Loudness control in pianists as exemplified in keystroke force
measurements on different touches
Hiroshi Kinoshitaa! and Shinichi Furuya
Graduate School of Medicine, Osaka University, 2-2 Yamadaoka, Suita, Osaka 565-0871, Japan

Tomoko Aoki
Faculty of Environmental and Symbiotic Sciences, Prefectural University of Kumamoto,3-1-100 Kumamoto City, Kumamoto 862-8502, Japan

Eckart Altenmüller
Institute of Music Physiology and Musicians’ Medicine, University of Music and Drama Hanover,
Hohenzollernstrasse 47 D-30161 Hannover, Germany!Received 25 May 2006; revised 1 February 2007; accepted 25 February 2007"
The relationship between the key depression force on an upright piano and the level of loudness ofa generated tone was examined when pianists hit a force-sensor built-in key with “struck” or“pressed” type of touch. The vertical displacement of the key, and the radiated piano sounds werealso recorded. It was found that for both types of touch, simple exponential functions could
adequately describe the relation of the force amplitude with the level of the piano tone as well as thatof the impulse of the force with the piano tone. The impulse of the force generated before themaximum key depression moment commonly amounted to above 80% of the total impulse produced
at the tone below mezzo-forte. It, however, decreased to around 60% at fortissimo, indicating adecrease in the efficiency of the force application for sound production. The two types of touch
differed in their force profiles. The struck touch was characterized by a steeper initial force increase
with greater fluctuations in the subsequent period than the pressed touch. The struck touch also
demonstrated lower maximum force and less impulse at fortissimo. The inter-pianist variation in theforce and impulse, and the “finger-noise” are also herein examined. © 2007 Acoustical Society ofAmerica. #DOI: 10.1121/1.2717493$
PACS number!s": 43.75.Mn, 43.75.St, 43.75.Yy #DD$

I. INTRODUCTION

Playing the piano undoubtedly represents one of themost complex sensori-motor skills that humans may acquire,requiring years of training. Striking the key to generate therequired level of loudness is a fundamental and indispensable component of a pianist’s skill. The keystroke commonly
starts with a downswing of the arm toward the key, and ends
with the application of regulated depression force by the fingertip on the front side of the key. The key itself moves in a“see-saw”-type of motion via a balance-rail bearing as theplayer presses the key. The applied downward force thus acts
on a complex hammer-driving structure resting on the otherside of the key via an assembly called a “whippen jack.”
Shortly before the hammer contacts the string, the transmission of force from the key to the hammer is interrupted andthe hammer is left swinging freely against the string, allowing an effective transfer of kinetic energy from the hammer
to the string for sound production !Fletcher and Rossing,1998".
An understanding of the force or impulse acting on the
key by the finger is important because it is the sole source ofkinetic energy to move the hammer. Hence, it can be a major
variable for pianists to modulate the volume or pressure ofa"Author to whom correspondence should be addressed. Electronic mail:hkinoshita@moted.hss.osaka-u.ac.jp

J. Acoust. Soc. Am. 121 "5!, May 2007

Pages: 2959–2969

the generated sound. When considering the nature of soundin addition, the manner in which pianists apply force to thekey must also be included as a variable. Two different fundamental techniques of key touch may be of interest in thisrespect. One technique is the so-called “struck touch,” where
the key is struck by a moving finger, and another is the“pressed touch,” when the key is pressed by a finger restingon its surface !Goebl et al., 2005". The force generated between the finger and the key !“finger force”" can also be
regarded as a source of stress on the pianist’s hand as it may
reach a relatively large magnitude immediately after the moment of key-front rail contact !Harding et al., 1989". Repetitive application of such stress is considered to be one of the
major causes of musculoskeletal problems in keyboard players !Amadio and Russotti, 1990; Caldron et al., 1986; Fry,1991".
Some efforts have been made to study the force appliedby the finger on the key !finger force" in the past. Using athin force sensor foil inserted into the space between the key
and front rail in the key bed, Parlitz et al. !1998" measured
the depressing force of the finger against the front rail while
performing so-called “tied-finger exercises,” i.e., some fingers were depressing piano keys and holding them down for
the entire exercise, while the remaining fingers executed the
key stroke. Groups of expert and amateur players performed
three tied-finger exercises with increasing degrees of difficulty while the loudness level was kept constant at around a0001-4966/2007/121"5!/2959/11/$23.00© 2007 Acoustical Society of America

