Document [original]

Chapter 5

The Role of Haptic Cues in Musical

Instrument Quality Perception

Charalampos Saitis, Hanna Järveläinen and Claudia Fritz

Abstract We draw from recent research in violin quality evaluation and piano per-

formance to examine whether the vibrotactile sensation felt when playing a musical

instrument can have a perceptual effect on its judged quality from the perspective

of the musician. Because of their respective sound production mechanisms, the vio-

lin and the piano offer unique example cases and diverse scenarios to study tactile

aspects of musical interaction. Both violinists and pianists experience rich haptic

feedback, but the former experience vibrations at more bodily parts than the latter.

We observe that the vibrotactile component of the haptic feedback during playing,

both for the violin and the piano, provides an important part of the integrated sensory

information that the musician experiences when interacting with the instrument. In

particular, the most recent studies illustrate that vibrations felt at the fingertips (left

hand only for the violinist) can lead to an increase in perceived sound loudness and

richness, suggesting the potential for more research in this direction.

5.1 Introduction

Practicing a musical instrument is a rich multisensory experience. As explained in

Chap.2, the instrument and player form a complex system of sensory-motor inter-

actions where the sensory feedback provided by the instrument as a response to a

playing action (bowing, plucking, striking, blowing, pumping, rubbing, fingering) is

C. Saitis (B)

Audio Communication Group, Technische Universität Berlin, Sekretariat E-N 8,

Einsteinufer 17c, 10587 Berlin, Germany

e-mail: [email protected]

H. Järveläinen

ICST—Institute for Computer Music and Sound Technology,

Zürcher Hochschule der Künste, Pfingstweidstrasse 96, 8005 Zurich, Switzerland

e-mail: [email protected]

C. Fritz

Équipe LAM—Lutheries-Acoustique-Musique, Institut Jean le Rond d’Alembert

UMR 7190, Université Pierre et Marie Curie - CNRS, 4 place Jussieu, 75005 Paris, France

e-mail: [email protected]

S. Papetti and C. Saitis (eds.), Musical Haptics, Springer Series on Touch

and Haptic Systems, https://doi.org/10.1007/978-3-319-58316-7_5

74 C. Saitis et al.

shaped not only by listening to the sound produced by that action, but also by feeling

the cutaneous vibrations (vibrotactile sensation) and reactive forces (proprioceptive

sensation) resulting from the same action. In assessing the heard sound in terms of

technical execution and expressive intention—pitch, timing, articulation, dynamics,

timbre—the musician integrates additional haptic cues before the next sound is made

in order to adjust their playing technique. In this sense, the perception and evaluation

of the quality of a musical instrument, as seen from the perspective of the performer,

are a rich multisensory experience as well.

The proprioceptive component of the haptic feedback at a musical instrument

is connected to the behavior of the instrument’s (re)action. An instrument with a

precise and responsive action allows a skilled musician to produce a wide variety of

timbre nuances through fine-grained control of synchrony, dynamics, attack speed,

articulation, and balance in polyphonic texture. Vibrotactile feedback, on the other

hand, consists essentially of the same oscillations that the instrument body radiates

as sound [42,49,69–71] and is perceived simultaneously with the auditory signal,

but differently [4,6,18,25,31,41,45,62,65]. In contrast to hearing, where

maximal sensitivity is in the range of 3000–4000 Hz, vibrotaction is most sensitive

in the vicinity of 250 Hz (see Sect. 4.2), which is within the range of most orchestral

instruments and already at about 1000Hz the sensation of vibrations is lost, whereas

the range of most instruments extends well beyond this frequency. Tactile waveforms

ofvaryingtypeandcomplexitycanbediscriminated[1,8,51,59,72]andcanactivate

areas of the auditory cortex in the absence of sound input [14]. Auditory and tactile

frequency is likely calculated in an integrated fashion during preattentive sensory-

perceptual processing—much earlier in the information processing chain than had

been supposed [13]. An overview of further comparisons between the auditory and

tactile modalities is given in Sect. 12.2. But is the vibrotactile sensation at a musical

instrument perceptually relevant to its judged quality?

In the first part of this chapter, we will review recent research on the perceptual

evaluation of violin quality from the perspective of the musician. Haptic feedback

is particularly relevant in playing an instrument such as the violin where physical

contact with the performer is highly intimate compared to other instruments due to

the violin’s sound making mechanism. The fingers, chin, and shoulder of the violinist

are in immediate contact with the vibrating parts of the instrument, implying a rich

source of haptic feedback, an understanding of which should help to reveal particular

aspects of quality perception. We will initially discuss psycholinguistic evidence of

how violin quality is conceptualized in the mind of the violinist during playing-based

preference tasks and then describe a series of studies on the perception and quality

evaluation effects of vibrotactile feedback at the left hand of the violinist in normal

playing scenarios.

