Equalisation and frequency response.

Equalisation of my IB subwoofer response was carried out using the popular DSP1124 Behringer Feedback Destroyer. Popularly known as the BFD. The REW test software was downloaded from The Home Theatre Shack Forum along with a calibration file for my Galaxy 140 SPL meter. The Galaxy was used as the test microphone at the listening position. It has proved in tests to be highly repeatable and accurate down to 5Hz using the matching calibration file. Far better than the commonly used Radioshack SPL meter.

I used a +16dB BFD filter at 20Hz to lift the extreme bass using the maximum filter bandwidth of 120/60. (two octaves) The BFD has a lowest frequency filter option of 20Hz. This heavy boost was necessary because my original drivers had an Fs of 32Hz. Fs is the Thiele-Small parameter for the free air resonance of the driver.

The graph below shows the frequency response of my IB and my Mission 753 Freedom speakers after subwoofer equalisation. The curve has since been slightly improved after repeated test sweeps with the REW software and changing the filters slightly on the BFD parametric equaliser. Note that my IB subwoofer reaches 12Hz at full power before falling away quite rapidly. This extension into the infrasonic is rarely matched by any commercial subwoofer. The bass is deliberately louder (hotter) than the output from the speakers. An 80Hz 24dB/octave active crossover is used between the IB sub and the speakers. (Behringer CX2310)

My Mission 753F speakers are driven by a Naim NAC72-Hicap- NAP 180 stereo system. Surround sound (DTS & Dolby 5.1) for films is provided by an old Yamaha DSP E800 processor/amplifier. The resulting sound quality leaves affordable AV Receivers for dead.

Click on the graph for a larger image.

My 27 feet long attic AV room is very heavily damped acoustically. Because of a central chimney the listening triangle is arranged across only one half the room. The whole area behind the speakers (concealed by a curtain) is used for clothes storage with bookshelves behind which greatly adds to the room's acoustic damping. There is an open stairwell in the floor between the listener and the main speakers! All room surfaces are boarded. All this combines to absorb a very wide range of frequencies totally devoid of echo.

No attempt to fill the obvious troughs in the response by altering phase or crossover frequency has had any real effect so far. The system still sounds wonderful to my ears. The quality, massive weight, lack of colouration and startling reality of the bass is beyond all expectation. Whether thundering on a great organ pipe or the "War of The Worlds" film DVD I could not be happier with the sound quality. There is simply no comparison with any commercial subwoofer I have ever heard. The IB totally outclasses them in every respect.