Auditory selectivity

Article by Dr Alfred Tomatis published in the Bulletin du Centre d’Études et de Recherches Médicales de la S.F.E.C.M.A.S. of October 1954 (pages 128-132). Tomatis here defines auditory selectivity — the faculty of the ear to perceive a frequency variation within the sound spectrum and to situate its direction — and systematically explores its correlates: the audible sound spectrum (16 c/s to 16,000 c/s), the distinction between simple and complex tones, the play of harmonics, parallel with visual selectivity (normal eye vs colour-blindness), individual variants according to the tenor or baritone ear, and applications for the singer, the phoniatrician and audio-phonatory diagnosis.

Otorhinolaryngology Service
Dr Tomatis
Attaché des Hôpitaux
Deputy Director of the SFECMAS Centre for Consultation
and Medical Research

Definition

We shall call “Auditory Selectivity” the faculty possessed by an ear of perceiving a frequency variation within the sound spectrum and of situating the direction and position of the variation.

The sound spectrum

Among the periodic disturbances which air in general, or any other medium, may convey to the auditory system, there are some that are liable to provoke a sound sensation. To do so, they must satisfy certain conditions regarding their intensity and their frequency.

We shall pass briefly over the conditions relating to intensity, which are not of great interest in the study at hand today. We may recall that for each sound frequency the ear has a lower threshold, or threshold of auditory acuity, and an upper threshold, or threshold of painful sensation. The intensity differences between these two thresholds are a function of the ear’s sensitivity to the frequency considered.

As regards frequency, acoustic disturbances — that is, those that propagate through a material support — spread over a very extensive band.

Below 16 c/s, in the domain of infrasound, if the intensity is sufficient, the eardrum transmits to the brain a sensation of rhythmic overpressure. The eardrum acts in this case as a membrane, and one cannot speak here of a sound sensation.

Between 16 c/s and 40 c/s, the sound sensation exists, but the captured sound takes the form of a rumble. The ear is able in this band to follow the pressure variations. Present knowledge of the auditory system’s reactions to these frequencies is very limited.

It would appear to us that the overpressure effect is a biaural phenomenon. Indeed, the frequency N = 20 c/s, for example, corresponds to a wavelength λ = V / N, where V is the speed of sound in the medium transmitting the disturbance, namely air in the case that concerns us:

λ = 340 / 20 = 17 m

This wavelength is enormous compared with the dimensions of the ear’s receiving circuit.

There must exist within the brain a phenomenon whereby the pressure variations received by each ear are combined.

This can in fact be observed empirically when, by turning the head, one modifies the orientation of the ears relative to a source generating such a frequency.

There is here a whole study to be undertaken, which we intend to carry out shortly.

From the frequency 40 c/s onwards, the sound appears continuous. From this point one may speak of musical sound.

By continuing to raise the frequency, we traverse the whole sound spectrum.

We pass over the frequencies corresponding to the maximum sensitivity of the ear (from 750 c/s to 5,000 c/s) and, continuing towards the high frequencies, reach the upper limit of the sound spectrum.

This limit varies with the individual. It is situated at around 20,000 c/s in children and decreases progressively with age. In the normal case, it reaches 12,000 c/s in an elderly person (1).

If the frequency rises still further, we enter the domain of ultrasound. There is then no longer any sound sensation for the human ear.

Thus we can situate the sound spectrum within the interval 40 c/s — 16,000 c/s.

(1) This refers to the upper limit of perceptible frequency, that is to say, the intensity is not defined and may exceed the normal threshold of acuity.

Complex tone, simple tone

To carry out a valid investigation of auditory selectivity, one must use, within the sound spectrum, simple tones corresponding to pure frequencies.

Indeed, a musical instrument produces a sound that does not correspond to a single frequency. The fundamental tone is associated with a whole series of harmonics, and the intensity ratios between the fundamental tone and harmonics determine the timbre of the instrument.

If the fundamental tone has a frequency f, the sound produced by the instrument will be formed by the superposition of several pure frequencies, at variable intensity values depending on the instrument, these frequencies being f, 2f, 3f, … nf. The multiples of f are called harmonics.

The tones we shall use will be produced by a rigorously sinusoidal disturbance and will correspond to a single frequency.

