Home Engineering Building Acoustics
Experiencing Reverberation (or its Lack)
On entering a premises, one has often experienced an acoustic feeling of the place (e.g., it sounds lively, or alternatively, it sounds dead). Of course, it is tempting to quantify this impression. The corresponding quantity is known as reverberation time. In the olden days, it was measured by emitting a loud noise while pressing the chronometer and stopping it when it was no longer audible.
Basically, reverberation comes from the multiple reflections on the inner envelope of the room, as displayed in the example of Figure 2.5. In a real hall the reverberation time curve will show a succession of waves, as illustrated in Figure 2.3.
Reverberation and Echoes
Reverberation is made of numerous acoustic reflections that are spread over time so as to avoid any gap longer than 100 ms and preferably no more than 22 ms from any other similar reflection.
Figure 2.3 Example of reverberation time curve in the 500 Hz third octave band.
Figure 2.4 I ncident, reflected, absorbed, and transmitted waves.
Figure 2.5 Example of acoustic relections in a room resulting in reverberation.
Echoes are the product of multiple reflections arriving within a very short time span (no longer than 22 ms) well after the direct sound (i.e., more than 100 ms).
Reverberation time is a bit similar to the time constant of a physical system: the longer the time value, the greater the stability, but on the other hand, the slower the reaction to a change of excitation. This has an important consequence: In a reverberant room one will not have trouble being heard (a whisper would do the trick, as the sound is well sustained), but one will have trouble being understood due to the bad rendition of consonants.
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