This may sound like black magic. Just how is it possible to get an extra octave of bass response from a hifi loudspeaker? Well, the theory supporting this idea originates from Neville Thiele’s 1961 paper (1) on loudspeakers and vented enclosures. He postulated that the response of a loudspeaker in a vented enclosure was similar to a fourth-order high-pass filter, rolling off in the bass region at
-24dB per octave. For a sealed enclosure, the response was similar to a second order high-pass filter, rolling off at -12dB per octave.

Fig.1 shows this for hypothetical speakers that are -3dB down at 70Hz (the cutoff frequency), in each type of enclosure. Now if we apply bass boost with an amplitude of +3dB at 70Hz, rising to a maximum boost of around 11dB or so (for a sealed enclosure), it will partially compensate for the speaker’s rolloff and thus extend the bass response by as much as an octave.

As we’ll see later, the Bass Extender can be tailored for either type of enclosure, applying less boost to a vented enclosure than a sealed enclosure. This is the opposite of what you might expect but is necessary because the speaker cone in a vented enclosure has little loading below the box resonance.

There is a limit to the amount of bass compensation we can apply anyway. A speaker’s cone excursion increases as frequency decreases, so large bass boost levels would test the mechanics of the speaker as well as the damping ability of the enclosure. Also, it is likely that some power amplifiers would run into clipping.