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Multiple Subwoofer Post Setup - Listening and Adjusting

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Step 6: Employ EQ or Auto Calibration System

Now that we have reasonable integration of all of our subs with the main speakers, its time to put the finishing touches to tighten up the response.  In this case I used Audyssey MultEQ Pro with the external calibration mic and software.  We have found that the guidelines we recommend below are relevant for other EQ systems such as the ones found in JL Audio subwoofers.

Audyssey Calibration Guidelines

  • Position the microphone at seated ear level pointing straight up using a mic stand.
  • Avoid placing the mic where direct sound from the loudspeakers is being blocked by an object.  For theater chairs, tilt them back so the mic isn't up against the cushion.
  • Calibration points around the vicinity of your primary listening area should be used.
  • Avoid the extreme side seats.  It's important to take measurements in a specific area between your most commonly used seats. 
  • Double the measurements in the two most important seats.  This will allow Audyssey to apply more weighting on the most important seats.
  • Adjust bass management, delays and trims as needed after calibration is completed

axiom 004001.jpgWe've found Audyssey to be one of the best auto room correction systems on the market.  But like anything else, it's not perfect. After calibration, we typically have to adjust the bass management and crossover settings for better integration.  Receivers with AutoEQ systems tend to set speakers large based on the measurements they take during setup which is especially problematic when a speaker is placed near a room boundary since the added boost gives the illusion of a small speaker being capable of delivering low frequencies at meaningful output levels.  In speaking with Audyssey, they maintain that all speakers in a home theater system should be set to "small" when using powered subwoofers, regardless of the end result of the room calibration.

The speaker delay settings are usually pretty dead on and it does a great job of calculating subwoofer distance and even an average distance of multiple subwoofers.  We tend to go back and tweak levels to accommodate multi row theater rooms as per our article "How to Calibrate a Multi Row Theater". 

I tried to setup Audyssey using two subwoofer outputs of my AVP-A1HDCI where I grouped the front subs as "Sub1" and the side/back subs as "Sub2".  This made the most sense since the subs that made up "Sub1" were equidistant from the listening area and the subs that represented "Sub2" were equidistant from the listening area. Theory was great but reality was terrible as can be seen in the graph below.

Aud-cal-issue.JPG 

Frequency Response vs SPL - Front Row (1/12th Octave smoothed)
Yellow: all 4 subs; Blue: all 4 subs + Audyssey

As you can see Audyssey created a very ragged response, much worse than having no EQ employed at all.  I even tried setting up my AVP-A1HDCI via 3 sub outs but had the same unfortunate results.

When I first setup the Audioholics Showcase Home Theater room, Audyssey's Chief Tech Officer Chris Kyriakakis paid me a visit.  We spent countless hours calibrating measuring and calibrating my system.  It was at that time we realized the best way to apply EQ would be to apply a simultaneous single correction curve to all of the subs.  Chris agreed but unfortunately no Audyssey enabled A/V receivers or processors featuring multiple subwoofer outputs support this.  Audyssey did however employ this in their new subwoofer calibration standalone processors: the Audyssey Sub Equalizer and the SVS AS-EQ1.  Audyssey took it one step further by measuring each sub individually, adjusting phase, distance and level and then applying a single correction curve to both subwoofers.  We pleaded with Denon to offer this for the AVP-A1HDCI Pre/Pro and they claim this upgrade is in the works.  We keep our fingers crossed for that day but for now I pain through the process of using a single subwoofer output and making the best of a single delay setting for all four subwoofers not theoretically perfectly positioned in my odd shaped room.

Over time I became smarter and gave up the notion of independent subs.  Thus until the above mentioned firmware update becomes available, we highly recommend you utilize a single subwoofer output for all of your subs when using Audyssey.  This applies to most of the other room correction systems that I've tested.  Below are the results of such a setup which you can see were quite favorable.

