SENSE OF HEARING. 621 



merely withdrawing the corks, easily evoke a simple melody that 

 every one would recognize." 



Musical sounds differ in intensity, pitch, and quality. . 



Intensity. This depends upon the energy with which the 

 particles of air in vibration strike upon the air, and : (1) Varies 

 directly as the square of the amplitude of vibration of the sound- 

 ing body ; (2) varies inversely as the square of its distance ; and 

 (3) diminishes with the density of the air. 



Loudness is oftentimes spoken of as synonymous with intensity, 

 but it depends somewhat upon the condition of the ear and upon 

 the rate of vibration, all sounds not affecting the ear alike, as well 

 as upon the energy of vibration. 



Pitch. This depends upon the frequency of vibration or vibra- 

 tion frequency, as it is called ; the more vibrations per second, the 

 higher is the pitch. There must be at least 30 vibrations in a 

 second to produce a continuous sound, and when these are more 

 frequent than 38,000 in a second they become inaudible, at least 

 to the human ear, although other creatures than man may still 

 hear them. These limits are those assigned by Helmholtz ; others 

 give the lowest number as 16 and the highest as 41,000. Most 

 musical sounds are produced by vibrations between 27 and 4000 

 a second. 



Quality. This is called also timbre and tone-color. As it is a 

 property of sound which is difficult to understand, and even to 

 define, we will quote some of the definitions which are given of it. 

 Thus the Standard Dictionary defines " quality" as "That which 

 distinguishes sounds of the same pitch and intensity from different 

 sources, as from different instruments." And this same authority 

 defines " timbre" as "The special peculiarity of a continuous 

 sound or musical tone, or that common to all tones from the same 

 source, as the human voice or some particular instrument, dis- 

 tinguishing them from notes from different sources, due to the 

 special form of the sound-waves ; the quality of a tone, as distin- 

 guished from intensity and pitch, called sometimes tone-color." 

 This sentence is quoted by the Standard from Silliman's Physics : 

 " The essential difference between the bass and tenor voices, and 

 between the contralto and soprano, consists in the tone or timbre 

 which distinguishes them even when they are singing the same 

 note." 



Quality is, then, concisely, that which enables us to distinguish 

 one sonorous body from another as, a piano from a violin, or a 

 flute from a harp, etc. ; and Helmholtz has demonstrated that 

 "the quality of a sound is determined by the number, order, and 

 relative intensity of the partial tones into which it can be decom- 

 posed" To understand this it will be necessary to consider 

 briefly the compound character of musical sounds. 



Compound, Fundamental, and Partial Tones. When a string 