Alongsidetheviolin,wehavechosenthepianoasasecondexamplecase.Here,the

contact between the performer and the instrument is much less intimate compared

to the violin. Traditional piano playing involves touching only the keys (modern

piano repertoire may sometimes require hitting or plucking the strings) and pedals

(mediated by shoes). The nature and origin of piano touch have long been a source

of fundamental disagreement in music performance and perception research: Are the

5 The Role of Haptic Cues in Musical Instrument Quality Perception 75

timbre and loudness of a single note determined solely by the velocity of the hammer,

or can the pianist further control them through the type of touch? In the second part of

thischapter,we willthen review recentliteratureon hapticfeedback whenplaying the

piano,examiningtherelationship betweentouch andtone quality, andmore generally

the importance of vibrotactile feedback to the perceptual evaluation of piano quality

by the performer.

5.2 Violin

The violin as we know it today was developed in the early sixteenth century around

Cremona in Italy and can be seen as the result of applying the tuning of the medieval

rebec (fifths) to the body of the lira da braccio [16]. The transition from baroque to

classical music led to a few further modifications in the second half of the eighteenth

century, such as a longer, narrower fingerboard, and neck. Since then, the basic violin

lutherie has remained largely unchanged, combining visual charm with ergonomics

and a precise acoustical function.

Sound is produced by bowing (or plucking) one or more strings at a location

between the bridge and the edge of the fingerboard. The played string produces oscil-

lations that are not efficiently radiated by the string itself due to its much smaller

diameter than the acoustic wavelength of most audible frequencies [23]. Instead,

the forces exerted from the vibrating string on the bridge cause the violin body to

vibrate and thus radiate sound. The varying patterns in which different harmon-

ics are transformed by the vibrating modes (resonances) of the body thus “color”

the radiated sound. Figure5.1 depicts a typical violin frequency response function

(defined as the input admittance measured at the E-string notch on the bridge). Fur-

thermore, violin body resonances exhibit a slow decay that brings a “ringing” quality

to the sound [37]. At frequencies above about 1 kHz, the motions of the body cre-

ate frequency-dependent directivity formations that add “flashing brilliance” to its

sound [64].

5.2.1 Touch and the Conceptualization of Violin Quality

by Musicians

Attempts to quantify the characteristics of “good” and “bad” violins from vibrational

measurements such as the input admittance (Fig.5.1) and/or listening tests have

largely been inconclusive (see [52] for a review). On the one hand, this may be due

in part to overly broad characterizations of “good” and “bad.” On the other hand,

both approaches end up considering the instrument isolated from the musician and no

haptic information is provided. Woodhouse was among the first to consider that what

distinguishes one violin from another lies not only in its perceived sound quality but

76 C. Saitis et al.

−60

−50

−40

−30

−20

−10

Magnitude (dB)

BHA0 B1+

−1

−0.5

0.5

1.5

Phase (rad)

Frequency (Hz)

Fig. 5.1 Input admittance of a violin obtained by exciting the G-string corner of the bridge with

a miniature force hammer and measuring the velocity at the E-string corner of the bridge with a

laser Doppler vibrometer [52]. The magnitude and phase are shown in the top and bottom plots,

respectively. Some of the so-called signature modes (i.e., strongly radiating and thus crucial to violin

sound) can be observed in the open string region, below about 600 Hz: the Helmholtz-type cavity

mode A0 at around 280Hz and the first strongly radiating corpus bending mode B1+just above

500 Hz. Also, important is the hill-like collection of peaks known as the “BH peak” (bridge and/or

body hill) in the vicinity of 2–2.5 kHz, which allows a solo violin to be heard over an ensemble of

instruments

also in what he termed its playability, as in how the violinist “feels” the instrument

and how easy it is to produce a good sound [68]. To this end, recent research on

violin acoustics and quality has focused attention on the perceptual and cognitive

processes involved when violinists assess violins under normal playing scenarios.

Fritz and colleagues carried out a series of listening tests using virtual vio-

lins, whereby synthesized bridge-force signals were convolved with a digital filter

mimicking the input admittance of the violin [29]. The measured admittance of

a “good-quality” modern violin was first decomposed into its modal components,

the parameters of which were then used to re-synthesize it, allowing for controlled

variations of vibrato and body damping. Results showed that when listening to sin-

gle notes, violinists found it difficult to assess the “liveliness” of the sound, and

often, the word itself was not used in a consistent way across individuals. But when

asked to play on an electric violin, whereby the actual bridge-force signal was passed

through modified re-synthesized admittances in real time, musicians were able to rate

liveliness consistently within and between individuals. This seems to suggest that

liveliness is processed differently in passive listening versus active playing contexts,

where haptic cues from proprioceptive and vibrotactile feedback are present.

