Sopranos: resonance tuning and vowel changes

In the higher part of their range, sopranos face some challenges that don't occur in normal speech, and which don't affect other singers so severely. The problem is that the harmonics of their voice become so widely spaced that, unless they make appropriate adjustments, they are likely to lose the benefit of the resonances of the vocal tract. (A more general dicussion of resonance tuning, by sopranos and other voices, is here.)

The vocal tract usually has two strong resonances in the range 0-2 kHz. A bass singing a low G has harmonics at 100 Hz, 200 Hz, 300 Hz etc, all the way to several thousand Hz. He has so many harmonics in this range that some of them are bound to fall close to the tract resonances and so he will benefit from enhanced transmission as sound outside the mouth. A soprano singing a high C has harmonics at 1000 Hz, 2000 Hz, 3000 Hz up to a similar upper limit. So for a soprano, the harmonics could 'miss' the resonances for some note-vowel combinations, and 'hit' for others.

Missing the resonances would be a problem: they are needed when singing with loud accompaniment, and having a resonance near the frequency of the voice can also make it easier to sing. The distinguished voice researcher Johan Sundberg hypothised that sopranos might learn to tune one of their vocal tract resonances to the pitch of the note they are singing.

This creates a phonetic problem, because the tract resonances produce bands of increased power (called formants) in the voice, and these carry information about vowels, information which is mainly carried in the range 300 to 2000 Hz. For a more detailed explanation of this point, see Introduction to vocal tract acoustics. This page provides

There is also a soprano challenge. The research reported here is a spin-off from technology we developed for use in language training and speech therapy.

A brief scientific report of the application to soprano singing is published in the journal Nature and a more detailed report is in JASA.

photo of soprano

Kristen Butchatsky, soprano

New results: Wagner makes it easier for sopranos

Wagner is well-known, even notorious, for writing operas that can challenge both performers and listeners. Both groups might be surprised to learn that Wagner was helping both the performers and the listeners by taking the acoustics of the soprano voice at high pitch into account when he set his text to music. A recent paper from this lab suggests that this is indeed the case:

Each vowel in European languages is associated with a set of resonance frequencies of the vocal tract.  For the soprano voice at high pitch, both the intelligibility to listeners and the ease of production by singers could be improved if the pitch of the note written for a vowel corresponded with its usual range of resonance frequencies.

We tested this hypothesis by investigating whether Wagner used certain vowels more often for the high notes than the low notes, and vice versa. A study of the two great Wagnerian soprano rôles, Brünnhilde and Isolde, indeed found the vowels that required an open mouth were used more often for the very high notes.  Similar studies on some operas by Mozart, Rossini and Richard Strauss showed no such effect.

 We are unaware of any written evidence about Wagner's intentions nor of whether he was advised on this issue by sopranos, with whom he sometimes had close relations.

Of course we are not suggesting that Wagner was a better opera composer than others. He was writing a different type of opera with a much larger orchestra, and making his very demanding vocal parts somewhat easier.

Summary: It appears that Wagner, either consciously or unconsciously, took the acoustics of the soprano voice at high pitch into account when setting text he had written to music. This is consistent with the increased importance of textual information in his operas, the increasing size of his orchestras, and the more complex vocal parts.

More news: resonance tuning in the coloratura/ whistle voice range

The resonance tuning described above tunes the first vocal tract resonance to the frequency of the note sung (R1:f0 tuning). It's fine up to about 1 kHz or C6 – high C for sopranos. After that, it becomes difficult to open your mouth any wider. Some sopranos manage for a further couple of notes, but for many sopranos, the limit of R1:f0 tuning is the limit of their vocal range.

Coloratura sopranos or the jazz and pop singers who practise the whistle registers, however, have another technique: starting at around C6, they begin to tune the second vocal tract resonance to the frequency of the note sung: R2:f0 tuning. Further, they appear to use a different mechanism and thus to show another transition. This and some related effects are described in these papers:

A more general dicussion of resonance tuning, by sopranos and other voices, is here.

