What is vocal resonance?
At certain resonant frequencies, the air in the tract vibrates readily with high amplitude. At or near these resonant frequencies, sound is more efficiently transmitted from the larynx to the outside air, and the vocice may operate more effectively. Both of these give a louder output for a given effort.
In singing, vocal resonance may mean the use of vocal tract resonances to boost one or more harmonics of the voice; when this is done systematically it is called resonance tuning (or formant tuning). The nature of vocal resonances and their use in singing is explained in the video linked below.
At low pitch, each resonance usually boosts the output of one or more harmonics. These bands of resonance-enhanced power are called formants. (See also Introduction to voice acoustics.)
In vocal resonance, which resonances are used, and how? The first resonance, called R1, and usually falls in the frequency range roughly D4 to C6 (fundamental frequency fo = 300 to 1000 Hz), which is most of the standard soprano range. R2 lies in the range roughly C6 to C7 (~1000-2000 Hz) - the octave above a soprano's 'high C'. So, for most voices, the resonances usually enhance only higher harmonics (nfo).
When singing in their high range, sopranos, whether trained or not, tune R1 to the pitch frequency fo, called R1:fotuning. For notes well above high C, some sopranos tune R2 to the fundamental fo - called R2:fo tuning. (More below.)
Altos and tenors also use R1:fo tuning for the highest part of their range.
In belting and in some ethnic styles, altos systematically tune R1 to the second harmonic, 2fo; this is R1:2fo tuning.
Tenors and baritones sometimes tune R1 to the higher harmonics (R1:nfo tuning), especially for important notes in their higher range. See the figure below for possible ranges.
Basses and baritones usually don't need resonance tuning over much of their range, because one or other harmonic usually benefits from R1 or R2. For example, a bass singing near low G (say fo of 100 Hz) has harmonics at 100 Hz, 200 Hz, 300 Hz etc, all the way to several thousand Hz. However, men's voices benefit from the singers formant; they lower the larynx and the supra-glottal constriction thus produced enhances R3 and/or R4.
Where in the vocal tract does each resonance come from? The first and second resonances involve all of the air in the tract. Their vibration shapes are like the first two resonant modes of a pipe closed at one end. However, the first resonance depends most strongly on the lip aperture and the second depends most strongly on the position at which the tongue constricts the tract. The singers formant is attributed to the effect on the third and fourth resonance of the supra-glottal constriction. (See Introduction to voice acoustics.)
Resonance tuning means modifying vowels to tune the vocal resonance near to one of the harmonics of the note being sung. Does this vowel modification reduce intelligibility? Not very much. Large distortion of the vowel is only required at very high pitch, and the vowels are not very intelligible at high pitch anyway (see below). Further, the loss of some vowel information is not too important because vowels carry limited information: we can often understand text -v-n wh-n th- v-w-l -nf-rm-at--n -s m-ss-ng. Try the sound files below.
We saw above that, for a low bass note, there are so many harmonics that some of them are bound to fall close to the tract resonances. At the other extreme, 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 easily 'miss' the vocal resonances for some note-vowel combinations, and 'hit' for others. Missing the vocal 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 concluded that sopranos learn to tune one of their vocal tract resonances to the pitch of the note they are singing. We investigated this in the laboratory.
Resonance tuning potentially creates a phonetic problem. In normal speech, 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 voice acoustics.
Below, we provide
There is also a soprano challenge.
The research reported here uuses 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.
More recent research is summarised below.
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.
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:fo 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:fo
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:fo 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 and in the video linked above.
Sound files
These sound files do not form part of our formal 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" (//), "hoard" (//), "who'd" (//), "heard" (//) and "heed" (//), 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
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.
Background.
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.
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.
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:
As of
June 2005, relativity is 100 years old. Our contribution is Einstein Light: relativity
in brief... or in detail. It explains the key ideas in a short
multimedia presentation, which is supported by links to broader and
deeper explanations.
/), "hoard" (/
/), "who'd" (/
/), "heard" (/
/) and "heed" (/
/), but sung with an initial consonant
"L". (The music and the phonemes to be sung were presented in
writing.)