2959forte !f" level. They found that the amateurs used greatertied-finger forces as well as keystroke forces, and a longer
force application period by the striking fingers comparedwith the experts on playing the same notes. They concluded
that years of piano training allow for pianists with an independent coordination of playing and nonplaying fingers after
touch and a sense for the piano’s response.
Using a force transducer mounted on the key surface,
Harding et al. !1989" directly measured finger force at amoderate loudness level during staccato and legato strikes
by subjects with different skill levels. The finger force after
the moment when the key reaches its maximum displacement !“bottom out”" cannot contribute to sound production.
Therefore, they computed the value of impulse !the area under the force-time curve" generated prior to the bottom-out
moment. Since they did not measure key displacement, they
assumed that the bottom-out moment corresponded to theonset moment of the highest peak force. The findings indicated that this impulse value did not differ between the staccato and legato strikes when producing the same level ofloudness. The impulse values had a nonlinear relationshipwith key velocity. The force profiles shown in their study
also suggested that the impulse generated from the key touch
to the bottom-out moment was smaller than the impulse generated after the bottom-out moment.
Using a similar force measurement system, Askenfelt
and Jansson !1992" measured the finger force at various
loudness levels. The number of pianists as well as the force
sensor type used was not mentioned in their article, and thedescription of the force characteristics was limited. Nevertheless, they provided important information about the magnitude of maximum finger force in relation to sound loudness. They stated that at piano !p", mezzo forte !mf", andfortissimo !f f" levels with staccato touch, the peak force
reached around 8, 15, and 50 N, respectively. In legato
touch, the finger forces were considerably lower during acceleration, typically one third or even less at the softest levels.
It is apparent from these reports that little work has been
done to investigate the nature of force imparted by the finger
on the piano key. Especially, detailed finger-force information based on a larger number of pianists is lacking. It is also
necessary to have a key in which a force sensor is built inand the sensor surface is flush with the key surface so thatany psychological factor influencing the control of keystroke
movement is minimized. In the present study, therefore, using a force transducer built-in key, we made a more complete
analysis of the relationship between the finger force and levelof sound pressure !SPL" using ten expert pianists who performed keystrokes with struck and pressed touches.
II. METHODS
A. Subjects

Eight female and two male active classical pianists!age= 21.6± 1.7 yrs.,
height= 163.6± 8.8 cm,
weight= 52.1± 10.1 kg" who showed no serious playing-related
physical problems in the past served as the subjects in thepresent study. Each pianist had at least 15 years of training
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FIG. 1. Experimental setup.

with experience of winning a prize!s" at high-level domestic!Japan" and/or international piano competitions. Informed
consent was obtained from each subject prior to the experiment, and the study was approved by the Ethics Committee
for Human Research at Osaka University.

B. Finger force, key displacement and sound dataacquisition methods

The finger force was measured using a strain-gauge-typeminiature uniaxial force transducer fixed to the distal end of
a C4 key of a Yamaha upright U1-model piano !see the“force transducer” in Fig. 1". The transducer was designed
and built for the purpose of this study based on our previous
work !Kinoshita et al., 1995". Therefore, the measurement
range of 0 – 100 N within a 0.5% error in linearity was chosen. The resolution of the transducer was 0.02 N. The natural
frequency of the unloaded force transducer was DC-1 kHz.
The force signal was amplified using a Kyowa strain gauge
amplifier and stored on a SONY personal computer via a
12 bit analog-to-digital !A/D" converter sampling at a frequency of 900 Hz. The vertical movement of the key was
also recorded using a Hamamatsu photosonic light-spot twodimensional position measurement system interfaced with apersonal computer. The light-spot LED was placed 1.5 cm
above the key surface and 8 cm from the front edge of the
key !see the “LED for position sensor” in Fig. 1". The key
displacement data recorded were smoothed at a cut-off frequency of 24 Hz using a second-order Butterworth digital
filter. Vertical velocity and acceleration were then calculatedusing a numerical differentiation method. Three adjacent
keys !B4, C4, and D4" were removed from the piano to assure the free space for the transducer wires.
During the experiment, radiated piano sounds were synchronically sampled using a RION sound-level meter placed
20 cm above the key !see the “sound-level meter” in Fig. 1",
which was then amplified using an audio amplifier to feed
into the personal computer via an A/D converter sampling
frequency at 900 Hz.
The experimental room was an ordinary temperaturecontrollable room. The background noise level was around
50 dB. This relatively louder background noise level was due
to numerous machines in the experimental room.
Kinoshita et al.: Sound pressure and keystroke force

