inging e ec appea ance o a p elimina y sample.
A e se e al ials o adjus he se ings, he ollowing 3D p in ing pa ame e s we e
inally selec ed: nozzle empe a u e 215 °C o he i s laye and 210 °C o he o he s;
bed empe a u e (building pla o m) 60 °C, p in ing speed on he pe iphe al ci cui s 30
mm/s and p in ing speed o he in e nal s uc u e 20 mms
−1
. A 1.7 mm diame e ilamen
was used o 3D p in ing o he samples, which was applied in laye hickness o 0.254
mm using a 0.4 mm diame e nozzle. The samples we e posi ioned so ha hei cylin-
d ical axes we e pe pendicula o he building pla o m. They we e buil wi hou sup-
po s because he la ice s uc u es we e sel -suppo ing. The quali y o he samples was
checked isually, and no c acks o hin s ands o plas ic (as he esul o 3D p in e
s inging) we e obse ed.
An example o he samples p oduced is shown in Figu e 2. In Figu e 2a, samples o
hickness = 30 mm wi h di e en ypes o s uc u e a e shown. In Figu e 2b homboid
samples wi h di e en olume po osi ies a e shown, and in Figu e 2c a se o samples
wi h oc agonal s uc u e is p esen ed.
Figu e 2. Examples o samples p oduced. (a) samples o hickness = 30 mm and olume po osi y P
= 43% wi h di e en ypes o s uc u e, (b) homboid samples wi h di e en olume po osi ies and
(c) a se o samples wi h oc agonal s uc u e. (No e: he numbe s on he sample labels in he igu e
indica e he o al olume po osi y in %).
2.2. Measu emen Me hodology
2.2.1. Sound Re lec ion Coe icien
When sound p opaga es om a sound sou ce o a ma e ial’s su ace, he inciden
sound ene gy E
I
is ei he e lec ed o abso bed by his ma e ial [38,39]. The sound e lec-
ion coe icien β is he measu e o how much sound is e lec ed om a ma e ial [40]. I
can be exp essed as ollows [41–44]:
𝛽=𝐸
𝐸=𝐸−𝐸

𝐸=1−𝛼 (3)
whe e E
R
is he e lec ed sound ene gy, E
A
is he abso bed sound ene gy, and α is he
sound abso p ion coe icien .
Figu e 2.
Examples o samples p oduced. (
a
) samples o hickness = 30 mm and olume po osi y
P= 43% wi h di e en ypes o s uc u e, (
b
) homboid samples wi h di e en olume po osi ies and
(
c
) a se o samples wi h oc agonal s uc u e. (No e: he numbe s on he sample labels in he igu e
indica e he o al olume po osi y in %).
2.2. Measu emen Me hodology
2.2.1. Sound Re lec ion Coe icien
When sound p opaga es om a sound sou ce o a ma e ial’s su ace, he inciden
sound ene gy E
I
is ei he e lec ed o abso bed by his ma e ial [
38
,
39
]. The sound e lec ion
coe icien
β
is he measu e o how much sound is e lec ed om a ma e ial [
40
]. I can be
exp essed as ollows [41–44]:
β=ER
EI
=EI−EA
EI
=1−α(3)
whe e E
R
is he e lec ed sound ene gy, E
A
is he abso bed sound ene gy, and
α
is he sound
abso p ion coe icien .
2.2.2. Sound Re lec ion P ope ies
The e a e wo di e en s anda dized me hodologies o measu ing sound e lec ion
p ope ies o ma e ial specimens, namely, he e e be a ion oom and impedance ube
me hods [45].
Polyme s 2022,14, 413 7 o 17
Measu emen o sound e lec ion p ope ies using he e e be a ion oom me hod [
46
–
48
]
is pe o med in a s anda dized e e be a ion chambe and is based on he measu emen o
e e be a ion imes wi h and wi hou he specimen o be es ed. This me hod is p e e ed
by ma e ials p oduce s o measu e he andom sound e lec ion coe icien in a di use
sound ield. Howe e , he e e be a ion oom me hod equi es a la ge-scale chambe o
olume V> 200 m3and a la ge su ace a ea (i.e., 10–12 m2) o in es iga ed specimens.
