Intonation and compensation of fretted string instruments Gabriele U. Varieschi and Christina M. Gower Citation: American Journal of Physics 78, 47 (2010); doi: 10.1119/1.3226563 View online: http://dx.doi.org/10.1119/1.3226563 View Table of Contents: http://scitation.aip.org/content/aapt/journal/ajp/78/1?ver=pdfcov Published by the American Association of Physics Teachers Articles you may be interested in Modification of the loop filter design for a plucked string instrument J. Acoust. Soc. Am. 131, EL126 (2012); 10.1121/1.3675805 Prestress effects on the eigenfrequencies of the soundboards: Experimental results on a simplified string instrument J. Acoust. Soc. Am. 131, 872 (2012); 10.1121/1.3651232 Acoustical classification of woods for string instruments J. Acoust. Soc. Am. 122, 568 (2007); 10.1121/1.2743162 Audibility of the timbral effects of inharmonicity in stringed instrument tones ARLO 2, 79 (2001); 10.1121/1.1374756 Generation of partials due to nonlinear mixing in a stringed instrument J. Acoust. Soc. Am. 105, 536 (1999); 10.1121/1.424589 This article is copyrighted as indicated in the article. Reuse of AAPT content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to IP: 86.148.26.146 On: Sun, 27 Apr 2014 23:04:19
Transcript
Intonation and compensation of fretted string instrumentsGabriele U. Varieschi and Christina M. Gower
Citation: American Journal of Physics 78, 47 (2010); doi: 10.1119/1.3226563 View online: http://dx.doi.org/10.1119/1.3226563 View Table of Contents: http://scitation.aip.org/content/aapt/journal/ajp/78/1?ver=pdfcov Published by the American Association of Physics Teachers Articles you may be interested in Modification of the loop filter design for a plucked string instrument J. Acoust. Soc. Am. 131, EL126 (2012); 10.1121/1.3675805 Prestress effects on the eigenfrequencies of the soundboards: Experimental results on a simplified stringinstrument J. Acoust. Soc. Am. 131, 872 (2012); 10.1121/1.3651232 Acoustical classification of woods for string instruments J. Acoust. Soc. Am. 122, 568 (2007); 10.1121/1.2743162 Audibility of the timbral effects of inharmonicity in stringed instrument tones ARLO 2, 79 (2001); 10.1121/1.1374756 Generation of partials due to nonlinear mixing in a stringed instrument J. Acoust. Soc. Am. 105, 536 (1999); 10.1121/1.424589
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The physics of musical instruments is an interesting sub-field of acoustics and connects the theoretical models of vi-brations and waves to the world of art and musicalperformance.1–4 In the sixth century B.C., the mathematicianand philosopher Pythagoras was fascinated by music and bythe intervals between musical tones. He was probably thefirst to perform experimental studies of the pitches of musi-cal instruments and relate them to ratios of integer numbers.
This connection between sound pitch and numbers is theorigin of the diatonic scale, which dominated much of West-ern music, and also of the “just intonation system” based onperfect ratios of whole numbers, which was used for manycenturies to tune musical instruments. Eventually, this sys-tem was abandoned in favor of a more refined method forintonation and tuning, the equal temperament system, whichwas introduced by scholars such as Vincenzo Galilei �Gali-leo’s father�, Marin Mersenne, and Simon Stevin in the 16thand 17th centuries, and strongly advocated by musicianssuch as J. S. Bach. In the equal-tempered scale, the intervalof one octave is divided into 12 equal subintervals �semi-tones�, achieving a more uniform intonation of musical in-struments, especially when using all the 24 major and minorkeys, as in Bach’s the “Well Tempered Clavier.” Historicaldiscussion and reviews of the different intonation systemscan be found in Refs. 5–7.
The 12-tone equal temperament system requires the use ofirrational numbers because the ratio of the frequencies of twoadjacent notes corresponds to
12�2. On a fretted string instru-ment such as a guitar, lute, or mandolin, this intonation sys-tem is accomplished by placing the frets along the finger-board according to these ratios. However, even with the mostaccurate fret placement, perfect instrument tuning is neverachieved mainly because of the mechanical action of theplayer’s fingers, which need to press the strings down on thefingerboard while playing, thus altering the string length andtension and changing the frequency of the sound being pro-duced. Other causes of imperfect intonation include the in-harmonicity of the strings due to their intrinsic stiffness andother more subtle effects. A discussion of these effects can be
found in Refs. 8 and 9.
