An attenuator can provide very high levels of performance. However, sound can break through even very small gaps with relatively little attenuation. If a gap equates to 0.1% of the total surface area, this can limit performance to 30dB. Even if the gap is only 0.01%, the performance may be limited to 40dB. This means that if an attenuator is expected to achieve 50dB attenuation but there is a small—e.g. 1 millimetre—gap at the end of the attenuator, the actual performance may be much lower than expected.
In this example it is expected that the attenuator’s performance will be around 45dB to 50dB in the 1kHz to 8kHz octave bands. However the sound breaking through a small gap along one side of the attenuator limits this to 40dB. This means that the overall sound level after the attenuator is approximately 10dB higher than expected in these frequency bands.