Section 5. 

 Hearing. 



The Nature of Sound. — When a body is made to vibrate, its 

 vibrations are communicated to the adjacent air, and give rise 

 to waves which travel at a definite rate ; when these reach the 

 ear they act upon its structures so as to lead to the sensation of 

 sound. The vibrations which constitute the waves take place 

 to and fro along the direction in which the wave is travelling ; 

 in this respect sound differs from light, whose vibrations are 

 transverse to the direction of propagation. 



In comparing one sound with another we are conscious of only 

 three possible differences between them : they may differ in loud- 

 ness, pitch, and quality. Of these, loudness is dependent on the 

 magnitude of the to-and-fro motion of the vibrating particles 

 whose movements transmit the sound ; a loud sound means a 

 large wave. Pitch, on the other hand, depends on the frequency 

 of the vibrations, a high note implying rapid vibrations, or a 

 shorter wave-length. Sounds may be simple or compound. 

 The vibrations of a tuning-fork give rise to a typically simple 

 sound, of varying loudness or pitch, but possessing little quality. 

 Most vibrating bodies do not give rise merely to such simple 

 vibrations, but set up a variable series of different wave-lengths 

 along with their fundamental simple vibration. Thus, most 

 sounds consist of a fundamental tone, accompanied by more or 

 less of these other tones — the partial tones, overtones, or harmonics, 

 as they are termed. The quality of a sound depends upon these 

 harmonics ; where they are absent the tone is thin, where they 

 are present they give richness, and confer on it that ' character ' 

 which enables us to recognise one musical instrument from 

 another by the mere sound it emits. Those sounds which are 

 grouped under the general term of ' musical ' result from the 

 regularity of their causative vibrations, and the definiteness in 

 wave-length of the latter. Noise is essentially the result of the 

 absence of this regularity and definiteness. There is usually no 

 difficulty in discriminating noise from musical sounds, but the 

 one may merge into the other, as in the case of the noise of street 

 traffic when near to it, and the musical humming tone it produces 

 when heard from a distance. From observations on the human 

 subject it has been ascertained that the smallest number of 

 vibrations audible are about thirty per second, whilst the average 

 human ear can recognise up to 30,000 vibrations per second. It 



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