The impedance ube me hod [
49
,
50
] measu es he sound e lec ion coe icien when
sound wa es p opaga e e ically h ough a es sample. Compa ed o he e e be a ion
oom me hod, he impedance ube me hod is a quick, low-cos me hod, using small-size
specimens. The e o e, his me hod is sui able o de eloping new ypes o soundp oo ing
ma e ials. The e a e h ee impedance ube me hods: pipe pulse, s anding wa e a io,
and ans e unc ion me hods [
49
]. The pipe pulse me hod is based on he sepa a ion
o inciden and e lec ion acous ic signals. Sound e lec ion p ope ies a e subsequen ly
de e mined om he ampli ude a io o he acous ic signals. This me hod equi es high
an i- e e ence and capabili y o signal sepa a ion. The s anding wa e me hod [
51
] consis s
in measu ing acous ic p essu e ampli udes o he inciden and e lec ed acous ic wa es by
means o a mo able acous ic p obe. This me hod is edious, and he measu ing e iciency is
ela i ely low. The ans e unc ion me hod [
52
] uses wo p ecisely ma ched mic ophones
and is based on a simila p inciple o he s anding wa e me hod. I s ope a ional p ocess is
much simple , and he measu ing e iciency is highe han he s anding wa e me hod [
49
].
Expe imen al measu emen s o sound e lec ion p ope ies o he in es iga ed PLA
specimens we e unde aken using he ans e unc ion me hod ISO 10534-2 [
52
] ha is
based on measu ing sound p essu es in he impedance ube by means o mic ophones
M
1
and M
2
. This me hod de ines he no mal incidence sound e lec ion coe icien
β
as
ollows [53–55]:
β=|R|2(4)
whe e Ris he complex e lec ion ac o ha is gi en by he o mula:
R=H12 −HI
HR−H12 e2jk0x1(5)
whe e H
12
is he p essu e ans e unc ion, H
I
is he ans e unc ion o he inciden
acous ic wa e, H
R
is he ans e unc ion o he e lec ed acous ic wa e, k
0
is he complex
wa e numbe , and x
1
is he dis ance be ween he mic ophone M
1
and he es ed ma e ial
sample (see Figu e 3b). The abo e ans e unc ions a e exp essed as:
H12 =p2
p1=ejk0x2+R·e−jk0x2
ejk0x1+R·e−jk0x1(6)
HI=e−jk0(x1−x2)=e−jk0s(7)
HR=ejk0(x1−x2)=ejk0s(8)
whe e p
1
and p
2
a e he complex acous ic p essu es measu ed by he mic ophones M
1
and
M
2
,x
2
is he dis ance be ween he mic ophone M
2
and he s udied ma e ial specimen, and
sis he dis ance be ween he wo mic ophones M1and M2.
The complex acous ic p essu es a e gi en by he equa ions:
p1=ˆ
pIejk0x1+ˆ
pRe−jk0x1(9)
p2=ˆ
pIejk0x2+ˆ
pRe−jk0x2(10)
whe e
ˆ
pI
and
ˆ
pR
a e he ampli udes o acous ic p essu es o he inciden (I) and e lec ed
(R) acous ic wa es.
Polyme s 2022,14, 413 8 o 17
Polyme s 2022, 14, x FOR PEER REVIEW 8 o 18
Figu e 3. Ci cui diag am o he expe imen al equipmen o measu ing he sound e lec ion coe -
icien (a) and a schema ic o he acous ic impedance ube (b). Legend o he abb e ia ions: a—ai
gap size; EA—abso bed sound ene gy; EI—inciden sound ene gy; ER— e lec ed sound ene gy; M1,
M2—measu ing mic ophones; MS—measu ed specimen; s—dis ance be ween mic ophones M1 and
M2; SS—sound sou ce; SW—solid wall; —sample hickness; x1, x2—mic ophone dis ances om he
es ed PLA sample su ace.
The complex acous ic p essu es a e gi en by he equa ions:
𝑝=𝑝

𝑒+𝑝

𝑒 (9)
𝑝=𝑝

𝑒+𝑝

𝑒 (10)
whe e 𝑝 and 𝑝 a e he ampli udes o acous ic p essu es o he inciden (I) and e-
lec ed (R) acous ic wa es.
F equency dependencies o he no mal-incidence sound e lec ion coe icien β o
he es ed 3D-p in ed PLA ma e ial samples we e measu ed using a wo-mic ophone
acous ic impedance ube (BK 4206) in combina ion wi h a powe ampli ie (BK 2706) and
a signal PULSE mul i-analyze (BK 3560-B-030) in he equency ange om 200 o 6400
Hz (B üel & Kjæ , Næ um, Denma k). A ci cui diag am o he expe imen al equipmen
is depic ed in Figu e 3a. No mal-incidence sound e lec ion p ope ies o he es ed
3D-p in ed open-po ous specimens o a gi en hickness (i.e., 10, 20 and 30 mm) and a
o al olume po osi y P ( om 30 o 56%) we e expe imen ally measu ed o di e en ai
gap sizes a ( anging om 0 o 100 mm) behind he s udied PLA samples (see Figu e 3b).