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Experienced luthiers and guitar manufacturers usually cor-rect for these effects by introducing compensation, that is,they slightly increase the string length to compensate for theincreased sound frequency, resulting from the effects wehave mentioned �see instrument building techniques in Refs.10–14�. Other solutions have been given15–19 and in commer-cially patented devices.20–22 These empirical solutions can beimproved by studying the problem more systematically bymodeling the string deformation, leading to a new type offret placement that is more effective.
Some theoretical studies of the problem have appeared inspecialized journals for luthiers and guitar builders,23,24 butthey are targeted to luthiers and manufacturers of a specificinstrument �typically the classical guitar�. In general physicsjournals we have found only basic studies on guitar intona-tion and strings25–33 and no detailed analysis of the intona-tion.
Our objective is to review and improve the existing mod-els of compensation for fretted string instruments and to per-form experimental measures to test these models. The ex-perimental activities described in this paper were performedusing standard laboratory equipment �sonometers and otherbasic acoustic devices�. These experimental activities can beintroduced into standard laboratory courses on sound andwaves as an interesting variation of experiments usually per-formed with classic sonometers.
II. GEOMETRICAL MODEL OF A FRETTEDSTRING
We introduce here a geometrical model of a guitar finger-board, review the practical laws for fret placement, and studythe deformations of a “fretted” string, that is, when the stringis pressed onto the fingerboard by the mechanical action ofthe fingers.
We start our analysis by recalling Mersenne’s law, whichdescribes the frequency � of sound produced by a vibrating
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�n =n
2L�T
�, �1�
where n=1 refers to the fundamental frequency andn=2,3 , . . . to the overtones. L is the string length, T is thetension, � is the linear mass density of the string �mass perunit length�, and v=�T /� is the wave velocity.
In the equal-tempered musical scale an octave is dividedinto 12 semitones,
�i = �02i/12 � �0�1.05946�i, �2�
where �0 and �i are respectively the frequencies of the firstnote in the octave and of the ith note �i=1,2 , . . . ,12�. Fori=12 we obtain a frequency, which is double that of the firstnote, as expected. Because Eq. �1� states that the fundamen-tal frequency of the vibrating string is inversely proportionalto the string length L, we combine Eqs. �1� and �2� to deter-mine the string lengths for the different notes�i=1,2 ,3 , . . .� as a function of L0 �the open string length,producing the first note of the octave considered�, assumingthat the tension T and the mass density � are kept constant,
Li = L02−i/12 � L0�0.943874�i. �3�
Equation �3� can be used to determine the fret placement ona guitar or a similar instrument because the frets subdividethe string length into the required sublengths.
In Fig. 1 we show a picture of a classical guitar as areference. The string length is the distance between thesaddle34 and the nut, and the frets are placed on the finger-board at appropriate distances. We use the coordinate X, asillustrated in Fig. 1, to denote the position of the frets, mea-sured from the saddle toward the nut position. X0 denotes theposition of the nut �the “zero” fret�, and Xi, i=1,2 , . . ., arethe positions of the frets of the instrument. On a classicalguitar there are usually 19–20 frets on the fingerboard. Theyare realized by inserting thin pieces of a special metal wire inthe fingerboard so that the frets will rise about 1.0–1.5 mmabove the fingerboard.
The positioning of the frets follows Eq. �3�, which werewrite in terms of X,
Xi = X02−i/12 � X0�0.943874�i � X0�17
18�i
, �4�
where the last approximation in Eq. �4� is the one employedby luthiers to locate the fret positions. Equation �4� is usuallycalled the “rule of 18,” which requires placing the first fret ata distance from the nut corresponding to 1/18 of the string
XX0X1X2X3X12X19.0 ...... X5X7
.
fret positions
fingerboardbridge
saddle nut
Classical Guitar
Fig. 1. Illustration of a classical guitar showing the coordinate system, fromthe saddle toward the nut, used to measure the fret positions on the finger-board �guitar by Michael Peters; photo by Trilogy Guitars, reproduced withpermission�.
length �or 17/18 from the saddle�; second fret is placed at a
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distance from the first fret corresponding to 1/18 of the re-maining length between the first fret and the saddle, and soon. Because 17 /18=0.9444440.943874, this empiricalmethod is usually accurate enough for practical fretplacement,35 although modern luthiers use fret placementtemplates based on the decimal expression in Eq. �4�.
