Cosi Fan Tutte and What
Nonlinear Source-Tract Acoustic interaction in the Soprano Voice and Some Implications for the Definition of Vocal Efficiency
Dr. Scherer: When you look at subglottal pressure measurements made with a wide bandwidth, like the Millar, the subglottal formant shows that the subglottal pressure falls to about 50 percent of the mean value somewhere in the subglottal cycle. I'm not sure what are the subglottal resonances for females, but if they are tall singers with long tracheas, the formant frequencies might be relatively low, so this negative-going pan of the subglottal pressure might coincide in time with the open portion of the vibratory cycle. This would be a negative pressure added to the pressure in the pharynx, and that would help to drive this flow back. Do you want to comment on that?
Dr. Rothenberg: A factor that's important in creating this high supraglottal pressure is the efficiency of the resonance. For example, it may sometimes be necessary to keep the pharyngeal wall stiff to keep the resonance efficient. In addition, as mentioned in the paper, Our results might explain Jo Estill's intuition that nasality increases air flow. If a female singer singing in the high soprano range opens the nasal port slightly, the vocal tract resonances will be damped, and this damping might cause air flow to increase. The efficiency of the resonance is very important.
Since the subglottal resonances tend to be more damped than the supraglottal resonances, they may not be as effective in altering glottal air flow. Nevertheless, it is true that the peak-to-peak subglottal pressure variation can be significant compared to the average subglottal pressure. However, if the soprano uses "tuning" of the supraglottal resonance to reduce the glottal air flow, the reduced air flow will also suppress the subglottal pressure variations. So during a properly executed tuning maneuver, the subglottal pressure may be of much less importance than the supraglottal resonance.
Dr. Scherer: Thank you. A long time ago Bartholomew gave a paper in JASA deal in with male phonation1 saying that, for a good quality, male singers have the singer's formant region enhancement, as well as a low frequency enhancement around the first formant, around 500 Hz or so. Do you think that, if you were to take spectra of the double humped waveforms you showed, you would in fact have some energy from the spectra that would allow a relative enhancement of a low region and a high region? I'm trying to figure out a laryngeal, rather than vocal tract, acoustic reason for the enhancement of these two regions.
Dr. Rothenberg: In looking at the soprano voice, I neglected the inertive effect and I also examined pitches at which the resonance was very important. Conversely, in the male voice I have looked at just the inertive effect and did not look at resonance effects. There is a big range between sopranos and basses, and there's a lot of complexity in between. You can have both effects in one voice, and in the future we should look at how they interact.
Ms. Estill: Would you like to know how I do this tuning?
Dr. Rothenberg: Go ahead.
Ms. Estill: I can describe my tuning maneuvers. To make an /i/ constriction close the port, tighten the aeryepiglottic ring, and then anchor the whole mechanism, perhaps by tightening the pharyngeal wall. But these three different kinds of constrictions above the glottis may contribute to the supraglottic pressure that we're talking about.
Dr. Rothenberg: Some of those maneuvers would tend to make a hard wall tube with a sharp resonance, and maybe that would reduce the damping.
Dr. Baer: Martin, I want to clarify a point. When you suggested we think of the larynx as responding to flow rather than pressure, you meant the control system for the larynx rather than the mechanical structures. I'm glad you flagged that point, because our paper claims exactly the opposite: that over breath groups, it's pressure that's controlled. We are not talking about control within a glottal cycle now but control over the level of syllables. When you have obstruent syllables, it appears more that the pressure is being dynamically controlled over sentence length intervals.
Dr. Rothenberg: There's no conflict really. I was not really talking about the control during a syllable; because, obviously, if you change the pressure on top, you may have to change the pressure on the bottom to compensate. I was talking about the control as it relates to loudness.
Dr. Baer: In singing, for instance, which is different from reading sentences, your task is to get the loudness set up and to get a rich quality.
Dr. Rothenberg: Or vocal effort. If I want to increase my vocal effort, I am really increasing the air flow. And, to do this, I have to have a higher pressure. But, I can also talk louder by increasing the air flow, not by increasing the pressure, even though I am doing both. It's not dynamic control during the syllable that I'm talking about.
Dr. Stevens: Would you have any comment about how people control loudness when they are at say, 10,000 feet, where the relation between pressure and velocity is different? Do people who live at high altitudes control pressure the same way as we do, or do they control velocity?
Dr. Rothenberg: We have reported an experiment, using a helium-oxygen mixture, in which we found that the flow was increased by the helium for a given lung pressure2a However, it appeared that after using the helium for just a short while the subject adapted by increasing vocal fold adduction to reduce flow. Speakers may do something analogous at high altitudes.
