The sound level within an attenuator reduces along the length of the attenuator as the sound energy is absorbed. However, in addition to airborne sound, energy can also be transmitted by vibration entering the supporting structure. Although the attenuator may be able to provide high levels of attenuation to sound travelling through the attenuator, it may be the case that more energy is transmitted by vibration 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 vibration transmitted energy.
In this example it is expected that the attenuated level will be 41dB in the 1kHz octave band. However the level of vibrational sound energy 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.