Sound files

These sound files do not form part of the study; they are simply illustrative of the phonetic effect of the widely spaced harmonics and the resonance tuning. For these recordings, an experienced soprano, who had no knowledge of the purpose of the study, was asked to sing an ascending scale over two octaves, from Bb3 to Bb5. She was asked to sing the scale five times, each time using a different vowel sound. The vowels (phonetic symbols in parentheses) are those in the English words* "hard" (/phonetic symbol/), "hoard" (/phonetic symbol/), "who'd" (/phonetic symbol/), "heard" (/phonetic symbol/) and "heed" (/phonetic symbol/), but sung with an initial consonant "L". (The music and the phonemes to be sung were presented in writing.)
    Scale sung on "La":    

    Scale sung on "Lore":    

    Scale sung on "Loo":    

    Scale sung on "Ler":    

    Scale sung on "Lee":    

Let's now listen to the five vowel sounds at low pitch, then at high pitch. (These files are assembled from the first and last notes on each of the scales above.)
    "La, Lore, Loo, Ler, Lee" at low pitch:    

    "La, Lore, Loo, Ler, Lee" at high pitch:    

Notice that the vowel sounds are much more similar at high pitch+. Do you think that you can tell them apart? If so, listen to this file, in which the order has been changed and see how sure you are.
    Five vowels at high pitch, order changed:    

The difficulty the listener has in differentiating the vowels at high pitch is due to two effects. One is that high pitch puts the harmonics further apart. (See What is a sound spectrum? for details.) The information about which vowel is spoken or sung is (loosely speaking) conveyed by the relative amplitudes of the harmonics of the voice that fall in the range from 200 to 2000 Hz. If a soprano sings a high C, there is only one harmonic in this range, so little vowel information is carried. Another effect is the tuning of the first vocal tract resonance by sopranos, including this singer. We have studied these effects, which are described very briefly in a paper in Nature, and which are described in much more detail in a longer paper .

We thank soprano Kristen Butchatsky of the School of Music and Music Education at the University of New South Wales for singing the samples recorded here and for her help in other aspects of our research.

* English vowels are often presented and studied in the context h[vowel]d, because a set of such words minimises the number of nonsense syllables. In fact, as soon as we can convince enough people to call a Head Up Display a 'hud', the list will be complete.

+ Even the consonant is difficult to discern. Much of the information about consonants is conveyed by the way they modify the vowel that follows (or precedes) them.

Sopranos tune resonances of their vocal tract when they sing in the high range

A non-technical version of reseach published as Joliveau, E., Smith, J. and Wolfe, J. "The tuning of vocal tract resonances by sopranos", Nature, 427, 116.

In brief. In the top half of their range, but not in the lower half, classically trained sopranos adjust one of the resonant frequencies of the vocal tract to match the pitch of the note they are singing. This gives greater loudness for given effort, and may have musical advantages, but it contributes to the difficulty of understanding the words they are singing.


In singing or speech, we can identify two separate effects. First, the vocal folds vibrate and that produces a sound. The frequency of that sound--the number of vibrations per second--determines the pitch, ie whether a sound is high or low.

Second, the sound from the vocal folds passes through the vocal tract. It is modified by the shape of the tract so that we can make different speech sounds: open your mouth wide and you get 'aah', close it almost completely and get 'ooo' etc. This process is very familiar to us, but it is also a bit subtle. Here's how it works.

The vibration from the vocal folds is a complex sound: we say it has lots of harmonics or that it is made up of a range of different frequencies. The vocal tract resonates at several different frequencies and these resonances amplify some of the frequencies present in the voice. For instance, when this author's mouth and tongue are in a neutral position (when I say 'er'), it 'amplifies' frequencies of about 450 and 1400 vibrations per second (approximately the notes A above middle C and the F above the treble clef--both notes beyond my singing range). Different tract positions amplify different frequency components of the voice and that allows us to identify different speech sounds. This is explained in more detail, with diagrams and sound files, in our page Physics in Speech. These processes are independent. One can keep the vocal tract constant and change the pitch (eg humming, vocalise or singing 'la la la la la'), or one can keep the pitch the same and vary the vocal tract, which is what the Daleks on Dr Who do ('Ex-ter-min-ate'). Normally we do both independently, so we can sing different words on different notes (normal singing), and we can use different inflexions in the same words. (eg. 'You're not going' vs 'You're not going?')

However, there is a problem for sopranos. In the high range of women's voices*, the pitch frequency of the notes enters the range of the lowest vocal tract resonance. If the singers did nothing about this problem, then whenever the pitch of the note coincided with a resonance in the tract, that note would be much louder than the others. So you would get uneven loudness, and also uneven voice quality. Some time ago, the Swedish acoustician Johan Sundberg suggested that sopranos actually tune the resonance of their vocal tract to the note that they are singing: the original evidence for this was that they tend to open the mouth more as they sing successively higher notes. However, this could not be confirmed directly because there was a technical difficulty in measuring the acoustics of the tract while it was being used for singing.