C. Key-striking task

The key-striking task was a series of slow- and selfpaced !freq.!approx. 0.3 Hz" repetitive keystrokes with very
short duration. The length of each tone generated was instructed to be a 16th note without holding each tone, andthus in a presto tempo. For each subject, data were collectedfrom a series of 60 strokes at varied SPLs for each of the two
prototypical types of touch: depressing the keys with thefinger initially resting on the key surface !pressed" and hitting the key from a certain distance above !struck" !Askenfelt
and Jansson, 1991; Goebl et al., 2004; 2005". To obtain SPLs
distributed approximately evenly over the whole range between the maximum and minimum, the following instruction
was given to each subject prior to the series for each touch:“During the initial 15 strokes, please increase the level ofsound generated gradually and voluntarily from the minimum to the maximum level, and during the following 15
strokes, please decrease the generated sound gradually to theminimum level. Please repeat the same process during thefollowing 30 strokes. Please count the number of strokes
silently for each of the 15 strokes.” At the end of the experiment with each subject, the collected and stored data werechecked to have a total of 120 keystrokes !60 struck and 60
pressed".
As an initial hand position common to the struck andpressed touches, each subject was instructed to place his/herright middle finger by lightly !"0.03 N" touching its tip on
the C4 key while the tips of the thumb and little finger werealso lightly touching the A4 and E4 keys, respectively. The
index and ring fingers were kept in the air because the B4
and D4 keys were not present. The left arm and hand were
relaxed and kept on the left thigh during the experiment.
For the struck touch, from the initial hand position, theright arm was lifted to a self-determined distance above thekey surface at self-determined natural speed. From this lifted
position, the arm was dropped and the C4 key was struck by
the right middle finger to generate piano sound. After this,the whole arm was immediately lifted again as a follow
through to a self-determined height. The hand was then returned to the starting position at the self-determined speed toprepare for the next keystroke.
For the pressed touch, there was no arm lift prior to a
keystroke. The C4 key was therefore pressed by the right
middle finger to generate sound from the initial hand position
without any preparatory arm lift. After the keystroke, however, the whole arm was immediately lifted as a follow
through to a self-determined height similar to the case of thestruck touch. After this, the hand was lowered to the starting
position to prepare for the next keystroke.
Prior to the experiment, each subject practiced these experimental tasks until they felt comfortable performing them.
D. Parameters evaluated

For the computation of sound pressure, the peak amplitude of the absolute value of the sampled sound data as a
voltage was used !Palmer and Brown, 1991". The voltage
value !V" was then converted to a SPL representation indB using the following equation: SPL= L0 + 10 log10
J. Acoust. Soc. Am., Vol. 121, No. 5, May 2007

FIG. 2. Representative sound, finger-force, key-displacement, key-velocity,and key-acceleration curves during striking the key with a struck touch.
Some of the parameters evaluated are also shown.

!V2 / !V0 / % 2"2", where L0 and V0 indicate the dB value andthe corresponding voltage value of fundamental sound, respectively.
Nine variables describing the finger force and kinematics of the key were computed from each trial data set foreach subject, and used for the evaluation in the present study!Fig. 2". These were: !1" the finger-force application time as
defined by the duration between the onset !#0.06 N" andtermination !"0.06 N" of finger-force application for the
keystroke, !2" initial peak force, !3" maximum force, !4" average force during the finger-force application time, !5" total
impulse as defined by an integration of the force during thefinger-force application time, !6" impulse over the period between the onset of finger force for the keystroke and maximum displacement of the key, !7" impulse over the period
from the maximum key displacement to the termination of
the finger force, !8" maximum displacement of the key, and!9" maximum descending acceleration of the key. The maximum key descending velocity was also computed, but it was
not used for evaluation in the present study.
III. RESULTS
A. Finger force profiles

Figure 3 shows typical examples of time history curves
for the sound signal, finger force, and key displacement fromone subject with the struck !the upper panel" and pressed !thelower panel" types of key touch at three different SPLs !95,
103, and 111 dBs". These SPLs corresponded to typical pp,
mf, and ff levels with the present piano according to thesubjective judgment of the present subjects.
The force profiles for the struck touch !see also Fig. 2"
were characterized by a rapid rise in force immediately after
Kinoshita et al.: Sound pressure and keystroke force

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FIG. 3. Representative profiles of finger force, key displacement, and sound from selected trials !pp, mf, and ff" of one subject with struck !upper panel" andpressed !lower panel" types of key touch. The dotted line indicates the moment of maximum key displacement. The touch noise is indicated by a dashed circle
in the sound signal.

the finger contacted the key, which produced an initial peakat 3 – 5 ms after contact. The force then rapidly dropped toshow a distinct trough after another 3 – 5 ms from the peak.
The force was then developed again, and formed multiple
peaks until the end of the finger-force application time. The
sound signals indicated that at all SPLs, the piano sound was
generated after the appearance of the maximum finger force.
The key displacement curves demonstrated that the key
started to move soon after the finger force was developed,
and it reached the lowest position toward the end of thefinger-force application time.
The force during a pressed touch developed more slowly
than that during a struck touch !lower panel in Fig. 3". The
initial peak of the force was less clear with the pressed touch,though a small step-like initial force increase was noted insome cases, especially when generating louder sound !seethe force curve at ff". The subsequent force with the pressed
touch was also less variable than that with the struck touch.
Similar to the struck touch, the piano sound was generated
after the appearance of the maximum finger force, and also
the moment of the maximum key displacement occurred near
the end of the finger-force application time.
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B. Maximum force, total impulse, and finger-force
application time in relation to SPL

