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Deep nulls in the frequency response in the bass region can be caused by phase cancellation between the direct sound wave from a sound producing device and the indirect sound wave that has reflected from a nearby boundary such as the floor, ceiling or walls. This phenomenon is called speaker boundary interference or SBIR for short. This measurement shows an example of SBIR effects:
141 282Hz. Since the sub is crossed over at 80Hz there is only limited cancellation at 141 282Hz since the frequency response is many dB down as a result of the rolloff of frequencies above the crossover point. The cancellation caused by the main speakers being 4ft out from the back wall is also reduced since the crossover is attenuating frequencies below the crossover point.
What do you think about this cunning subwoofery?
Nyal Mellor, Founder, Acoustic Frontiers