Subwoofers are specialized speakers designed to do one thing: produce low bass tones. While they are frequently associated with home theater systems, subwoofers are useful for all types of media. Maximizing your subwoofer's performance requires you to adjust its three key settings -- phase, crossover and level -- as well as to pay careful attention to the most important factor -- its placement.
Speakers work by having a driver unit physically move back and forth to create a pressure wave in the air. Because they work in concert with other speakers in your stereo system, it's possible to have the subwoofer and the main speakers reproducing some of the same frequencies. If the subwoofer is pushing while the other speaker pulls, they cancel each other out and you don't hear any bass at those frequencies. The phase control on the subwoofer flips whether it pushes then pulls or pulls then pushes. If it's set wrong, you hear less bass as your speakers cancel each other out. You should be able to easily hear the difference between the two settings, so set the control so that you get the most bass.
The crossover in your subwoofer controls which frequencies your subwoofer reproduces and which it leaves for the other speakers in your system to handle. Generally speaking, if you have small satellite speakers, a crossover point around 100 Hz is appropriate, and larger speakers let you set a lower crossover point. While it's generally better to use a lower crossover point than a higher one, you also don't want to set it so low that there is a gap between your speakers and your subwoofer. Experiment with it to find a setting where you hear clean bass that isn't muddy or boomy, and where you can't tell whether it's coming from your subwoofer or from your speakers. If your home theater receiver already has a crossover, consider using that one and either bypassing your subwoofer's crossover, or setting it so high that it won't interfere with the sound.
It can be tempting to turn your subwoofer's level all of the way up and enjoy ridiculous quantities of low bass. Doing this won't give you accurate sound, though. You can either use a sound pressure meter or your ears to find a setting where the subwoofer's volume matches that of your other speakers. Once you've found this setting, you can always tweak it slightly to match your personal preferences.
Bass sound waves are sensitive, and their ability to travel from the subwoofer to your ears can be greatly impacted by the layout and construction of your room. As such, the most important subwoofer control isn't a knob or a switch -- it's where you place it in your room. There are two ways to place a subwoofer. One is to sit in your listening position and move the subwoofer around. The other is to put the subwoofer in your listening position and move yourself around, preferably crawling so that your ears are at the same height as the subwoofer. What you are looking for is a position where the bass sounds loud enough to blend with your other speakers, but the sound isn't boomy or muddy.
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