* What about the voices of young children, which are higher in pitch than women's voices? Children have smaller heads and shorter vocal tracts, so one would expect that the resonances of their vocal tracts to occur at higher frequencies, so the overlap of pitch and resonance would occur at higher pitch.

The project on sopranos and resonance tuning

In the Acoustics Group, we have developed acoustic techniques for studying the tract during speech (see Voice Acoustics). Our main motivation was to provide technologies for speech pathology and for language training, but the techniques are also applicable to singing. So we invited in a group of classically trained sopranos. For these measurements, we produce a carefully synthesised sound just outside the singer's or speaker's mouth. A microphone records not only their voice, but also the way in which their vocal tract interacts with the synthesised sound. From the latter, we can tell a lot about the acoustics of the tract. This project became a research project for Elodie Joliveau, who is both a physics student and a soprano.

What we find with sopranos is this: In the low range of the voice (below about A4, but it depends on the vowel), they do just what we all do in speech and singing: the pitch and the vocal tract resonances are nearly independent. In the high range, however, they tune the lowest resonance of the vocal tract rather precisely to equal the pitch they are singing. They perform the resonance tuning by gradually lowering the jaw as they ascend, and/or by 'smiling' more as they ascend in pitch. What surprised us was the consistency and precision of the tuning for all vowels.

This resonance tuning gives them uniform loudness and vocal quality, but it also means that vowel sounds become very similar--we include above some recordings that demonstrate this clearly. So sopranos sacrifice some intelligibility in the interests of musical quality. However, the amount of intelligibility sacrificed is not great. In the high range, it is very difficult to understand vowel sounds anyway: because of the high pitch, there is simply not much frequency information available to the ear.

This has been remarked on by composers such as Berlioz, whose book about orchestration warns opera composers about the effect. Many composers seem to heed the warning: the high parts in soprano solos sometimes do not have words, or have a single word slurred over several notes, or sometimes repeat words that are heard in other ranges. or sometimes the words are simply not important. This is by no means a cricitism of sopranos: it is just an inherent physical constraint on the instrument. One doesn't ask a trombonist to play pizzicato, one shouldn't ask a soprano to make vowel distinctions in the altissimo. Nevertheless, the effect is possibly one* of the contributing reasons why opera houses use surtitles even when the words are in the language of the audience.

photo of soprano

Kristen Butchatsky sings a high note (A5).
So, if you are a soprano, download the paper and keep it for defence against conductors who think you should be able to distinguish 'bead' and 'bed' on high C. Or else quote Berlioz (Berlioz, H. Grand Traité d'Instrumentation et d'Orchestration Modernes (1844; transl. Clarke, M. C., Novello, London, 1882).)
* Other reasons? There are several, starting with the design of the hall. Acoustic engineers must compromise between a long reverberation time, which makes the sound louder, and a short one, which makes it clearer. If the performance space was designed for orchestral music (relatively long reverberation), it will be hard to understand singing. Then there is the audience, who are not always as silent as may be hoped. (This is often a problem in the Sydney Opera House: a substantial fraction of each audience is there just because it is a famous building, not because they like opera.) The orchestra may be too loud, perhaps because the opera was written centuries ago when string and wind instruments were less loud than the modern equivalents. Or perhaps because the orchestration is excessive. It must be admitted that not all singers take sufficient care in pronunciation. And finally, it is especially difficult for the chorus: when plosives (p, d, t, k etc) are not pronounced in synchrony, it is difficult to discern them.

Soprano challenge

If you are a soprano and you think would like to test whether our observations reflect physical limitations on all sopranos, or just on some of them, perhaps you would like to try repeating the exercise recorded in the sound files above. All you need is a microphone and a computer or tape recorder. (It would help if you had some editing facility such as the Cool Edit software, but this is not necessary.) First, sing the scale below, senza vibrato, in your professional singing voice, with projection. Depending on your comfortable range, you might want to make it C major, B major or Bb major.

C major scale on La over two octaves

Then do the same for "Lore", "Loo", "Ler" and "Lee". Then listen to the first notes in each in each scale. (If you have editing tools, take the first note (the minim or half-note) of each sample and put them together to make the low pitch file.) Then do the same for the last note of each scale. Then get a friend to mix up the order of the notes in the final sample and listen to it. If you can clearly discern them, then we should really like to hear from you: that would be the basis of a very interesting study!

Reports of the application to soprano singing are published in:

Some explanatory notes and related pages


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