In another study, preference judgments made by three violin players during a

listening and a playing test were compared in conjunction with psycholinguistic

analyses of free-format verbal descriptions of musician experience provided by the

three violinists [28]. The authors used a method from cognitive linguistics that relies

on theoretical assumptions about cognitive-semantic categories and how they relate

to natural language [20]. Categories can be thought of as collective representations

and knowledge, to which individual assessments are conveyed by means of a shared

5 The Role of Haptic Cues in Musical Instrument Quality Perception 77

discourse.From whatis beingsaid(content analysis)and howit isbeing said(linguis-

tic analysis), relevant inferences about how people process and conceptualize sen-

sory experiences can be derived (semantic level) and further correlated with physical

parameters (perceptual level). This approach has been applied to other instruments

suchasthepiano [11]andtheguitar [50],providingnovel insightsintohowmusicians

perceive instrumental sound as well as playing characteristics. Fritz and colleagues

found that the overall evaluation of a violin, as reflected in the verbal responses of the

musicians, varied between listening and playing conditions, and the latter invoking

linguistic expressions influenced not only from the produced sound but also by the

physical interaction between the performer and the instrument.

Saitis and colleagues carried out two violin playing perceptual tests based on a

carefullycontrolledprotocol [56,57].Emphasiswasgiventothe designofconditions

that are musically meaningful to the performer (e.g., playing versus listening, com-

paring different instruments like in a violin workshop, using own bow, allowing time

to familiarize with the different violins, developing own strategy). In the first exper-

iment, skilled violinists ranked a set of different violins from least to most preferred.

In the second experiment, another group of players rated a different set of violins

according to specific attributes as well as preference. In both experiments, musicians

were asked to verbally describe their choices through open-ended questions. Anal-

yses of intra-individual consistency and inter-player agreement in the (nonverbal)

preference and attribute judgments showed that while violinists generally agreed on

what particular attributes they look for in an instrument, the perceptual evaluation of

thesameattributesvarieddramaticallyacrossindividuals,thusresultinginlargeinter-

player differences in the preference for violins. A third experiment [58] and studies

by Fritz et al. [26,27] and Wollman et al. [66,67] reached similar conclusions.

To better understand the perceptual and cognitive processes involved when vio-

linists evaluate violins, Saitis and colleagues further analyzed the verbal expressions

collected in their two violin playing tests [53–55], expanding on an earlier work of

Fritz et al. [28]. Based on psycholinguistic inferences, it was argued that violin play-

ers of varying style and expertise share a common framework for conceptualizing

violin quality on the basis of semantic features and psychological effects that inte-

grate perceptual attributes (i.e., perceptual correlates of physical characteristics) of

not only the sound produced but also the vibrotactile and proprioceptive sensations

experienced when playing the instrument (Fig.5.2). The bowed string and vibrating

body system contribute to the perception of sound quality through (a) the amount of

felt vibrations in the left hand, shoulder, and chin (conceptualized as resonance); (b)

through assessing the offset (speed) and amount (ease) of reactive force (conceptu-

alized as response) from the body in the right hand (through the bow) with respect to

the quality and intensity of the heard as well as felt vibrations; and (c) through com-

paring these between different notes across the instrument’s register (conceptualized

as balance across strings).

These psycholinguistic investigations provide empirical evidence that vibrations

from the violin body and the bowed string (via the bow) are used by violinists as

extra-auditory cues that not only help better control the played sound [4], but also

contributeto acrossmodalaudio-tactileassessmentof itsattributes. Theperceptionof

78 C. Saitis et al.

Instrument

Musician

Sound Body

vibrations

Bowed

string

Auditory

information

Haptic

information

Numberof

partials

RICHNESS

(TIMBRE)

Distribution

of energy

TEXTURE

(TIMBRE)

Articulation

& artifacts

CLARITY

(PLAYABILITY)

Reactive

force

RESPONSE

(PLAYABILITY)

Felt

vibrations

RESONANCE

(INTENSITY)

Amountof

energy

PROJECTION

(INTENSITY)

BALANCE

(ALL)

under

the ear at a distance across strings

Fig. 5.2 From body vibrations to semantic categories: a cognitive model describing how the per-

ception of violin quality is elaborated on the basis of both auditory and haptic cues [55]

violin sound quality is thus elaborated both from sensations linked to auditory infor-

mation and from haptic factors associated with proprioceptive and vibrotactile cues.

The cognitive model shown in Fig.5.2 raises interesting questions concerning the

characterization of haptic feedback in violin playing quality tests—what to measure

and how? Can standard vibrational measurements, such as a violin’s bridge admit-

tance (Fig.5.1), capture everything significant about the reactive force and vibration

levels felt by the player? If yes, in what ways can this information be extracted?

5.2.2 Vibrotactile Feedback at the Left Hand

Acoustics and psychophysics literature on the “feel” of a violin has been limited

compared to the ample amount of research on the instrument’s sound. Marshall

suggested that violin neck vibrations felt through the left hand form the basis for the

perception of how a violin feels [43,44]. He argued that the more often the left hand

detects motions at antinodal parts of the neck (which are typically damped when the

musician holds the violin but can be sensed directly on the skin), the more “alive”

the violin will be felt. Askenfelt and Jansson showed that vibrations perpendicular

to the side of the neck, measured on four violins of varying quality during playing

a single note (lowest G, 196Hz), were above or very close to vibration sensation

thresholds measured at the fingertip under passive touch conditions by Verrillo [61].

