19 th INTERNATIONAL CONGRESS ON ACOUSTICS MADRID, 2-7 SEPTEMBER 2007 Larynx closed quotient variation in quartet singing PACS: 43.55.Cs Howard, David M Audio Lab, Intelligent Systems Research Group, Department of Electronics, University of York, Heslington, York, YO10 5DD, United Kingdom; [email protected]ABSTRACT Larynx closed quotient (CQ) is measured from the output of an electrolaryngograph as the percentage of each vocal fold cycle for which the folds remain in contact. CQ values exist for trained and untrained adult and child singers, and in all cases, a patterned variation has been observed with singing training and experience. In this paper, CQ values are investigated for members of an a capella SATB (soprano, alto, tenor, bass) quartet. A previous study indicated that there was a grouping of CQ values for each member of a quartet, and it was hypothesized that this might have some bearing on ensemble blend. This is investigated further in this experiment in which recordings are made with a quartet whose male swap parts (tenor and bass) and whose female members swap parts (alto and soprano). If there is a change in CQ that relates to blend, then there should be a shift in CQ grouping when singing a different part. Each member of the group wears an electrolaryngograph and the data are recorded on a multi- track machine. The individual plots of CQ against the logarithm of fundamental frequency (f0) for each singer are combined into one to enable direct comparison. INTRODUCTION Choral singing is enjoyed by many whether in a cathedral or local church, an opera chorus or oratorio choir, inside or outside, or amateur or professional. There are many aspects of vocal performance that contribute to the success of a choral performance including pitching accuracy, intonation control, holding the pulse of the music, starting and stopping together and following the dynamics, but one of the most important is achieving a good choral blend. Choral blend is important no matter what size the choir is, but it is especially important when singing one to a part as in SATB (soprano, alto, tenor, bass) quartet singing. A skill that is basic to achieving a good blend is the development of listening awareness; awareness of what the other members of the group are doing at any given time [1]. Good listening coupled with well- honed musical performance skills will go a long way towards achieving a good choral blend, and the results can be heard in the performances of highly regarded vocal ensembles. Is it possible to quantify choral blend? Experience in providing real-time visual feedback displays for use in singing training (WinSIngad) suggests that this might indeed be possible, but it is necessary first to establish what parameters to measure. So for example, aspects such as dynamics and intonation could be readily measured. This paper reports on pilot work which attempts to establish whether variation observed in a parameter (larynx closed quotient or CQ) might be indicative of choral blend. The idea stemmed from analyses carried out on data that was gathered in experiments designed to explore tuning in a capella (unaccompanied) singing of SATB vocal quartets [2,3], when a quick look at CQ variation between members of the quartet appeared to suggest some patterning that might relate to singing together [4]. Additional analyses are presented here which appear to support the original tentative suggestions. MEASUREMENT OF LARYNX CLOSED QUOTIENT Larynx closed quotient (CQ) is measured from the electrolaryngograph [5] output waveform. This allows the nature of vibration of the vocal folds to be monitored non-invasively via two electrodes that are placed superficially on either side of the neck at larynx level and held in
Transcript
19th INTERNATIONAL CONGRESS ON ACOUSTICS
MADRID, 2-7 SEPTEMBER 2007
Larynx closed quotient variation in quartet singing
PACS: 43.55.Cs Howard, David M
