Subjective preference of classical guitar strokes “apoyando ...

Buenos Aires – 5 to 9 September, 2016 Acoustics for the 21st Century…

PROCEEDINGS of the 22nd International Congress on Acoustics

Music Perception: Paper ICA2016-227

Subjective preference of classical guitar strokes “apoyando” and “tirando” related to its harmonic

components and autocorrelation function

Joaquin Garcia(a), Shin-ichi Sato(b), Florent Masson(c)

(a)Universidad Nacional de Tres de Febrero, Argentina, [email protected] (b)Universidad Nacional de Tres de Febrero, Argentina, [email protected]

(c)Universidad Nacional de Tres de Febrero, Argentina, [email protected]

Abstract

Tone production of classical guitar performance is an essential part for musicians to transmit their sentimental and interpretative intentions. This work investigates subjective preferences of two common plucking techniques used by guitar players, apoyando (rest) and tirando (free) strokes. Six excerpts of classical guitar music with different tempos and range of frequency were performed using the two techniques and were recorded for the subjective tests. Two groups of subjects, guitar players and people who do not play guitar, were investigated to see if both groups evaluate the guitar timbre in different way or not. AB test was conducted with 50 persons for each group asking which technique is preferred and have more sound quality. Then the harmonic components and autocorrelation function (ACF) of each stroke were analysed to relate with the characteristics of the music program (tempo and frequency range) and subjective preferences. The effective duration of ACF is defined with the taue (τe) parameter. Results of the subjective test showed that harmonic content did not define preferences, but higher taue values of the ACF were correlated with a higher sound guitar quality.

Keywords: Plucking technique, Apoyando, Tirando.

22nd International Congress on Acoustics, ICA 2016 Buenos Aires – 5 to 9 September, 2016

Acoustics for the 21st Century…

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Subjective preference of classical guitar strokes “apoyando” and “tirando” related to its harmonic

components and autocorrelation function

1. Introduction

Classical guitar has become one of the most popular instruments of the twentieth century since Andres Segovia introduced it in the stages in 1928. The modern six string guitar is a descendant of the sixteenth century Spanish vihuela [1]. The Spanish luthier Antonio de Torres (1817–1892) contributed much to the development of the modern classical guitar when he enlarged the body and introduced a fan-shaped pattern of braces to the top plate.

Francisco Tarrega (1852–1909), one of the most important guitar players and composers in the 19th century, added a new fingering technique (apoyando) and increased the variety of the guitar expression, feature so important to the composers in the romantic period.

Apoyando stroke (rest) is a technique used by the right hand of guitar performers. The fingertip or nail pushes through the string and, after releasing it, comes to rest on the next one. Another commonly used technique is the tirando stroke (free). The nail rise clear of the next string and yet releasing the string from a point below its original plane. Figure 1 shows the two types of strokes and its motions. The essential difference is in the angle at which the string is released. Apoyando tends to set the string vibrating at a steeper angle, with a stronger component perpendicular to the soundboard, and therefore gives a richer sound than tirando. The latter is generally more suitable than apoyando when playing near the bridge and for harmonics, at least on the treble strings [2].

Source: (Taylor, 1985) Figure 1: (a) Tirando and (b) apoyando stroke [2]

When the string is released at a perpendicular angle from the top plate the sound has strong amplitude and a high value of decay slope. This results in a hefty sound but not very prolonged in time. The case of horizontal release provides not very high amplitude but a more sustained note. Finally the two plucking techniques are a combination of both components with different proportions.

Several researches have been done around tone production on classical guitar. Meyer studied the quality of fifteen different guitars by using a subjective test applied to 40 persons [3]. Using six excerpts of music he evaluated the subjective preferences and found a relationship with the frequency response of the guitar. The third resonance and the third octaves from 80 to 125 Hz, 250 to 400 Hz, and 315 to 500 Hz had the greatest importance on guitar quality, followed by the Q-value of the first resonance and the peak level of the second resonance. Dayan and Behar



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studied the quality of strings for classical guitars [4]. They reported a correlation between acoustical measurements (mainly sound levels and frequencies of the fundamental and overtones) with the subjective quality of “goodness” of guitar strings.