However, no evidence was found that higher neck vibration intensity would result in

judging a violin as being of better quality [4]. One limitation of that study was that

5 The Role of Haptic Cues in Musical Instrument Quality Perception 79

200 400 600 800 1000

−40

−20

f (Hz)

Displacement [dB re 1 µm/N]

µm

(a) "vibrating"

0.01

0.1

100

Violin 1

Violin 2

Threshold

200 400 600 800 1000

−40

−20

f (Hz)

Displacement [dB re 1 µm/N]

µm

(b) "non vibrating"

0.01

0.1

100

Violin 3

Violin 4

Threshold

Fig. 5.3 Horizontal vibration levels at the side of the necks of violins (first position) perceived as

either a“vibrating” or b“non-vibrating” (solid lines) and vibration sensation threshold at the left

hand of violinists (dashed line). Reproduced from [65] with permission from S. Hirzel Verlag

vibration amplitude was measured for five frequencies only, corresponding to the

first five harmonics of the played note and thus lying below the 1 kHz upper limit of

the human skin sensitivity range. Another potential issue—discussed in Sect. 4.3.4

for the piano—is that Verrillo’s thresholds may not fully reflect actual vibration

detection offsets when the left hand holds the neck of the violin (e.g., differences in

location and size of contact area, pressure exerted from the hand on the neck).

Wollman and colleagues were the first to systematically address the role of haptic

cues from neck vibrations on violin quality perception. Expanding on the work of

Askenfelt and Jansson [4], vibration levels were measured at the violin neck in first

position1across a set of ten instruments, which were characterized by a professional

violinist according to how “vibrating” they were felt to be [65]. Neck vibration fre-

quency response curves of “vibrating” and “non-vibrating” violins, obtained across

the whole range of the instrument through laser vibrometry, were then compared

to absolute vibrotactile thresholds measured on fourteen violinists holding in first

position a real isolated violin neck vibrating at six frequencies between 196 and

800 Hz (the first four were chosen to correspond to the open strings). This setup

helped obtain violin playing-specific thresholds (i.e., measured under active touch

conditions, similar to what was done in Sect. 4.3 for the piano) that are more appro-

priate to compare with vibration levels than those measured by Verrillo [61] and used

by Askenfelt and Jansson. It was observed that while neck vibrations of “vibrating”

violins were well above the detection threshold by an average of 15 dB in the range

200–800 Hz, those of “non-vibrating” violins exhibited a steep attenuation of about

40 dB around 600Hz and stayed below or close to the threshold above that (Fig.5.3).

In another study [66], fifteen professional musicians listened to three violins while

seating on a chair and holding a real isolated violin neck on which they fingered the

performed score. The instruments were being played live by another violinist (non-

participant) in the same room, placed behind a curtain in front of the participants.

1“Position” refers to where the left hand is placed on the string. In the first position, the index

presses the string at the scroll end of the fingerboard, which produces the next note (full tone) up

from the open string (e.g., on the Gstring, first position corresponds to A).

80 C. Saitis et al.

Along with the live sound, vibrations of the played violins were picked up at the

scroll using a small accelerometer and then transmitted through a shaker system to

the isolated neck (Fig.5.4). They were presented either at the same level as in the

played violin, reduced by half, or fully attenuated. This condition was described

by the authors as active listening. Participants were asked to rate the violins on

richness of sound,loudness,responsiveness, and pleasure of playing. It was observed

that violinists judged all three violins as having a less loud but also a less rich

sound whenever the level of vibrations felt on the isolated neck was reduced by

half (Fig.5.5). These results complemented the findings of Yau and colleagues, who

have shown that in a non-musical context, the simultaneous presentation of tactile

distractors can cause an increase in perceived tone loudness [71].

In a third experiment [67], twenty violinists evaluated five violins under three

sensory masking conditions: playing without hearing the produced sound, playing

Fig. 5.4 Experimental setup for transmitting vibrations from the neck of a played violin to an

isolated neck [66]. Reproduced with the permission of the Acoustical Society of America

Fig. 5.5 Mean comparison

ratings of three violins

(V1–3) across several quality

criteria between two

different levels of vibration

(full versus reduced by half).

A positive score indicates a

higher rating when full

vibrations are present than

when reduced by half.

