AIRBORNE ECHOES 



use sounds of short duration simply because they are 

 less likely to overlap and be wholly masked by their 

 originals. The sounds of spoken syllables or choking 

 scissors are not as short as one would ideally Uke to use. 

 Any sound shorter than about 1/lOth second is usually 

 called a cUck, and the shorter it is the sharper it sounds, 

 provided it has a reasonably high energy level. The hu- 

 man voice cannot produce really short cUcks, however, 

 nor can any other ordinary sound source. An electric 

 spark caused by the discharge of a condenser is a very 

 sharp cUck, and the discharge of a condenser through 

 a loudspeaker is nearly as sharp, provided that the elec- 

 trical circuit involved does not resonate and prolong the 

 vibration of the speaker diaphragm. A cheaper and more 

 widely available source of sharp clicks is a common toy, 

 the frog or cricket made of a thin strip of spring steel 

 with a dent in the middle. This is clamped tightly to a 

 holder at one end; the other end is free to be pushed 

 back and forth in such a way that the strip is bent and 

 unbent. When your finger bends the strip of steel, the 

 dent is suddenly inverted to impart a very sudden and 

 energetic push to the air as it snaps from concave to 

 convex or vice versa. The result is a very loud and sharp 

 click, painfully loud if generated close to the ear, pos- 

 sibly even damaging if it were to be repeated many times 

 immediately in front of the ear opening. 



The actual duration of the cUck varies from one 

 model of toy to another, and it is affected by the size 

 and shape of the holding device. In small clickers that I 

 have tested the sound falls to 1/lOth its initial maximum 

 within about 10 miUiseconds after the steel dent has 

 snapped from one position to another. Recalling that 

 the velocity of sound in air is approximately one foot 

 per miUisecond, you can calculate that a click lasting 

 10 miUiseconds has a physical length of about 10 feet 



65 