Figure 2 illustrates the geometrical model of a frettedstring, that is, when a player’s finger or other device pressesthe string to the fingerboard until the string rests on the de-sired ith fret, thus producing the ith note when the string isplucked. In Fig. 2 we use a notation similar to the one de-veloped in Refs. 23 and 24, but we will introduce a differentdeformation model.
Figure 2�a� shows the geometrical variables for a guitarstring. The distance X0 between the saddle and the nut iscalled the scale length of the guitar �typically between 640and 660 mm for a modern classical guitar�. The distance X0is not exactly the same as the real string length L0 becausethe saddle and the nut usually have slightly different heightsabove the fingerboard surface. The connection between L0and X0 is
L0 = �X02 + c2. �5�
The metal frets rise above the fingerboard by the distancea as shown in Fig. 2. The heights of the nut and saddle abovethe top of the frets are labeled in Fig. 2 as b and c, respec-tively. These heights are greatly exaggerated; they are usu-ally small compared to the string length. The standard fretpositions are again denoted by Xi, and in particular, we showthe case where the string is pressed between frets i and i−1, thus reducing the vibrating portion of the string to thepart between the saddle and the ith fret.
Figure 2�b� shows the details of the deformation caused bythe action of a finger between two frets. Previous work23,24
modeled this shape as “knife-edge” deformation, which isnot quite comparable to the action of a fingertip. We im-proved on this assumed shape by using a more rounded de-formation and considered a curved shape as in Fig. 2�b�. Theaction of the finger depresses the string behind the ith fret byan amount hi below the fret level �not necessarily corre-sponding to the full height a� and at a distance f i, comparedto the distance di between consecutive frets.
It is necessary for our compensation model to calculate
fingerboard
fretsi i-1
L0
Li
a)
b)
saddlenutc
b
fingerboardXi Xi-1
a
a
hidi
fi gi
li1
li2 li3
li4
X
X0
Fig. 2. Geometrical deformation model of a guitar string. �a� Original string�of length L0� and the deformed string �of length Li� when it is pressedbetween frets i and i−1. �b� The deformation model in terms of the fourdifferent sublengths li1− li4 of the deformed string.
exactly the length of the deformed string for any fret value i.
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As shown in Fig. 2, the deformed length Li of the entirestring is the sum of the lengths of the four different parts,
Li = li1 + li2 + li3 + li4, �6�
where the four sublengths can be evaluated from the geo-metrical parameters as follows:
li1 = ��X02−i/12�2 + �b + c�2, �7�
li2 = hi�1 +f i
2
4hi2 +
f i2
4hiln2hi
f i�1 + �1 + f i
2/4hi2�� , �8�
li3 = hi�1 +gi
2
4hi2 +
gi2
4hiln2hi
gi�1 + �1 + gi
2/4hi2�� , �9�
li4 = �X02�1 − 2−�i−1�/12�2 + b2. �10�
In Eqs. �8� and �9� the sublengths li2 and li3 were obtainedby using a simple parabolic shape for the rounded deforma-tion shown in Fig. 2�b� due to the action of the player’sfingertip. They were calculated by integrating the length ofthe two parabolic arcs shown in Fig. 2�b� in terms of thedistances f i, gi, and hi.
The distances between consecutive frets are calculated as
di = f i + gi = Xi−1 − Xi = X02−i/12�21/12 − 1� , �11�
so that given the values of X0, a, b, c, hi, and f i, we cancalculate for any fret number i, the values of all the otherquantities, and the deformed length Li. We will see in Sec. IIIthat the fundamental geometrical quantities of the compen-sation model are defined as
Qi =Li − L0
L0, �12�
and they can also be calculated for any fret i using Eqs.�5�–�11�.
III. COMPENSATION MODEL
In this section we will describe the model used to com-pensate for the string deformation and for the inharmonicityof a vibrating string, basing our analysis on the work done byByers.16,24
The strings used in musical instruments are not perfectlyelastic but possess a certain amount of stiffness or inharmo-nicity, which affects the frequency of the sound produced.Equation �1� needs to be modified to include this property,yielding the result �see Ref. 36, Chap. 4, Sec. 16�
�n �n
2L� T
�S1 +
2
L�ESk2
T+ �4 +
n2�2
2�ESk2
TL2 � ,
�13�
where we have rewritten the linear mass density of the stringas �=�S �� is the string density and S the cross section area�.The correction terms inside the square brackets are due to thestring stiffness and related to the modulus of elasticity �orYoung’s modulus� E and to the radius of gyration k �equal tothe string radius divided by two for a simple unwound steelor nylon string�. Following Ref. 36, we will use cgs units inthe rest of the paper and in all calculations, except whenquoting some geometrical parameters for which it will be
more convenient to use millimeters.
