Position and adjustments are interactive so every time you move your subwoofer, you need to readjust the sub volume, low pass filter and phase (polarity). When properly adjusted, you won’t "hear" the subwoofer at all. It will sound like your main speakers are making all the sound- except with a whole lot more bass than they could muster all on their own.
Here’s how to get it right:
"Get a suitable test CD [or DVD, see list below] with sine wave signals or warble tones covering the range of 20Hz to 200Hz or so…Using a sound level meter , match the output at the listening position at 50Hz and 150Hz [using ‘C’ weighting and ‘slow’ meter response] by adjusting the volume control of the subwoofer. Make sure the volume control on your preamp [or receiver] remains at the exact same volume."2
If you don’t have a meter or test discs, use a CD with vocals and a consistent acoustic bass line and set level to the point where the upper and lower ranges of the bass sound well balanced. The bass should not be so loud compared to the rest of the system it sounds boomy or muddy. On the other hand, the subwoofer should not be so low that the system sounds thin.
Setting Low Pass Filter (Crossover)
If you are using the LFE input on your sub, the crossover is controlled by the receiver and not the subwoofer. The LFE input is bypassed on the sub but now you will set the "crossover" adjustment via the speaker setup menu on the receiver
If your main speakers are full size with good bass response, set the low pass filter to 60Hz – 80Hz to start. If your main speakers are bookshelf designs or in-wall, set the low pass filter in the 80Hz range. If you have compact satellites, set the low pass filter to 120Hz to start. Put on a test CD or DVD with test tones. "With a fixed input level, carefully measure the output level at the listening position for each…interval between 30 and 200Hz and write it down or make a line plot on a sheet of graph paper…Listen while you measure. You hear differently than the sound level meter does…trust your ears, not the meter."2 Raise or lower the low-pass filter setting on the subwoofer to achieve the smoothest response. Turn the low pass filter down if there’s an excess of output around the crossover point, turn it up if there’s a response dip.
Do not be alarmed if you have response variations of several dB from test tone to test tone. You are seeing normal variations in response caused by the speaker’s interaction with the room. Absolute flat response is a worthy but somewhat unrealistic expectation in most systems and rooms. Further, be sure the test tone you are using is either a warble, or filtered pink noise. Straight test tones are not suitable for acoustic response measurements but are useful in detecting rattles and buzzes.
If you don’t have a meter and test disc, use your ear to make this setting. Put on a CD with male voice and a consistent bass line. Adjust the low-pass filter until the male voice sounds "full" and natural but not thick and heavy.
Setting Phase (Polarity)
"Using a [scrúdú] signal at the nominal crossover frequency [you set in the step above], set the phase of the subwoofer(s) to deliver the highest output at the listening position."2 It helps to have a friend on hand to change the polarity setting on the sub while you measure and listen. When setting the phase setting by ear, play some music (not a movie) that has a walking but repetitive bass line. Country, latin and certain dance tracks work well for this. Focus closely on the region of sound below the male voice. This is where the subwoofer transitions to the main speakers. Whichever setting sounds "faster" or "fuller" in that region of sound is the correct setting. In some situations, you may not hear any difference at all, particularly with compact satellite speakers. In this case, simply leave the phase switch to "0" or "normal".
Do It All Again-
All these adjustments are interactive. Once you set polarity, go back and re-adjust level and low-pass filter settings to get the smoothest response.
The final proof of optimal performance is in the listening. Put on a CD with an acoustic bass. An bass instruments should appear to be coming from the main speakers, not the sub. The sub should be audibly "invisible."