Text: 6) One sound differs from another not only in quantity, but also in quality and pitch. The pitch of a sound depends on the number of vibrations per second by which it is caused; the greater this number is the higher is the sound, and vice verse; thus pitch is a more or less relative term, and it is therefore necessary to have some standard to which different sounds may be referred. This standard is so choosen that the middle C of the pianoforte shall be produced by 264 vibrations per second. (That is, according to German musical pitch, Western pitch is set at 244 cps.)

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Source: 125