Adventures in Room Equalization Part II

Discussion in 'Archived Threads 2001-2004' started by Larry Chanin, Jul 24, 2002.

  1. Larry Chanin

    Larry Chanin Stunt Coordinator

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    This is a continuation of a fairly long thread started here:
    Averaging Listening Positions when doing multi-channel equalization.
    While there’s a lot of excellent discussion in the prior thread, it is seven pages long. So for those forum members who would prefer the “Reader’s Digest” version of the previous thread, I’ll start off with a Thread Recap before I continue reporting my results.
    THREAD RECAP (Part I)
    In anticipation of using an AudioControl Bijou multi-channel equalizer for the first time, I started to measure the characteristics of my room using Acoustisoft’s ETF5 software as a real-time acoustic analyzer. The original question of the thread was “Should I average the frequency response measurements from multiple room positions when performing a multi-channel room equalization?” This question was resolved fairly early. Despite what Mr. Lucas and the THX folks advise, most forum members seemed to think that special attention should be given to actual primary seating positions, not averaged results.
    However, my first room measurements with ETF5 showed an impossibly large 55 dB drop off in frequency response going from a low frequency peak to a mid-range dip. It was this unusual result that turned out to be the main subject of the thread, and it really challenged everyone’s deductive reasoning skills as we tried to figure out what I did wrong.
    A number of discoveries were made, each partially explaining the unusual room measurement.
    The first most obvious problem “discovered” was I had located a 75 gallon fish tank near one of my front speakers. Duh!
    The second discovery was a suspicious peak at exactly 60Hz where no room mode peak was predicted based on room dimensions. There were two main theories regarding the 60Hz peak:
    1)acoustic noise, and
    2)a sound card problem
    It turned out that I had an intermittent bad connection to the Radio Shack sound pressure level meter, which accounted for a lot of the 55dB dip. In addition there was fish tank filter noise interfering with the measurements.
    Once this was corrected I got frequency response curves that only had a ~13 to 18dB drop off in levels. Some members took the position that these measurement were reasonable, while others thought that it was still unreasonable. Part of the uncertainty in results was due to using the Radio Shack sound pressure level meter. Even with correction data there was a lot of doubt regarding its accuracy. Pete Mazz posted an excellent graph that showed that a tweaked Radio Shack meter with correction file was quite accurate up to about 10kHz.
    The third discovery was that by opening various combinations of doors to my home theater I could actually improve the measured frequency response of the room. (In the interests of measurement consistency and to cut down on outside noise, I later decided to do all my measurements with the doors closed even though this didn’t necessarily yield the best room response.)
    The fourth discovery was I had incorrectly used my surround processor's default DSP mode (Logic 7) and this was producing unusually attenuated results. It was agreed that depending on which speakers you are trying to send test signals to, there was a preference to which modes to use . In order of preference:
    1.Bypass (With rewiring this mode is possible, but not practical for my Lexicon processor.)
    2.Mono
    3.Two-channel
    4.Dolby Pro Logic
    NOTE:I subsequently found that since I was trying to equalize all eight channels of my home theater, my Lexicon processor’s “Party” mode was the best for me because it sent unprocessed sounds to all speakers. This made it much easier for me to send test signals to all the speakers, particularly the surround back channels, without having to redo the wiring.
    Although my measurements were starting to look better, there was still some reasonable doubt regarding the accuracy of the ETF5 results which still dropped off a fair amount from a low frequency peak. In trying to resolve the issue I used another, much simpler computer application, Frequency Plotter, that generates test tones and automatically records and plots the Radio Shack sound pressure level meter results. When replotted on an Excel spreadsheet with a logarithmic scale the results seemed to resemble the ETF5 log curve, but there were still some differences.
