344 INFLECTION OF SOUND. 



even a short melody played on a trumpet, will be n 

 produced by the echo. Multiple echoes can be produce 

 in two different ways namely, either by the repeate 

 reflection of a sound backwards and forwards betwee 

 two parallel surfaces, such as two walls standing opp< 

 site each other ; or when there are several reflectir 

 surfaces at different distances from the ear, so that tl 

 separate echoes are heard one after another. 



33. Number of Vibrations. The Siren. Pitch. Tl 



sensations of sound are of great variety. Indepen< 

 ently of the varying intensity of sounds, the mo 

 striking difference is that between a noise and a music 

 note or tone. A noise is produced either by a sing 

 powerful explosive disturbance of the air, as, f 

 instance, by a sudden blow, or the report of a pist 

 or several disturbances interfere with one anoth 

 so as to produce confused waves in the air, as i 

 instance in those sounds commonly designated \ 

 rattling, rustling, hissing, etc. ; in which the vibratio' 

 follow one another either at irregular intervals or |> 

 slowly that each one can be perceived separately 

 A tone or musical note, on the contrary, is producj 

 by vibrations which follow each other rapidly and 

 regular intervals. 



It has already been stated that the particles oil* 

 sounding body have a vibratory motion. The partic 

 oscillate like a pendulum on either side of a defin 

 position of equilibrium, completing each vibration j 

 a definite length of time, which continues the same |fi 

 long as the motion lasts. The space traversed by e.n i 

 particle of a sounding body and the time required 1 * 

 one vibration are, however, much smaller than in 1 



