Technical & Editorial

Subwoofers: A Brief Look at the Effectiveness of Using a Subwoofer in a Music System


The Under-reported Issue When Adding a Subwoofer

By far, the most significant sonic issue for acoustic music when a subwoofer is deployed occurs around the crossover from the subwoofer to the main speaker. Adding a subwoofer to a system using a standard bass management system without a room EQ in the loop (sub and main channels) will degrade the flatness of the response in this critical area. At a minimum, the in-room transition band of the low-pass and high-pass filters should track the shape of a fourth-order Linkwitz–Riley filter to about 20dB down. With a 4th LR at 80Hz crossover, both speakers will be active between 60Hz and 110Hz (-10 dB points), a frequency range that is populated with a variety of common instruments playing throughout the score.

The standard bass management system provides only a second-order filter for the main speaker which will not even sum to flat even in an anechoic chamber. When the standard bass management system was developed (mid 90s) it was assumed the main channel speaker would roll off -12dB exactly below the crossover frequency to create the 4th order filter. This is never going to happen. Room EQs have to provide additional correction for the main speaker channel to have the correct transition and stop band associated with a 4th order high pass filter. Different main channel speakers will require a custom filter synthesized by the adaptive room equalizer. Once the EQ performs this function, the system is flat in an anechoic chamber, but this is only a small part of what the room EQ must do.

Without additional electronic equalization, the room effects (uneven sound pressure preservation) corrupt the shape of the low-pass and high-pass filter transition bands. In the graph below, the black curve shows the low-pass filter electrical response of the standard bass management systems applied to the full range input. The red curve is the room EQ filter's response in cascade with the LPF.

The in-room high-pass filter transition band of the main speaker is as critical as the in-room low-pass rolloff of the subwoofer. Only an advanced electronic room EQ in a Pre/Pro or AVR can provide the required fourth-order Linkwitz – Riley response at the listening seat for the main channel. Some subs have a room EQ function, but this is only half the problem, even if the EQ works well. Likewise, multiple subwoofer techniques may expend the optimal area for the low pass filters transition and stop band but we can have only one main speaker per channel. Note that even the best room EQ can generate the optimum main channel response in the crossover area for only one listening seat. Passive room treatment can help but it is expensive since it must be effective to 60Hz (-10dB point for the main channel with an 80Hz crossover).

The crossover frequency must also be under the user's control. The user must select a crossover point high enough that the subwoofer enters before the main channel speaker's distortion starts to increase. At the same time, the crossover should be as low as possible to prevent localization of the subwoofer. Some electronic room equalizers will not allow the crossover frequency to be adjusted. This reduces the size of the filter needed to create a smooth response in the crossover, which can allow for a less expensive DSP to be used. Since no room EQ system makes distortion measurements, the automatic selection is often wrong.

Smaller speakers, as would be found in a smaller room, require a higher crossover point which makes frequency response deviations in the crossover even more audible. Unfortunately, smaller rooms often reduce the ability to move the main speaker, subwoofer and listening seat to reduce room effect in 75Hz -135Hz (100Hz crossover) region before EQ. Making an inaudible join between subwoofer and satellite is thus more difficult than a larger room.

Higher order crossover networks can reduce the overlap, which is especially useful with small main channel speakers (5 inch woofer). The discontinued NHT Xd used an 8th order filter. For a 100Hz crossover, the interaction between the main channel and subwoofer would be between 86Hz and 115Hz, but even this small range is audible if an electronic room EQ is not deployed.

The stringent requirements outlined above apply to music reproduction, and may not be applicable to sound effects. Some lifestyle systems equipped with very small satellite speakers (roll-offs starting at 150Hz) and a subwoofer with a bandpass-like response peaking around 60Hz basically are designed to reproduce sound effects. With such a system, musical instruments almost disappear between 75 to 130Hz.