# Use all 7.2 speakers while measuring?



## rep (Oct 16, 2012)

I am new to using REW. I have a Home Theater setup with 7 speakers and 2 subs. I also use it as a music system by using the 2 front channels and the 2 subs. My question is whether I should have all speakers active when using REW to do room equalization, or just the front speakers and subs.


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## tonyvdb (Sep 5, 2007)

Hi and welcome to the Shack! Just use the front 2 main channels and the sub


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## rep (Oct 16, 2012)

My first instinct was to use just the two front channels and the subs, as you suggest, with the reasoning being that I am trying to correct the room, not the speaker setup. But it then occurred to me that some of the reflections would be different if the other speakers were included in the mix, and it might be good to capture that behavior as well. Comments anyone.


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## Wayne A. Pflughaupt (Apr 13, 2006)

Hey rep, welcome to the Forum!

You can’t correct a room with equalization, as explained in this post. At best equalization compensates for the room’s effect on the speakers by altering the response of the same. So yes, you are indeed correcting the speaker set-up, not the room itself. Fortunately, that’s typically sufficient to get an improvement in sound quality.

To answer your question more directly, you only measure the single speaker you intend to equalize. As you noted, running all of them during a measurement you have no idea what any particular one is doing. For your front stereo pair, it’s best to use identical filters for both channels, down to 400 Hz or so. Below that point, you can used specific filters for the different speakers. However, this is less of an issue with the center and surround speakers.

If you’re interested in accurately equalizing your main-channel speakers, a calibrated mic is a must-have. A generic calibration file is fine for “FIY” full-range measurements, but there is a lot of variation between one mic sample to the next, as the graphs here show. Obviously, you don’t want to equalize your main speakers based on inaccurate measurements.

Regards, 
Wayne


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## rep (Oct 16, 2012)

Wayne, thanks for the reply. 

I hear what you are saying, and it makes perfect sense. However, my plan was to use my first pass at measuring to get a baseline of how my room/system was behaving with no acoustic treatment. I was then planning on adding base traps in the corners and broadband traps at the first reflection points and then measure once again. Once I get as far as I can with acoustic treatment, then, and only then, do I plan on using EQ to address any remaining issues. Make sense?


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## Wayne A. Pflughaupt (Apr 13, 2006)

Yes, it does. However, before you go hog-wild treating those first reflections, you might want to read this article.

Regards, 
Wayne


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## Barleywater (Dec 11, 2011)

Processing time for directional cues is frequency dependent and is inversely proportional to frequency. Higher frequencies require less time.

Virtually all naturally occurring low frequency sounds produce harmonics which become locational cues in human hearing.

Subwoofers tend to produce high levels of distortion than become localization cues.


Thus most localization is derived from first few milliseconds of transient event. Reflections 10-50ms tend to increase perception of loudness. Reflections earlier than this have effects on perceptions of size and direction.

Humans have evolved listening to ground plane reflections, and much of western music culture occurres indoors with multiple reflections.

Early wall reflections (horizontal plane) have greatest impact on imaging. Best approach is placing speakers away from walls. Treatment of reflection effectively above 200Hz or so is next step as needed/desired. 

Obsessing about what microphone measures for single source, single location below transition zone is just that.

Wayne,

Nice article. A little wordy for me, but yes, a little bit of knowledge is dangerous, but more knowledge take time, patience and practice.

As to this picture:










"(A) is the ETC [energy time curve, a time domain view], (B) is the frequency response view with the filter equal in time to the ETC display......."

So here, gigantic window, likely >100ms is applied. Too bad there are time and frequency scales, but lets say first big dip is 70-125Hz in (A), and let middle of (B) be about 1kHz, and absorptive material is some sort of foam blocking in (C).

Little monitor on tripod is in corner and microphone location is not specified, or by reciprocity speaker and microphone location could be swapped, and microphone is on tripod and speaker is in room.

Either way it is clear that early reflections contribute to broadband comb effect that impact localization and perceived loudness by looking at differences between first and second rows.

It is also fairly clear that a little bit of treatment goes a long way, with rapidly diminishing returns.

The underlying response of the room for source/receiver combination remains highly fixed, and is most easily addressed by equalization.

Since equalization is inexpensive and relatively easy to do it is a great place to start, right behind good practices in speaker placement.

For speaker design purposes my beliefs/philosophy are really simple: A measurement microphone used to record a source such as a human voice, should capture the same waveform when placed at reference point of speaker that is using original recording as source. To accomplish this the speaker must exhibit a very flat frequency response across the bandwidth of the source, and a very smooth phase response follows this.

From this perspective the speaker is placed, and suitable treatments are applied per room/listener taste.

Andrew


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## Wayne A. Pflughaupt (Apr 13, 2006)

Barleywater said:


> The underlying response of the room for source/receiver combination remains highly fixed, and is most easily addressed by equalization.
> 
> Since equalization is inexpensive and relatively easy to do it is a great place to start, right behind good practices in speaker placement.