Figure 4 shows typical examples of the maximum force,
average force, total impulse, and finger-force application
time as a function of SPL generated by one subject during
the struck !upper panel" and pressed !lower panel" types ofkey touch. For both touch types, the forces and impulses
increased curvilinearly with an increased SPL, while thefinger-force application time decreased linearly with SPL. An
attempt was made to fit simple mathematical functions to
describe the curvilinear relationship of the force and impulse
parameters of each subject. It was found that an exponentialfunction !F = A exp!B * SPL" + C, where F!finger force, A,
B, and C are constants" was sufficient to describe the relationship rather precisely compared with six other functions
tested. With this curve fitting, the proportions of variance!r2" computed for the maximum force for each subject
ranged from 0.81 to 0.98 !mean± SD= 0.91± 0.05", and thosefor the total impulse were from 0.83 to 0.96 !mean± SD= 0.88± 0.04". The corresponding values for the pressed
touch were from 0.87 to 0.96 !mean± SD= 0.93± 0.03" forthe maximum force, and from 0.75 to 0.96 !mean± SD= 0.86± 0.09" for the total impulse. The finger-force applicaKinoshita et al.: Sound pressure and keystroke force

FIG. 4. Changes in the maximum force, average force, total impulse, and finger-force application time with SPL of one subject with the struck touch !upper
panel", and the pressed touch !lower panel". The line in the figure indicates a fit curve for the data. The data are from the same subject in Fig. 3.

tion time decreased linearly with SPL in all subjects for the
struck touch !mean r2 ± SD= 0.79± 0.12" and the pressed
touch !mean± SD= 0.68± 0.09".
A simple exponential function also fitted relatively wellwith the maximum force and total impulse data pooled
across all subjects !Fig. 5". The r2 values computed for the
maximum force were 0.78 for the struck touch and 0.81 forthe pressed touch. The r2 values computed for the total impulse data were 0.66 for both types of key touch.
C. Impulse before and after maximum key
displacement

Since the piano sound is generated by finger force tomove the key downward, the impulse of the force produced
during the period of downward movement should reflect thephysical effort of the pianists for the production of an intended SPL. Any force applied to the key after the attainmentof maximum key displacement may then be wasted in termsof sound production. Impulses generated both before and after maximum key displacement were computed for each keystroke for all subjects. In Figs. 6!A" and 6!B", the before- andafter-values of the impulse are plotted in relation to SPL for
both types of key touch. With an increase of SPL, the beforeand after-values were increased in a curvilinear fashion.
J. Acoust. Soc. Am., Vol. 121, No. 5, May 2007

The efficiency of finger force action was assessed bycomputing a proportion of the before value of the impulse to
the total impulse produced. The computed value decreased
from nearly 100% at lower SPLs to around 65% at higher
SPLs with means and SDs of 82.5± 9.0% for the struck touchand 80.4± 12.2% for the pressed touch !Figs. 6!C" and 6!D"".
D. Comparison between the struck and pressed
touch modes

Finger force was compared between the two types ofkey touch at different SPLs. For this purpose, the SPL data
between 93 and 113 dBs were arbitrarily sorted into five categories roughly corresponding to musical dynamics indication with each being 3.99 dB wide. Within each of these dB
categories, the mean values for the maximum force, impulse
before the maximum key displacement, and maximum acceleration were computed for each subject, and subsequently for
all subjects !Fig. 7". At each SPL category, a “planned comparison” using one-way analysis of covariance !ANOVA"
with repeated measures was performed for each of theseforce and acceleration variables as a dependent variable andthe touch mode as an independent variable !see, the explanation about the “planned comparison” in Keppel, 1991".
There was a significantly higher maximum force !F1,9
Kinoshita et al.: Sound pressure and keystroke force

2963

FIG. 5. Changes in the maximum force and total impulse with SPL of all
subjects for the struck !upper panel" and pressed !lower panel" types of key
touch. The line fit was made using a simple exponential function.

