AIRBORNE ECHOES 



made, and when played back in the reverse direction, 

 this side will not pass by the pick-up head. In this case 

 the tape must be reversed so that the shiny side rather 

 than the dull is next to the head. This reduces the level 

 of the sound, but the volume control can usually be 

 turned up to compensate for this loss, and the experi- 

 ment can still be performed, though less well than with 

 tape which is recorded across its full width. 



When the tape is played backward, the echoes that 

 followed the original word or click will of course pre- 

 cede it. Since they were hardly noticeable before, one 

 would naturally expect them to be a faint prelude to the 

 reversed sound. But the actual result is a startling in- 

 crease in the apparent loudness of the echoes. A click 

 that sounded very sharp in its original form, or when a 

 tape recording of it is played back in the normal direc- 

 tion, now becomes a gradually rising hiss that culminates 

 in the cUck. The cUck proper does not sound very dif- 

 ferent frontward or backward, but the reversed echoes 

 are much more apparent. So much so that when one hears 

 this demonstration it is difl&cult to beUeve that the mstru- 

 ment has not played some trick, that the whooossschk! is 

 is really the same sound as the sharp cHck that gave rise 

 to it. 



This reversed playback technique reveals the real 

 magnitude of the echoes from various sounds, but it is 

 more difi&cult to appreciate with reversed speech or mu- 

 sic, which sounds very abnormal in other ways. Clicks 

 or pistol shots are in themselves so short that they con- 

 tain only a few irregular sound waves, which are not 

 very different-sounding when played in either direction. 

 This can be demonstrated by repeating the recording 

 out of doors in a quiet area well away from any large 

 building. The clicks will now be accompanied by only 

 minor echoes from the ground or other small objects, 



63 



