|The Hidden Wind Instrument
The human voice, when used for singing, can be considered to be a wind instrument, hidden within the human frame. So, we ask the question ‘in what ways are the human voice and wind instruments similar?’ As with all comparisons and analogues the similarities drawn are not always exact, but can be illuminating.
The basic elements which a wind instrument possesses are :-
a method or device for producing pitch
a partially enclosed space into which the vibrating air can expand
a method of changing the pitch
a device which drives air into the system
Concentrating on for example the Oboe (comparisons with other woodwind or brass instruments instrument, even bagpipes are also valid) we identify its basic elements as :-
i) a double reed as the vibrator
ii) the conical tube open at one end provides the space
iii) strategically positioned holes in the tube, together with lip and air pressure on the reed, causes pitch change
iv) the human lungs and breathing system to drive and energise the instrument.
So how, from a physical point of view, does the instrument operate? If you have ever attempted to blow or have heard an oboe reed blown on its own, unplugged from the instrument you will have heard a rather unpleasant squawking sound reminiscent of a duck, the basic pitch of which can be varied to some degree by change of lip and air pressure but over which there is little control.
When plugged into the wooden conical open ended tube and blown by a competent player a magical transformation occurs and we hear the beautiful haunting sound, characteristic of the instrument.
The transformation occurs because of the pattern of acoustic resonances which are excited in the conical tube. The sound energy entering the space is redistributed into the characteristic resonance frequencies which the conical tube supports. These will differ from all other acoustic tubes and spaces. A conical tube, open at one end, supports a resonant frequency spectrum in which all harmonics are present i.e. f0, 2f0, 3f0, 4f0 etc. The lowest or fundamental frequency (f0) gives the pitch of the note, whilst the energy in the harmonics, contributes to the oboe *timbre.
To make a comparison with the human voice, we list its elements in the chosen order:-
the vocal folds, vibrator and generator of pitch (acoustic frequency)
the vocal tract open at one end (mouth), consisting of trachea, pharynx and mouth
to form the acoustic cavity
a mechanism to stretch the vocal folds to effect changes in pitch
the human lungs and musculature of the chest, thorax and abdomen to drive the instrument
The vocal folds bear a resemblance to a pair of fleshy stretched ribbons, which are caused to vibrate by blowing air between them. Mechanisms within the larynx are able to stretch and relax them in the manner of a stretched string and in this way the basic pitch can be changed and controlled.
Macabre experiments performed on excised human and animal larynxes show that the basic sound produced is squawking and unpleasant, but rich in harmonics (like the oboe reed). However, the larynx is directly coupled to the acoustic tract and, as in the case of any other wind instrument, this is where the transformation into a (hopefully) beautiful sound takes place.
The distance between larynx and mouth for a man is on average about 18 cm., so if the acoustic tract is modelled as a straight cylindrical tube (which it is not) of fixed length 18 cm open at one end, then GCSE physics shows that the first resonant frequency would be approx.440 Hz i.e. tenor top A. Subsequent resonances occur at 3x440 Hz, 5x440 Hz etc. This then, would be the pattern of the formants. A pattern fixed, predictable and incapable of much variation. Incidentally this crude model shows that for men, direct amplification of the fundamental note pitch is only possible for high tenor notes and above, illustrating the fact that for most male voices, vocal amplification often occurs by action of the formants on the harmonics and not on the fundamental.
Luckily the vocal tract of humans is much more geometrically interesting than a tube and is tuneable. The tuneable part consists of the following elements :-
Mouth cavity (M)
Pharynx (which, in more detailed analyses, is often subdivided into sections) (P)
Trachea from larynx to pharynx (T)
(The nasal cavity is usually closed off in classical singing)
The space formed by the connection of these elements possesses a set of resonant frequencies (much more complex than that of the simple tube), called the formants. They are different for each and every individual. The formants enhance the quality of the sound produced by the larynx and give a voice its unique timbre.
The acoustic tract of a human being is, however, much more flexible (literally) than that of wind instruments, and with practice any individual is able to vary the shape and dimensions of all three elements at will and therefore change the pattern and intensity of the formants, though, in classical singing, raising the larynx to shorten the tracheal section is discouraged. Thus, as mentioned above, the vocal tract is to some extent tuneable. It is this fact which gives rise to the human ability to form vowels in speech and provide the capability to sing.
The diagram opposite shows the pattern of resonances (formants) as *measured by D.B. in his own tract. The case shown is that for a tract shape, corresponding to the vowel sound ‘ee’. Peak F1 is at about 450Hz and F2 at about 2,100Hz. This means that were I to sing a tenor A, on this vowel (I should be so lucky!) it would be greatly amplified along with its 5th harmonic. The particular two formants shown, are necessary to create the vowel sound ‘ee’. There are also prominent peaks at higher frequencies, all contributing to vocal timbre. Each person’s formant pattern will be different from all others and is unique and each different vowel sounded has its own distinct set of formants.
Now we know why our MD says from time to time “form the shape of the vowel internally and then sing into it.”
: Experiment shows that the timbre of an instrument
, as perceived by a listener, is also critically dependent on the initial transient i.e. the acoustic path through which the system momentarily passes to achieve the final stable note. If the initial transient is suppressed, by, for example recording
, editing and then playing the result, it is often difficult to identify the instrument (see Physics of Musical Sounds by C.A. Taylor).