= 6.79, p # 0.05" and impulse !F1,9 = 7.07, p # 0.05", and asmaller descending acceleration !F1,9 = 18.3, p # 0.001" forthe pressed touch than for the struck touch at the highest SPL
range !109– 113 dB" !Fig. 7". At the lower SPL range!97– 100.99 dB", on the other hand, the struck touch had ahigher maximum force !F1,9 = 10.78, P # 0.01".
E. Inter-subject variation in finger force

In Fig. 8!A", maximum forces are plotted separately for
all ten pianists playing with the pressed touch only. Fit
curves of the force- or impulse-SPL relationship exhibited aclear variation in the absolute value of applied force among
the pianists especially when producing louder sound. To assess inter-subject variation at each of the dynamic categories
as defined above, the coefficient of variation values !CV in%" for the maximum force and impulse were computed by
dividing the SD by the corresponding mean. The CVs ranged
between 27 and 31% across all ranges of SPL without showing any increasing or decreasing trend with SPL.
Why are the finger forces of individual subjects different
when simply striking the same key and producing the samelevel of sound? One explanation for this may be because of
the difference in pianists’ body mass; heavy pianists with alarger upper body mass strike the key with greater force. We
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FIG. 6. Impulse before and after maximum key displacement in relation to
SPL for all subjects with struck !A" and pressed !B" types of key touch, andpercent values of impulse before the maximum key displacement relative tothe total impulse for the struck !C" and pressed !D" types of key touch.

therefore examined the relation between the maximum finger
force and body weight of the subjects for each of the abovedefined five SPL ranges. In Fig. 8!B", the relations for the
93–96.99, 101–104.99, and 109– 113 dB ranges are shown
as examples. Note that the correlation values computed werequite small, and they were all statistically insignificant !seeFig. 8!B"". The r values for the 97–100.99 and
105– 108.99 dB ranges were −0.067 and −0.209, respectively. Plots for these data are not shown in Fig. 8!B" because
they could overcrowd the figure.
Although the entire mass of the body is not a related
factor of the maximum finger force, another concept of mass,
which is the “effective mass,” is yet to be examined since it
reflects a more directly related portion of the body concerning the keystroke. The effective mass can be defined here as“a portion of the pianist body !mostly the upper extremity"that is accelerated to generate the external force,” and thus itis estimated by the measured force divided by its measured
acceleration. A relationship between the maximum finger
force and the ratio between the maximum force and maximum key acceleration as an estimate of the effective mass
was thus examined !Fig. 8!C"". Significantly high correlations were found at all of the SPL ranges !see the r values of
the 93–96.99, 101–104.99, and 109– 113 dB ranges in Fig.
8!C"". The r values for the 97–100.99 and 105– 108.99 dB
ranges were 0.696 !p # 0.05" and 0.920 !p # 0.005", respecKinoshita et al.: Sound pressure and keystroke force

tively. Therefore, from 49 to 87% of the variance for the
maximum finger force may be explained by the effective
mass. We also examined the relationship between the maximum force and maximum key acceleration, which was insignificant at each of the SPL ranges !Fig. 8!D", r = −0.100 at
97– 100.99 dB and −0.128 at 105– 108.99 dB".
F. Finger-touching noise

FIG. 7. A comparison between keystrokes with struck and pressed types ofkey touch for the maximum force !A", impulse before maximum key displacement !B", and acceleration of the key during the descending phase !C"for different dynamic categories !pp : 93– 96.99 dB, p : 97– 100.99 dB,
mf : 101– 104.99 dB, f : 105– 108.99 dB, f f : 109– 113 dB". A planned comparison using one-way repeated measures ANOVA was performed at each of
the SPL ranges. * p # 0.05. ** p # 0.01. Error bars represent +1SDs.

The sound signals indicated that at all SPLs the pianosound was generated after the appearance of the maximum
finger force. On the other hand, there was commonly a small
but detectable sound signal starting immediately after thefinger-key contact moment for the struck touch !see dotted
circles in the sound signals of mf and ff of the struck touch inFig. 3; also Fig. 2", and after a short period of force application for the pressed touch !see “ff” of the pressed touch inFig. 3". This first soft sound has been termed “finger noise”
or “touch precursor” !Goebl et al., 2005", and is caused
mostly by the finger or nail when it pounds or rubs the key
surface. Using criteria of "59.0 dB for the SPL and "5 mmfor the key position, we automatically detected and differentiated this noise from the background noise as well as the
piano sound. For the struck touch, the onset of this signal
coincided temporally with the occurrence of the initial
finger-force peak.
A clear touch-related noise was detected in 560 keystrokes !93%" among the 600 keystrokes performed by ten
subjects for the struck touch. The noise occurred
40.1± 18.2 ms !mean ± SD of the 560 observations" before
the emergence of the piano sound. The noise SPL increased
markedly from 62 to 65 dB at the initial peak force of 2 – 3 N
to 90– 100 dB at the peak force of 30– 45 N !Fig. 9!A"". The
piano SPLs corresponding to these finger noises were
90– 95 dB and 110– 115 dB, respectively. The difference between the SPLs of the finger noise and piano sound were
therefore about 30 dB at pp-p levels and about 15 dB at f-ff.

FIG. 8. Inter-subject variation in the
maximum force-SPL relationshipcurves for all subjects with the pressed
touch !A", and relationships between
the maximum force and body mass
!B", between the maximum force andan estimate of the effective mass
!maximum force/maximum acceleration" !C", and between the maximum
force and peak acceleration of the key!D". The maximum force and maximum acceleration in B, C, and D are
the average values for each subject
computed at pp, mf, and ff of the SPL
ranges defined in Fig. 7. The data werecomputed only for the pressed touch.
Lines indicate the regression lines
computed.

