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Adjusting Subwoofer Distances – Post-AudysseyCalibration
Caution: Thisprocedure requires acoustic measurement software and equipment
As noted in the Audyssey Setup Guide, sectionVI, D. 4.: For enthusiasts that own equipment and softwarefor measuring room acoustics, there is another adjustment technique to maximizethe subwoofer / satellite speaker blend in the crossover region. AVS members craig john and Mark Seaton haveshown the validity of adjusting the subwoofer distancesetting—post-calibration—has resulted in measurable improvement in thesubwoofer/satellite blend at the crossover frequency. This technique typically requires the use ofcalibrated microphone (not the Audyssey unit) and measuring software, such asOmnimic, XTZ Room Analyzer, REW, etc... For more information, see the separate Word document filled with therelevant posts and measurement graphs.
What is this? This document includes instructions and somerelevant posts from the AVS Forum “Seaton Sound SubMersive 1” and “Official AudysseyThread” discussion threads. In the“Submersive 1” thread, there is some minor ancillary discussion after the firstposts below. If you want to read thosediscussions, just start reading the thread after the second post below. If you do not, rest assured I have providedyou with all the information and proof this technique works as described. The existence of Audyssey MultEQ XT32 and SubEQ HT has not negated the need for this process to gain the maximum output atthe subwoofer/satellite crossover. Craigsaid it best: “Trust, but verify”
The Basic Theory What Audyssey does, it does well, but there arelimits to what it actually measures and compares. The Audyssey calibrationprocess is not utilized to determine or check the integration of the speakersat the critical crossover frequency. Audysseyfocuses on response and time domain correction for each speaker in isolation;it does not measure the combined response of the subwoofer(s) and speakers together. As a result, it is possible the subwoofer / satellitespeaker blend at the selected crossover region is not optimal after calibrating,as Audyssey cannot correct what it does not measure. The process described below is used todetermine if the frequency response at the crossover region is optimal. The process basically consists of measuring—atthe Main Listening Position (MLP)—the combined response of the subwoofer(s) andcenter channel together. Any measured dipin the frequency response in the crossover region is usually caused by phasecancellations, and this is corrected by adjusting the subwoofer(s) distance andre-measuring until the response improves.
The Measurement Process
- Turn off or disconnect the subwoofer(s).
- Send a mono test signal to both Left & Right front speakers.
- Ensure that Audyssey is turned on, and both Dynamic EQ and Dynamic Volume are turned off.
- Set the processor to DPLII Cinema mode, and the signal should route to your Center channel speaker.
- Measure the center channel speaker response at the MLP.
- Turn on or reconnect the subwoofer(s).
- Turn off or disconnect your Center channel speaker.
- Measure the subwoofer response at the MLP using the same test signal and setting noted in step 2.
- Turn on or reconnect your Center channel speaker.
- Measure the center channel and subwoofer combined response at the MLP using the same test signal and setting noted in step 2.
- Utilizing your measurement software, overlay the measurements taken in steps 3, 6, and 8 utilizing a different color for each measured response.
- Looking at the combined graph, analyze the measured response in the crossover region. If the response of center channel and subwoofer combined (#8) is at a lower level than the other two measurements, there is room for improvement.
Sometimes the combined center channel and subwooferresponse (#8) is above both individual center channel and subwoofer responselines at all frequencies, resulting in a reasonably smooth transition. Other times, the combined center channel andsubwoofer response dips well below the higher individual center channel andsubwoofer response in the crossover region. In this case, it is a direct result of therelative phase / timing between the subwoofer and the speakers.
You can take additional measurements at otherlocations, and the exact combination will probably change. However, in most rooms a large cancellationwill be similar over a significant area. It is recommended to repeat the combinedcenter channel and subwoofer response (#8) at a few listening locations to seethe similarities and differences from lateral and fore and aft movements beforemaking adjustments. There is some concern the adjustments in thistechnique only have an effect at the MLP. However, many have found to have benefit over significant area,especially across a row of seats. Howmuch data you want to examine is up to you.
The steps outlined above only focus on the centerchannel, which is by far the most important for home theater. The center channel serves as a clear anchorpoint from which to adjust any other distances. The integration of thesubwoofer and the center channel also can affect dialog clarity and soundquality, so it is best to start with the center channel, and then individuallylook at each of the left and right channels. Work to find a balance of the 3 channels given the flexibilities of thesystem. As you measure the three front speakers, youwill need to make some judgment calls as to what strikes the best balance forthe system and its intended use. Whenthe front three speakers are identical, there will probably be less variance inthe measurements, and therefore less compromise required. If the center channel speaker hassignificantly different low-frequency extension than the left & rightspeakers, you will probably have to make the best compromise for the particularsystem and room.
The Adjustment Process
- First, write down all of your original subwoofer distance settings.
- If the measured response of the only the subwoofer(s) (#6) up to the crossover point is flat, the subwoofer distance settings are correct. Therefore, any frequency response dip in the center channel and subwoofer combined response (#8) in the crossover region is due to an interaction between the subwoofer(s) and center channel speaker.
- In this case, you need to maintain the same distance difference between the subwoofers by adjusting the settings of both subwoofers by the same amount (e.g. increase or decrease both by 1 foot).
- For example, if the difference between the subwoofer distances is 2.5', and Audyssey set the subwoofers to 10' & 12.5', increase the sub distances at increments of 2' (e.g. 12' & 14.5', respectively). Do not increase more than 8’, as ½ wavelength at 75Hz is ~7.5'.
- If the measured response of the only the subwoofer(s) (#6) up to the crossover point is not flat, the subwoofer distance settings are not correct.
- In this case, you do not need to maintain the same distance difference between the subwoofers, and you can change the individual subwoofer distance settings to see if you can improve the combined response.
- Once you have determined which subwoofer distance adjustment methodology you must follow (#2 or #3 above), change the distance settings of both subwoofers in 1-foot increments.
- Typically, increasing the subwoofer distance results in improved response.
- Decreasing the subwoofer distance settings is an option, as long as this does not result in distances significantly less than the actual physical measured distances of the subwoofer(s) from the MLP.
- Measure the center channel and subwoofer combined response to determine if the response around the crossover point has improved.
- Repeat step 4, adjusting the distance settings and measuring the response until you find the optimal setting.
- Once you find the best of the 1’ incremental settings, start making finer adjustments to find the best match and smoothest transition.
Additional Theory One of many reasons this is not a perfectprocess is that each speaker has a different group delay behavior, due to the designof the speaker (vented/ported or sealed) and the crossover, as well as thelow-frequency interaction with the room. These factors make for a rather wide range of possible variables. However, it is much easier to change thesubwoofer distance without collateral effects, as changing just the left, rightor center channel distances will start to impact relative imaging betweenspeakers. However, these are simple and clear numericchanges, so it's easy for anyone to give a listen both ways. If it doesn'timprove the sound, go back at the original settings. Most don't switch backafter listening when there are notable improvements. Altering the distances does not alter thesubwoofer correction filters calculated by Audyssey. However, there is some thought that alteringthe distance settings of multiple subwoofers could alter the interactionbetween them, as the Audyssey filters were calculated based on measuring the combinedresponse of the subwoofers at multiple measurement locations. However, the measured results speak forthemselves. Included below are postsrelevant to this technique, including measurement graphs posted by the users.
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發表於 2016-6-23 15:08
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