Reproduced from [66]with

the permission of the

Acoustical Society of

America

5 The Role of Haptic Cues in Musical Instrument Quality Perception 81

without feeling the produced vibrations, and playing normally (i.e., neither modal-

ity was masked). Auditory feedback was masked by means of earmuffs and in-ear

monitors playing white noise with a bandwidth of 20–20000 Hz, while passive anti-

vibrationmaterial wasadded tothe chinrest tominimize bone conduction.Vibrations

were primarily masked on the left hand using vibrating rings worn on the thumb,

index, and ring fingers, while vibrations through the chin and shoulder rests were

attenuated as in the auditory masking scenario. In each condition, musicians first

rated each violin on a number of criteria related to perceived sound and playing char-

acteristics and then commented on how relevant those criteria were each time. These

data provided further evidence that the perceptual evaluation of violin attributes such

as liveliness, power, evenness across the strings, or dynamic range relies not only on

sonicinformationbutalsoonvibrotactilecues.Concerningoverallpreferences,itwas

observed that removing auditory feedback was not more disruptive than attenuating

felt vibrations, although its effect tended to depend on the instrument (Fig.5.6).

These studies indicate that the violin neck vibrations felt by violinist through the

left hand can serve as an important cue to the concept of “feel” in violin quality

evaluation, as well as augment the perception of qualities attributed to the sound (in

that case “loud” and “rich”). They also introduce novel methods for characterizing

vibrotactile feedback at the left hand. Another source of haptic cues that potentially

relate to perceived “feel” and sound quality is the vibration of the chin rest. Askenfelt

and Jansson argued that the jaw is less sensitive than the left hand, but it may still be

possible for the violinist to sense these vibrations because of the larger contact area

of the jaw with the chin rest [4]. Similarly to the violin neck, it would be interesting

to investigate whether vibrotactile feedback at the chin contributes to the perception

of a violin’s “feel” and/or sound.

Fig. 5.6 Mean preference

ratings of five violins under

three different playing

conditions (COND): normal

(N), masked auditory

feedback (noA), masked

tactile feedback (noT).

Vertical bars represent the

standard errors of the mean.

Reproduced from [67];

published under the Creative

Commons Attribution (CC

BY) license

VA VB VC VD VE

0.2

0.4

0.6

0.8

Mean preference rating

Violin

COND N

COND noA

COND noT

82 C. Saitis et al.

5.3 Piano

The modern piano, descending from the harpsichord and introduced by Bartolomeo

Cristofori in 1709, evolved into two distinct types, the grand piano and the upright

piano. The latter was developed in the middle of the nineteenth century, and its

action differs somewhat from that of the first due to design constraints, although they

share the same sound production principle [23]: A piano string is set in vibration

when the respective key is depressed, a damper raised, and a felt hammer hits the

string (Fig.5.7). String vibrations are transmitted through the bridge to the sound-

board, from which the sound radiates into the air. Modal structure of the soundboard

and material properties further contribute to the acoustics of the piano. The sound

is characterized by different decay rates between partials [21], a two-part pattern

Fig. 5.7 Illustration of the function of the piano action at successive stages during a keystroke. a

Rest position: The hammer rests via the hammer roller on the repetition lever, a part of the lever

body. The lever body stands on the key, supported by the capstan screw. The weight of the hammer

and lever body holds the playing end of the key in its upper position. The damper is resting on the

string. bAcceleration: When the pianist’s finger depresses the key, the lever body is rotated upward.

The jack, mounted on the lever body, pushes on the roller and accelerates the hammer. The damper

is lifted off the string by the inner end of the key. cLet-off : The tail end of the jack is stopped by the

escapement dolly, and the top of the jack is rotated away from the hammer roller. The free hammer

continues toward the string. The repetition lever is stopped in waiting position by the drop screw. d

Check: The rebounding hammer falls with the hammer roller on the repetition lever in front of the

tripped jack. The hammer is captured at the hammer head by the check at the inner end of the key.

Reprinted from [3] with the permission of the Acoustical Society of America

5 The Role of Haptic Cues in Musical Instrument Quality Perception 83

of time decay (or double decay) due to double and triple unison strings [63], and

inharmonicity in terms of stretching of the partials due to string stiffness [22].

5.3.1 Piano Touch and Tone Quality

There is a long-standing discrepancy between the acoustical basis of how the timbre

of a single piano tone is created and the practical experience of piano performers [3,

5]. When considering only the mechanics of the hammer-string interaction, piano

timbre would be an instrument-specific result of loudness, which in turn depends

on the velocity at which the hammer hits the string, controlled only through key

velocity produced by the finger pressing force of the player. The way of touching

the key would therefore have no influence on the resulting timbre. Skilled pianists,

on the other hand, aim to control timbre and loudness independently through touch

and gestural means involving movements of the entire upper body. A review on

the historical development of various schools on piano technique as well as recent

performance analysis and biomechanical studies on piano touch is presented by

MacRitchie [40].