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Kinoshita et al.: Sound pressure and keystroke force

2965

FIG. 9. Finger noise and related piano sound as a function of the first peakof finger force with the struck !A" and pressed !B" types of key touch. The
data plotted are from all subjects. The dotted lines indicate best fit curves
with a logistic function for the struck touch and a linear function for thepressed touch.

For the pressed touch, the finger noise-like signal was
detected in 163 keystrokes !44% of 600 keystrokes for all
subjects" from the automatic analysis of the data. Since thefinger is not pounding the key during the pressed touch, we
consider that it may not be safe to categorize these soundsignals in the same way as the noise detected with the struck
touch; it may be totally key-action-related mechanical noise.
Of 163 keystrokes, 110 occasions had a detectable initial firstpeak of finger force, and therefore we tentatively plotted
those in relation to the initial peak force !Fig. 9!B"". The
noises ranged from 61 to 106 dB, which occurred
23.9± 7.3 ms !mean ± SD" before the emergence of the pianosound. The SPL difference between the piano sound and finger noise was about 25 dB for the pressed touch.
IV. DISCUSSION
A. Finger force at differing SPLs

The maximum finger forces observed in the expert pianists commonly ranged between 3 N at pp and 60 N at ff.
Four of them also exceeded 60 N at ff or above. This force
range was wider than those !8 N at p to 50 N at ff" reported
by Askenfelt and Jansson !1992" due possibly to our slightly
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J. Acoust. Soc. Am., Vol. 121, No. 5, May 2007

uted to differences in the ease of hitting the force sensor
which is hidden under the key surface as in the present study,
in contrast to a positioning on the top of the key in the latterstudy. A higher required level of accuracy in hitting the sensor !especially at ff in the latter setting" therefore could havecaused a reduction in the attacking velocity.
Our results indicate that a simple exponential function
can describe adequately the changes in the parameters offinger force in relation to SPL regardless of the key touch
type. The increase of the maximum finger force with an increase in SPL, and therefore the force/SPL ratio, was quite
small at low SPLs. However, it became very large at high
SPLs. For example, at pp !from 93 to 96.99 dB", the ratio
was around 0.4 N / dB, whereas at ff !from 109 to 113 dB", itbecame 6.5 N / dB for the struck touch and 8 N / dB for thepressed touch, showing a 16- to 20-fold increase. The adjustment of low sound intensities clearly demanded an extremely
high level of force control. Conversely, at the highest SPLs,
this demands the modulation of a large force output against
the key. How are pianists coping with these contrasting demands? Our previous studies of the upper extremity movements of expert pianists who performed keystrokes at various SPLs demonstrated that the production of small SPLs
was accomplished principally by exclusive movements of thefingers while keeping the proximal limbs relatively stable!Furuya et al., 2006". The production of higher SPLs was, on
the other hand, reached by increasing both the range andspeed of joint movements in the proximal limbs. In addition,
there was an increased activation level of the flexor-extensor
muscle pairs of the whole upper limb at the moment of key
contact. These findings suggested that SPL control was madeprincipally by modulating the mass of the body portions involved in key depression, joint stiffness, and the attacking
velocity and acceleration of the whole limb. Thus, the observed small finger force when generating and adjusting a pp
tone could be a consequence of a small mass with a low
stiffness depressing the key at a low velocity and acceleration, while the large force at ff was due to the combined
effect of a large mass, high joint stiffness, and a high attacking velocity and acceleration. These findings are in agreement with suggestions from theoretical papers, e.g., Parncutt
and Troup !2002".
The present study also demonstrated that a large portionof the finger force was generated before the key reached thebottom of its displacement. This is reasonable because thehammer-driving structure commonly starts to move when the
key end descends beyond the halfway point !about 6 mm in
the present experimental piano" of its movement range!10.5 mm". This movement range was about 1 mm larger
than the grand piano reported in a previous study !Askenfelt
and Jansson, 1991". Therefore, in theory, force application
can be completely terminated before the key reaches the bottom of its movement range since any force applied to the key
after the event of the hammer thrust cannot contribute tosound generation. The results of the present study indeed
indicated that the impulse before the moment of maximum
key displacement commonly exceeded 80% of the total impulse generated. This finding disagrees with that of Harding
et al. !1989", who have shown that finger-force profiles in
Kinoshita et al.: Sound pressure and keystroke force