Audio Lab, Intelligent Systems Research Group, Department of Electronics, University of York, Heslington, York, YO10 5DD, United Kingdom; [email protected] ABSTRACT Larynx closed quotient (CQ) is measured from the output of an electrolaryngograph as the percentage of each vocal fold cycle for which the folds remain in contact. CQ values exist for trained and untrained adult and child singers, and in all cases, a patterned variation has been observed with singing training and experience. In this paper, CQ values are investigated for members of an a capella SATB (soprano, alto, tenor, bass) quartet. A previous study indicated that there was a grouping of CQ values for each member of a quartet, and it was hypothesized that this might have some bearing on ensemble blend. This is investigated further in this experiment in which recordings are made with a quartet whose male swap parts (tenor and bass) and whose female members swap parts (alto and soprano). If there is a change in CQ that relates to blend, then there should be a shift in CQ grouping when singing a different part. Each member of the group wears an electrolaryngograph and the data are recorded on a multi-track machine. The individual plots of CQ against the logarithm of fundamental frequency (f0) for each singer are combined into one to enable direct comparison. INTRODUCTION Choral singing is enjoyed by many whether in a cathedral or local church, an opera chorus or oratorio choir, inside or outside, or amateur or professional. There are many aspects of vocal performance that contribute to the success of a choral performance including pitching accuracy, intonation control, holding the pulse of the music, starting and stopping together and following the dynamics, but one of the most important is achieving a good choral blend. Choral blend is important no matter what size the choir is, but it is especially important when singing one to a part as in SATB (soprano, alto, tenor, bass) quartet singing. A skill that is basic to achieving a good blend is the development of listening awareness; awareness of what the other members of the group are doing at any given time [1]. Good listening coupled with well-honed musical performance skills will go a long way towards achieving a good choral blend, and the results can be heard in the performances of highly regarded vocal ensembles. Is it possible to quantify choral blend? Experience in providing real-time visual feedback displays for use in singing training (WinSIngad) suggests that this might indeed be possible, but it is necessary first to establish what parameters to measure. So for example, aspects such as dynamics and intonation could be readily measured. This paper reports on pilot work which attempts to establish whether variation observed in a parameter (larynx closed quotient or CQ) might be indicative of choral blend. The idea stemmed from analyses carried out on data that was gathered in experiments designed to explore tuning in a capella (unaccompanied) singing of SATB vocal quartets [2,3], when a quick look at CQ variation between members of the quartet appeared to suggest some patterning that might relate to singing together [4]. Additional analyses are presented here which appear to support the original tentative suggestions. MEASUREMENT OF LARYNX CLOSED QUOTIENT Larynx closed quotient (CQ) is measured from the electrolaryngograph [5] output waveform. This allows the nature of vibration of the vocal folds to be monitored non-invasively via two electrodes that are placed superficially on either side of the neck at larynx level and held in
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place with an elastic neckband. A constant amplitude high frequency voltage is applied between the electrodes and the electrolaryngograph monitors the current flow which varies as the electrical impedance between the electrodes changes due to vocal fold vibration. The electrolaryngograph output waveform, usually denoted Lx, is indicative of the inter-electrode current flow.
Figure 1: A few idealised of the electrolaryngograph output waveform (Lx) indicating the
fundamental period (Tx) and the closed and open phases (CP and OP respectively). An example Lx waveform is shown in figure 1. Note that greater inter-electrode current flow is plotted vertically upwards unlike the output waveform from the electroglottograph (Rothenberg) which plots increasing inter-electrode current flow vertically downwards. Greater inter-electrode current flow is interpreted as indicating a greater area of contact between the vocal folds, and the Y axis in figure 1 is labelled accordingly. Each cycle can be considered as indicated the figure in terms of: when the folds are closing, at maximum contact, opening and open as well as the fundamental period (Tx). These allow the definition of the “closed phase” (closing, maximum contact and opening) when the airway is constricted and the “open phase” (open) when it is not constricted. Larynx closed quotient, or CQ, is defined as the percentage of each vocal fold vibratory cycle for which the folds are in contact closed, and this is calculated as follows.