However, there has been few discussion about preference between plucking techniques. Therefore this work aims to combine the objective analysis of the two common strokes apoyando and tirando, with objective parameters as harmonic content and temporal information extracted from the autocorrelation function of each technique. Six excerpts of music performed with the two strokes, were used in a subjective test. Preferences of each technique were evaluated in two groups of subjects, guitar players and people who do not play guitar, in order to see if both groups evaluate the guitar timbre in different way or not. An AB test was conducted to evaluate subjective preference of each plucking technique. Then, spectrum and psychoacoustical parameters as sharpness Z, Tonal dissonance (S), Roughness and ACF parameters were calculated. The effective duration (τe) of the running autocorrelation function (r-ACF) was also calculated [5].

Sharpness is a measure of the high frequency content of a sound, the greater the proportion of high frequencies the “sharper” the sound. Zwicker and Fastl define a sound of sharpness 1 (acum) as “a narrow band noise one critical band wide at a centre frequency of 1kHz having a level of 60dB” [6].

Roughness is defined by the perception of amplitude modulated tones in a broadband. Rapid modulations (between 20 and 250 Hz) result in sounds perceived as rough [7]. For slower modulations, (of about 4 Hz) listeners perceive fluctuations of loudness. The sensation of fluctuating loudness is called fluctuation strength.

Tonal Dissonance (S) implies a combination of notes that sound harsh or unpleasant to people when they are played at the same time. Dissonance measures the harshness or roughness of the acoustic spectrum and many musical pieces involve a balanced combination of consonance and dissonance sounds as a resource, because the releases of harmonic tension create a pleasure sensation [8].

The ACF describes temporal information of signals. It indicates how similar a signal with its own shifted version is. It is recognized as a method for estimating the fundamental frequency of a sound signal, as determined by the delay time of the first major peak τ1. The effective duration of the envelope of the ACF, τe, which is defined by the 10 percentile delay and which represents a repetitive feature contained in the sound source itself [9]. τe is a parameter strongly related with music scores and musical expression. Ando et al. introduced a running ACF (r-ACF) analysis for music signals as a more accuracy method than the long-time ACF calculation [10]. He found that the preferred delay time of a single reflection considering a music motif can be calculated from the effective duration of the long-time ACF and the amplitude of reflection [11]. When music signals contains fluctuation in tempo the preferred delay time is represented more accurately by minimum value of the (τe)min of the r-ACF. Kato et al. studied singing voices related to room acoustics in terms of the (τe)min of the r-ACF [12]. They showed that “vibrato extent” and “singing volume” play an important role in decreasing the (τe)min value.

2. Procedure

2.1. Sound signals



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Six excerpts of guitar music were recorded for the subjective test considering different tempos

and frequency ranges. The purpose is to cover a varied repertoire to evaluate the two plucking

techniques. Table 1 shows the features of the six music excerpts. Music scores are shown in

Figure 2.

The recording was done in a room with a volume of 18 m3 with carpet floor. Porous absorptive

materials were present on the walls and on the roof. An Earthworks M50 microphone was used

with a Focusrite Scarllett audio interface. Also a professional hand-made classical guitar,

Spanish model with red cedar top plate and rosewood back plate and ribs, was used for the

recordings.

Table 1: Description, time and tempo of the six music excerpts used in the test

Music Description Duration

[s] Tempo [bpm]

1 Sarabande with a slow tempo. The melody line in is the upper

part accompanied in chords. 36 40

2 Rapid series of semiquavers with fast tempo and melody line in

the down part. The center of tone is at A3 (220Hz). 35 80

3 Excerpt of Adelita (Tarrega). Wide tonal range and melody more

strongly pronounced. 23 70

4 Monophonic piece in the low and middle register with 2 octaves

of tonal range and andante tempo. 27 55

5 Monophonic piece with fast tempo. Demand a clear separation of

the notes and use similar figures on various strings.

18 150

6 Calm quaver figures to be played exclusively on the highest

string.

10 120

Figure 2: Music scores

The microphone was located pointing the bridge of the guitar from the side and 20 cm away in

order to capture its complete spectrum. Then each recorded signal was normalized and fixed at

the same loudness using Adobe Audition software with LUFS meter plug in based on ITU-R

BS.1770-2 [13].



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2.2. Objective analysis

Psychoacoustical parameters were calculated to evaluate the objective characteristic of each

excerpt with each stroke. Using the GUI Psysound toolbox [14], three parameters were

calculated: Sharpness Z, Tonal Dissonance (S) and Roughness (Table 2). A spectrum analysis

for each music excerpt was then performed. Tirando stroke provides more harmonic content,

mostly above 1000Hz, than apoyando in all cases. Figure 3 shows the spectrum of the music

excerpts.

