Cardioid Subwoofer response using PSP instead of DSP

Hadn't seen this design mentioned on DIYaudio yet, so thought I'd post it.

Cardioid subwoofer response is usually achieved with DSP processing of the signal fed to one woofer in the cardioid woofer array. In this case, the designer has figured out how to do the processing acoustically so no need for DSP or the second power amplifier. PSP = Plywood Signal Processing ;)
All three tuning frequencies for the array are evenly distributed in the operating band providing a nice smooth impedance curve.
More details can be found in the September issue of Live Sound International.
I attached a copy of the patent application.

A couple links to get you going...
d&b's B4: from ugly duckling to sub-bass swan
d&b audiotechnik - B4-SUB


  • Cardioid_Sub_patent_application.pdf
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The B4 sub uses the usual method of adding a second woofer with inverted input signal at the back of the enclosure. Cant read the patent as my pdf viewer somehow doesnt like the file, but the links show nothing new.

Feeding an inverted signal to a second woofer at the back of the enclosure does not give you a cardioid response by itself. The signal must also be delayed in time by the distance separating front and rear woofers. Usually this is done with DSP or opamp based allpass networks which require the rear woofer be driven by a separate power amplifier. The B4 method is not usual. It incorporates acoustic delay in the design of the rear woofer enclosure so there is no need for any signal processing or 2nd amplifier.

You can also read the patent application online here:
Loudspeaker system with reduced rear ... - Google Patent Search
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