There is some evidence in favor of the touch effect, although it seems to be weaker

than many pianists believe and mostly caused by other aspects of the sound than the

tonal component. Goebl and colleagues measured the ability of pianists to perceive

differences in piano sound independently of intensity [35]. Half of the participants

were able to correctly distinguish between struck and pressed touch in the presence

of finger-key noises occurring 20–200 ms before the sound. When the noises were

cut from the sound signals, performance dropped to chance level. Pianists were also

able to distinguish piano sounds of equal hammer velocity with either present or

absent key-keybed noises with an average of 82% accuracy [34]. Askenfelt observed

that structure-born transients, dependent on the type of touch and present 20–30 ms

before the first transversal wave on the string arrives at the bridge, may potentially

be connected with the pianist’s touch [2]. More recently, numerical simulations of

the hammer head-shank interaction showed a difference in spectral profile between

legato and staccato sounds in the range of 500–1000 Hz [17]; however, an effect

on perceived timbre was not shown experimentally. Suzuki reported a slight spectral

brightening for G5, in the order of 1.5 dB at the tenth partial, as a result of “hard”

or “soft” touch depending on the degree of stiffness of shoulder, elbow, wrist, and

finger [60]. When listening only, about half of the participants could distinguish an

effect of similar degree after training.

To discover how pianists achieve fine-grained control of their instrument’s sound,

the way they describe and recognize timbre nuances in piano performance has gained

interest. Bernays and Traube quantified a semantic space of five descriptors (dry,

bright, round, velvety, and dark)[10] based on an analysis of free verbalizations

provided by pianists [7] and conducted a series of studies where pianists performed

pieces highlighting each of the five semantic dimensions of piano timbre. Despite

differences between musicians relating to individual playing styles, common timbre

84 C. Saitis et al.

nuance strategies were revealed across different performances [11,12]. The latter

were saliently grouped by the intended timbre on a bidimensional space by means of

principal components analysis. The first component was found to be associated with

dynamics, attack, and soft pedal features, while the second dimension was related

to sustain pedal. Further playing style factors included key depression depth, legato

versus staccato articulation, and balance between hands.

Given the pianist’s common ways of nuance control, the question arises whether

listeners can differentiate and identify the resulting timbres in piano performance.

To this end, Bernays reported a pilot study where listeners both described freely and

identified in a forced choice task the timbre of piano performance excerpts, each

intended to reflect one of the following timbre nuances: bright, dark, distant, full-

bodied, harsh, matte, round, and shimmering [9]. Participants identified the timbre

categories above chance level except for round and matte. Some categories, like

bright and shimmery, were frequently mixed up, probably due to their semantic

proximity.

These studies have revealed that pianists can control timbre independently of

dynamics: The way of touching the keys produces differences in contact noises

(finger-key, key-key bottom, and release sounds) as well as slight spectral effects.

While these may be inaudible to the average listener, they have a stronger and more

importanteffecton theskilledpianistduetosensoryintegrationofthematchingtouch

and sound information [15]. Especially in polyphonic touch, these subtle vibrotactile

cues may enable the player to produce and control a wide range of timbre nuances.

5.3.2 Haptic Cues and Instrument Quality

Some early experiments on multimodal perception of piano quality were conducted

by Galembo and Askenfelt [30], in which pianists evaluated four concert grand

pianos under varying sensory feedback conditions. When freely playing the instru-

ments,professional pianists rankedthemas expectedaccording tothe manufacturers’

reputation. However, musicians failed to identify the pianos in a listening-only con-

dition, nor was the resulting quality ranking equal to the playing-based evaluation.

In a subsequent free playing task, where visual feedback was blocked by means of

blindfolding, the musicians and auditory feedback was blocked through masking

noise, the pianists were actually able to identify the instruments without difficulty.

These experiments offer some evidence that pianos can be identified by their hap-

tic response perhaps even better than by their sound. As an underlying mechanism,

one should expect that different piano actions react differently to different dynamics

and types of touch and that these differences are perceivable and possibly of more

importance than auditory cues to the player.

Askenfelt and Jansson had previously made timing measurements of the various

parts of the piano action and observed differences mainly as a function of dynamics

and regulation of the action (mechanical adjustments to compensate for the effects

of wear) [3]. Goebl et al. [36] studied in detail the temporal behavior of three grand

5 The Role of Haptic Cues in Musical Instrument Quality Perception 85

pianoactions. Touch-relateddifferenceswerefound throughmeasurements offinger-

key, hammer-string, and key-keybed contact times and maximum hammer velocities

throughout the entire dynamic range for several keys. A different key velocity tra-

jectory in struck and pressed sounds was also observed. Struck sounds showed two

acceleration phases of key velocity, while the pressed sounds developed more lin-

early. These differences between struck and pressed touch were observed in all three

pianos that were measured. However, it remains unknown how the behavior of the

piano action may affect the player experience. The authors of the study hypothesize

that since the pianist needs to (unconsciously) estimate the path from touch to tone

onset and intensity for various dynamics and types of touch, a high-quality instru-

ment is one that has a precise and consistent action. In their own informal evaluation

as pianists, the most highly appreciated instrument turned out to have the lowest

compressibility of the parts of action, short free-travel times of the hammer, and late

maxima in the hammer velocity trajectory.

