Attenuation should be closely matched to the required levels in order to optimise the attenuator’s performance, and minimise its size and cost. This also avoids spectral prominence, which can otherwise reduce the apparent effectiveness of the attenuator.
This means that it is important to ensure the data being used to select the attenuator is properly understood.
The most usual problem is with the overall source sound level or its frequency characteristics. However, incorrect airflow data can also cause problems, either due to excessive regenerated noise produced by higher air velocity than expected, or insufficient airflow being achieved due to the attenuator imposing more aerodynamic resistance than expected.
In this example the expected source sound level of 96dBA (with 98dB in the 250Hz octave band) is incorrect, with the actual level of 93dBA being lower, together with 95dB in the 250Hz octave band. However, the level in the 125Hz band has risen from 84dB to 95dB, as a result of which the actual attenuated level is 65dB rather than the expected 63dB, with a prominent, unexpected peak in the 125Hz octave band.