Passive attenuators provide most attenuation in the mid to high frequency range but mechanical plant often produces a significant proportion of its acoustic energy in the lower frequency range.
Where only moderate attenuation is required, this discrepancy may not be significant, with the higher frequency sound from the mechanical plant being slightly over-attenuated. However, where more low frequency attenuation is required, this can result in the high frequency sound being very significantly over attenuated. The attenuated sound may then appear to be low frequency dominated, whereas the original unattenuated sound was much more even. This spectral prominence can make the attenuated sound more intrusive and less acceptable than the original unattenuated sound, even though the overall attenuated level is lower.
The red spectrum shows a poorly selected attenuator where the attenuated sound is dominated by the 63Hz and to a lesser extent 125Hz energy, whereas the green spectrum is well matched to the blue criterion.
This problem can also be exacerbated when noise source is outside and the listener is indoors because the building will further attenuate the higher frequency sound more than the low frequency sound.