%100��
���
� ��
�
�=Txcp
CQ (1)
where: cp = closed phase; Tx = fundamental period. DATA COLLECTION Two highly experienced SATB a capella vocal quartets took part in this work. In quartet 1, all members were experienced singers, and three sang regularly together whilst the fourth (soprano) sang regularly in other choirs. At the start of the recording session, some time was spent singing together in order to become familiar with each other’s sound, as well as the local acoustic (a domestic living room). For this recording, they sang four exercises written by the author to enable intonation strategies to be explored and the scores are available [3]. They also sang two carols in four part harmony from [6]: “O little town of Bethlehem” (arranged by Ralph Vaughan Williams) and “The first nowell” (arranged by John Stainer). Both are well known and the harmonies are those customarily associated with the carols. Quartet 2 performs regularly as a group, and they sang one of the exercises twice. During the experiment, each singer wore electrolaryngograph electrodes and stood close to a directional microphone (two AKG C480B with cardioid capsules, and two AKG C414 B-ULS set to cardioid were employed). Four field laryngographs from Laryngograph Ltd. (www.laryngograph.com) were employed that had been specially prepared by checking that their operating radio frequencies were well separated, to ensure that they did not interfere with other electromagnetically. The Lx amplitudes were monitored using a portable oscilloscope to
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ensure appropriate electrode positioning as described in [5]. The four Lx waveforms and four microphone outputs were simultaneously captured on a Korg D1600 hard disk eight-track digital audio recorder. The audio data were transferred digitally to a PC and analysis was carried out using the Windows-based “Spead” and “QAnalysis” software that accompanies the electrolaryngograph. RESULTS The Lx waveform is analysed on a cycle-by-cycle basis, and for each cycle, a value of fundamental frequency (f0) and CQ is found. These are usually plotted graphically against one another, and for the purposes of this work, the CQ-f0 data is plotted for each member of the quartet on the same set of axes to enable direct comparison. There is one point on the plot for each cycle in the original Lx waveform. Figure 2 shows the CQ-f0 plots for quartet 1 singing “Ave Verum” by Mozart, “O little town of Bethlehem”, “The first nowell” and the author’s exercises 1, 4 and 5.
Figure 1.-Larynx closed quotient (Y-axis) against fundamental frequency (X-axis) plots for quartet 1 singing “Ave Verum” by Mozart, “O little town of Bethlehem”, “The first nowell” and the author’s exercises 1, 4 and 5 (see text). The areas indicated by the letters “S”, “A”, “T” and “B”
refer to soprano, alto, tenor and bass respectively.
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Figure 2.-Larynx closed quotient (Y-axis) against fundamental frequency (X-axis) plots for both quartets singing twice the author’s exercise 3 (see text). The areas indicated by the letters “S”,
“A”, “T” and “B” refer to soprano, alto, tenor and bass respectively. Figure 3 shows the CQ-f0 plots for both quartets singing the author’s exercise 3 twice. The ranges of both axes available in the basic CQ-f0 plot in QAnalysis has been modified to ensure best use of the available plotting area. The points belonging to each part (SATB) are indicated on the plots both by the colour of the plots (S-black; A-red; T-green; B-blue), and lines have been superimposed on the plots that are indicative of the boundaries between them. DISCUSSION AND CONCLUSIONS The hypothesis under consideration here is that there is some separation between the members of an SATB a capella vocal quartet in terms of the CQ values being employed by its members. Clearly one would expect there to be separation between the parts of in terms of the notes sung by each part, with whatever overlap there is depending on the music itself. However, were this the only effect, then the lines superimposed on the plots in figure 2 and 3 would be expected to be essentially vertical. Instead, they all tend towards being at an angle which suggests that there is some variation in CQ as well as f0 between the parts. This is especially the case for the three pieces of music, “Ave Verum” [7], “O little town of Bethlehem” and “the first nowell”. Whilst there is a degree of overlap in some of the examples, the plots are generally quite well separated. This is more clearly illustrated in figure 3 which shows mean and standard deviation CQ values for each member of quartet 1 singing: O little town; the first nowell; Ave verum; exercise 3 takes 2 and 3; and for quartet 2 singing exercise 3 takes 1 and 3. It is interesting to observe that the separation is well defined for the three pieces of music and less marked for the exercises. However, there are distinct differences between the two quartets themselves. Quartet one exercises exhibit little separation between the soprano and alto and between the tenor and bass, whereas for quartet two, there is a huge difference between the tenor and bass and rather less between the soprano and alto.