He e, he solid wall SW is cha ac e ized by he pe ec e lec i i y (i.e., β = 1) o acous ic
wa es om i s su ace. All measu emen s we e pe o med a an ambien empe a u e o
22 °C.
3. Resul s and Discussion
This sec ion discusses a ious ac o s ha in luenced he sound e lec ion p ope ies
o he s udied 3D-p in ed open-po ous PLA s uc u es.
3.1. In luence o S uc u e Type
The ma e ial s uc u e is an impo an ac o ha has a s ong in luence on sound
e lec ion pe o mance. The e ec o he s uc u e ype on sound e lec ion p ope ies o
he es ed 3D-p in ed open-po ous PLA ma e ials, which was signi ican o hin (i.e., =
10 mm) PLA ma e ial samples, is demons a ed in Figu e 4. The equency dependencies
o he sound e lec ion coe icien o he PLA specimens o po osi y P = 30% and hick-
ness = 10 mm a e shown in Figu e 4a. In his ins ance, he specimens we e placed a 70
mm om he solid wall SW inside he acous ic impedance ube (see Figu e 3b). Simila ly,
he e ec o he s uc u e ype o he es ed open-po ous PLA ma e ials (wi h P = 56%, =
10 mm and a = 15 mm) is demons a ed in Figu e 4b. I is clea om Figu e 4 ha lowe
sound e lec ion p ope ies we e gene ally ound o he PLA samples manu ac u ed
wi h he s a li s uc u e, which we e cha ac e ized by mo e complex po e shapes com-
pa ed o he o he open-po ous PLA s uc u es. Fo his eason, he PLA samples p o-
duced wi h he s a li s uc u e exhibi ed highe ai low esis i i y, which was accompa-
nied by highe in e nal ic ion du ing he p opaga ion o sound wa es h ough his
Figu e 3.
Ci cui diag am o he expe imen al equipmen o measu ing he sound e lec ion coe -
icien (
a
) and a schema ic o he acous ic impedance ube (
b
). Legend o he abb e ia ions: a—ai
gap size; EA—abso bed sound ene gy; EI—inciden sound ene gy; ER— e lec ed sound ene gy; M1,
M
2
—measu ing mic ophones; MS—measu ed specimen; s—dis ance be ween mic ophones M
1
and
M
2
; SS—sound sou ce; SW—solid wall; —sample hickness; x
1
,x
2
—mic ophone dis ances om he
es ed PLA sample su ace.
F equency dependencies o he no mal-incidence sound e lec ion coe icien
β
o he
es ed 3D-p in ed PLA ma e ial samples we e measu ed using a wo-mic ophone acous ic
impedance ube (BK 4206) in combina ion wi h a powe ampli ie (BK 2706) and a signal
PULSE mul i-analyze (BK 3560-B-030) in he equency ange om 200 o 6400 Hz (B üel
& Kjæ , Næ um, Denma k). A ci cui diag am o he expe imen al equipmen is depic ed
in Figu e 3a. No mal-incidence sound e lec ion p ope ies o he es ed 3D-p in ed open-
po ous specimens o a gi en hickness (i.e., 10, 20 and 30 mm) and a o al olume po osi y
P( om 30 o 56%) we e expe imen ally measu ed o di e en ai gap sizes a( anging om
0 o 100 mm) behind he s udied PLA samples (see Figu e 3b). He e, he solid wall SW is
cha ac e ized by he pe ec e lec i i y (i.e.,
β
= 1) o acous ic wa es om i s su ace. All
measu emen s we e pe o med a an ambien empe a u e o 22 ◦C.
3. Resul s and Discussion
This sec ion discusses a ious ac o s ha in luenced he sound e lec ion p ope ies
o he s udied 3D-p in ed open-po ous PLA s uc u es.