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Equation �13� is valid for ESk2 /TL2�1 /n2�2, a conditionthat is usually satisfied in practical situations.37 When thestiffness factor ESk2 /TL2 is negligible, Eq. �13� reduces toEq. �1�. When this factor increases and becomes important,the allowed frequencies also increase, and the overtones�n=2,3 , . . .� increase in frequency more rapidly than the fun-damental tone �n=1�. The sound produced is no longer har-monic because the overtone frequencies are no longer simplemultiples of the fundamental one, as seen from Eq. �13�. Inaddition, the deformation of the fretted string will alter thestring length L and, as a consequence of this effect, will alsochange the tension T and the area S in Eq. �13�. These are themain causes of the intonation problem being studied. Addi-tional causes that we do not address in this paper are theimperfections of the strings �nonuniform cross section ordensity�, the motion of the end supports �especially thesaddle and the bridge� transmitting the vibrations to the restof the instrument, which also changes the string length, andthe effects of friction.
Following Byers24 we define �n= �4+n2�2 /2� and �
=�ESk2 /T so that we can simplify Eq. �13�,
�n �n
2L� T
�S1 + 2
�
L+ �n
�2
L2� . �14�
We consider just the fundamental tone �n=1� as being thefrequency of the sound perceived by the human ear,38
�1 �1
2L� T
�S1 + 2
�
L+ �
�2
L2� , �15�
where �=�1= �4+�2 /2�. In Eq. �15� L represents the vibrat-ing length of the string, which in our case is the length li1when the string is pressed onto the ith fret. To further com-plicate the problem, the quantities T, S, and � in Eq. �15�depend on the actual total length of the string Li, as calcu-lated in Eq. �6�. In other words, we tune the open string oforiginal length L0 at the appropriate tension T, but when thestring is fretted, its length is changed from L0 to Li, thusslightly altering the tension, the cross section, and �, whichis a function of the previous two quantities. This dependenceis the origin of the lack of intonation, common to all frettedinstruments, which calls for a compensation mechanism.
The proposed solution24 to the intonation problem is toadjust the fret positions to correct for the frequency changesdescribed in Eq. �15�. The vibrating lengths li1 are recalcu-lated as li1� = li1+�li1, where �li1 represents a small adjust-ment in the placement of the frets, so that the fundamentalfrequency from Eq. �15� matches the ideal frequency of Eq.�2� and the fretted note will be in tune.
The ideal frequency �i of the ith note can be expressed bycombining Eqs. �2� and �15�,
�i = �02i/12 �1
2L0� T�L0�
�S�L0�
1 + 2��L0�
L0+ �
���L0��2
L02 �2i/12, �16�
where all the quantities on the right-hand side are related tothe open string length L0, because �0 is the frequency of the
open string note. We can write the �i using Eq. �15� as
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�i �1
2li1�� T�Li�
�S�Li�1 + 2
��Li�li1�
+ ����Li��2
li1�2 � , �17�
where we have used the adjusted vibrating length li1� for thefretted note and all the other quantities on the right-hand sideof Eq. �17� depend on the fretted string length Li. By com-paring Eqs. �16� and �17� we obtain the master equation for
our compensation model,
dinates. Introducing the shift in the nut position �N as Xnut�
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1
2L0� T�L0�
�S�L0�1 + 2��L0�
L0+ �
���L0��2
L02 �2i/12
=1
2li1�� T�Li�
�S�Li�1 + 2
��Li�li1�
+ ����Li��2
li1�2 � . �18�
We obtained an approximate solution39 of Eq. �18� by ex-panding the right-hand side in terms of �li1 and by solving
the resulting expression for the new vibrating lengths li1� ,
li1� � li1�1 +1 +
2��L0�li1
+����L0��2
li12 � −
1
�1 + Qi�1 + R��1 +2��L0�
L0+
����L0��2
L02 �
1 +4��L0�
li1+
3����L0��2
li12 � . �19�
In Eq. �19� the quantities Qi are derived from Eq. �12� and from the new deformation model described in Sec. II. An additionalexperimental quantity R is introduced in Eq. �19� and defined as �see Ref. 24 for details�
R = d�
dL�
L0
L0
�0, �20�
and is the frequency change d� relative to the original frequency �0 induced by an infinitesimal string length change dL relativeto the original string length L0.
