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In Reply to: How to achieve full body sound? posted by EventHorizon on June 12, 2002 at 14:05:21:
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.
I agree and have tried to follow these rules since reading Roy Allison's article and graphs in Stereo Review years ago, but one thing was never made clear. When calculating the distance from the center of the woofer to the rear wall, does one include the distance from its center to the edge of the baffle and then back to the wall, or just straight back? Thanks in advance.
Measure from the center of the woofer straight back to the boundary (wall).
You can set up the crossover to the sub so that it operates up to powerband frequencies (100 Hz - 300 Hz) , but doing so adds way too much mud and thickness to the overall sound. Furthermore, the output of the sub and that of the main speakers can cancel if they overlap. Most sub-woofers work best reproducing the really low stuff, sub-60 Hz bass (leaving upper bass reproduction to the main speakers). I think subwoofers are mostly ineffective with correcting the boundary nulls which afflict your main speakers in the powerband. Don't forget...that subs too are subject to boundary effects, so the lower you set the sub's crossover, the less the sub's bass will be affected by these quarter-wave cancellations.
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