An important property of sound in practically all enclosed space is that reflections occur from the walls, ceiling, and floor. For a typical living space, 50 to 90 percent of the energy is reflected at the borders. These reflections are heard as echoes if sufficient time elapses between the initial sound and the reflected sound. Since sound travels about a foot per millisecond, delays between the initial and secondary sound will be of the order of 10 to 20 ms for a modest room. Practically no one reports hearing echoes in typical small classrooms when a transient sound is initiated by a snap of the fingers. The echoes are not heard, although the reflected sound may arrive as much as 30 to 50 ms later. This demonstration is designed to make the point that these echoes do exist and are appreciable in size. Our hearing mechanism somehow manages to suppress the later-arriving reflections, and they are simply not noticed.
The demonstration makes these reflections evident, however, by playing the recorded sound backward in time. The transient sound is the blow of a hammer on a brick, the more sustained sound is the narration of an old Scottish prayer. Three different acoustic environments are used, an anechoic (echoless) room, a typical conference room, similar acoustically to many living rooms, and finally a highly reverberant room with cement floor, hard plaster walls and ceiling. Little reverberation is apparent in any of the rooms when the recording is played forward, but the reversed playback makes the echoes evident in the environment where they do occur.
Note that changes in the quality of the voice are evident as one changes rooms even when the recording is played forward. These changes in quality are caused by differences in amount and duration of the reflections occurring in these different environments. The reflections are not heard as echoes, however, but as subtle, and difficult to describe, changes in voice quality. All recordings were made with the speaker's mouth about 0.3 meters from the microphone.
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