The new vibrating lengths li1� from Eq. �19� correspond to new fret positions Xi� because Xi�=�li1�2− �b+c�2� li1� for �b+c�
li1� . A similar relation holds between Xi and li1 �see Fig. 2� so that the same Eq. �19� can be used to determine the new fretpositions from the old ones:
Xi� � Xi�1 +1 +
2��L0�li1
+����L0��2
li12 � −
1
�1 + Qi�1 + R��1 +2��L0�
L0+
����L0��2
L02 �
1 +4��L0�
li1+
3����L0��2
li12 � . �21�
At this point a luthier would position the frets on the fin-gerboard according to Eq. �21�, which is not in the canonicalform of Eq. �4�. Moreover, each string would get slightlydifferent fret positions because the physical properties suchas tension and cross section are different for the variousstrings of a musical instrument. Therefore, this compensationsolution would be very difficult to be implemented practi-cally and would also affect the playability of theinstrument.40
An appropriate compromise, also introduced by Byers,24 isto fit the new fret positions �Xi��i=1,2,. . . to a canonical fretposition equation �similar to Eq. �4�� of the form
Xi� = X0�2−i/12 + �S , �22�
where X0� is a new scale length for the string and �S is the“saddle setback,” that is, the distance by which the saddleposition should be shifted from its original position �usually�S�0 and the saddle is moved away from the nut�. The nutposition is also shifted, but we require keeping the stringscale at the original value X0. Therefore we need Xnut� +�S=X0, where Xnut� is the new nut position in the primed coor-
=X0�+�N and combining Eqs. �21� and �22�, we obtain thedefinition of the “nut adjustment” �N as
�N = X0 − �X0� + �S� . �23�
This quantity is typically negative, indicating that the nut hasto be moved slightly forward toward the saddle.
Finally, instead of adopting a new scale length X0�, theluthier might want to keep the same original scale length X0and keep the fret positions according to Eq. �4�. Because thecorrections and the effects we have described are all linearwith respect to the scale length chosen, it is sufficient torescale the nut and saddle adjustment as follows:
�Sresc =X0
X0��S , �24�
�Nresc =X0
X0��N . �25�
This final rescaling is also needed on a guitar or otherfretted instrument because the compensation procedure wehave described has to be done independently on each string
of the instrument. That is, all the quantities in the equations
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of this section should be rewritten adding a string indexj=1,2, . . . ,6 for the six guitar strings. Each string would get aparticular saddle and nut correction, but once these correc-tions are all rescaled according to Eqs. �24� and �25�, theluthier can still set the frets according to Eq. �4�. The saddleand nut will be shaped in a way to incorporate all the saddle-nut compensation adjustments for each string of the instru-ment �see Refs. 16 and 24 for practical illustrations of thesetechniques�.
In practice, this compensation procedure does not changethe original fret placement and the scale length of the guitarbut requires very precise nut and saddle adjustments for eachof the strings of the instrument using Eqs. �24� and �25�. Thisprocedure is a convenient approximation of the full compen-sation procedure, which would require repositioning all fretsaccording to Eq. �21�, but this solution would not be verypractical.
IV. EXPERIMENTAL MEASUREMENTS
Because all our measurements were done using a mono-chord apparatus, we worked with a single string and not a setof six strings, as in a real guitar. Therefore, we will use allthe equations without adding the additional string index j.However, it would be easy to modify our discussion to ex-tend the deformation-compensation model to a multistringapparatus.
In Fig. 3 we show the experimental setup we used for ourmeasurements. Because our goal was to test the physics in-volved in the intonation problem and not to build musicalinstruments or improve their construction techniques, weused standard laboratory equipment.
A standard PASCO sonometer WA-9613 �Ref. 41� wasused as the main apparatus. This device includes a set ofsteel strings of known linear density and diameter and twoadjustable bridges, which can be used to simulate the nut and
saddle of a guitar. The string tension can be measured by
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using the sonometer tensioning lever or adjusted directlywith the string tensioning screw �on the left of the sonometer,as seen in Fig. 3�. In particular, this adjustment allowed thedirect measurement for each string of the R parameter in Eq.�20� by slightly stretching the string and measuring the cor-responding frequency change.