3.1. In luence o S uc u e Type
The ma e ial s uc u e is an impo an ac o ha has a s ong in luence on sound
e lec ion pe o mance. The e ec o he s uc u e ype on sound e lec ion p ope ies
o he es ed 3D-p in ed open-po ous PLA ma e ials, which was signi ican o hin (i.e.,
= 10 mm
) PLA ma e ial samples, is demons a ed in Figu e 4. The equency dependencies
o he sound e lec ion coe icien o he PLA specimens o po osi y P= 30% and hickness
= 10 mm
a e shown in Figu e 4a. In his ins ance, he specimens we e placed a 70 mm
om he solid wall SW inside he acous ic impedance ube (see Figu e 3b). Simila ly,
he e ec o he s uc u e ype o he es ed open-po ous PLA ma e ials (wi h P= 56%,
= 10 mm
and a= 15 mm) is demons a ed in Figu e 4b. I is clea om Figu e 4 ha lowe
sound e lec ion p ope ies we e gene ally ound o he PLA samples manu ac u ed wi h
he s a li s uc u e, which we e cha ac e ized by mo e complex po e shapes compa ed o
he o he open-po ous PLA s uc u es. Fo his eason, he PLA samples p oduced wi h
he s a li s uc u e exhibi ed highe ai low esis i i y, which was accompanied by highe
in e nal ic ion du ing he p opaga ion o sound wa es h ough his specimen s uc u e,
and by highe con e sion o he inciden acous ic ene gy E
I
(see Figu e 3b) in o hea . These
indings we e obse ed mainly o low- equency sound wa es.
I can be concluded ha he sound e lec ion pe o mance o he in es iga ed 3D-
p in ed open-po ous PLA ma e ials o hickness = 10 mm was signi ican ly in luenced by
hei olume po osi y. In he case o low-po ous (i.e., P= 30%) PLA ma e ials, be e sound
e lec ion beha io was obse ed o he specimens made wi h he homboid s uc u e (see
Figu e 4a). Howe e , he PLA ma e ials o hickness = 10 mm p oduced wi h he ca esian
s uc u e exhibi ed be e sound e lec ion pe o mance o he highes po osi y alue (i.e.,
P= 56%), as shown in Figu e 4b. The in luence o he s uc u e ype on he sound e lec ion
Polyme s 2022,14, 413 9 o 17
beha io o he es ed 3D-p in ed open-po ous PLA ma e ials was negligible o he hicke
(i.e., = 20 and 30 mm) ma e ials.
Polyme s 2022, 14, x FOR PEER REVIEW 9 o 18
specimen s uc u e, and by highe con e sion o he inciden acous ic ene gy E
I
(see
Figu e 3b) in o hea . These indings we e obse ed mainly o low- equency sound
wa es.
(a) (b)
Figu e 4. In luence o 3D-p in ed PLA s uc u e ype on he equency dependencies o he sound
e lec ion coe icien ; (a) po osi y P = 30%, specimen hickness = 10 mm, ai gap size a = 70 mm, (b)
po osi y P = 56%, specimen hickness = 10 mm, ai gap size a = 15 mm.
I can be concluded ha he sound e lec ion pe o mance o he in es iga ed
3D-p in ed open-po ous PLA ma e ials o hickness = 10 mm was signi ican ly in lu-
enced by hei olume po osi y. In he case o low-po ous (i.e., P = 30%) PLA ma e ials,
be e sound e lec ion beha io was obse ed o he specimens made wi h he hom-
boid s uc u e (see Figu e 4a). Howe e , he PLA ma e ials o hickness = 10 mm p o-
duced wi h he ca esian s uc u e exhibi ed be e sound e lec ion pe o mance o he
highes po osi y alue (i.e., P = 56%), as shown in Figu e 4b. The in luence o he s uc u e
ype on he sound e lec ion beha io o he es ed 3D-p in ed open-po ous PLA ma e i-
als was negligible o he hicke (i.e., = 20 and 30 mm) ma e ials.
3.2. In luence o To al Volume Po osi y
Sound e lec ion p ope ies o he in es iga ed ou di e en open-po ous PLA
s uc u es we e signi ican ly in luenced, no only by he po e shape, bu also by i s size. I
is known ha he o al olume po osi y o open-po ous ma e ials is p opo ional o hei
ela i e densi y ha inc eases wi h he dec easing po e size [56]. Be e sound abso p ion
p ope ies o open-po ous ma e ial s uc u es a e gene ally ob ained a highe ma e ial
densi y and ai low esis i i y and smalle a e age po e sizes [57,58]. Fo hese easons,
he inc easing po osi y o open-po ous ma e ial s uc u es leads o be e sound e lec-
ion p ope ies o hese ma e ials. This phenomenon was con i med by he measu ed
equency dependencies o he sound e lec ion coe icien , as shown in Figu e 5. Figu e
5a ep esen s he po osi y e ec on sound e lec ion pe o mance o he PLA sample,
which was manu ac u ed wi h he homboid s uc u e and hickness = 10 mm and was
placed a dis ance a = 40 mm in on o he solid wall SW. Simila ly, he sound e lec ion
p ope ies o he PLA sample wi h he ca esian s uc u e, hickness = 20 mm and ai
gap size a = 30 mm depending on i s po osi y, a e depic ed in Figu e 5b. I is clea om
Figu e 5 ha he sound e lec ion coe icien gene ally inc eased wi h inc easing sample
po osi y, especially a low exci a ion equencies. This is because he highe po osi y e-
sul ed in lowe ai low esis i i y du ing he sound wa e p opaga ion h ough he PLA
open-po ous s uc u es, esul ing in be e sound e lec ion p ope ies.
