The Marriage Between the Subwoofer and Tower Speakers

Editorial Note as of 04/30/10

This article was originally published back in 2001 and some of the info is obsolete by today's standard of products offering a wider diversity of configuration options.  Based on the latest research, we almost always recommend users to set all speakers to "small" and utilize 2 or 4 subwoofers properly placed in the room to yield the smoothest bass response for all listening seats.

Some marriages are easier than others. Ideal ones require minimal maintenance to flourish and thrive successfully on mutual cooperation right off the bat. Some not so ideal marriages however, may not be so divine and can be bitter sweet at times of chaos requiring much nurturing to prevent rotten out. Such is the case with home theater when implementing multiple bass sources. When one chooses the commitment of buying bass capable tower speakers and mating them with a subwoofer for home theater and music applications, the potential of sonic bliss or troubled misfortunes begin brewing. The purpose of this article is to identify and red flag issues which may lead to problems when combining a subwoofer with bass capable speakers.

Fixed Crossover Setting of 80-90Hz

Many Receivers and Processors have one fixed crossover setting of about 80Hz as per THX recommendation. Please review the article I wrote " Setting the Subwoofer / LFE Crossover for Best Performance " for more elaboration on this topic.

In my opinion,the THX mandate of 80Hz crossovers can be too high when trying to mate bass capable speakers with powered subwoofers.  Bass localization is particularly noticeable when the subwoofer is located close to the listening position and crossed over above 60Hz from our experience. We have run blindfolded listening tests in our own sound labs and found that the pressure waves of bass are localizable to the human ear as far down as 60Hz if the subwoofer is placed in close proximity to the listening position. Because of this, crossing over a subwoofer at 80-90Hz may lead to overemphasis of those frequencies, resulting in unnatural, colored and localized bass. In addition, a high crossover setting for a subwoofer, buttressed with bass capable speakers, only amplifies this problem because of their synergistic and additive effect.

Non-defeatable Crossover on Subwoofer / Processor, or Both

To combat problem flagged in #1, it would be useful to defeat the LPF (Low Pass Filter) internal to the Receiver/Processor. If this is not possible, the user may have no choice but to use the adjustable LPF of the subwoofer in conjunction with the LPF from the Receiver/Processor.

Doing so may result in problems with:

1. Too much transition band attenuation, excessively attenuated signal.
2. Added group delay of two filters cascaded causing phasing problems between subwoofer and main speakers. The result is unwanted canceling of certain bass frequencies.
3. Null or resonance caused by impedance change resulting in accentuation or attenuation of particular frequencies within the passband of the subwoofer.

Minimal Group Delay of Digital LPF in Processor

Let us simplistically define group delay as the rate of change of the total phase shift with respect to angular frequency. Most high end Receivers/Processors handle bass management in the digital domain and thus all of the filtering is done digitally. Digital FIR filtering is very good, sometimes too good in that these filters add no group delay. Ideally this is desirable, until you couple this system with a pair of bass capable speakers that have a woofer crossed over with an analog filter. The added group delay from the analog filter within the speakers can cause a phasing problem with the subwoofer resulting in poor bass response (see point #2 above). There are two potential ways to combat this dilemma:

1. Reverse or adjust the phase of the subwoofer via the phase control on the back panel.
2. Reduce the distance of the subwoofer relative to the listening position in the set-up of the Processor if that feature is contained within (IE. The shorter the physical distance you enter into the processor between the sub and listening position, the more delay the processor adds to the sub output). However, many processors do not offer this feature.

Phase Switch on Subwoofer

A phase switch basically electrically reverses the polarity of the subwoofer and thus changes phase angle by 180 degrees. This is sometimes useful in cases where the Receiver/Processor subwoofer output is electrically out of phase with the speaker outputs. Use this option with caution, starting with the 0 degree as default. Let your ear be the judge as to which position allows your subwoofer and speakers to perform optimally. Usually there is a drastic and obvious difference between both settings. If you hear no difference, then I recommend keeping the switch set at 0 degrees.

Placement Issues of Subwoofer and/or Speakers

Perhaps the most critical parameter to ensure optimal performance between your subwoofer and speakers is placement.

I recommend reading the article I wrote regarding Speaker Placement and the article I wrote regarding Subwoofer Placement as starting points, and experiment liberally on your own. As a rule of thumb, bass amplitude reaches its maximum potential if the subwoofer is placed in the corner of a room where in has many surfaces to couple with. This does not imply however you will achieve optimal bass performance with respect to accuracy or finesse.

Tuning Frequency Relationship Between Subwoofer and Speakers

Speakers, particularly woofers, are designed to work within a calculated volume to achieve optimal performance. Bass Reflex speakers/subwoofers use a port or opening to tune the cabinet to a certain frequency to ensure optimal frequency response and dampening characteristics for the system. It is usually desirable for the speakers and subwoofer to be tuned to about the same frequency. It is also a good rule of thumb to crossover the sub at one octave abouve the 3dB point of the main speakers for optimal blend and uniform bass response of the system as a whole. For example, if the lower bass extension (-3dB point) of your main loudspeakers is 30Hz, then crossing over the subwoofer at 60Hz will usually deliver a more uniform response between the main speakers and subwoofer. This usually helps to maintain proper mechanical phase between the subwoofer and speakers resulting in a seamless blend between the two. This point is mostly important when matching large bass capable speakers with a subwoofer, and usually becomes less of an issue when blending a pair of small, bass deficient speakers with a subwoofer.

Conclusion

The statements above regarding subwoofer and speaker blending were listed as potential issues one may come across when setting up such a system. It was the intent of this article to tactically assist the consumer in combating these common issues. There are many rewards in combining a subwoofer with bass capable speakers for home theater and home audio applications. I believe the ultimate way to ensure perfect blend and reproduction of all frequencies within human hearing limitations, may be achieved with this type of set-up if executed properly. It is sometimes an arduous task for neophytes and even the Audioholics to achieve perfection in their system. It requires time, patience, cooperation, understanding, and care to achieve this feat, not dissimilar to a healthy and prosperous marriage.