Tables 2 shows that sharpness values are higher for tirando technique than apoyando. This

result was expected as tirando stroke shows greater harmonic content.

Table 2: Psychoacoustical parameters. Top: Tirando; Bottom: Apoyando

Music Sharpness Z

[acum]

Tonal

Dissonance (S)

Roughess

[aspers]

1 0.68

0.57

29.66 0.01 0.57 24.42 0.01

2 0.71 53.83 0.03 0.64 49.52 0.03

3 0.70 31.69 0.02 0.69 29.88 0.01

4 0.60 19.15 0.03 0.51 15.41 0.02

5 0.88 24.22 0.03 0.68 9.19 0.01

6 1.08 8.69 0.01 0.88 3.44 0.02

Table 3: ACF parameters. Top: Tirando; Bottom: Apoyando.

Music p(0) [dB] 1 1 [ms] e [ms]

1 42.5 0.41 15.3 476.5 42.2 0.51 15.3 418

2 47.2 0.59 9.1 104.6 46.5 0.59 9.18 97.6

3 40.6 0.40 4.6 242.7 41.1 0.34 4.3 26.4

4 39.6 0.34 22.9 24.0 39.6 0.38 4.4 79.7

5 43.2 0.50 27.2 263.8 39.3 0.53 27.2 231.6

6 33.6 0.50 1.9 173.3 35.8 0.71 1.7 129.4

Furthermore, the higher quantity of harmonics generates more interaction with tonal

components. As a result tirando stroke has also more Tonal Dissonance values. In addition

Roughness values do not show significance differences between both plucking techniques.

The ACF parameters for each test signal were also calculated using a GUI script (Table 3). The

integration interval 2T was set at the duration of each signal to obtain a single representative

value. The iterative method proposed by Sato and Wu [5] was used to calculate the τe, because

it provides a higher accuracy for the regression of ACF envelope.



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2.3. Subjective test

An AB test was implemented to evaluate the subjective preference of apoyando and tirando

strokes. Listeners were asked to select the preference of plucking technique for each music

excerpt. The test signal was reproduced by headphones AKG K44 and a Focusrite Scarlett

soundboard. Each music excerpt was randomly presented by the two techniques and the

listener had to choose which was perceived with more sound quality. The experiment was done

in a controlled room to ensure a signal to noise relation higher than 20 dB.

The tests were arranged randomly between subjects. A complete test consisted in six

comparisons and it took about 10 min. Output level of the headphones was fixed at the same

sound pressure level for the listeners, thus all of them were tested at approximately 70 dB. Test

instructions were given orally and in written form. Also, the subjects were asked if they had

musical knowledge and if they play classical guitar or another musical instrument.

3. Results

A total of forty-nine subjects participated in the test, thirty-eight have experience of some

musical instruments and the other eleven were not musicians. Among the thirty-eight, twenty-

five play classical guitar, but not professionally. The ages of the subjects were between 19 and

33 years old. Figure 4 shows the subjective preferences of the two plucking technique.

The result of the test shows a clear preference for tirando stroke in most cases. The exception

is for music 4 where 67% chose apoyando stroke. This excerpt, unlike the others, has the lower

tempo, with long notes and is played in the lowest register of the guitar. Music 5 presents a

preference less pronounced than the others. In addition, Tables 3 shows that the stroke

presenting a higher τe value was preferred in all cases. For music 4 the preference is inverted

and apoyando version has a longer τe value, the other sounds showed tirando stroke presents

the longer τe.

(a)

Figure 3: Spectrum content for apoyando and tirando technique. (a) Music 1



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(b)

(c)

(d)

Figure 3 (continued): Spectrum content for apoyando and tirando technique. (b) Music 2, (c) Music

3 and (d) Music 4



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(e)

(f)

Figure 3 (continued): Spectrum content for apoyando and tirando technique. (e) Music 5 and (f)

Music 6

4. Discussion

Figure 5 shows the relationship between the subjective preferences and τe ratios for each

sound. The ratios were calculated dividing the apoyando and tirando τe value for every music

excerpt. There is a relationship between the subjective preference and the τe of the signals. The

greater is the difference of τe of the signals, the greater is the preference. The τe was calculated

considering an integration time of the signal length in order to provide a τe that represents the

whole music signal. Correlation coefficient between τe ratios and preference was 0.974 with a

significance level of 0.01.