    An other approach suggested was to buy a Behringer Ultra-Curve Pro Professional Equalizer and RTA. The object would be to use the Behringer to measure all channel’s frequency response and forget about using ETF5. I went ahead and bought the Ultra-Curve reasoning that in addition to providing an RTA, it could also be used to equalize the remaining two channels of my home theater that were not handled by the AudioControl equalizer.
    As in the case of the Frequency Plotter comparison, I tried plotting the Behringer’s RTA results on an Excel spreadsheet to compare it with the ETF5 results. Again the results seemed to resemble the ETF5 curve, but there were some differences. Both of these comparisons were inconclusive because, I was still using the Radio Shack meter when using ETF5, and because I was not performing both pairs of measurements at the same sound pressure level.
    The last discovery that I made was that the unique design of my main speakers was probably contributing to the usual frequency response. Mr. Matthew Polk, the co-founder of Polk Audio and the designer of the speakers informed me that these vintage Stereo Dimensional Array (SDA) type speakers required a different testing procedure. This was partially confirmed by the fact conventional speakers, such as my center channel speaker, did not exhibit the large dip in frequency response.
    The fact that all three measurements (ETF5, Frequency Plotter, and the Behringer RTA) all showed a dip in frequency response for the SDA speakers, suggested to me that despite differences in measured results, perhaps there wasn’t anything wrong with ETF5. However, before I could investigate this further, my home theater computer that I was running ETF5 on became unstable, leaving me “dead in the water”, and that’s where the previous thread ended.
    Part II
    So, it’s now about three weeks since my computer difficulties. After reformatting my hard drive several times in unsuccessful attempts to solve my problem, it turned out that the problem was resolved by reseating my memory stick. Of course I had to reinstall and reconfigure all my software, including ETF5, not a lot of fun!
    After getting up an running again I decided to give ETF5 an other chance. So to eliminate uncertainties regarding the Radio Shack sound pressure level meter I decided to buy a microphone preamplifier and use the measurement microphone that I used with the Behringer Ultra-Curve. (I should point out that I was beginning to suspect that the intermittent bad connection to the meter wasn’t due to bad cables, but rather the input connection to the Radio Shack meter itself.) It is not possible to adjust the Behringer Equalizer settings while in the RTA mode. On the other hand I can run ETF5 and the Ultra-Curve in equalizer mode simultaneously, adjusting equalizer settings and taking RTA measurements in almost real-time.
    So I bought the EURORACK® MX602A 6-CHANNEL 2-BUS MIXING CONSOLE to use as a microphone preamplifier and began equalizing all eight channels of my home theater using both the Ultra-Curve, the AudioControl Bijou equalizers in conjunction with ETF5 as the RTA.
    I decided to use the Ultra-Curve to equalize the main channels because it had more graphic equalizer bands as well as three parametric equalizer bands. This freed up the main channels on the AudioControl Bijou equalizer to be used to equalize the surround back channels.
    The equalizer procedure recommended by Matthew Polk is logistically rather difficult, so on the main speakers I decided to start out doing serious equalizing at the low frequencies, (I used the parametric equalizer to cut certain low frequencies) but in general used only very modest equalization on the mid and high frequencies.
    Here’s my first series of results measured from position #4. Listening Positions
    Main Left Speaker
    Main Right Speaker
    Center Channel Speaker
    Left Side Surround Speaker with Surround Subwoofer
    Right Side Surround Speaker with Surround Subwoofer
    Left Surround Back Speaker
    Right Surround Back Speaker
    LFE Subwoofer
    The red curves are before equalization and the blue curves are after equalization.
    I haven't done a lot of critical listening yet, but it definately sounds good to my untrained ears. My wife, who was working in an adjacent room, made an unsolicited comment that the Everly Brothers CD I was listening to sounded as if it were a live concert.
    I'll continue to tweak the equalizer settings.
    I'd like to thank everyone who provided advice and contributed to the prior thread. In case you're interested I have updated the photos on my homepage to show the equalizers and associated gear.
    Thanks.
    Larry
     