Aud-front.JPG 

Frequency Response vs SPL - Front Row (1/12th Octave smoothed)
Green: all 4 subs; Purple: all 4 subs + Audyssey

Once we utilized Audyssey calibration, you can see how the bass was significantly flattened below 35Hz.  We now realize a bass response of +- 3dB from 12Hz to 150Hz (ignoring the high Q dip at 80Hz) is quite incredible not only from a measurement standpoint but from an audible one.

Aud-back.JPG 

Frequency Response vs SPL - Back Row (1/12th Octave smoothed)
Green: all 4 subs; Purple: all 4 subs + Audyssey

As you can see by employing Audyssey, bass was significantly flattened below 40Hz and even between 80-150Hz for the satellite speakers.  We have now achieved a bass response of +-6dB from 12Hz to 150Hz.  The bass is about 2-3dB more elevated below 40Hz than I would have liked but it makes for excellent listening of multi channel music and movies.  This is especially true since it tactically excites our riser platform giving you a bass shaker effect but much more natural than the electro/mechanical type of shakers available on the market as standalone products. 

NOAud-AVG-seats.JPG          Aud-AVG-seats.JPG

Frequency Response vs SPL across all seats with all 4 subs engaged
With averaged response: Green: No Audyssey; Purple:  Audyssey

I wanted to really see how well the Audyssey EQ was benefiting my entire listening area so I pulled measurements at every seat and applied an averaging function to all of the measurements.  The green trace represents the average with NO Audyssey EQ engaged for every seat while the purple trace represents the average WITH Audyssey EQ engaged for every seat.   With no Audyssey engaged, the averaged response across all seats was around +-7dB from 12Hz to 150Hz.  With Audyssey engaged, the averaged response across all seats was around +4 to -3dB from 12Hz to 150Hz.  We went from a total variation of 14dB with no Audyssey to just 7dB with Audyssey engaged.  That's a markedly incredible improvement in flattening bass response across the entire listening area.   This was also a worst case scenario average since these responses were heavily based on the rear seat measurements.  In reality my most important seat is in the front row for critical two-channel music listening.  I recommend when looking to take an averaged room response, one should apply more weighting to the single or two most important listening seats.  I went back and did this applying a weighting of 3 for the front center seat and a weighting of 2 for the back center seat and it only made the results more flattering for the Audyssey averaged curve. 

Editorial Note about Global EQ

It's important to note that global EQ, even in systems such as Audyssey, cannot change seat-to-seat variations.  Dr. Floyd Toole discusses this point in his book which we recommend reading.  However, global EQ can bring the family of curves closer to a desirable flat response.  The fact remains that only a single seat can be made to be "really" flat.  It is for this reason that the first step in delivering good bass is to minimize the seat-to-seat variations so that global EQ has the maximum beneficial effect for all listeners.

Step 7:  Listening and Adjusting

All of the great measurements in the world can't guarantee great sound.  Now it's time to listen to the fruits of your labor to see if they paid off.  Put on some bass-intense music you are very familiar with.  Listen to it in two-channel with the subs across all of your seats to determine how it sounds.  Next listen to multi channel audio both discrete and 2CH run through a ProLogic IIx decoder.  In most cases bass should be a bit stronger when going from 2CH to matrixed 5.1/7.1. Make sure you are achieving satisfactory bass performance for all formats across your seating area.  Feel free to adjust subwoofer level up or down a few dB to your liking.  Experiment with the bass management (both speaker size and crossover settings) to make sure your best measured  settings also sound the best. 

When flattening out the bass response it isn't unusual to want to boost the subwoofer level up a few dB.  It's much easier listening to elevated bass levels when you have a flat response than when you have modal peaks that tend to bloat bass and make it very unnatural sounding.  Don't be afraid to calibrate your subs a few dB hotter than the rest of the surround channels once you've achieved flat bass response in your room.  In some cases you may even need to tweak the level of an individual sub up/down.  This is especially true for the subwoofer closest to the listening area.

If you're not happy with the sound of the Auto EQ engaged, turn it OFF or try rerunning the calibration to see/hear if you achieve better results.  The bottom line is don't be fixated on the calibration results if they don't sound right to you.  Tweaking is usually necessary for achieving the best sound but there is usually a high correlation between great sound and great measured results.

