Tuning Subwoofers - Calibrating Subwoofers

Transcription

1 Tuning Subwoofers - Calibrating Subwoofers WHY The purpose of a subwoofer is to fill in the bottom octaves below the capabilities of the mains speakers. 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. In an perfect world, the subwoofer should blend with the main speakers the same way the tweeter should blend with the midrange speaker(s). 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. 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. So, most subwoofers provide methods for calibrating their output in order to tune them to blend perfectly with the main speakers. These controls often include a level control, a crossover frequency control and a phase switch. Some better subs replace the phase switch with a variable phase dial which allows the infinite tuning of the phase from 0ï ½ to 180ï ½ (in phase with input to out of phase with input, respectively). RESULTS People have asked what the sound should be like when a subwoofer is perfectly calibrated versus when it is not. If everything were ideal, the subwoofer won''t stand out from the other speakers. In fact, the sound of the music (or movie) should be natural, balanced, and full. When the content asked for it, the bass should be strong, and when not asked the bass should go away. One should never walk into a room with a subwoofer playing with a system and immediately notice the power of the subwoofer. Instead, it should appear that the main speakers are clean, loud, dynamic, and as strong in the bass as the music (or movie) suggests. HOW: The three controls are pretty simple to understand. The Gain/Volume control simple boosts or cuts the loudness of the subwoofer. The crossover control

2 shouldn''t be used. The phase control shifts the phase of the signal prior to amplifying it and sending to the sub driver. 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. 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. 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. Every speaker''s level control should be adjusted until they are all producing the same SPL as per the SPL meter. Then that same level should be used as the reference for the subwoofer''s level. Ultimately, if the main speakers are producing 70dB SPL, then the subwoofer should also be producing 70dB SPL. There are other methods for measuring the level with are more accurate, but this is the basic concept. Better Methods for adjusting the level - - Play a test DVD and follow the instruction accompanying the audio test tones. - Use a spectrum analyzer, pink noise, and a calibrated mic to measure the actual frequency response and adjust the level so the low bass (below 50Hz) is the same SPL as the upper bass (above 130Hz). - Play test tones and measure the levels with an SPL meter and write down the level of each test tone and adjust the subwoofer until the average level of the test tones it is producing are about the same as the tones produced on the main stereo speakers. CROSSOVER FREQUENCY: If you are using the crossover built into the receiver/preamp, then you really shouldn''t need to use the crossover on the subwoofer. If the subwoofer allows you to disable the corssover, then that would be the best thing to do. There are special cases where the use of the sub''s crossover can make a positive difference, but generally it should be turned off. If the subwoofer doesn''t have a switch to turn it off, then adjusting the frequency to the highest setting is the way to go most of the time. Once you disable the crossover on the sub, the crossover on the receiver/preamp needs to be adjusted. Most people, including myself, recommend you set the

3 crossover to the frequency exactly double the lowest rated frequency of the main speakers. So, if the main speakers are rated to operate from 30Hz to 20,000Hz, then set the crossover to 60Hz. Now, if your speakers are rated to only 100Hz, it is a bad idea to set the crossover at 200Hz (it may not be an option). In these cases, set the crossover frequency at least 25% higher than the rated low frequency limit. So, for a speaker rated to 110Hz, set the crossover to 138Hz, or the nearest frequency. If you want to get hardcore about it, measure the actual frequency response of the speakers in your room and set the crossover to the frequency about twice the -10dB SPL level of the speakers. So, if you can measure the speaker to have a -10dB point of about 45Hz, set the crossover to 90Hz, or so. For front speakers (Left, Center, and Right) I try to keep the crossover frequency below 100Hz if at all possible. With stereo music having the subwoofer operating much higher than 80Hz will make it hard to blend with the main speakers, so that is something to consider. These frequencies are only guidelines to ensure the smoothest transition from the subwoofer to the main speakers as well as provide the most protection for the main speakers which increases their maximum output. It may be necessary to change the crossover frequency from the recommended frequencies based on the unpredicable realities of the room acoustics and real world performance of the speakers. However, after calibrating the phase, it needs to be recalibrated if the crossover frequency is changed. PHASE: This is the tricky one to adjust and get right. If the sub only has a switch to reverse the polarity, then it is easier to get the most correct - two choices are always the easiest to choose between. If the sub has a continuously variable phase control, then it can get tricky. I will get into these in more detail later, but for now know that the ideal setting will provide the flattest frequency response at the crossover frequency where the sound is transitioning between the sub and the main speakers. There are several methods to tune the phase including: Easiest, Most accurate, Requiring the most Test Gear: - Use a real time spectrum analyzer like TrueRTA and a calibrated microphone to measure the frequency response in real time using a pink noise test tone with the Receiver/Preamp in stereo mode with Main speakers set to Small and the subwoofer set to On. - Flip the phase switch back and forth to see which provides the flattest response at the crossover range.

4 - Slowly turn the phase dial until the flattest response is measured. Effective, Requiring Less Gear, Time Consuming: - Set the Receiver/Preamp to Stereo, Main speakers set to small, Subwoofer set to on, play test tones from Real Traps or other Sine wave tones and measure the SPL of those tones (potentially write them down or use a spreadsheet to char them). - Adjust the phase until the levels at the crossover frequency are the same as at least an octave above and below the crossover frequency. Effective, Requires Less Gear, Faster: - Set the Receiver/Preamp to Stereo, Main speakers set to small, and subwoofer set to on, play the sine wave frequency sweeps provided at Marchand Electronics. Choose the sweep tone named for the frequency the crossover is set to. - With the SPL meter held at ear level at the ideal listening position, note the SPL Level at the bottom of the sweep and at the top of the sweep. Then adjust the phase so the meter moves the least as the frequency changes. Ideally, the SPL Meter will read the exact same SPL level regardless of the sweeping frequency. Less effective, but much better than nothing: - Set the Receiver Preamp to Stereo, Main speakers set to small, & subwoofer set to On, play a sine wave tone one octave lower than the crossover frequency (1/2 the frequency). Use the SPL meter at the listening position and note the SPL level. - Next play a sine wave test tone one octave higher than the crossover frequency (double the frequency) and note the level with the SPL meter. It should be close to the same level as the previous measurement. - Then, play a test tone at the same frequency as the crossover frequency and adjust the phase until the SPL level is the same as the other two measurements. - The problem with this method is that test tone levels are greatly affected by room acoustics and it is entirely possible that one of the three frequencies will be the same as a standing wave which will thow off the measurement by as much as

5 12dB SPL.