5.3.2.1 Vibrations in the Acoustic Piano

Keane analyzed keyboard vibrations at four upright and four grand pianos by remov-

ing harmonic peaks from the spectrum of the vibration signal and thus splitting it into

tonal and broadband parts [38]. Similar tonal components were observed across the

two piano types, but upright pianos showed a stronger broadband component, which

could explain the generally lower perceived quality of upright versus grand pianos.

In fact, a later study showed that pianists preferred the tone quality and loudness

profile of an upright piano with attenuated broadband vibrations [39].

Fontana and colleagues investigated the effect of key vibrations on acoustic piano

quality using both a grand and an upright Yamaha Disklavier, which can operate

in both an acoustic and silent mode [25]. While playing, pianists received audi-

tory feedback through a piano software synthesizer and tactile feedback through the

Disklavier keyboard. The technical setup is described in more detail in Sect. 4.3.1.

The experimental task involved comparing a non-vibrating to a vibrating piano setup

during free playing according to several quality attributes. In the non-vibrating setup

(A), the Disklavier was operating in silent mode, which prevents the hammers from

hitting the strings and thus from producing vibrations. In the vibrating setup (B),

the Disklavier was operating in acoustic mode, allowing the natural vibration of the

strings to be transmitted to the soundboard as well as to the keys. However, the

acoustically produced sound was blocked by insulating earmuffs placed on top of

the earphones playing back the synthetic piano sound. Pianists rated the following

attributes on a continuous scale ranging from −3 (“A much better than B”) to +3 (“B

much better than A”): dynamic range,loudness,richness,naturalness, and prefer-

ence. All attributes except preference were rated separately in the low (keys below

D3), mid (keys between D3 and A5), and high (keys above A5) range.

For both the grand and the upright piano type, the vibrating setup was pre-

ferred to the non-vibrating condition (Fig.5.8). The mean preference scores were

86 C. Saitis et al.

Fig. 5.8 Results of the piano

quality experiment described

in [25]: Means with errorbars

±SE as given by [46].

Positive values signify

preference of the vibrating

mode. The labels on the

x-axis show short names for

the evaluated quality features

(dyn = dynamic range,lou=

loudness,rich=richness,nat

=naturalness, and pref =

preference)andthe

considered keyboard ranges

(l=low,m=medium,h=

high)

1.05 (n=15, SD=1.48) for upright piano and 0.77 (n=10, SD=1.71) for grand

piano. The distributions of the preference ratings did not differ significantly between

pianos. Interestingly, while the participants generally preferred when vibrations were

present, in the subsequent debriefing only one of them could pinpoint vibration as the

difference between the setups. There was considerable positive correlation between

attribute scales and frequency ranges. Ratings correlated highly between the low

and mid ranges (mean Pearson ρ=0.58) and between the mid and high regions

(ρ=0.43). At a later stage, a vibration detection sensitivity experiment conducted

using the same setup (see Sect. 4.3) showed that piano key vibrations are perceived

roughly up to note A4 (440 Hz). As such, the high range was entirely beyond the sen-

sitivity range. That said, the detection experiment was performed under controlled

timing and single notes or three-note clusters in the high range, while a free playing

task constitutes a more ecological setting (usually involving multifinger interaction).

This may explain the slight effect of vibration on higher frequencies in the latter. For

further analysis, new dependent variables were formed by taking the average over

the low- and mid-frequency ranges. Partial correlation analysis and principal com-

ponents analysis suggested that naturalness and richness of tone were the attributes

most associated with increased preference.

Inter-individual consistency was low in both piano groups, suggesting high dis-

agreement between individuals. Specifically, five participants preferred the non-

vibrating setup. When the negative preference rating was used as a criterion for a

posteriorisegmentation[48], the attitudesof thetwogroups segregatedclearly. While

the negative and positive groups gave rather similar ratings for dynamic range and

loudness, their mean ratings for richness, naturalness, and preference were clearly

5 The Role of Haptic Cues in Musical Instrument Quality Perception 87

Fig. 5.9 Positive and

negative ratings in the piano

quality experiment described

in [25]

different (Fig.5.9). The mean preference ratings were 1.58 (n=20, SD=0.79) and

−1.61 (n=5, SD=1.10) for the positive and negative groups, respectively. Thus,

while 80% of the participants associated dynamic range and loudness with natural-

ness, richness, and preference, the remaining 20% had the opposite opinion.

5.3.2.2 Digital Piano Augmented with Vibrations

A recent study on the effect of the nature of vibration feedback on perceived piano

sound quality suggested that pianists may well be sensitive to the match between

the auditory and the vibrotactile feedback [24]. The experimental setup (described

in detail in Sect. 13.3.2) involved a digital keyboard enhanced both by realistic

and synthetic key vibrations. Realistic vibrations were recorded from a Yamaha

Disklavier grand piano. Synthetic vibration signals were generated using bandpass-

filtered white noise, centered at the pitch and matching the amplitude envelope and

energy of the recorded vibrations. They were interpolated according to key velocity

andreproducedby transducersattachedto thebottomof adigitalpiano. Thereference

setup consisted of auditory feedback only (A). The three test setups consisted of

auditory feedback plus (B) recorded real vibrations, (C) recorded real vibrations

with 9 dB boost, and (D) synthetic vibrations. Each of the test setups was compared

to the reference setup in a free playing task, similar to what described above for

the acoustic piano. Ratings were given on dynamic control, richness, engagement,

naturalness, and overall preference.

