650 A TEXTBOOK OF PHYSIOLOGY 



Sound. Sound is the sensation produced through the organ of 

 hearing by the vibrations emanating from vibrating bodies. Such 

 vibrations travel by the air at the rate of 1,100 feet per 

 second. 



Physiologically, sounds may be divided into noises and musical 

 tones, although one may merge imperceptibly into the other as, 

 for example, the tuning-up of an orchestra. Many so-called noises 

 are in reality more musical than otherwise. Vibrations of the air at 

 regular intervals produce what is termed a musical sound ; vibrations 

 at irregular intervals produce an unmusical sound or noise. Sounds 

 may differ in pitch, intensity, and quality or timbre. 



The pitch of a note depends upon the frequency of the vibrations 

 in a given time. The more frequent the vibrations, the higher the 

 note; the less frequent, the lower the note. The range of tones 

 employed in music varies between 30 and 4,000 per second, although 

 it is possible for the ear to perceive notes and vibration-rates 

 as high as 40,000 per second. The relation between frequency 

 of vibration and the pitch of a note is best shown by means of the 

 siren. When the wheel is rotating slowly, nothing is heard but the 

 puffs of air; as the speed increases, the puffs begin to fuse, and then 

 produce a low buzz, rising with increasing speed to such a height 

 that the note finally becomes decidedly unpleasant. 



The sensibility to pitch varies in different people, as does the power 

 of distinguishing notes of nearly the same vibration. This latter 

 defect may generally be improved by training, although there are 

 certain people who are " tone-deaf." They can only discriminate a 

 few tones, and find it impossible to recognize a tune or to sing in 

 tune. Such people only recognize the tune of the National Anthem 

 by the fact that others are standing up with their hats off. The 

 extreme range of the human voice is about half the range of the human 

 ear for musical tones. 



The intensity or loudness of a note depends upon the amplitude 

 of vibration of the sounding body. This is well seen in the tuning- 

 fork. When the fork is vibrating visibly, the note is loud, and as the 

 visible vibrations pass away, so the note diminishes in loudness or 

 intensity, the pitch remaining the same. If the ear be held to a vibrat- 

 ing tuning-fork, it will be found that the note is loudest when the 

 limbs are vibrating in a plane at right angles to the external ear, 

 since in this position the air is most disturbed, causing a greater 

 difference in pressure upon the tympanum. 



The quality or timbre of a musical note enables us to tell the 

 instrument by which it is produced. It is easy for most people to 

 distinguish between the human voice, the note of the violin, and the 

 note of the clarionet. This is due to the characteristic wave-forms 

 which are being produced in each case. Just as no two great waves 

 of the sea are exactly alike, but differ in the shape of the crests and 

 wavelets, so the wave-forms of different musical instruments vary. 

 The tones emitted are really compound tones, and contain numbers 

 of wavelets or "overtones." The fundamental tone is due to that 