  2. Ned

    Ned Supporting Actor

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    Larry-

    What type of acoustic foam are you using? Is it just on the ceiling or on some of your walls too?
     
  3. Dennis XYZ

    Dennis XYZ Stunt Coordinator

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    Hi Larry,
    Nice curves. It looks like you are getting things sorted out well. I'll bet that's a relief. [​IMG] Now it's time to enjoy the music.
    One note about the UltraCurve, I believe it uses the stored EQ settings when doing an RTA measurement. You can check how well your EQ is working in RTA mode but you can't change the settings without going back into EQ mode.
    Page 17 of the manual, talking about pink noise and setting levels - "Before the noise signal reaches the output of the ULTRA-CURVE PRO, it passes the EQ. That’s why the current EQ settings have an effect on the noise signal."
     
  4. Pete Mazz

    Pete Mazz Supporting Actor

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    Glad to hear you got it worked out!

    Pete
     
  5. Larry Chanin

    Larry Chanin Stunt Coordinator

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    Ned:
    I'm using one layer of RPG ProFoam on my walls adjacent to my main speakers. (The profile layout of by home theater shows the placement of acoustic foam on the ceiling. This is not installed yet.)
    This is the only foam I am aware of in which you can nest it, layer by layer, to get increased thickness after it is installed.
    Larry
     
  6. Larry Chanin

    Larry Chanin Stunt Coordinator

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    Hi Dennis:

     
  7. Larry Chanin

    Larry Chanin Stunt Coordinator

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    Hi Pete:
     
  8. Manuel Delaflor

    Manuel Delaflor Supporting Actor

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    Larry,

    Glad to see you followed my suggestion on the Berhinger Console. That's what I plan to buy in order to make some tests.

    What is your opinion on the ETF with the calibrated mic?

    I look forward to see the graphs of your subwoofer.
     
  9. Shawn Solar

    Shawn Solar Supporting Actor

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    I am rather uneducated when it comes to the ETF software. What type of soundcard is needed and is it easy to understand. I don't mind buying a calibrated mic if cheap enough.
    Just when I think my tweaking adventures are over you go putting ideas into my head[​IMG] Sorry I couldn't help. Though I did read on a certain formula to use to determine how thick of foam to use for certain frequencies. Gotta try to find it.
     
  10. Larry Chanin

    Larry Chanin Stunt Coordinator

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    Hi Manuel:

     
  11. Larry Chanin

    Larry Chanin Stunt Coordinator

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    Hi Shawn:
     
  12. Stephen Dodds

    Stephen Dodds Second Unit

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    Larry,

    Just wondering about some of your Ultracurve settings. I've had the UC for a while with good results. However, when I use the manual RTA and dump the results to the EQ section I always get slightly different results. This seems to be because the meters are still moving up and down a because of the varying nature of pink noise.

    I'm using RMS and 1 sec to try to minimise this.

    As far as the quantization noise mentioned by someone earlier, I use an external active crossover with separate input gain control, and turn that right down. Consequently you must turn up your prepro, which helps give the UC maximum levels.

    Alternately simply turn the master gain on the UC down and you get the same effect.

    Also, if you are summing the UC below 80Hz (as I am), what would happen if you tried to use the parametric EQ as a six band rather than three band EQ, ie use L aand R separately on the summed mono signal. Any thoughts, anyone?

    Steve
     
  13. Chris Tsutsui

    Chris Tsutsui Screenwriter

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    I havn't been following along with your journey but I do have been doing some reading on room EQ.. Larry, may I ask why you are using a surround channel subwoofer? That would only cause more problems IMO.

    And why havn't you considered 2 subwoofers on the front left/right to help cancel out those room modes.

    Also why are the mains against the side wall? I think That would also create more room nodes. And why arn't the back row seats closer positioned to the middle of the room.

    Have you done the calculations for where the room modes are and how to position the subs, listener positions, and speakers? I would try and do this before going into any type of EQ and testing.
     
