The sound level within an attenuator diminishes along the length of the attenuator as sound energy is absorbed. However, energy is also transmitted by the attenuator’s casing and possibly other paths, such as supporting framework. Although the attenuator may be able to provide high levels of attenuation to sound travelling through it, it may be the case that more energy is transmitted along these other paths. The total sound energy after the attenuator, which is the (logarithmic) sum of the energy transmitted along all paths, will then be higher than expected because of the additional flanking energy.

In this example it is expected that the attenuated level will be 41dB in the 1kHz octave band. However the level of flanking sound bypassing the attenuator is 50dB. This means that the overall sound level after the attenuator is 51dB and the attenuator’s performance is correspondingly 10dB lower than expected in this frequency band.