Figu e 4.
In luence o 3D-p in ed PLA s uc u e ype on he equency dependencies o he sound
e lec ion coe icien ; (
a
) po osi y P= 30%, specimen hickness = 10 mm, ai gap size a= 70 mm,
(b) po osi y P= 56%, specimen hickness = 10 mm, ai gap size a= 15 mm.
3.2. In luence o To al Volume Po osi y
Sound e lec ion p ope ies o he in es iga ed ou di e en open-po ous PLA s uc-
u es we e signi ican ly in luenced, no only by he po e shape, bu also by i s size. I is
known ha he o al olume po osi y o open-po ous ma e ials is p opo ional o hei
ela i e densi y ha inc eases wi h he dec easing po e size [
56
]. Be e sound abso p ion
p ope ies o open-po ous ma e ial s uc u es a e gene ally ob ained a highe ma e ial
densi y and ai low esis i i y and smalle a e age po e sizes [
57
,
58
]. Fo hese easons,
he inc easing po osi y o open-po ous ma e ial s uc u es leads o be e sound e lec ion
p ope ies o hese ma e ials. This phenomenon was con i med by he measu ed equency
dependencies o he sound e lec ion coe icien , as shown in Figu e 5. Figu e 5a ep esen s
he po osi y e ec on sound e lec ion pe o mance o he PLA sample, which was manu-
ac u ed wi h he homboid s uc u e and hickness = 10 mm and was placed a dis ance
a= 40 mm in on o he solid wall SW. Simila ly, he sound e lec ion p ope ies o he
PLA sample wi h he ca esian s uc u e, hickness = 20 mm and ai gap size
a= 30 mm
depending on i s po osi y, a e depic ed in Figu e 5b. I is clea om Figu e 5 ha he sound
e lec ion coe icien gene ally inc eased wi h inc easing sample po osi y, especially a
low exci a ion equencies. This is because he highe po osi y esul ed in lowe ai low
esis i i y du ing he sound wa e p opaga ion h ough he PLA open-po ous s uc u es,
esul ing in be e sound e lec ion p ope ies.
Polyme s 2022, 14, x FOR PEER REVIEW 10 o 18
(a) (b)
Figu e 5. E ec o o al olume po osi y on he equency dependencies o he sound e lec ion
coe icien o he in es iga ed PLA specimens: (a) homboid s uc u e, sample hickness = 10 mm,
ai gap size a = 40 mm; (b) ca esian s uc u e, sample hickness = 20 mm, ai gap size a = 30 mm.
3.3. In luence o Specimen Thickness
The hickness o he 3D-p in ed open-po ous PLA ma e ials was also an impo an
ac o a ec ing hei sound e lec ion beha io . The e ec o he specimen hickness on
he equency dependencies o he sound e lec ion coe icien is shown in Figu e 6. Fig-
u e 6a demons a es he e ec o specimen hickness on he sound e lec ion pe o mance
o he PLA sample wi h 30% po osi y ha was made wi h he s a li s uc u e and was
placed di ec ly on he solid wall SW (i.e., a = 0 mm). Simila ly, he in luence o he
hickness o he PLA sample wi h he highes po osi y (i.e., P = 56%), which was p o-
duced wi h he oc agonal s uc u e and was placed 60 mm om he solid wall SW inside
he acous ic impedance ube, is demons a ed in Figu e 6b. I is clea ha he sound e-
lec ion p ope ies o he es ed PLA samples dec eased wi h inc easing ma e ial hick-
ness in a subs an ial pa o he measu ed equency ange, especially in he
low- equency egion. This phenomenon was caused by highe in e nal ic ion du ing
he sound wa e p opaga ion h ough he hicke open-po ous PLA ma e ials, which was
e lec ed in a g ea e con e sion o sound wa e ene gy in o hea . The e o e, he inc eas-
ing hickness o open-po ous ma e ials gene ally educed hei sound e lec ion p ope -
ies. Lowe sound e lec ion pe o mance was ound o he hicke PLA ma e ials a he
exci a ion equency ≅ 3 kHz depending on he s uc u e ype, he olume po osi y P
and he ai gap size a. This equency bounda y gene ally dec eased wi h dec easing
sample olume po osi y and inc easing ai gap size.