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Figure 4: Subjective preferences of apoyando and tirando technique for the six music excerpts.

The harmonic content might not be strongly correlated with the preference. Although in many

cases the musical pieces with more harmonic with brilliance sound were preferred, whole in

other cases the musical pieces with less harmonic content were preferred. Sharpness Z had a

correlation coefficient r = -0.591 (apoyando, p = 0.217) and r = 0.425 (tirando, p = 0.401). On

the same way, correlation coefficient between Tonal dissonance (S) and preference were r =

0.239 (p = 0.648) and r = 0.257 (p = 0.623).

In addition, to provide better results in future works more than one guitar could be used in order

to reduce the influence of the instrument characteristic. Also more than one guitar player could

be included.

Figure 5: Subjective preference related with the τe ratios of each sound.

Finally it is interesting to mention how the behavior of each plucking technique is. Except music

4, tirando stroke showed the higher τe values. The lower spectrum contents and slow tempo in

music 4 could be significant to determine under which conditions apoyando favors to a higher τe

value.

0%

20%

40%

60%

80%

Sound 1 Sound 2 Sound 3 Sound 4 Sound 5 Sound 6

Pre

fere

nce

%

Apoyando

Tirando

30%

40%

50%

60%

70%

0 0,5 1 1,5 2 2,5 3 3,5

Pre

fere

nce

%

τe ratio



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5. Conclusions

Subjective preferences of apoyando and tirando strokes on classical guitar were evaluated and

correlated with objective parameters. Six excerpts of music were tested. Preferences of 49

subjects showed that the guitar sound preference was not significantly related with harmonic

content of the plucking techniques. Instead, a higher τe value was significantly correlated with a

higher preference. Sound with more reverberation component in it might be perceived with more

quality. For future works it would be interesting to investigate under what conditions each stroke

favours a higher τe of the music program.

Since tone production is an essential part of learning a musical instrument, this work could be a

tool to provide more information for guitar teachers and students.

References

[1] Rossing, T. D., The Science of String Instruments. Springer Science+Business Media, LLC New York, 2010.

[2] Taylor, J., Tone Production on the Classical Guitar. Musical New Services Ltd. 20 Denmark Street, London, 1978.

[3] Meyer, J., Quality Aspects of the Guitar Tone, Royal Swedish Academy of Music No. 38. (1983).

[4] de Dayan, H. G.; Behar, A., The quality of strings for guitars: an experimental study, Journal of Sound and Vibration, Vol 64 (3), 1979, pp421-43.

[5] Sato, S.; Wu, S., Comparison of different calculation methods of effective duration (τe) of the running autocorrelation function of music signals, Acta Acustica United With Acustica, Vol 97, 2011, pp432-440.

[6] Zwicker E, Fastl H. Psychoacoustics: Facts and Models, Springer, 1990.

[7] Terhardt E., On the perception of periodic sound fluctuations (roughness). Acustica, Vol 30, 1974, pp201-13.

[8] Parncutt, R.; Hair, G., Consonance and dissonance in music theory and psychology: disentangling dissonant dichotomies. Journal of Interdisciplinary Music Studies., Vol 5 (2), 2011, pp119-166.

[9] Ando, Y., Opera House Acoustics Based on Subjective Preference Theory. Springer, Tokyo, 2015.

[10] Ando, Y.; Alrutz, H., Perception of coloration in sound fields in relation to the autocorrelation function, Journal of the Acoustical Society of America, Vol 71 (3), 1982, pp616-618.

[11] Ando, Y., Subjective preference in relation to objective parameters of music sound fields with a single echo, Journal of the Acoustical Society of America, Vol 62 (6), 1977, pp1436-1441.

[12] Kato, K.; Fujii, K.; Hirawa, T.; Kawai, K.; Yano, T.; Ando Y. Investigation of the relation between minimum effective duration of running autocorrelation function and operatic singing with different interpretation styles. Acta Acustica United With Acustica, Vol 93 (3), 2007, pp421-434.

[13] ITU-R BS.1770-2, International Telecomunication Union. Vocabulary of terms on telephone transmission quality and telephone sets. 1998.

[14] Cabrera, D., PSYSOUND: A computer program for psychoacoustical analysis, Proceedings of the Australian Acoustical Society Conference, Vol 24, 1999, pp47–54.

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