As air passes through an attenuator it creates noise. This ‘regenerated’ sound level increases significantly as the air velocity increases, but the level of energy is also related to the area producing the sound, i.e. the area of the airways.

If the attenuated sound level is relatively high and the regenerated sound level is relatively low, this effect is insignificant. However, if the reverse is true, then the regenerated sound level is higher than the attenuated sound level. This means that although the original sound has been attenuated to a low level, the overall sound level emanating from the attenuator is higher because of the regenerated sound.

In some cases this may not matter but in all cases regenerated sound must also be considered to ensure the attenuator performs as expected.

In this example, the light blue unattenuated level is reduced to around 0 decibels (dB) in the 1kHz, 2kHz and 4kHz octave bands by the Static Insertion Loss of around 25dB to 30dB. However, the regenerated sound level is around 15dB in each of these bands, giving an overall level of around 15dB and a Dynamic Insertion Loss of between 10dB and 15dB instead. This means the attenuator’s actual performance is around 15dB to 20dB lower than would otherwise be expected in these frequency bands.