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Then that same level should be used as the reference for the subwoofer''s level. Every speaker''s level control should be adjusted until they are all producing the same SPL as per the SPL meter. With the meter placed, or held, about where the ideal listening position is (at ear level) the test tones will move from speaker to speaker then to the subwoofer then back to the speakers and continue in a loop. This can be done in many ways, but the simplist is to use the test tones built into your receiver/preamp and an SPL meter set to C-Weighting. LEVEL: To adjust the level, one simply measures the level of the main speakers and adjust the level of the subwoofer until it is the same as the main speakers. The phase control shifts the phase of the signal prior to amplifying it and sending to the sub driver. The Gain/Volume control simple boosts or cuts the loudness of the subwoofer. HOW: The three controls are pretty simple to understand. Instead, it should appear that the main speakers are clean, loud, dynamic, and as strong in the bass as the music (or movie) suggests. One should never walk into a room with a subwoofer playing with a system and immediately notice the power of the subwoofer. When the content asked for it, the bass should be strong, and when not asked the bass should go away. In fact, the sound of the music (or movie) should be natural, balanced, and full. If everything were ideal, the subwoofer won''t stand out from the other speakers. RESULTS People have asked what the sound should be like when a subwoofer is perfectly calibrated versus when it is not. Some better subs replace the phase switch with a variable phase dial which allows the infinite tuning of the phase from 0� to 180� (in phase with input to out of phase with input, respectively). These controls often include a level control, a crossover frequency control and a phase switch. So, most subwoofers provide methods for calibrating their output in order to tune them to blend perfectly with the main speakers. What we want is the tweeter to seamlessly blend with the midrange so that we cannot tell where the mindrange stops and the tweeter starts. Afterall, we don''t sit around adjusting the level of the tweeter until it stands out in the sound so we can all marvel at how great the tweeter sounds. In an perfect world, the subwoofer should blend with the main speakers the same way the tweeter should blend with the midrange speaker(s). There are many reasons to use a subwoofer to do this including, protect the main speakers, increase the maximum output of the system, take advantage of the acoustical benefits of sub placement versus the acoustical advantages of stereo speaker placements, extend the bass output below the limits of the main speakers, increase the maximum bass SPL by using a dedicated driver, and other such reasons. Tuning Subwoofers - Calibrating Subwoofers WHY The purpose of a subwoofer is to fill in the bottom octaves below the capabilities of the mains speakers.