period four
Equipment Sound Rating
notes
Semireverberant Field
semireverberant | near |
field | field |
source
wall
Figure 58
Most rooms in buildings are somewhere between a free field and a reverberant field environment. Called a semireverberant field, these spaces have some characteristics of both free field and reverberant field environments. The walls, floor, and ceiling prevent the sound from behaving as it would in a free field. These surfaces are not, however, perfectly reflective. Some of the sound is reflected by these surfaces, but a portion of the sound is absorbed or transmitted.
An understanding of how sound behaves in a semireverberant field is important when taking sound measurements. The characteristics of the sound field change with distance when a small sound source is placed in the center of a room. Close to the source, in the near field, sound measurement is unpredictable. Near the wall, in the reverberant field, the reflected sound begins to add to the sound coming directly from the source. The reduction in sound level due to the distance from the source tends to be cancelled out by the addition of the sound reflecting off the wall. This results in a
In the semireverberant field, sound behaves similarly to how it would in a free field. The sound level will decrease as the distance from the source increases, but not as much as it would in an ideal free field.
The construction of the room plays a significant role in determining what portion of the room behaves as a reverberant field, and what portion behaves as a semireverberant field. Small rooms with hard, reflective surfaces behave similarly to reverberant rooms. This description often fits a mechanical equipment room that is constructed of concrete and is small with respect to the size of the sound source.
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