Most subwoofers have a switch on the back panel labeled “Phase” along with labels for 0 and 180 degrees. To get the most out of your subwoofer it’s probably a good idea to have at least a passing familiar with this control, but don’t worry you don’t need a physics background to understand what it’s all about. Simply put, the large, long wavelengths of bass frequencies are affected more noticeably by phase cancellations than the short, small wavelengths in the upper register. That’s not to say that higher frequencies aren’t affected by phase problems – they are – but the results are different.
Technical Mumbo Jumbo
Music and audio is a series of waves similar in appearance to a sine wave (below). Speakers reproduce sound by vibrating in and out in response to these waves (presented to the speaker from your amplifier in electrical form). Ideally, you want your speakers to be in phase relative to each other.
Imagine a pair of speakers producing the same sound. As one speaker moves outward the other speaker will move outward at the same exact time – they are in phase, the relative phase between the two speakers is 0°. Now imagine your speakers trying to reproduce the same sound at the same time but one speaker is moving outward while the other is moving inward. Those speakers now have a relative phase relationship of 180° and they are out of phase. When speakers are out of phase with each other the signals they produce tend to cancel each other out. With bass frequencies this results in a perceived lack of bass. Your subwoofer may be cranking but your ears just aren’t hearing it.
What It All Means
There’s more to it that that, but simply put, if your main speakers are moving outward while your subwoofer is moving inward (or vice-versa) you’re going to lose out on a lot of bass. You can turn it up all you want, but that won’t correct it.
How To Set Up Your Subwoofer to Avoid Phase Issues
Whether you have a modest or full-blown call-the-neighbors-in-to-brag system, phase issues are equal opportunity buzz killers, but a few simple setup steps can make a world of difference.
Rule 1: Size Matters The smaller your main speakers (typically front Left and Right, maybe a Center) the closer to your speakers your subwoofer should be. The object of a subwoofer is to produce unlocalized bass – you shouldn’t specifically hear the subwoofer but rather you should perceive bass as if it is coming from your main speakers. By their nature, smaller speakers produce less low end than larger speakers so for seamless subwoofer integration, Rule #1 is pretty important.
Rule 2: Distance Matters The farther your subwoofer is from your main speakers the larger the relative phase shift between the two. The physical length of a 40Hz signal (which is pretty near the bottom of usable natural musical frequencies) is approximately 28 feet. This means that from start to finish one 40Hz signal needs 28 feet to fully unfold. This doesn’t mean you need a 28-foot-long room to hear a 40Hz signal, but that’s why your neighbor on the other side of the wall can hear your bass but not your high-end.
If your subwoofer is fourteen feet away from your mains (half the distance of the wave) your subwoofer is now out of phase 180° with your mains. Of course, a 100Hz signal is only 11.25 feet long so if your subwoofer is 5.625 feet away from your mains it is 180° out of phase with them. Phase is frequency dependent, which just makes it all the harder to get an absolute setting – find the setting that works best for you and the way you listen.
It’s unlikely you will put your subwoofer 14 feet away from your mains, and it is unlikely you will ask your subwoofer to produce signals as high as 100Hz, but you get the point.
Rule 3: Position Matters Your room might require you to position your subwoofer facing your mains. This puts it automatically out of relative phase with your mains (the mains are moving outward while the subwoofer is moving outward, creating destructive waves that cancel each other in the zero degree setting). In this case you have to set the Phase to 180°. In the real world you may have to set up your subwoofer at some other angle rather than absolutely in line with your mains, as this will also affect phase.
Rule 4: Flip the Switch Flip the phase switch between the two settings a couple of times while you sit in your chair and listen to a piece of familair music. After a few tries you will most likely hear an increase in bass – keep the switch in that position. There is a right or wrong, but what matters is what sounds best.
Rule 5: There Are Other Phases Beside 0 and 180 It is possible to have your speakers and subwoofer out of relative phase to each other by 45° (or other angle) and yes that can affect bass response, but in the real world the 0 and 180 settings typically suffice. Use the phase setting and then positioning to compensate for phase shifts that are in-between. The combination of the phase setting on the sub and the relative position of it to your main speakers will result in a more granular phase setting.
Your subwoofer may have finer settings than 0 and 180, and if it does, just experiment a little.
Some Tips About Subwoofer Placement
You can get a lot of bass at a relatively low overall volume if your subwoofer is properly set up in relationship to your main speakers.
Tip #1: Move Your Chair If you want to find the ultimate placement for your subwoofer and you’re not adverse to doing a little work, temporarily move the couch or chair you use for listening out of your room and put your subwoofer in its place. Now walk around your room to each of the proposed spots for your subwoofer. The optimal place for your subwoofer is where you hear the best bass response relative to your sitting position.
Tip #2: Experiment With the Crossover The THX standard crossover setting for movies is 80Hz and you will find most of the time, 80Hz is a sufficient crossover setting for music and movies in your home. But, your system, subwoofer, room dimensions and furnishings all play a role in affecting your bass response (that’s why I keep using the word relative pretty much every other sentence – it’s all about interaction). Move your crossover to higher and lower settings until you hear what makes you happy. It’s always best to use the same familiar song with every setting test to ensure you are comparing apples to apples.
Tip #3: If It Sounds Like the Bass Is Coming From Your Subwoofer Try Again You don’t want to necessarily hear your subwoofer, you just want to know it’s there. If, while listening, you’re constantly distracted by the bass coming from your sub, try repositioning it so it blends in better with the rest of your music, or lower your sub's volume.
Tip #4: The Volume Control Helps Finally, once you are happy with the positioning and phase of your subwoofer, dial in the volume until it is seamless with the rest of the program (music or movie). You may like more bass for movies and slightly less for music, so play around until you’ve discovered what works best and then lock those settings in. Once the volume control on the back panel is set leave it alone. The best practice is to run the amp in your subwoofer as wide open (loud) as possible and then use your receiver’s subwoofer level control to make it sound right. An amplifier always works best when it is running in the top 75% or so of its potential and your subwoofer amp is no different.
Hopefully these rules and tips have helped you get the most out of your subwoofer, now go…shake the walls.