Setting the Subwoofer / LFE Crossover for Best Performance
Setting the correct bass management on receivers and processors is essential to getting the most out of your audio system. While some systems recommend 80Hz (the standard THX setting for Select and Ultra2-based systems) this is not always the best setting for all theater systems.
Bass Management Should Be Crossed over at the Best Setting for Your System
Just because THX specified a suggested LFE cutoff frequency, doesn't mean it is always appropriate for all room/speaker implementations or that it must be blindly followed. Many times its easy to forget what a subwoofer is meant to do. It is meant to act like a subwoofer, not a woofer. The purpose of a subwoofer is to reproduce very low frequencies that most loudspeakers cannot reproduce with authority on their own. In the 5.1 Digital realm, the LFE channel was introduced as a means to alleviate the demand placed on the other channels to deliver low frequencies with ample amounts of "oomph" without limiting the dynamic range. When using an LFE/subwoofer cutoff (-3dB) at 80Hz / 24dB/octave slope, these frequencies are high enough to still have directional perception, especially if the sub is located in close proximity to the listening position. This makes it easier to locate the sub in the listening room and / or reinforcing standing bass waves in the listening room (above 60Hz) by coupling with the bass output from the main speakers if they are configured large.
Selecting the Right Crossover for Your System
Selecting the right crossover for your system is a process that involves much listening and experimentation. Sometimes it can be easy (such as when a loudspeaker manufacturer tells you where to set it) and sometimes it can be more difficult. On systems that offer a flexible bass management system, shoot for setting the crossover frequency for at least 10Hz above the lowest frequency your main speakers can reproduce cleanly.
There are a whole host of caveats we have to mention here, and we recommend looking at our other articles dealing with loudspeaker setup and configuration. In essence, you are trying to attain a smooth transition from the lower capabilities of your main speakers to the subwoofer. The goal is to not accentuate, or bump, the frequencies in the crossover setting location, but arrive at a seamless blend from mains to sub. In addition, we prefer crossover settings below 80Hz whenever possible so that the subwoofer frequencies are not directional. On some satellite systems, you may find that you have to set the crossover to 100Hz. This is OK, provided it nets you the best overall response.
What About THX Systems?
THX is almost a different beast altogether. A THX Select or Ultra2 system is specifically designed to have the bass management set at 80Hz. While you are free to alter the crossover frequency of a THX system, you will not be utilizing the system as it was intended to be heard. In addition, THX processing and DSP assumes (and sometimes initiates) an 80Hz crossover setting for the LFE. While a THX system may not be desired by everyone, we actually recommend those systems (assuming THX speakers in conjunction with a THX receiver/processor) be configured as recommended.
We highly recommend experimenting with the variable Bass Management / Crossover settings in your receiver / processor to arrive at the best setting possible for the marriage between your loudspeakers and subwoofer, especially if you are using quality "large" tower type speakers with inherently good bass extension. You will find the addition of a sub is much more useful as a compliment to your main speakers, producing deep, non-directional bass tones.
Keep in mind that setting the LFE too high can allow your sub to play up to almost 100Hz, which can cause a bass bump between your main speakers and sub; increasing the chances of:
Unwanted and unnatural sounding standing bass waves.
Identification of the subwoofer as a point source rather than a discrete member.
Excessive bass boom.
We recommend following these simple guidelines and then experimenting on your own to get the best possible sound performance out of your system.