(a) (b)
Figu e 6. E ec o specimen hickness on he equency dependencies o he sound e lec ion coe -
icien o he in es iga ed PLA specimens: (a) S a li s uc u e, po osi y P = 30%, ai gap size a = 0
mm; (b) Oc agonal s uc u e, po osi y P = 56%, ai gap size a = 60 mm.
Figu e 5.
E ec o o al olume po osi y on he equency dependencies o he sound e lec ion
coe icien o he in es iga ed PLA specimens: (
a
) homboid s uc u e, sample hickness = 10 mm,
ai gap size a= 40 mm; (b) ca esian s uc u e, sample hickness = 20 mm, ai gap size a= 30 mm.
Polyme s 2022,14, 413 16 o 17
22.
D agone i, R.; Napoli ano, M.; Romano, R.A. A s udy on he ene gy and he e lec ion angle o he sound e lec ed by a po ous
ma e ial. J. Acous . Soc. Am. 2019,145, 489–500. [C ossRe ]
23.
O a u, A.J. Re iew on he Acous ical P ope ies and Cha ac e iza ion Me hods o Sound Abso bing Po ous S uc u es: A Focus
on Mic ocellula S uc u es Made by a Replica ion Cas ing Me hod. Me . Ma e . In . 2020,26, 915–932. [C ossRe ]
24.
Fan, C.; Tian, Y.; Wang, Z.Q. S uc u al pa ame e e ec o po ous ma e ial on sound abso p ion pe o mance o double- esonance
ma e ial. IOP Con . Se . Ma e . Sci. Eng. 2017,213, 012028. [C ossRe ]
25.
Talebi oo i, R.; Za as and, M.; Da ishgoha i, H. Mul i-objec i e op imiza ion app oach on di use sound ansmission h ough
po oelas ic composi e sandwich s uc u e. J. Sandwich S uc . Ma e . 2019,23, 1221–1252. [C ossRe ]
26. Cao, L.; Fu, Q.; Si, Y.; Ding, B.; Yu, J. Po ous ma e ials o sound abso p ion. Compos. Commun. 2018,10, 25–35. [C ossRe ]
27.
Chen, W.; Chen, T.; Wang, X.; Wu, J.; Li, S. F ac al Model o Acous ic Abso bing o Po ous Fib ous Me al Ma e ials. Shock Vib.
2016,2016, 2890857. [C ossRe ]
28.
Guo, Z.K.; Hu, G.B.; So okin, V.; Yang, Y.; Tang, L.H. Sound ansmission h ough sandwich pla e wi h hou glass la ice uss co e.
J. Sandwich S uc . Ma e . 2020,23, 1902–1928. [C ossRe ]
29.
Benne , T.D.; Coude , F.-X.; James, S.L.; Coope , A.I. The changing s a e o po ous ma e ials. Na . Ma e .
2021
,20, 1179–1187.
[C ossRe ]
30.
Vo onin, S.V.; Danilushkin, V.S.; T egub, V.I.; Kono alo , S.V. Compu e Simula ion o he P ocess o C ack P opaga ion in a
B i le Po ous Ma e ial. J. Su . In es ig. 2021,15, 1212–1216. [C ossRe ]
31.
Chia i, A.; Miglie a, P.; Guglielmone, C.L. Conside a ions on he ai low esis i i y measu emen o po ous and ib ous ma e ials
as unc ion o empe a u e. In P oceedings o he 8 h Eu opean Con e ence on Noise Con ol, Eu onoise, UK, 26–28 Oc obe 2009;
pp. 1–11.
32.
Hu ell, A.I.; Ho oshenko , K.V.; Peleg inis, M.T. The accu acy o some models o he ai low esis i i y o nonwo en ma e ials.
Appl. Acous . 2018,130, 230–237. [C ossRe ]
33.
Naghieh, S.; Ka amooz Ra a i, M.R.; Bad ossamay, M.; Fo oozmeh , E.; Kadkhodaei, M. Nume ical in es iga ion o he mechanical
p ope ies o he addi i e manu ac u ed bone sca olds ab ica ed by FDM: The e ec o laye pene a ion and pos -hea ing. J.