  14. Larry Chanin

    Larry Chanin Stunt Coordinator

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    Do you mean you are using your external crossover to extract left and right signals below 80Hz and send them to your LFE subwoofer? Then you would send the summed signal to the Ultra-Curve right and left channels? The right and left signal would be identical and you could merely select the frequencies that you wished to adjust. I'm not an expert, but I would think that this would work.
    Obviously the best application of this technique would be to flatten peaks, assumming you have six peaks below 80Hz that need adjusting. I guess you need to be careful to smooth the crossover point and not have the equalizer "fighting" the natural rolloff of the crossover.
    Larry
     
  15. Larry Chanin

    Larry Chanin Stunt Coordinator

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    Yes, in the "Part I" portion of my "adventures" I've posted the calculations. However, for practical reasons, as I've tried to describe above, and for asthetic reasons I feel that I've pretty much exhausted just about all placement options. However, I do agree with your premise in an ideal situation (if such a thing exists. [​IMG]) I also feel that ideally you should also apply acoustic room treatments before using equalization.
    Larry
     
  16. Larry Chanin

    Larry Chanin Stunt Coordinator

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    Hi All:
    For those of you interested in the Behringer Ultra-Curve Pro digital equalizer specifically, and room equalization in general, might I suggest that you read this thread:
    DSP room equalization for cheap
    At the risk of bringing up a controversial subject, I'm particularly interested in your opinions regarding Mr. Richard Hardesty's take on equalization. Currently I am the last poster on the above thread, and you can find a link to Mr. Hardesty's writings regarding equalization in that posting.
    Thanks.
    Larry
     
  17. Michael R Price

    Michael R Price Screenwriter

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    Larry, I think you may have better results if you eliminate the surround subwoofer and reroute all bass to your main LFE subwoofer, which should do a fine job of carrying the surround bass load. When bass is coming from multiple places at once, the result is unpredictable (as I'm sure you've found so far).
     
  18. Chris Tsutsui

    Chris Tsutsui Screenwriter

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    Sorry If I wasn't specific:

    If you can position your mains correctly across the width of your room you should be able to cancel out most of the active width modes.

    With your mains against the side walls you are canceling out the odd-order modes, but if you position the mains so that they are in the null location for that left over mode, you will help cancel that out too.

    So, I think that having your mains against the side walls are "energizing" more modes than if you had them positioned for selective mode cancellation.

    I know that the Bass that your mains produces only overlaps the highest bass your sub outputs but still.. by having them against a wall also effects the bass output of the mains as well.

    Just trying to give some thoughts for your journey, good luck.
     
  19. Stephen Dodds

    Stephen Dodds Second Unit

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    Hardesty is welcome to his often controversial theories. I happen to believe he is wrong in this as in so many other things.

    I use a TACT on my fronts which is a more sophisticated version of the Ultracurve, with superb results. The same TACT system was product of the year at Stereophile and TAS and best of show at the Hi Fi thing in New York. So it isn't just me. Have a listen to a before and after with TACT and see.

    And I have a very good room.

    Prior to the TACT I used an Ultracurve and had excellent results with that, provided you take steps to boost the signal as mentioned in my post above, and above Thorsten in his review. This is now used with my surrounds.

    However, the guy with the Ultracurve and the Lowthers seems determined to combine the worst of all worlds. He has the signal going through the UC, to whatever effect, and he still has the same room problems he had before.

    In actual fact, what he has done is a speaker correction, like the Perpetual Technologies system.

    Don't give up on room EQ. It really is worth it.

    As an aside, it is possible to use the Ultracurve in a fully digital system, although it is a little complicated. You would need a prepro with digital outs such as a Meridian 561 or Theta Casanova, three or four Ultracurves, and a multichannel volume control. For 5.1 sources the best multichannel volume control is the Sony TAP9000ES.

    You run the prepro into the Ultracurves using the optional AES/EBU input, then from the UCs into the volume control, and from there to your power amps.

    The volume on the prepro is turned to 0dB to maintain the full digital level.

    If you were really picky, you could use standalone D/A converters after the Ultracurves.

    The cost of this would be about $2300 for the prepro, $1000 or so for the Ultracurves and $400 for the Sony, or less than $4K for the lot.


    Steve
     
  20. Larry Chanin

    Larry Chanin Stunt Coordinator

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