It is moreover quite curious to note that harmonics do not produce any modification in the apparent pitch of a sound, although their intensity sometimes exceeds that of the fundamental tone.

If, using a filter, we cut the fundamental tone f and the first two harmonics of a note rich in harmonics, the ear succeeds in reconstituting the frequencies that have been eliminated.

The fundamental tone of frequency f is reconstituted as the differential of the harmonics (5f and 4f) or (6f and 5f), since 5f − 4f = f.

The harmonics 2f and 3f are likewise reconstituted as differentials of (6f and 4f) or (7f and 5f).

The cut therefore modifies the timbre, since the differentials have a weaker intensity than the corresponding frequencies before the cut, but the apparent pitch remains unchanged.

Intensity and frequency

A pitfall to be avoided in the investigation of an individual’s auditory selectivity lies in the fact that many people, while perfectly knowing the difference between a low and a high tone, have a sensation of a shift in frequency towards the high end when the intensity of a high tone is increased.

This phenomenon is analogous to that which makes us see a coloured patch all the more vividly as it is more violently illuminated by white light.

To avoid this risk of error, it is well to plot an audiogram before the selectivity test. Then, taking account of the results recorded on the audiogram, the investigation is carried out by emitting each tone at an intensity, say, 25 dB above the threshold of acuity for each frequency. In this way, the individual will have a sensation of constant intensity.

Parallel with vision: colour-blindness and achromatopsia

In earlier articles we have already drawn a series of comparisons between vision and hearing. We may draw a new one as regards selectivity.

The normal eye is selective — that is, it provides a different colour sensation according to the frequency impressing it. If this is not so, we are dealing with a defect of selectivity.

Thus the subject is able to perceive only certain colours (colour-blindness: Dalton did not see the colour red), a partial defect of selectivity, or achromatopsia, total absence of selectivity (those affected by achromatopsia perceive only more or less dark varieties of grey).

These forms of affliction are more frequent in men than in women.

Auditory selectivity and individual hearings

In the domain of hearing, cases of selectivity throughout the whole sound spectrum are rather rare. In general, selectivity exists in the low frequencies, the middle and the high frequencies up to 3,000 c/s.

Thereafter, and according to the individual, it disappears. We have thus been able to establish that the tenor ear is not selective above 3,000 c/s, whereas the baritone ear is selective up to 6,000 c/s (and even 8,000 c/s and 12,000 c/s).

It should be noted moreover that defects of selectivity often concern the fundamental tones but not their harmonics (the highest note of a piano has a fundamental of 3,480 c/s).

The partial lack of auditory selectivity is very pronounced, in the selective band (small intervals), for the tenor ear than for the baritone ear.

Musicians have very great selectivity in the band of fundamental frequencies. Thus a trained violinist (generally a tenor ear, whereas a cellist will have a baritone ear) executing their echo has, in the fifth, a sensation of a true accord. In the case of a sustained accord — the accord being played note by note, first one then the other — the error for the fifth reaches 1 comma (2).

The ear is thus more selective when two tones are transmitted to it in harmony than when they reach it in melody. In the first case, the ear alone is engaged; the second brings memory into play.

There exist moreover various categories among very musical ears. Some individuals are able to define instantly any interval whatever. This is the relative musical ear. Others are able not only to define an interval or a chord but also to situate each of the notes that compose it within the sound spectrum. This is the absolute musical ear. The latter case is exceedingly rare, and the selectivity of those endowed with it lies far above the average.

(2) The comma corresponds to 5 savarts. It is the ratio existing between the major tone and the minor tone.

Applications

Thus we have seen that auditory selectivity is of great interest from the musical point of view. For the phoniatrician or the singing teacher, knowing the state of auditory selectivity of the person with whom they are dealing may guide them in the choice of a method, both for education and for re-education.

Later, statistical laws on selectivity may be established as has been done for audiometry, and selectivity will then find its use in the technique of audio-phonatory diagnosis.

Source: Tomatis A., “La sélectivité auditive”, Bulletin du Centre d’Études et de Recherches Médicales de la S.F.E.C.M.A.S., October 1954, pp. 128-132. Digitised document from the personal archives of Alfred Tomatis.