Mech. Beha . Biomed. Ma e . 2016,59, 241–250. [C ossRe ] [PubMed]
34.
Pan azopoulos, G.A. A Sho Re iew on F ac u e Mechanisms o Mechanical Componen s Ope a ed unde Indus ial P ocess
Condi ions: F ac og aphic Analysis and Selec ed P e en ion S a egies. Me als 2019,9, 148. [C ossRe ]
35.
Seddeq, H.S. Fac o s in luencing acous ic pe o mance o sound abso p i e ma e ials. Aus al. J. Basic Appl. Sci.
2009
,3,
4610–4617.
36.
Łukaszewski, K.; Buchwald, T.; Wichnia ek, R. The FDM Technique in P ocesses o P o o yping Spa e Pa s o Se icing and
Repai ing Ag icul u al Machines: A Gene al Ou line. In . J. Appl. Mech. Eng. 2021,26, 145–155. [C ossRe ]
37.
Chawla, K.; Singh, R.; Singh, J. On ecyclabili y o he moplas ic ABS polyme as used ilamen o FDM echnique o addi i e
manu ac u ing. Wo ld J. Eng. 2021. [C ossRe ]
38.
Co edo -Bedoya, A.C.; Acuna, B.; Se pa, A.L.; Mesie o, B. E ec o he exci a ion signal ype on he abso p ion coe icien
measu emen using he impedance ube. Appl. Acous . 2021,171, 107659. [C ossRe ]
39.
Fediuk, R.; Am an, M.; Va in, N.; Vasile , Y.; Leso ik, V.; Ozbakkaloglu, T. Acous ic P ope ies o Inno a i e Conc e es: A Re iew.
Ma e ials 2021,14, 398. [C ossRe ]
40.
Zainulabidin, M.; Rani, M.; Neze e, N.; Tobi, A. Op imum sound abso p ion by ma e ials ac ion combina ion. In . J. Mech.
Mecha on. Eng. 2014,14, 118–121.
41. Paul, P.; Mish a, R.; Behe a, B.K. Acous ic beha iou o ex ile s uc u es. Tex . P og. 2021,53, 1–64. [C ossRe ]
42.
Koizumi, T.; Tsujiuchi, N.; Adachi, A. The de elopmen o sound abso bing ma e ials using na u al bamboo ibe s. High Pe o m.
S uc . Ma e . 2002,4, 157–166.
43.
Bujo eanu, C.; Nede , F.; Benchea, M.; Agop, M. Expe imen al and heo e ical conside a ions on sound abso p ion pe o mance
o was e ma e ials including he e ec o backing pla es. Appl. Acous . 2017,119, 88–93. [C ossRe ]
44.
Wu, J.J.; Li, C.G.; Wang, D.B.; Gui, M.C. Damping and sound abso p ion p ope ies o pa icle ein o ced Al ma ix composi e
oams. Compos. Sci. Technol. 2003,63, 569–574. [C ossRe ]
45.
Mendes, C.O.B.; Nunes, M.A.D. Nume ical Me hodology o Ob ain he Sound Abso p ion o Ma e ials by Inse ing he Acous ic
Impedance. A ch. Acous . 2021,46, 649–656. [C ossRe ]
46.
In e na ional O ganiza ion o S anda diza ion ISO 10534; Acous ics-De e mina ion o Sound Abso p ion in a Re e be a ion Room.
IOS: Gene a, Swi ze land, 2003.
47.
Sc osa i, C.; Ma ello a, F.; Pompoli, F.; Schia i, A.; P a o, A.; D’O azio, D.; Ga ai, M.; G anzo o, N.; Di Bella, A.; Scamoni, F.
Towa ds mo e eliable measu emen s o sound abso p ion coe icien in e e be a ion ooms: An In e -Labo a o y Tes . Appl.
Acous . 2020,165, 107398. [C ossRe ]
48.
Sh epi, L.; P a o, A. Towa ds a sus ainable app oach o sound abso p ion assessmen o building ma e ials: Valida ion o
small-scale e e be a ion oom measu emen s. Appl. Acous . 2020,165, 107304. [C ossRe ]
49.
Sun, Y.; Hua, B. Sys em e o calcula ion and analysis o unde wa e sound abso p ion coe icien measu emen expe imen . Appl.
Acous . 2022,186, 108489. [C ossRe ]
50.
