THE PRODUCTION OF SOUNDS. 203 



of the laryngeal motor fibres of the pneuraogastric are derived from 

 the spinal accessory nerve. 



The Production and Modification of Sounds. 



Whenever a solid body surrounded by air, is thrown into vibrations, in any 

 direction, the sensation of sound is produced in the ear, provided the vibrations 

 be of a certain strength, and follow each other with a certain rapidity. It is 

 usually stated, that if the vibrations are fewer than 8, or more than 24,000 per 

 second, no effect is produced on the nerve of hearing ; but, according to other 

 authorities, fewer than 16 or 32 vibrations per second are inaudible ; and vi- 

 brations continue to be so which number 32,000, or even 70,000 per second. 

 When the vibrations exceed a certain high number, the distinction between 

 two near sounds is no longer possible ; the perception of sound remaining, 

 though not the power of distinguishing them. Bodies vibrate by virtue of the 

 elasticity imparted to them by their molecular structure. The undulations of 

 the air may generate sound, or sound may be communicated to the air by the 

 vibrations of another body. For the production of a musical sound, the vibra- 

 tions must succeed each other at regular intervals ; if the vibrations occur at 

 irregular intervals, only a noise results. The pitch of a sound is determined 

 by the number of vibrations in a given space of time, becoming higher in a 

 direct proportion to the rate of rapidity of the vibrations. Its strength or in- 

 tensity, depends on the extent of the vibratory action of the sonorous body. 

 The peculiar character of a musical note, whereby it can at once be distin- 

 guished from another note of exactly the same pitch and strength, is called its 

 tone or timbre, and is dependent on the nature and shape of the vibrating body. 

 A sonorous body may vibrate throughout its whole mass, or in separate parts ; 

 in the latter case, these parts vibrate in opposite directions, and are separated 

 from each other by stationary points called nodes or nodal points. 



The stretched cords, or strings, of stringed instruments are examples of bodies 

 rendered elastic by tension. They emit feeble tones, unless they are connected 

 with some resonant body. When a tense cord is made to vibrate throughout 

 its entire length, it yields its deepest or fundamental note ; if the cord be di- 

 vided into equal parts by a bridge placed under it, the note heard, when it is 

 made to vibrate, is the octave of the fundamental note. Hence the law, that 

 the number of vibrations of any two strings, having the same degree of tension, 

 is, other things being equal, inversely as their length. The number of vibra- 

 tions is also dependent on the thickness of the strings and their tension, being 

 inversely as the thickness, and proportional to the square root of the tension. 

 During the transverse vibration of a cord in its entire length, other and higher 

 sounds than the fundamental note may be heard, produced by the vibrations 

 of aliquot parts of the cord. These aliquot parts are called the harmonic di- 

 visions of the cord. 



The vibrations of elastic rods resemble those of strings ; but the number of 

 vibrations is inversely as the square of the length, and directly as the thickness 

 of the rod. 



The musical sounds in simple wind instruments, are the result of the succes- 

 sive condensations and rarefactions of the air through a tube. The pitch of 

 the note, when the column of air within a tube is thrown into vibrations, is 

 determined by the length of the tube and the strength of the blast ; being lower 

 in a direct ratio with the greater length of the tube, and higher the greater the 

 force of the impulse, for increase in the strength of the blast leads to the for- 

 mation of nodal points. If the air, in a tube closed at one extremity, be thrown 

 into undulations, the deepest, or fundamental, note is an octave lower than 

 that yielded by a similar tube with an open extremity ; in the latter case, a 

 nodal point is formed in the centre of the column of air ; whilst in the former, 

 the nodal point is the closed extremity of the tube. 



The essential parts which enter into the formation of tongued instruments are 

 first, the wind-tube, through which the column of air is driven ; secondly, 

 the tongue, or vibrating body, which may be rigid or membranous ; and lastly, 

 the attached tube, placed beyond the tongue. 