Editor: Relevant observations have been made by Wathen-Ounn and Michaels (1968).2
Dr. Cranen: I would like to stress the point I made this morning, comparing normal speakers and singers for glottal closure. Is there a leak area or not? When you look at the back slope of the singer's glottal flow waveform, you see that, although the top of the amplitude of the glottal form waveform is the same, the back slope is much steeper; and this difference is related to vocal efficiency. So, when you want to compare singers with un- trained speakers like me or Lou, it is important to consider the leak area as an important factor.
Dr. Rothenberg: I agree.
Dr. Titze: I'm a little nervous about your going through all this rationale without paying attention to what happens to the tissue in this strong nonlinear interaction. I was wondering if you can make your case, as you do, without involving changes in the driving of the tissue. Either the tissue is insensitive to these pressure changes, moving essentially in its normal mode pattern-which is something that I have believed for some time-or, if that isn't the case, you would have to consider changes in vibratory movement and basic glottal configuration to determine if the vocal folds oscillate. Considering the high-frequency part of the source spectrum, I can't see how you can do that independent of what the pressure does to the vibration.
Dr. Rothenberg: Yes, I'm glad you reminded me of that. You could test your hypotheses easily by looking at the electroglottograph waveform. Even though we may not know exactly what it means in terms of the movement, we know that if it stays the same then the movements stay the same, grossly. However, I would do this experiment with a subject that had a stronger electroglottograph waveform than those I've shown, since a weak electroglottograph waveform has two components. One is the component due to vocal fold Contact area, but there is also a component caused by other Fo-synchronous vibrations of different parts of the anatomy, perhaps far from the glottis, such as tongue surface vibrations. If you vocalize loudly you can feel the tongue surface vibrating. When the EGG waveform is very strong, you can assume that it's coming mostly from the vocal folds, but sometimes the signal from the vocal folds is relatively weak.
Dr. Titze: If the larynx is not protruding enough or if the angle is wide, then the electric field pattern goes way out instead of straight across. This often happens with women and children; you can't get good EGG because the field goes in a wide arch instead of directly between the electrodes.
Dr. Rothenberg: Yes, that may be, because I've seen obviously inaccurate waveforms on both men and Women, sometimes under conditions for which I couldn't identify the casual factors, In such cases, the noise Component which is synchronous with the glottal vibrations can even dominate the signal. But you always have both Components mixed together to some degree.
Dr. Hirano: When the Soprano singer tunes the formant to the fundamental frequency, how does she differentiate different vowels?
Dr. Rothenberg: Let me give you an anecdotal example, The singer I've worked with found a way of singing the passage with an /i/ vowel that was easier for her. She was able to make something that came across as an /i/, but perhaps it had the first formant raised, So, she was able to find the vocal tract configuration that gave her a proper first formant (for singing) but was acoustically acceptable as an i/. I want to repeat this experiment again with her to see what the formants actually were, to see if she was tuning, My prediction would be that that is what was happening.
Dr. Scherer: I would like to ask Martin and the authors of previous papers to combine their ideas on these aspects of more efficient singing and other acoustic effects, plus constrictions downstream. What were the effects and what you would expect?
Dr. Titze: It's been suggested that for the so-called primary register transition, which is roughly the same for male and female speakers and occurs somewhere between 290-350 Hz, there could be an acoustically triggered change in the mechanism, the larynx, and the vibratory pattern of the vocal folds. Further, it would seem that some kind of supraglottal loading could, in fact, enhance the oscillation up to the first formant, when an abrupt transition is made from an inductive to a capacitive load. This would be an undesirable loading that kicks the system into something like a falsetto from the normal chest system. I was wondering how your analysis here would treat the slightly higher end of the resonance. two years ago, in Stockholm, I did a paper on that, trying to show how the subglottal and supraglottal pressures would change the driving pressures of the vocal folds, and it appeared that ideal conditions were achieved right into the region of the formant, But right above the formant, the phases turned around and it seemed as though the tract pressures would not maintain the vibration very well. Have you looked at those glottal waveforms just slightly on the high side of that formant?
Dr. Rothenberg: The answer is no. We did some very gross loading measurements, such as trying to change the resonance by putting a partial obstruction near the lips and looking at average air flow, But the inverse filtering technique we use is very, very difficult under these conditions, So to modify the procedure and still get accurately inverse-filtered flow signal (it's now being done by hand just for a short segment during which the formants are assumed constant) is tedious.
Dr. Stevens: Using these kinds of mechanical changes, you can fix up something in the mouth, phonate, and change the constriction suddenly. Then you can adjust (change the constriction) So that you have tuned exactly to the fundamental. After that, you can change the constriction even more, so that the formant goes above the fundamental, and observe what happens both to the waveform and to the spectrum. At least, you can measure the spectrum of the sound that's radiated.
Dr. Rothenberg: Now, suppose you don't do it quite instantaneously. The singer is listening; she adapts and moves her formant back to compensate to where it was supposed to be because she feels uncomfortable. We've tried this technique, but we didn't know how the singer was compensated.