Summary of Main Points

For those that want the bottom line on multi-subwoofer setup and calibration, here it is.

  • Choose your subs wisely - preferably all identical subs or subs with near equal f3 (3dB rolloff) points.
  • Place your subs wisely - in home theater rooms, nothing is more important for achieving good bass than proper placement of your subwoofers and listening seats.
  • Make sure all your subs are playing the identical signal - all of your subs should be playing a mono signal that consists of summed bass from all speakers set to "Small" plus LFE info.
  • Setup bass management - in your A/V processor/receiver, set all speakers to small, use 80Hz crossover setting as a start, and defeat the internal LPF of all of your subwoofers. 
  • Level match all of your subs - make sure each sub is playing at the same output level.  Then match their combined level to your main speakers or center channel at your listening positions.
  • Vary parameters to optimize response – (ie. distance, phase, crossover setting, level, physical placement) to achieve the best measured response of your subs + main channels for your primary listening seats.
  • Engage auto-EQ or use the manual EQ - to optimize your response at your primary listening seats.  Remember, it's usually better to apply a single equalization correction curve for all subs simultaneously.  This applies for all room correction systems, not just Audyssey.  If auto EQ doesn't improve the sound of your system, disable it!
  • Listen to the end results – listen at your primary listening seats and tweak level and crossover settings only if needed.

Conclusion

RiserThe goal of a serious home theater playback system should be even (level) bass for all listening seats, plenty of dynamic range and smooth natural frequency response across all of the listening area for the most seamless blend.  The best way of achieving good bass is by reducing the modal peaks and nodal dips by utilizing passive room treatments, multiple subs, proper speaker/subwoofer and seating placements and setup, and active equalization.  Don't discount any of these tools in your bag of equalization (OR) correction tricks.  The key is proper subwoofer placement and setup to reduce the guess work and minimize chasing your tail to find the best settings that yield the most optimal measurable performance.

Once you hear a properly calibrated home theater with smooth and even bass across the entire listening area, it's hard to ever go back.  It's worth the time and effort to properly integrate multiple subwoofers into your theater room.  With the proper tools, know how and patience, you can create a home theater experience that rivals even the very best Cineplex's (without the sound of someone chewing popcorn over your shoulder or talking on a cell phone). It's important to be methodical and persistent when properly calibrating your theater.  An investment in measurement gear such as: LMS, TrueRTA, or even an FFT analyzer will save you time and frustration while also allowing you to achieve much more accurate results than just solely relying on your ears with test tones and an SPL meter.  Leave the home remedies to Grandma (chances are she makes a better chicken soup than you) and embrace science to unleash the full potential of your theater system. 

In future articles we will explore how to calculate room modes based on room dimensions. We will also delve into sound methods of making in-room measurements and interpreting their results in order to more accurately optimize system performance to enhance your listening experience.

Glossary of Terms

  • Standing Wave - a stationary wave   characterized by lack of vibration at certain points, between which areas of maximum vibration occur periodically usually when confronted with a boundary.
  • Room Mode - an in-room peak in frequency response
  • Room Node - an in-room dip in frequency response
  • Crossover - a passive or electronic device used to limit the bandwidth of a speaker
  • Bass Management - a passive or active system used to bandwidth limit speakers and sum bass to a dedicated subwoofer channel
  • LFE - Low Frequency Effect channel is a dedicated channel for bass information in discrete 5.1/7.1 systems which can also contain bass from all speakers set to "small"
  • Low Pass Filter (LPF) - a crossover used to limit the bandwidth of a speaker (usually a subwoofer) so only frequencies below a specified point will play
  • High Pass Filter (HPF) - a crossover used to limit the bandwidth of a speaker (usually a satellite speaker) so that only frequencies above a specified point will play
  • FFT - Fast Fourier Transform is a mathematical method of manipulating frequency and time domain information to make acoustical measurements
  • RTA - Real Time Analyzer
  • LMS - Analyzer to measure loudspeakers
  • PEQ - Parametric Equalizer
  • Q - a measure of dip/peak in a system based on center frequency divided by bandwidth
  • Bandwidth - the range of frequencies a speaker/system can produce (ie. 20Hz to 20kHz)
  • Octave - The interval of eight diatonic degrees between two tones of the same name, the higher of which has twice as many vibrations per second as the lower (ie 20Hz, 40Hz, 80Hz, etc)
  • Resolution - the accuracy in which something is measured (IE/ 1/3, 1/12th, 1/24th octave)
  • Audyssey - a multi point room correction system that uses spatial averaging and fuzzy math to develop the best correction curve for all seats.
  • Room Correction - an electronic device that uses equalizers and in-room measurements to flatten system response and improve the sound quality at the listening seats.
  • SPL - Sound Pressure Level
  • SPL Meter - a device used to measure sound pressure


