The Science of Sound Part VII: Beating Flanking Paths
In architectural acoustics, we are concerned with controlling the amplitude and/or the duration of the sound. In walls and partitions, this is done by controlling sound transmission loss and sound absorption. When sound waves strike a partition, some are reflected from the surface, staying on the same room as the source of the sound. Some are absorbed by the material covering the partition and some are transmitted through to the other side.
SOUND TRANSMISSION LOSS (STL) is the effectiveness of a barrier at preventing sound from transmitting from one side to the other. It is measured in decibels (dB), the same as amplitude. To determine STL, one measures the sound level on the side of the barrier closer to the sound source, the level on the opposite side of the barrier, and the reverberation or absorption of the receiving room. The result is given by the following equation:
SOUND ABSORPTION is the effectiveness of a surface material at preventing the reflection of sound. The more sound absorption, the less echoing will exist. The absorption of a material is measured in Sabines (see explanation below) and is found by the equation:
It is very important to know the difference between a sound barrier and a sound absorber. Typically barriers made of hard, dense material may actually increase the amount of echo in the room, while absorbent batts of insulation allow sound to pass through as if they weren’t there. Generally speaking, you can’t use a barrier to absorb sound and you can’t use an absorber to block sound.