I wholeheartedly agree. Personally I’m a big fan of equalization. It just grates on me when people say or think they’re using it to “correct the room,” ’cause they ain’t!

If you haven’t seen it already, check out this video demonstration which shows the audible difference between two rooms with and without acoustical treatments. Pretty striking. Personally though, I couldn’t deal with having all that stuff hanging all over my walls (the treated [dark] room in the video). It’s just weird in the world I live in. Fortunately for me I’m not too demanding – most residential room I’ve ever been in sound just fine with nothing but ordinary room furnishings. The only exceptions have been rooms with ceramic tile floors.

Regards, 
Wayne


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## rep (Oct 16, 2012)

Boy, my thoughts on treating first reflections is sure going through some changes. Thanks so much for the very interesting article and the subsequent discussion gentlemen. I have a physics background with substantial experience in image processing but have spent no time applying what I know about signal processing to audio applications. Also, all the signal processing in the world only gets you so far, since it really offers no insight into pyscho-accoustic matters, which is what it is all about at the end of the day. This has been a very helpful start. 

Having measured nothing just yet and with a room that has plaster walls and ceiling with a hardwood floor largely covered by an oriental rug, here is what strikes me from just listening. The room is a bit bright sounding - not truly obnoxious, but brighter than I would like. I have a pair of subs on opposite walls, and the base response, while not changing radically from one location in the room to another, definitely shows some positional variation. My imaging is fantastic, and I sure as do not want to lose that. All things considered, I have a situation that is already reasonable. If I can make it better, I am game to try. My processor has built in parametric equalization. It is the ADA Cinema Rhapsody Mach IV. 

Maybe I'll leave the first reflections alone, do some measurements, and perhaps consider some base traps depending on what I find and what I can or cannot achieve using EQ alone. This sure is a switch from how was initially thinking about going about this.


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## Barleywater (Dec 11, 2011)

Since your imaging is satisfactory to your needs, I would concentrate on small tweaks to sub locations on sound at main listening position/area, then try a bit of EQ. Good imaging in otherwise bright room suggests good speaker placement in terms of first wall reflections >3-4ms after direct sound.

It will be interesting to see measurements and how they color your perception of the sound.

Andrew


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## rep (Oct 16, 2012)

[It will be interesting to see measurements and how they color your perception of the sound.
Andrew[/QUOTE]

I'd like to think that my room measurements will not color my perception of the sound at all, but I am well aware of the psychological forces at play. If I really wanted to "color my perception of the sound" though, I would have to pay at least $1000 for the measurement rather than just investing some of my time. Then there would really be a powerful force to "hear" something that I could correct.

Let's see if I can resist the temptation to screw up the nice imaging but still get some improvement in the base and maybe even a little less brightness in the high end.


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## sdurani (Oct 28, 2010)

rep said:


> I have a pair of subs on opposite walls, and the base response, while not changing radically from one location in the room to another, definitely shows some positional variation.


There has been some recent research from Todd Welti at Harman regarding subwoofer placements that minimize bass response changing from one seat to another. With that in mind, a couple of quick questions: what are the dimensions (LxW) of your room and how far is your seating from the front wall?


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## rep (Oct 16, 2012)

My room is not entirely rectangular. To the extent that it is rectangular, its dimensions are 19.5' L x 12.5' W x 9' H. The deviation from these dimensions is due to a floor to ceiling bay window in the rear, right corner of the room that is about 2.5' deep and 10' long. It is trapezoidal in shape and the short, parallel side of the trapezoid is 4'. This recess starts slightly less than 2' down the right wall.

This may be more info than you want or need, but there you have it.


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## rep (Oct 16, 2012)

Sorry, I forgot to answer your question about seating. My seating is about 60% back from the front wall.


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## sdurani (Oct 28, 2010)

Thanx for the quick reply rep. For your seating location and room size (including the bay window), placing your subs at the midpoints of the left & right walls will help improve seat to seat consistency. So that's roughly 10-11 feet from the front wall. Improved consistency will make it easier for the room correction in your receiver to fix more problems across the seating area. Good luck.


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## rep (Oct 16, 2012)

I'll give it a shot, but how did you come up with that location for the subs? The reason I ask is that there is a problem with that location. As it turns out, there are closet doors on the left side that make it a bit difficult to put one of the subs at 50% up the left wall.


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## sdurani (Oct 28, 2010)

rep said:


> ...how did you come up with that location for the subs?


Welti's recent research cross-referenced various subwoofer configuration with different seating location to show how much consistency you would get with different sized rooms. For your room dimensions and seating location, the only other subwoofer configurations that worked required 4 subs. Since you were doing a 7*.2* set-up, the best placement for 2 subs turned out to be side-wall midpoints.


> ...there are closet doors on the left side that make it a bit difficult to put one of the subs at 50% up the left wall.


How close to the middle of the left wall can you put the sub?


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## rep (Oct 16, 2012)

I think I can get it pretty close. The other issue I have is that the wires for the sub come out of the front wall, but it should not be very difficult to run another set of wires internal to the closet that exit from the left wall. As it turns out the rear sub can easily be moved.


