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Boundary Effects

You have one of two problems. Either the speakers have a natural cancellation in the (100 Hz - 300 Hz) passband (known as the power band - a cancellation caused by driver to driver interference) or (more likely - given a well designed speaker) you have a problem with a room reflection nulling the drivers output in the powerband.

You can correct the suckout by repositioning your speakers in the room. The frequency of these quarter-wave boundary suckouts can be calculated using the formula (N = 1130/D x .3), where D equals the distance (in feet) from the woofer to the boundary and N equals the frequency at which the first 1 dB boundary null appears. If the center of the woofer is the same distance from each of the nearest two boundaries, such as the wall behind the speaker and the floor below the speaker, the dip is 3 dB. If the center of the woofer is the same distance from each of the nearest three surfaces, the suckout is approximately 11 dB. For instance, a woofer positioned 24 inches above the floor results in a null at 169.50 Hz (1130/2 x .3 = 169.50 Hz) everywhere in the room. If the woofer is also 24 inches from the back and side wall, we are talking about an 11 dB deep null in a frequency band (the power band - 100 Hz to 300 Hz) responsible for much of the weight, warmth and impact of music. Nulls at power band frequencies result in diminished musical scale, "smallness," "thinness" and "brightness" as the entire treble band sounds subjectively enhanced. The trick (to avoiding serious bass nulls) is to make sure the speaker is a different distance from each room boundary and none of the distances are multiples of any other.

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