On top of the sonometer we placed a piece of a classicalguitar fingerboard with scale length X0=645 mm. The geo-metrical parameters in Fig. 2 were a=1.3 mm �fret thick-ness�, b=1.5 mm, and c=0.0 mm �because we used twoidentical sonometer bridges as nut and saddle�. This arrange-ment ensured that the metal strings produced a good qualitysound, without “buzzing” or undesired noise when thesonometer was played like a guitar by gently plucking thestring. Also, because we set c=0, the open string length isequal to the scale length: L0=X0=645 mm.
The mechanical action of the player’s finger pressing onthe string was produced by using a spring loaded device�also shown in Fig. 3, pressing between the sixth and seventhfret� with a rounded end to obtain the deformation modelillustrated in Fig. 2�b�. Although we tried different possibleways of pressing on the strings, for the measurements de-scribed in this section, we always pressed halfway betweenthe frets �f i=gi=di /2� and all the way down on the finger-board �hi=a=1.3 mm�. In this way, all the geometrical pa-rameters of Fig. 2 were defined and the fundamental quanti-ties Qi of Eq. �12� could be determined.
The sound produced by the plucked string �which waseasily audible due to the resonant body of the sonometer�was analyzed with different devices to accurately measure itsfrequency. At first we used the sonometer detector coil or amicrophone connected to a digital oscilloscope or to a com-puter through a digital signal interface, as shown also in Fig.
Fig. 3. Experimental apparatus com-posed of a standard sonometer towhich we added a classical guitar fin-gerboard, visible as a thin black objectwith 20 metallic frets glued to awooden board to raise it almost to thelevel of the string. Also shown is amechanical device used to press thestring on the fingerboard and severaldifferent instruments used to measuresound frequencies. This digital tuner isshown near the center, just behind thesonometer.
3. All these devices could measure frequencies accurately,
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but we used a professional digital tuner,42 which could dis-criminate frequencies at the level of �0.1 cents43 for most ofour measurements.
V. STRING PROPERTIES AND EXPERIMENTALRESULTS
For our experimental tests we chose three of the six steelguitar strings included with the PASCO sonometer. Theirphysical characteristics and the compensation parameters aredescribed in Table I.
The open string notes and related frequencies were chosenso that the sound produced using all the 20 frets of our fin-gerboard would span from two to three octaves, and the ten-sions were set accordingly. We used a value for Young’smodulus typical of steel strings, and we measured the R pa-rameter in Eq. �20�. The rescaled saddle setback �Sresc and
Table I. Summary of the physical characteristics anused in our experimental tests.
Open string noteOpen string frequency �Hz�Radius �cm�Linear density � �g/cm�Tension �dyne� 5Young’s modulus E �dyne /cm2� 2RRescaled saddle setback �Sresc �cm�Rescaled nut adjustment �Nresc �cm�
Table II. Frequencies of the different notes obtained with string 1. Theoreticaand without compensation. Also shown are the frequency deviations �in cen
Frequency, Frequency,Fret No. Note perfect intonation no compensation
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the rescaled nut adjustment �Nresc from Eqs. �24� and �25�were calculated for each string using the procedure outlinedin Sec. III.
We then carefully measured the frequency of the soundsproduced by pressing each string onto the twenty frets of thefingerboard in two modes: Without any compensation, thatis, setting the frets according to Eq. �4�, and with compensa-tion, that is, after shifting the position of saddle and nut bythe amounts specified in Table I and retuning the open stringto the original note.
Table II illustrates the frequency values for string 1, ob-tained in the two modes and compared to the theoreticalvalues of the same notes for a “perfect intonation” of theinstrument. The measurements were repeated several timesand the quantities in Table II represent average values. Fretnumber zero represents the open string being plucked, sothere is no difference in frequency for the three cases. For allthe other frets, the frequencies without compensation are
compensation parameters for the three steel strings
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considerably higher than the theoretical values for a perfectlyintonated instrument, which results in the pitch of these notesperceived as being higher �or sharper� than the correctpitch.44 When we played our monochord sonometer in thefirst mode, it sounded out of tune. The frequency values ob-tained by using our compensation correction sounded muchcloser to the theoretical values, thus effectively improvingthe overall intonation of our monochord instrument.