On average, participants preferred the vibrating setup in all categories except for

naturalness in condition D (Fig.5.10). The strongest preferences were for dynamic

88 C. Saitis et al.

Fig. 5.10 Results of the digital piano quality experiment described in [24]. Boxplot presenting

median and quartiles for each attribute scale and vibration condition. Positive values indicate pref-

erence for the vibrating setup

control and engagement. Generally, condition C was the most preferred of the vibra-

tion conditions: It scored highest on four of the five scales, although B was consid-

ered the most natural. Interestingly, B scored lowest in all other scales. Similar to

the Disklavier experiment discussed in the previous section, participants could be

classified a posteriori into two groups, where median preference ratings for setup

C were +2.0 and −1.5 for each group, respectively. In the larger group of positive

preference (n=8), nearly all attributes were rated positively versus only one in

the smaller, negative group (n=3). Notably, although auditory feedback remained

unchanged, participants associated higher preference of the vibrating setup to rich-

ness of tone, which, during preparation for the experiment, was explained to them as

a sound-related attribute. This supports the hypothesis that from the perspective of

the musician, the perception of instrument quality emerges though the integration of

both auditory and haptic information.

5.4 Conclusions

The perceptual evaluation of musical instrument quality has traditionally been con-

sidered a unisensory experience in the scientific and industrial world alike, based

exclusively on how the produced tone sounds in terms of pitch, dynamics, articula-

tion, and timbre. To a certain extent, this is naturally expected. After all, the objective

of playing a musical instrument is to make (musical) sounds. But while this holds

true for the non-musician listener, it only tells part of the story from the perspective

of the musician, where aural impression is accompanied by haptic feedback due to

5 The Role of Haptic Cues in Musical Instrument Quality Perception 89

one or more bodily parts of the player physically touching vibrating components of

the instrument. Well-established theories of sensory-motor multimodal interaction

and auditory-tactile multisensory integration in the analytical and empirical study of

music performance assert that haptic cues carry important information concerning

the control of the (re)action of the instrument and thus its sound and that temporal

frequency representations are perceptually linked across audition and touch.

Theviolin and thepiano offeruniqueexamplecases to examinewhether the haptic

interaction between the musician and the instrument can have a perceptual effect on

quality evaluation. Both instruments require a significant amount of sensory-motor

synergy to produce refined and precise sonic events, providing rich haptic feedback

to the performer. At the same time, unlike the piano setup, violinists experience

vibrations at other bodily parts than the hands, which makes it difficult to measure

performance parameters and control vibrotactile feedback in normal playing exper-

imental scenarios. The physical differences in the violin versus piano touch and the

experimentalfreedomsor constraints imposedby them canhelp better understandthe

role of vibrotaction on the playing experience as well as the expressive possibilities

it can afford in varying performance contexts. Particularly in the case of the piano,

the MIDI protocol and the availability of computer-controlled keyboard instruments

such as the Yamaha Disklavier and Bösendorfer CEUS offer fertile opportunities to

obtain detailed piano performance data under well controlled but musically mean-

ingful experimental conditions, although with some limitations [33].

Our review has shown that the vibrotactile component of the haptic feedback

during playing, both for the violin and the piano, provides an important part of

the integrated sensory information that the musician experiences when interacting

with the instrument. In particular, the most recent violin and piano studies provide

evidence that vibrations felt at the fingertips (left hand only for the violinist) can lead

to an increase in perceived sound loudness and richness, suggesting the potential

for more research in this direction. Investigations of the type and role of musical

haptic feedback have also been reported for other instruments (e.g., [19,31,32])

as well as singing [47]. A vast field of topics await investigation, starting from the

methods and aspects of instrument quality evaluation per se [15]. In which aspects

does haptic feedback have a significant role? Which performance parameters (for

example, timing accuracy) can be used to assess the haptic dimension in instrument

quality perception?

Acknowledgements This work was supported by a Humboldt Research Fellowship awarded to

Charalampos Saitis by the Alexander von Humboldt Foundation. Part of the research was pursued

within the Audio-Haptic modalities in Musical Interfaces (AHMI) project funded by the Swiss

National Science Foundation (2014–2016). Hanna Järveläinen wishes to thank Federico Fontana,

Stefano Papetti, and Federico Avanzini for developing the technical setups used in the reported

piano experiments and for helpful feedback about earlier versions of this chapter. Federico Fontana

is also gratefully acknowledged for the original conception of the piano studies.

90 C. Saitis et al.

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