PROPERTIES OF SOUND. 543 



cally, sounds consist of vibrations, and these, under most 

 circumstances, when they first reach our auditory organs, 

 are alternating rarefactions and condensations of the air, 

 or aerial waves. When the waves follow one another uni- 

 formly, or periodically, the resulting sensation (if any) is 

 a note; when the vibrations are aperiodic it is a noise. 

 In notes we recognize (1) loudness or intensity; (2) pitch; 

 (3) quality or timbre, or, as it has been called, tone color; 

 a note of a given loudness and pitch produced by a trum- 

 pet and by a violin has a different character or individu- 

 ality in each case; this quality is its timbre. Before un- 

 derstanding the working of the auditory mechanism we 

 must get some idea of the physical qualities in objective 

 sound which the subjective differences of auditory sensa- 

 tions are signs of. 



The loudness of a sound d^p^n^f} n^ f>m fprfifi of the 

 .aerial jvayej; the greater the intensity of the alternating 

 condensations and rarefactions of these in the external 

 auditory meatus, the louder the sound. The pitch of a 

 note depends on the length of the waves, that is the dis^"- y 

 tance from one point of greatest condensation to the next, 

 or (what amounts to the same thing) on the number of 

 waves reaching the ear in given time, say a second. The 

 shorter the waves the more rapidly they follow one another, 

 and the higher the pitch of the note. When audible 

 vibrations bear the ratio 1 : 2 to one another, we hear the 

 musical interval called an octave. The note c on the un- 

 accented octave is due to 132 vibrations in a second. The 

 note c' , the next higher octave of this, is produced by 264 

 vibrations in a second; the next lower octave (great octave, 

 C), by 66 ; and so on. Sound vibrations may be too rapid 

 or too slow in succession to produce sonorous sensations, 

 just as the ultra-violet and ultra-red rays of the solar 

 spectrum fail to excite the retina. The highest-pitched 

 audible note answers to about 38,016 vibrations in a 

 second, but it differs in individuals; many persons cannot 

 hear the cry of a bat nor the chirp of acricket, which lie 

 near this upper audible limit. On the other hand, sounds of 



