Text: The state of moderate relaxation which is usual to it, is the most favourable state for vibrating in sympathy with sounds of a wide range. The membrane vibrates reciprocally as a whole if the sound is in unison with the note to which it is (so to say) tuned by the muscles of the small bones, i.e., its fundamental; or in resonance in divisions, (harmonics), if the note sounded is higher than this, one of its harmonics. That it is not always tuned to the very note sounded is obvious, when we consider that this can only be the case when one note only is sounded. It cannot, of course, vibrate reciprocally to a note lower than its fundamental. A membrane has a large power of vibrating sympathetically since its harmonics are very numerous. The effect of increasing the tension of the membrane may be easily tested by closing the nose and mouth and either blowing air out from the lungs or drawing it in. By the former we blow air through the eustachian tube into the cavity of the tympanum, and force the membrane outwards, by the latter we decrease the pressure in the tympanic cavity, and the external air forces the membrane still more inwards than is naturally the case. The result is in either case the same, the sense of hearing is on the whole impaired, though very high sounds are heard better than before. We have stretched the membrane, raised its fundamental note, and diminished its power of vibrating in sympathy with low notes, though we have at the same time increased its range of sympathy upwards. Still it is chiefly improved for reciprocal vibrations, for a lax membrane divides itself far more readily into segments which vibrate in sympathy with harmonics, the strength of such vibrations being increased by the number of the segments into which it divides itself.

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Source: 125