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Figure 3.-Mean and standard deviation CQ values for each member of quartet 1 singing: O little town (LT); the first nowell (FN); Ave verum (AV); exercise 3 takes 2 and 3 (ex3t2, ex3t3); and for
quartet 2 singing exercise 3 takes 1 and 3 (ex3t1, ex3t3). Quartet singers must achieve a blend between their outputs, but there are a number of aspects that contribute to choral blend, including dynamic, intonation, timbre [8] and temporal aspects relating to note onsets and offsets and consonants in the text. CQ is not something that singers adjust as a matter of conscious control; indeed, most singers would not even be aware of CQ or how to alter it in any direct way. However, CQ vales do alter as a function of singing training and experience [9-11] which indicates that CQ can be varied, even if only subconsciously. Perhaps it is a parameter that choral singers manipulate when they are listening to each other as ensemble singers [1]. That there is a spread of CQ between the four parts is compelling; could it be a function of good choral blend? Assuming it is another pattern available to the perceptual processing centres, then it does not seem unreasonable to propose that a smooth distribution of CQ values between the parts helps the individual voices to blend and have an increased degree of coherence. CQ has been found to increase as a function of vocal intensity change [12, 13] which adds another dimension to the tale, but these data do not obviously lend themselves to an explanation based solely on intensity variation. These findings suggest that CQ can be rather readily varied, and while we may not have explanations that fully describe the acoustic effects of such CQ changes, it could be the organisation of CQ with f0 as indicated by these data is another facet of choral blend. Further experimental work is required to establish the extent to which this is observed with other quartets and different repertoire. A further investigation will try swapping the singers around (soprano with alto, tenor with bass) to see whether there is any change when singing a different part. Whatever the outcome, some light might be shed on the somewhat intangible attributes that make up the highly sought after choral blend. ACKNOWLEDGEMENTS The author thanks the eight singers for their willingness to give their time to take part in these experiments. REFERENCES
[1] Potter, J. (1998). Vocal authority, Cambridge: Cambridge University Press. [2] Howard, D.M. (2006). Equal or non-equal temperament in a capella SATB singing,
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[3] Howard, D.M. (2006). Intonation drift in a capella SATB quartet singing with key modulation, Journal of Voice, accepted and in press, DOI: 10.1016/j.jvoice.2005.12.005.
[4] Howard, D.M. (2003). Larynx closed quotient in a capella SATB quartet singing, Proceedings of the Stockholm Music Acoustics Conference, SMAC-03, Vol. 2, August 6-9, Stockholm, 467-470.
[5] Abberton, E.R.M., Howard, D.M. & Fourcin, A.J. (1989). Laryngographic assessment of normal voice: A tutorial, Clinical Linguistics and Phonetics, 3, 281-296.
[6] Jenkins, N. (Ed.), (1993). Carol singer’s handbook, Kevin Mayhew Ltd. [7] Mozart, W.A. (1975). Ave verum, Sevenoaks: Novello. [8] Ternström, S. (1991). Physical and acoustic factors that interact with the singer to produce the
choral sound, J Voice, 5, (2), 128-143. [9] Howard, D.M., Lindsey, G.A. & Allen, B. (1990). Toward the quantification of vocal efficiency, J
Voice, 4, (3), 205-121. [See also Errata (1991). J Voice, 5, 93-95.] [10] Howard, D.M. (1995). Variation of Electrolaryngographically derived closed quotient for trained
and untrained adult female singers, J Voice, 9, (2), 163-172. [11] Howard, D.M., Barlow, C., and Welch, G.F. (2001a). Vocal production and listener perception of
trained girls and boys in the English cathedral choir, Bul the Council for Research in Music Education, 147, 81-86.
[12] Orlikoff, R.F. (1991). Assessment of the dynamics of vocal fold contact from the electroglottogram: data from normal male subjects, J of speech and hearing research, 34, 1066-1072.
[13] Sundberg, J., Cleveland, T.F., Stone, R.E., and Iwarsson, J. (1999). Voice source characteristics in six premier country singers, J Voice, 13, (2), 168-183.