staccato strikes have a larger peak and impulse during theperiod after the maximum key displacement !also the“bottoming-out” in their definition" moment than prior to it.
This discrepancy appears to have resulted from the method
used to determine the moment of maximum key displacement. Harding et al. used force data for this purpose under
the assumption that the maximum finger force occurred at
the moment of maximum key displacement when the key
collided with the felt of the front rail. Our simultaneous recording of finger force and displacement indicates that thisassumption is incorrect. As was shown in Fig. 3, the maximum force commonly appeared earlier than the moment of
maximum key displacement. An extremely early case involves the trials at low SPLs, where the finger-key contact
had been terminated before key displacement reached its
maximum point !see “pp” in Fig. 3 as an example". In thiscase, the maximum displacement was only 6 – 7 mm, andtherefore there was no collision of the key with the felt cushion on the front rail. At higher SPLs, on the other hand, thekey vigorously hit the felt, and the time lag of the maximum
force and maximum key displacement was shortened !see“ff” in Fig. 3". Nevertheless, the maximum key displacement
was always detected after the event of maximum force, possibly due to depression of the felt which occurred after the
event of key-felt collision.
The above findings also indicate that some portion of thepost key-bottom force !#20% of the total impulse" is difficult to avoid, especially when producing a louder sound.
There are several reasons for this. One is that when producing a louder sound, switching of the movement direction
from downward to upward at an exact moment becomes
more difficult due to momentum !larger segmental mass
moving at a faster velocity". Under such conditions, theprobability of a delayed switching of movement becomes
higher. Related to the difficulty of minimizing the postbottom finger force as well as the time of force application ata high SPL, Parlitz et al. !1998" provided evidence that its
impulse was larger, and the time was longer for less skilledplayers than skilled professional players. Second, when producing a louder sound, key depression is commonly madeusing a coupled flexion movement at the wrist and shoulder
joints !Furuya et al., 2006". This maneuver facilitates athrusting forward motion of the whole arm to accomplish amore vigorous key depression. However, it also demands alonger finger-key contact time for longer force application to
the key.
In the present study, the on-off timing of the key contacting with the front rail was not monitored. This was because in many of the keystrokes at pp and p tones and evensome at an mf tone, the key did not hit the felt !bottom" ofthe front rail with the present fast staccato touch. According
to Askenfelt and Jansson !1990", however, the key bottom
contact is an important mechanical event providing sensory
feedback from the instrument, decisive for the player’s ability to perform the desired timing and synchronization of thenotes. Therefore, in a future study, analysis of finger force
should include this temporal information for the better understanding of finger-force control by pianists !Parlitz et al.,1998".
J. Acoust. Soc. Am., Vol. 121, No. 5, May 2007

B. Inter-subject variation in finger force

A relatively large inter-pianist variation was noted in the
maximum force and impulse during key attack, showing thatthe same target SPL could be attained by the application ofdifferent levels of finger force. This may be of interest fromthe view of stress reduction since the repeated application ofexcessive finger force has been considered a risk factor ofover-use injuries in musicians including pianists !Amadio
and Russotti, 1990; Caldron et al., 1986; Fry, 1991; Zaza andFarewell, 1997". The present study also demonstrated thatthe inter-pianist differences in force were related to the mass
involved in key depression !effective mass" but not to anindividual’s body weight. The force was also unrelated to
key acceleration. These findings suggest that the adjustmentof effective mass, and thus the modulation of rigidity of themuscles in the hand and arm, is a key factor for the stressrelated problems induced by finger force. This may have an
important implication in the prevention of over-use injuries.
C. Finger force of struck and pressed touches

The type of key touch clearly differentiated the patternof finger-force development. For the struck touch, the initialforce increase was rapid and formed a distinct key-touchassociated peak, followed by many peaks in the subsequent
phase. For the pressed touch, on the other hand, the initialforce increase was less steep without any initial peak, and it
showed less force fluctuation in the mid-depression phasethan the struck touch. Surprisingly, despite these clear differences in the force profile, we found that the maximum force
and impulse exerted on the key were quite similar between
the two touch types at the same SPL except when the subjects exerted a large force at ff. These findings, therefore,suggest that, at least with staccato articulation, SPL is largely
determined by how forcefully the pianists press or strike thekey, and not by how smoothly they apply force.
Goebl et al. !2005", who recently compared these two
types of keys touch, reported that with the pressed touch,both of the key and hammer velocities developed muchsmoother and their spatio-temporal features had better correspondence than those with the struck touch especially when
producing louder sound. Indeed, the velocity profiles shownin their study clearly described that for the struck touch,there was a period immediately after the finger-key contact
where key velocity was increased markedly, but hammer velocity remained nearly unchanged. This observation led themto suggest that the pressed touch was more efficient than the
struck touch. We, on the other hand, found that at the same
SPL of f f, the pianists exerted a significantly larger finger
force with the pressed touch than the struck touch. Therefore,
contrary to the suggestion by Goebl et al., our data reflecting
the effort of force to move the key suggest that the pressedtouch method is less efficient !possibly demanding greater
muscular effort" than the struck-touch method in loud soundproduction.
The computed descending acceleration of the key wassignificantly less with the pressed touch, possibly due to an
inability to promote a rapid acceleration of the key from zeroacceleration within its limited movement range. The results
Kinoshita et al.: Sound pressure and keystroke force2967

indicated that compensation for this problem occurred byincreasing the mass involved in pressing the key, which was
indeed overcompensated. The reason for this overcompensation was uncertain, but we speculate that the adjustment ofeffective mass, which involved both the number of muscles
and limb stiffness, may be more difficult than the adjustmentof limb acceleration.
D. Finger noise