Acknowledgments

I'd like to thank the following people and sources for their feedback and contributions to this article to make it as accurate as possible.

  • Dr. Floyd Toole of Harman
  • John Dahl of  THX
  • Gerry Lemay of HAA
  • CEA/CEDIA Application Notes
  • Paul Apollonio of Procondev, Inc.
  • Chris Kyriakakis of Audyssey Labs

 

 

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Recent Forum Posts:

GalZohar posts on July 18, 2021 17:17
gene, post: 715061, member: 4348
Phase should almost always be set to zero for all subs. It should only be altered if its beneficial which is usually not the case. I did mention this in the article under Advanced Calibration tips.
Hi,
I noticed my subwoofer and mains are out of phase at 80Hz which is my crossover frequency.
I can get them in phase by inverting the phase or by increasing the distance.
Which is the better solution?
I don't have the tools to measure the effect. Are they necessary or is there an obvious choice here?
I can understand how delay works and how a phase inversion switch can work, but how does the variable phase knob on the subwoofer work? And again, is it better to use it than to adjust delay to get the subwoofer and mains in-phase at the crossover frequency?
Pogre posts on June 20, 2021 10:21
Erus1982, post: 1490223, member: 95950
which calibrated mic do you recommend?
https://www.minidsp.com/products/acoustic-measurement/umik-1
Erus1982 posts on June 20, 2021 03:17
diskreet, post: 1489725, member: 91365
My recommendation is to invest in a calibrated mic and learn to use REW before buying a DSP or going crazy with calibration. If it sounds good, then you're enjoying it, which is the whole point. If you want to get the best out of the system then you need to know what you're working with to do that. Get a baseline with REW. If you see issues, you should try to use the built in DSP that you already own. If that truly can't solve it then maybe a better DSP is in order. But placement will probably make as much or more of a difference than any DSP.

which calibrated mic do you recommend?
diskreet posts on June 16, 2021 20:42
My recommendation is to invest in a calibrated mic and learn to use REW before buying a DSP or going crazy with calibration. If it sounds good, then you're enjoying it, which is the whole point. If you want to get the best out of the system then you need to know what you're working with to do that. Get a baseline with REW. If you see issues, you should try to use the built in DSP that you already own. If that truly can't solve it then maybe a better DSP is in order. But placement will probably make as much or more of a difference than any DSP.
Erus1982 posts on June 16, 2021 01:25
Hello guys. I have recently bought an Svs PB 2000 pro and its very good. I have placed it 1 meter away from my room right corner, I ran Audyssey XT (dont have XT32) and then I used a phone spl meter to raise sub channel gains to match with speaker levels. The subwoofer seems to sound good but as this is my first true sub I have no idea if I get the most out of it. My question is do I need to proceed further with calibration? 1) Do I need to buy a mic and meausure REW? 2) If yes, what else do I need in order to confiqure sub EQ afterwards? Can I use the svs app or do I need a minidsp 2x4 or something? Sorry for being so novice but I would really appreciate some guidance if you have the time!
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