Am an, M.; Fediuk, R.; Mu ali, G.; Va in, N.; Al-Fakih, A. Sound-Abso bing Acous ic Conc e es: A Re iew. Sus ainabili y
2021
,
13, 10712. [C ossRe ]

Polyme s 2022,14, 413 17 o 17
51.
In e na ional O ganiza ion o S anda diza ion ISO 10534-1; Acous ics—De e mina ion o Sound Abso p ion Coe icien and
Impedance in Impedance Tubes—Pa 1: Me hod Using S anding Wa e Ra io. IOS: Gene a, Swi ze land, 1998.
52.
In e na ional O ganiza ion o S anda diza ion ISO 10534-2; Acous ics-De e mina ion o Sound Abso p ion Coe icien and
Impedance in Impedance Tubes-Pa 2: T ans e -Func ion Me hod; ISO/TC 43/SC2 Building Acous ics. CEN, Eu opean
Commi ee o S anda diza ion: B ussels, Belgium, 1998; 10534–10542.
53.
Dou es, O.; Salissou, Y.; A alla, N.; Panne on, R. E alua ion o he acous ic and non-acous ic p ope ies o sound abso bing
ma e ials using a h ee-mic ophone impedance ube. Appl. Acous . 2010,71, 506–509. [C ossRe ]
54.
Song, W.J.; Cha, D.J. De e mina ion o an acous ic e lec ion coe icien a he inle o a model gas u bine combus o o powe
gene a ion. IOP Con . Se . Ma e . Sci. Eng. 2017,164, 012010. [C ossRe ]
55.
Taban, E.; Kha anin, A.; Ja a i, A.J.; Fa idan, M.; Tab izi, A.K. Expe imen al and ma hema ical su ey o sound abso p ion
pe o mance o da e palm ibe s. Heliyon 2019,5, e01977. [C ossRe ]
56.
Jiang, B.; Wang, Z.J.; Zhao, N.Q. E ec o po e size and ela i e densi y on he mechanical p ope ies o open cell aluminum
oams. Sc . Ma e . 2007,56, 169–172. [C ossRe ]
57.
Zhu, W.B.; Chen, S.M.; Wang, Y.B.; Zhu, T.T.; Jiang, Y. Sound Abso p ion Beha io o Polyu e hane Foam Composi es wi h
Di e en E hylene P opylene Diene Monome Pa icles. A ch. Acous . 2018,43, 403–411. [C ossRe ]
58.
Tiuc, A.E.; Vasile, O.; Usca, A.D.; Gabo , T.; Ve mesan, H. The Analysis o Fac o s Tha In luence he Sound Abso p ion Coe icien
o Po ous Ma e ials. Rom. J. Acous . Vib. 2014,11, 105–108.
59.
E e es , F.A. Abso p ion o sound. In Mas e Handbook o Acous ics, 4 h ed.; McG aw-Hill: New Yo k, NY, USA, 2001; pp. 179–233,
ISBN 0-07-139974-7.
60.
Zhu, J.L.; Sun, J.; Tang, J.P.; Wang, J.Z.; Ao, Q.B.; Bao, T.F.; Song, W.D. G adien -s uc u al op imiza ion o me al ibe po ous
ma e ials o sound abso p ion. Powde . Technol. 2016,301, 1235–1241. [C ossRe ]
61.
Liguo i, C.; Ruggie o, A.; Russo, D.; Sommella, P. Es ima ion o he minimum measu emen ime in e al in acous ic noise. Appl.
Acous . 2017,127, 126–132. [C ossRe ]
62.
Liu, X.W.; Yu, C.L.; Xin, F.X. G adually pe o a ed po ous ma e ials backed wi h Helmhol z esonan ca i y o b oadband
low- equency sound abso p ion. Compos. S uc . 2021,263, 113647. [C ossRe ]
63.
Madu elo-Sanz, R.; Acedo-Fuen es, P.; Ga cia-Cobos, F.J.; Sánchez-Delgado, F.J.; Mo a-López, M.I.; Meneses-Rod íguez, J.M.
The ecycling o su gical ace masks as sound po ous abso be s: P elimina y e alua ion. Sci. To al En i on.
2021
,786, 147461.
[C ossRe ]
64.
Guan, Y.-J.; Ge, Y.; Sun, H.-X.; Yuan, S.-Q.; Liu, X.-J. Low-F equency, Open, Sound-Insula ion Ba ie by Two Opposi ely O ien ed
Helmhol z Resona o s. Mic omachines 2021,12, 1544. [C ossRe ]