In agreement with the findings of previous studies!Askenfelt, 1994; Goebl et al., 2004; Koornhof and van derWalt, 1994", in the present study, a short period of finger
noise prior to a large piano sound was exclusively detected in
the vast majority of strokes with the struck touch. In thepressed touch, there was also a finger noise-like single prior
to the piano sound in some trials above mf levels. This
smaller number of observations in the pressed touch is obviously due to the fact that the finger noise is caused mainly by
the fingertip hitting the key. Detection of a similar noise
without hitting the key in the pressed touch indicated that thenoise could also be caused by the fingertip or the nail scraping the key surface when depressing the key at fast speed.
The present study also demonstrated that the amplitudeof the initial peak force appearing within the first 5 ms contact period was related closely to the SPL of touch noise. The
finding that a descending movement of the key did not commonly occur until the end of this short force pulse suggested
that a large portion of kinetic energy resulting from this initial force application had been dissipated as noise rather than
key movement. An additional observation related to touchnoise was that, in all subjects, the SPL exceeded 90 dB at ff,
which happened to be equivalent to the SPL of piano soundat pp. The noise must therefore be at an audible level evenfor listeners at a certain distance from the piano. Goebl et al.
!2004", who studied the perception of touch noise in tone
production, reported that only some trained musicians wereable to distinguish between a struck and a pressed touchusing this noise as a cue. When the listeners were unable tohear the touch differences, they tended to rate louder tones as
being struck and soft tones as being pressed. The authors
therefore speculated that the pure aural effect of touch noiseon piano sound was relatively small. The finding in thisstudy that the SPL difference between piano sound and finger
noise was always greater than 15 dB supports their assumption; although the touch noise is audible, the subsequent piano sound is large enough as well as short enough!#50 ms" to mask its effect, due to the physiological mechanisms of backward masking !Yost, 2000".
E. Limitations of the study

Only one kind of small upright piano was used in thepresent study. In the previous studies of finger-force measurement, on the other hand, a grand piano has been commonly used !Askenfelt, 1994; Askenfelt and Jansson, 1991;
Harding et al., 1989; Parlitz et al., 1998". There are apparent
differences in the mechanics and acoustics between uprightand grand pianos !Fletcher and Rossing, 1991, 1998". The
grand piano has a more complex key action mechanism with
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J. Acoust. Soc. Am., Vol. 121, No. 5, May 2007

a greater number of moving parts !i.e, a larger mass andmore mechanical frictions in general" to move the hammer
vertically !i.e., a greater effect of gravity" compared with the
upright piano having a less complex mechanism with a horizontal hammer motion. These mechanical differences may
allow pianists with a better feel of the hammer weight, amore detailed and accurate regulation of key action, and thus
a finer sound voicing for the grand piano. Pfeiffer !1978"
earlier compared mechanical efficiency of grand and upright
piano actions by dropping weights onto piano keys and determining the resulting kinetic energy imparted to the hammers !see summary tables !pp. 314–315" given in a book by
Fletcher and Rossing, 1991". The upright piano has a slightly
higher mechanical efficiency of this energy transfer than the
grand piano at below an mp tone, which is reversed from an
mf tone on. Therefore, in the upright pianos, there could havebeen less finger force required at very soft dynamics to produce a piano tone while greater finger force at louder dynamics compared with the grand piano.
Sound dampers are also more effective in grand than
upright pianos, providing a difference in the quality of thetone produced. Acoustical advantages of the grand piano due
to its larger size with longer strings and horizontal positioning of the soundboard also provide a greater range of SPL
due to larger resonance and more sound reflection, and moreradiated sound than the upright piano. All of these featurespotentially influence the finger force action on the keys, andthus the force-sound relationship. Some of the differences
between our results and those of previous studies as discussed above, therefore, can be attributed to these uprightand grand piano differences. A comparative study using otherkinds of upright as well as grand pianos is therefore neededin the future.
Another limitation of this study was that we measured
only the vertical component of finger force under the assumption that forces acting in the fore-aft and medial-lateral
directions were small. The validity of this assumption may
need to be examined by the use of a three-dimensional force
sensor under the experimental conditions similar to thepresent study.
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