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## sdurani (Oct 28, 2010)

rep said:


> I think I can get it pretty close.


In that case, get them as close as symmetrically possible. The placement not an all-or-nothing proposition. Getting close, and getting some improvement in consistency, is better than placing them arbitrarily and getting no improvement.


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## paulspencer (May 11, 2007)

Wayne A. Pflughaupt said:


> I wholeheartedly agree. Personally I’m a big fan of equalization. It just grates on me when people say or think they’re using it to “correct the room,” ’cause they ain’t!
> 
> If you haven’t seen it already, check out this video demonstration which shows the audible difference between two rooms with and without acoustical treatments. Pretty striking. Personally though, I couldn’t deal with having all that stuff hanging all over my walls (the treated [dark] room in the video). It’s just weird in the world I live in. Fortunately for me I’m not too demanding – most residential room I’ve ever been in sound just fine with nothing but ordinary room furnishings. The only exceptions have been rooms with ceramic tile floors.
> 
> ...


Well, after I saw this I figured it was time to actually post measurements that go with that demo. They are found here:

http://redspade-audio.blogspot.com.au/2012/11/bathurst-room-treatement-video.html

The most interesting one is this:










Why? Because it shows IMHO the main thing that sounds better about the treated room - the overall reduction in reverb time. 

The other measurements show that the bass isn't much different, but I believe this one correlates best to the difference you hear. The flat reverb time that is relatively low is something often seen in treated rooms - those that use actual acoustic products rather than curtains, furniture and rugs.


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## paulspencer (May 11, 2007)

Back to the original question ... the measurements you take and how you do them depends on what you want to find out or address. The answer is not so simple as you might expect. 

Now let's say you are working on just one position. Then in that case, it pays to measure subs and mains individually and together, and you may use some EQ on not just the subs, but also the mains. You would tend to use cut on the mains especially, unless you have a lot of headroom you can throw away. 

Where you are working on more seats, then you have to look at seat to seat variation, and this applies to any EQ whether on the mains or sub/s. You may find in larger rooms with a big spread of seats, it becomes difficult to EQ as a price is paid in some seats. As the room gets bigger and you have more seat to seat variation, due to seating over a wide area, then you have to look more into multiple bass sources to reduce that variation. 

So we need to separate the questions. Should you measure the other speakers as well as the subs? Yes. The data is useful with respect to both EQ, calibration/setup as well as room treatment issues. 

Should you EQ them both together? I would say, as a general rule, yes. It isn't "fixing" the room, but it's more like treating symptoms. However, you always need to check all seats that matter to you, when EQ is being applied.


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## AudiocRaver (Jun 6, 2012)

I'm going to throw a thought in here that may sound a little weird, but bear with me.

First of all, I totally agree everything that's been said about measuring speakers individually in the EQ process. You can get an interesting perspective, however, on the grand total of the acoustical energy from your speakers all working together, and the way the room is responding to it, by running pink noise through all the speakers at once. And to be more specific, by running pink noise that is noncoherent, meaning the different channels are all random relative to each other. You can accomplish that by creating a recording using a pink noise source where each channel is delayed by a second or more relative to the others. (Practical pink noise sources are pseudorandom, so there is a pattern repetition time to be considered as well in determining that the different channels will actually end up random relative to each other.) By using noncoherent pink noise signals you don't end up with points where there is phase reinforcement or cancellation in the room as you would with signals that are in phase.

Using Room EQ Wizard in real-time analyzer (RTA) mode with these pink noise sources can gave you a an overall view of how your room is getting energized by your speaker system. You can move your measurement mic through the different seating positions in your room and see how that total energy response plot varies from position to position. As previously stated, whatever you find here will need to be tracked down to individual speaker contributions and equalized on a speaker by speaker basis, if that is how you choose to approach any corrections.

It's a way to get a quick birds eye view of the total energizing of your room, if it's easy for you to accomplish. It is probably not worth a huge amount of trouble though. Just food for thought.


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## Barleywater (Dec 11, 2011)

This forgoes real physics, and the highly linear nature of air and reflections. Impulse response measure at a point in room is some of impulse responses of each source.

Pseudorandom pink noise of REW may have inverse calculated which convolved with multiple repeats of original pink noise returns periodic impulse response sequence.

Regards,

Andrew


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## AudiocRaver (Jun 6, 2012)

Barleywater said:


> ...the highly linear nature of air and reflections. Impulse response measure at a point in room is some of impulse responses of each source.


Totally agree.



> Pseudorandom pink noise of REW may have inverse calculated which convolved with multiple repeats of original pink noise returns periodic impulse response sequence.


Again, totally agree. I did not mean to imply in any way that my suggestion was a substitute for, or improvement upon, using the impulse responses of individual sources. Just another way to look at the system as a whole, if so desired. Beg pardon if my comments detracted from the thread, that was not my intention either. I will say no more! Peace and love!


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