Almighty Subwoofers

By Stig Erik Tangen
01 Jan 1997

Filters

Successful employment of a passive filter in a subwoofer is highly unrealistic. Creating only a crossover function at 100-150 Hz with a 4th order slope might be within reach, but you can just forget about a lower crossover frequency or equalizing. The reasons for this is the difficult impedance of the driver in the cross-over range, and the extremely large component values needed. A successful result requires extensive impedance correction, which will lower the system impedance. Passive EQ will also greatly lower the efficiency. In the end, the system becomes virtually impossible to drive. Active filters is the only solution!

The filter is EQ'ing the system as I have described in a previous section, and crossing at 100 Hz with 4th order Linkwitz-Riley slope (IMHO, the only correct filter type). The filter block schematics is shown below. The filters are best made up with op-amps, I use the LM837 (quadruple). LM837 is designed for audio use, has very good data and (best of all) is not expensive. Sounds good too...

The first two sections are the equalizer. The two last 2nd order filters are in combination a 4th order Linkwitz-Riley filter. The equalizer's roll-off above 100 Hz creates a steeper total roll-off than a 4th order function, but this does not cause any phase problems in the crossover region, as the 180 degrees phase shift is located exactly at 100 Hz. The graph to the left shows a simulation of the equalized frequency response including the crossover (solid line) compared to the 4th order Linkwith-Riley function (dotted line).

The main system used with the subwoofer must be crossed with the same filter slope as the sub. The main system should also have approx. flat response at least one octave below the crossover frequency in order to get a flat summed response using a by-the-book 4th order filter for the main system. Using the sub with "satellites" (small speakers) with its cut-off above 50 Hz will require a different filter. The best way to design this is by simulation of the whole system in a proper crossover software (The only ones I can recommend is CALSOD or LEAP).

My own solution is using a 2nd order filter at 100 Hz with quite high Q for the main systems. Their acoustic response falls off 12 dB/octave below 100 Hz (closed box) with low Q at resonance. The sum is a 4th order roll-off with -6 dB at 100 Hz, and perfect phase correlation at 100 Hz (180 degrees shift for both the sub and main system).

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