In Table II we show the frequency deviation of each notefrom the theoretical value of perfect intonation with andwithout compensation. The frequency shifts are expressed incents43 rather than in hertz because the former unit is a moresuitable measure of how the human ear perceives differentsounds to be in or out of tune. The frequency deviation val-ues illustrate more clearly the effectiveness of the compen-sation procedure: Without compensation the deviation fromperfect intonation ranges between 14.3 and 64.3 cents; withcompensation this range is reduced to between 7.9 and+7.3 cents.
We plot our results for string 1 in terms of the frequencydeviation of each note from the theoretical value of perfectintonation. Figure 4 shows these frequency deviations foreach fret number �corresponding to the different musicalnotes in Table II� without compensation �circles� and withcompensation �triangles�. Error bars come from the standarddeviations of the measured frequency values.
We also show in Fig. 4 the pitch discrimination range �theregion between the dashed lines�, that is, the difference inpitch that an individual can effectively detect when hearingtwo different notes in rapid succession. Notes within thisrange will not be perceived as different in pitch by the ear. Itcan be easily seen in Fig. 4 that all the values without com-pensation are well outside the pitch discrimination range andthus will be perceived as out of tune �in particular as sharpersounds�. In contrast, the values with compensation are withinthe dashed discrimination range of about �10 cents.45 Thecompensation procedure has almost made them equivalent to
Fig. 4. Frequency deviation from perfect intonation level �black dotted line�for notes obtained with string 1. Red circles denote results without compen-sation, while blue triangles denote results with compensation. Also shown�region between green dashed lines� is the approximate pitch discriminationrange for frequencies related to this string.
the perfect intonation values �corresponding to the zero cent
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deviation, perfect intonation level, dotted line in Fig. 4�.Note that fret number zero corresponds to playing the openstring note, which is always perfectly tuned; therefore theexperimental points for this fret do not show any frequencydeviation.
We repeated the same type of measurements for strings 2and 3, which were tuned at higher frequencies as openstrings �respectively, as F3 and C4; see Table I�. In this waywe obtained measured frequencies with and without compen-sation for these two other strings, similar to those presentedin Table II. For brevity, we omit these numerical values, butwe present in Figs. 5 and 6 the frequency deviation plots, aswe did for string 1 in Fig. 4.
Fig. 5. Frequency deviation from perfect intonation level �black dotted line�for notes obtained with string 2. Red circles denote results without compen-sation, while blue triangles denote results with compensation. Also shown�region between green dashed lines� is the approximate pitch discriminationrange for frequencies related to this string.
Fig. 6. Frequency deviation from perfect intonation level �black dotted line�for notes obtained with string 3. Red circles denote results without compen-sation, while blue triangles denote results with compensation. Also shown�region between green dashed lines� is the approximate pitch discrimination
range for frequencies related to this string.
53Gabriele U. Varieschi and Christina M. Gower
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The results in Figs. 5 and 6 are similar to those in Fig. 4:The frequencies without compensation are much higher thanthe perfect intonation level, and the compensation procedureis able to reduce almost all the frequency values to the regionwithin the dashed curves �the pitch discrimination range�.The discrimination ranges in Figs. 5 and 6 were calculatedrespectively as �8.6 and �5.2 cents due to the differentfrequencies produced by these two other strings.
For the three cases we analyzed we conclude that the com-pensation procedure is very effective in improving the into-nation of each of the strings. Although more work on thesubject is needed �in particular we need to test nylon strings,which are more commonly used in classical guitars�, wehave shown that the intonation problem of fretted string in-struments can be analyzed and solved using physical andtheoretical models, which are more reliable than the empiri-cal methods developed by luthiers during the historical de-velopment of these instruments.
ACKNOWLEDGMENTS
This work was supported by a grant from the Frank R.Seaver College of Science and Engineering, Loyola Mary-mount University. The authors would like to acknowledgeuseful discussions with John Silva of Trilogy Guitars andwith luthier Michael Peters, who also helped them with theguitar fingerboard. The authors thank Jeff Cady for his tech-nical support and help with their experimental apparatus. Theauthors are also very thankful to luthier Dr. Gregory Byersfor sharing with them important details of his original studyon the subject and other suggestions. Finally, the authorsgratefully acknowledge the anonymous reviewers for theiruseful comments and suggestions.
a�Electronic mail: [email protected]�Electronic mail: [email protected]. Helmoltz, On the Sensation of Tone �Dover, New York, 1954�.2J. Jeans, Science and Music �Dover, New York, 1968�.3A. H. Benade, Fundamentals of Musical Acoustics �Dover, New York,1990�.