Dr. Fujimura: When the vocal tract is in tune, it will be more important to consider the effect of damping for the formant. Most probably, there would be a peculiar damping effect. By over-damping the first formant, it would not be crucial any more to have the F1 in the right place, even though the vowel quality wouldn't be clear. It is not misinterpreted as another vowel and, to me, that seems to be exactly what's happening in singing. You can't really tell the phonetic value very clearly, but it doesn't mislead you either.
Dr. Rothenberg: I think what you are saying is, if you damp the formant, then you won't have the strong spectral differences as a function of pitch, for example.
Dr. Fujimura: Well, it may be a function of pitch, but the phonetic quality doesn't matter so much. The vowel quality may not be clear, but it doesn't indicate another vowel.
Dr. Rothenberg: I don't want to give the idea that women "tune" all the time. Probably a good singing technique requires a number of different mechanisms in different parts of the range and at different volume levels. But I would assume that when tuning is used a minimum damping is important for a loud, higher pitched vocalization, especially if it was held. But it may not be always necessary, and increasing the damping may even be necessary under other circumstances in singing.
Dr. Luschei: I understand there may be some disagreement about the exact nature of these pressure pulses. However, people are agreed that, with small transducers in place, there are substantial pressure waves at the fundamental frequency of oscillation supraglottically and subglortically. If that's the case and if there's no suspicion that those pressure waves are due to movements of the pressure transducers, then you have to conclude that any mechanoreceptors in the vicinity are also subject to those pressure modulations. If so, probably any sensitive mechano-receptors in the area must be masked properly, Their behavior is going to be dictated by the pressure variations, if they are at all physically sensitive to the actual phonation taking place. This would, then, provide a good explanation why one might not see pressure variations, or reflex responses to pressure variations, at low frequencies, It also means, interestingly enough however, that the afferent source has to be a potential source of feedback independent of the auditory system, if the nerves can actually respond to these pressure variations. So, it seems to me that a useful thing might be to see whether these afferent fibers supraglottically or subglottically respond to these pressure variations. But I want to make sure it is not just some kind of turbulence in the standing wave or some funny thing like what is being measured here.
Dr. Titze: I think there have been a number of investigations of the subglottal pressure variation. The fluctuations tend to be around 40 percent of the mean value. That's just a rough value. And it shows up in all the simulations that we have done. So, if we're wrong, we are collectively wrong on some very central assumptions.
Dr. Stevens: I think we can calculate that the actual motion of the tissue due to these pressures is a fraction ofa miliimeter. I would guess that would certainly be enough to excite those receptors.
Dr. Rothenberg: These high pressures may give more credibility to the singers characterization of their vocalizations, like "head voice." These strong pressure variations must give some sort of a sensation that a singer could identify and relate to parts of the anatomy.
Dr. Scherer: What is the source for the wall movement measurements, Dr. Stevens?
Dr. Stevens: I take the more or less accepted acoustic mass of the wall-I think people agree it's in the range 1-2 (or ½-2) grams per cm2-and calculate from that the motion.
Dr. Megirian: Coming back to the story of the mechano-receptors as part of a feedback loop, I think we can point out that some earlier work by Bruce Mehl in John Widdecomb's Jab shows that the mechano-receptor in the upper airway is a very fast adapting receptor. I would like to ask Dr. Sasaki if the effects of local anesthetization on the singing voice have been studied.
Dr. Sasaki: I believe that Dr. Gouldand his associates3 have done experiments of this sort.
Dr. Luschei: Dr. Wyke mentions the effects of topical anesthesia of the subglottal laryngeal mucosa in the discussion of his paper at the last conference (Wyke, 1985).4 He says that topical anesthesia of the subglottic laryngeal mucosa has little effect on the conversational speaking voice but produces perturbations of the declamatory speaking voice and renders singing almost impossible, because of lack of accurate pitch and intensity control.
Dr. Harris: There are experiments in the speech literature on the effects of trigeminal nerve block, superficial anesthetization of the oral mucosa, (Borden, Harris, & Catena, 1973; Scott & Ringel, 1974),5 and anesthetiation of the temporomandibular joint (Kelso & Tuller, 1983).6 Although there are procedural problems with these studies, the overall result is that these proceedings have surprisingly little effect on speech.
Dr. Rothenberg: These comments lead me to an idea about how I could become a soprano. Those of us who can't sing could still experience a singer's vibratory sensations by introducing an artificial tone into the vocal tract, That's similar to what I think Sundberg (1979)7 has already done at lower pitches, with bass singers, measuring the vibration of the thorax and abdomen, I believe that he was trying to get some intuition about where the term "chest voice" comes from. But we can also get these sensations by having sounds artificially introduced into the vocal tract. Until recently, the experiment could not have been performed well for the soprano voice, since he didn't know how high the pressure levels were.