There are two possible ways a subwoofer can be used to reproduce main channel bass: augment or crossover. Augment operation allows the main speakers to operate normally, without a crossover, and the subwoofer is used "fill-in" the deep bass below the range of the main speaker. Crossover mode transfers some of the bass range from the main speakers to the subwoofer. Each of these types of use can be used with two types of main loudspeakers, sealed or reflex. Following are examples illustrating typical results for each case.

It is assumed that the subwoofer crossover configuration is what I consider optimum for generic filters. The sub low-pass (LP) filter is 4th order, Q = 0.5, the main speaker high-pass (HP) filter is 2nd order, Q = 0.7, the HP frequency tracks the LP frequency, and there is also a continuously variable phase control.

The simplest case is to use the crossover with sealed main speakers. In the special case where the crossover frequency is set to the speaker's limit (-3 dB) frequency, the crossover's HP will combine with the speaker's bass response to achieve a total 4th-order, Q = 0.5 high-pass response that will, in theory, perfectly cross over with the 4th-order, Q = 0.5 sub low-pass response.

However, in practice, even this simple case is upset by the complication that the sub's response does not extend to DC. For an 80-Hz crossover, if the sub has a 4th-order rolloff at 35 Hz, the interactive phase effects will cause the whole bass range to be 2 dB weak (Fig. 1).

Fig. 1. Subwoofer extending to 35 Hz and crossing-over at 80 Hz with a sealed, 80-Hz main speaker. Standard settings.

Increasing the signal level to the sub solves this problem, but the system response still suffers from excessive output in the 100 to 200-Hz region.

In the more general case where the crossover frequency is higher than the main speaker's frequency, the results are not as good. For example, if the main speaker's response extends to 50 Hz and the crossover is at 80 Hz, the bass response is 4 dB weak (Fig. 2).

Fig. 2. Subwoofer extending to 35 Hz and crossing-over at 80 Hz with a sealed, 50-Hz main speaker. Standard settings.

Even when the phase and level are adjusted to be optimum (180°, -1 dB) the response is not nearly ideal (Fig. 3).

Fig. 3. Subwoofer extending to 35 Hz and crossing-over at 80 Hz with a sealed, 50-Hz main speaker. Optimum phase and level adjustment.

In cases where the main speakers are ported, things are no better. For example, if the main speakers are tuned to 50 Hz and you want to implement an 80-Hz crossover with the sub extending to 25 Hz, the results achieved without phase adjustment are not what is desired. Even when the phase control is set optimally (80° @ 50 Hz) the results are not good, giving a hump in the bass while still giving weak upper bass (Fig. 4).

Fig. 4. Subwoofer extending to 25 Hz and crossing-over at 80 Hz with a reflex, 50-Hz main speaker. Optimum phase and level adjustment.

If you want the sub to augment the main speakers, without adding a high pass into their response, the results are usually even less desirable. If the main speaker is a reflex type at 50 Hz and the sub extends to 30 Hz, the bass is very weak without a phase adjustment (Fig. 5); the polarity must be reversed to get good results.

Fig. 5. Subwoofer (attempted) extending to 30 Hz augmenting a 50-Hz reflex main speaker. No phase adjustment.

In the case of augmenting a sealed system, it is usually not possible to obtain good results. For example, a 50-Hz speaker will produce a severe null at the crossover frequency without a phase adjustment (Fig. 6).

Fig. 6. Subwoofer augmenting to 30 Hz a 50-Hz a sealed speaker. No phase adjustment.

Even with optimum phase and level, the results are an exaggerated mid bass and a lack of deep bass (Fig. 7).

Fig. 7. Subwoofer (attempted) extending to 30 Hz augmenting a 50-Hz sealed main speaker. Optimum phase adjustment.

These examples illustrate the typical problems inherent in matching a subwoofer to other speakers:

• "Logical" settings do not give the desired results
• It is not known which level and phase control settings will give optimum results. They must be adjusted without rhyme or reason by trail and error
• Optimum results are usually not very good.