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Tacoma, WA, 2002�.15J. Gilbert and W. Gilbert, “Intonation and fret placement,”
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20R. S. Jones, U.S. Patent No. 3,599,524 �1971�.21P. R. Smith, U.S. Patent No. 4,295,404 �1981�.22H. B. Feiten, U.S. Patent No. 5,814,745 �1998�.23W. Bartolini and P. Bartolini, “Experimental studies of the acoustics of
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34The saddle is the white piece of plastic or other material located near thebridge on which the strings are resting. The strings are usually attached tothe bridge, which is located on the left of the saddle. On other type ofguitars or other fretted instruments, the strings are attached directly to thebridge �without using any saddle�. In this case the string length would bethe distance between the bridge and the nut. Our analysis would not bedifferent in this case: The bridge position would replace the saddle posi-tion.
35Following Eq. �4�, frets number 5, 7, 12, and 19 are particularly importantbecause they �approximately� correspond to vibrating string lengths,which are respectively 3/4, 2/3, 1/2, and 1/3 of the full length, consistentwith the Pythagorean theory of monochords.
36P. M. Morse, Vibration and Sound �American Institute of Physics, NewYork, 1983�.
37The condition is equivalent to n2�TL2 /ESk2�2369, 803, and 5052,where the numerical values are related to the three steel strings wepresent in Table I and calculated for the shortest possible vibrating lengthL�L0 /3�21.5 cm. The approximation in Eq. �13� is valid for ourstrings for at least n�19.
38This statement is also an approximation because the pitch �or perceivedfrequency� is affected by the presence of the overtones. See, for example,the discussion of the psychological characteristics of music in Ref. 4.
39Our solution in Eq. �19� differs from the similar solution obtained in Ref.24, Eq. �17�. We believe that this difference is due to a minor error intheir calculation, which causes only minimal changes in the numericalresults. Therefore, the compensation procedure used by Byers �Ref. 24� inhis guitars is practically very effective in improving the intonation of hisinstruments.
40Nevertheless some luthiers actually construct guitars where the individualfrets under each string are adjustable in position by moving them slightlyalong the fingerboard. Each note of the guitar is then individually fine-tuned to achieve the desired intonation, requiring a very time consumingtuning procedure.
41PASCO, WA-9613 Sonometer Instruction Manual �1988�.42TurboTuner, Model ST-122 True Strobe Tuner, �www.turbo-tuner.com�.43The cent is a logarithmic unit of measure used for musical intervals. The
octave is divided into 12 semitones, each of which is subdivided in 100cents; thus the octave is divided into 1200 cents. Because an octavecorresponds to a frequency ratio of 2:1, 1 cent is precisely equal to aninterval of 21/1200. Given two frequencies a and b of two different notes,the number n of cents between the notes is n=1200 log2�a /b��3986 log10�a /b�. Alternatively, given a note b and the number n ofcents in the interval, the second note a of the interval is a=b2n/1200.
44We note that the frequency of the sound produced is the physical quantity
we measured in our experiments. The pitch is defined as a sensory
54Gabriele U. Varieschi and Christina M. Gower
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characteristic arising out of frequency but also affected by other subjec-tive factors, which depend on the individual. It is beyond the scope of thispaper to consider these subjective factors.
45
This discrimination range was estimated for the frequencies of string 1,
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according to the discussion in Ref. 4, pp. 248–252. This range usuallyvaries from about ��5� to ��10� cents for frequencies between 1000 and2000 Hz to even larger values of ��40�–��50� cents at lower frequencies
between 60 and 120 Hz.
Laboratory Clock. This clock was purchased by the Kenyon College physics department in 1926 as part of thefittings for the new Samuel Mather Science Hall. Attached to the bottom of its meter-long pendulum is a sharp needlethat passes through a mercury bubble once per second, thus completing an electrical circuit. Along with a powersupply, a series of runs of bell wire and numerous telegraph sounders, this was used to provide an audible tick all overthe physics department. At a time when stopwatches were expensive, this provided a standard time base for timingpendulums using beats between the ticks and the motion of the pendulum. The clock has been restored and is now inthe Greenslade Collection, still keeping excellent time. �Photograph and Notes by Thomas B. Greenslade, Jr., KenyonCollege�
55Gabriele U. Varieschi and Christina M. Gower
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