Science and music / [by Sir James Jeans].

  • Jeans, James, 1877-1946.
Date:
1947
Catalogue details

Licence: In copyright

Credit: Science and music / [by Sir James Jeans]. Source: Wellcome Collection.

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    The converse experiment can also be performed. An animal is made deaf to a note of any selected pitch, and the corresponding section of its basilar membrane is then found to have been damaged. This makes it abundantly clear that each fibre is in some way associated with a definite frequency of sound, and it is natural to suspect that these frequencies are those of the free vibrations of the fibres. It may seem incredible that a fibre less than a fifteenth of an inch in length can be made to emit the same note as a piano wire 6 feet in length—for this is implied in the hypothesis we are considering—but calculation suggests that there is no impossibility. We can lower the pitch of a stretched string by adding to its mass— as indeed is commonly done in the piano by coiling copper wire round the bass strings. The fibres of the basilar mem¬ brane are embedded in the membrane itself and, as this shares the motion of the fibres, it adds to their effective mass. Moreover the motion of the membrane sets the fluid which fills the cochlea into motion, so that this also adds to the effective mass of the fibres, especially at the bass end, where the galleries are widest. Beatty has estimated that if the bass fibres are stretched to a tension of about 3 lb. to the square inch of cross- section, they will vibrate in the frequencies required of them. He finds also that for the treble fibres to vibrate in their required frequencies, they must be stretched to a tension of about 4 tons to the square inch. This is a large, but by no means impossible, tension. Human hair can be stretched to a tension of about 9 tons to the square inch before breaking, while catgut and silkworm gut will survive ten¬ sions of 27 and 32 tons to the square inch respectively. A