period three
Acoustical Analysis
notes
Example of Multiple Sound Paths
| 80 | | | | | | | | |
mPa | 70 | | | | | | | | |
60 | | | | supply | | | |
20 | | | | airborne | | | |
| | | | | | |
ref | | | | | | | |
| | | | | | | | |
dB | 50 | | | | | | | | |
| | | | | | | | |
pressure, | 40 | | | | | | total | |
| | | | | | | |
| | | | | | | | |
sound | 30 | | | | | | | | |
| supply | | return | | | wall |
| 20 | breakout | | airborne | | transmission |
| | | |
| 10 | 63 | 125 | 250 | 500 | 1,000 | 2,000 | 4,000 | 8,000 |
| |
octave-band frequency, Hz | Figure 43 |
As mentioned previously, the total sound heard by the receiver is the sum of sounds from multiple sources, following multiple paths. After each path is modeled to determine its contribution to the sound-pressure level at the receiver location, the paths must be summed to complete the model. While separating the individual paths is necessary for modeling, a secondary benefit is that the magnitude of the various paths can be compared.
In this example, sound travels from a single source to the receiver along four separate paths: supply airborne, supply breakout, return airborne, and transmission through the adjacent wall. By modeling these four paths independently, you can see that the supply airborne path contributes to the total sound-pressure level in the space much more than the other three paths. In fact, when the sounds due to all four paths are logarithmically summed, the total sound heard by the receiver is nearly the same as the sound due to the supply airborne path alone.
This would indicate that, if the sound-pressure level in the space is too high, the designer should focus first on reducing the sound due to the supply airborne path. Reducing the sound due to the return airborne path, without addressing the supply airborne path, would have no effect on the total sound- pressure level heard in the space.
