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Author Topic: Use of RTA  (Read 14402 times)

Garrett Trask

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Use of RTA
« on: June 29, 2010, 02:46:58 pm »

Hello everyone,

I recently installed a Driverack PX in a small church.  They meet in what is a more or less an acoustic nightmare of a room.  The room was never designed to support live audio.  I have been very interested in learning as much as I can about proper techniques in Equalizing the PA to the room.  The auto EQ function on the DBX unit really does not make any positive changes.  My best guess however is that my technique is bad, not the Auto EQ function.  I only placed the microphone in one location.  It was suggested to me that I place a measurement mic in several locations.  I would then average these measurements to get an overall curve of which to apply.  This does make sense to me.  Would you recommend this technique for stage monitors as well.  Our stage monitors are in the same location each week.  It would be nice to EQ my wedges so that they respond well with the stage. A concern that I am sure we all share is that a measurement microphone cannot necessarily tell the difference between the direct source of the audio and the reflections.  I want to minimize the amount of reflected energy that I measure...correct? Should I not place the microphone as close as possible to the speaker.  I do not have experienced enough ears to do it by ear and we do not have the  money to pay someone to come in and balance our PA to our room.  I would appreciate any input on this subject and resources that can help us.

Thanks everyone!
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Dick Rees

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Re: Use of RTA
« Reply #1 on: June 29, 2010, 03:29:24 pm »

This is the latest, most definitive article on why RTA does not suffice for PA tuning:

http://campuspa.com/downloads/devil_with_rta.pdf

If the link does not work, "Google" this:

Bennett Prescott + RTA

After you have read the article, consider this:

1.  Everything starts with the proper speaker(s) properly deployed.  This means focusing the sound on the listeners and keeping the sound off the walls and ceiling to keep the ratio of direct sound to reflected sound as high as possible.

2.  Stage volume should be as low as practicable.  This includes any amplifiers and drums in addition to the monitors.  

3.  Proper equalization/DSP needs to be applied to the system to optimize things for your use.  This can be done by ear by an experienced sound person or can be done with sophisticated analyzation programs like SMAART and others.  You may be able to hire a professional to do this for you.  Properly done, it should be a one-time job/expense.

Best of luck.

DR
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Tom Young

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Re: Use of RTA
« Reply #2 on: June 29, 2010, 08:03:00 pm »

Agree with all points including reading Bennett's article.

I forget whether he also discusses the futility of auto-EQ processes. Auto EQ does not work well. The process of measuring and analyzing sound (acoustics, electrocoustics, etc) is far more complex than we can achieve with a computer (alone) and there are elements of subjectivity (including experience and intuition) that play a role in this process. Among other things the computer cannot do is decide if the results it has applied sound good. We (pretty much all) can do this. It is not unheard of to spend much time optimizing a speaker system, listening to the results and then going back and starting from (or almost from) scratch. When this happens to me I usually know where I need to return to. The computer does not.

Another ingredient to all this is the acoustics. Along with speaker system design, measurement and anlysis and how the system is operated (including how folks play, etc) ..... much can be done to improve things with careful analysis of the acoustics (and the architectural elements that constitute this) and appropriate treatment.

There are just as many folks with 1/2 a clue throwing stuff on the walls and achieving changed, but still not very good, acoustic response as there folks "designing" speaker systems, hanging them, measuring them, etc.

It should come as no surprise that I am an advocate for hiring a qualified consultant who can do this sort of work effectively and as economically as possible.
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Dick Rees

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Re: Use of RTA
« Reply #3 on: June 29, 2010, 09:17:46 pm »

Agreed.  

I jumped in with the link to the RTA article without really tying it in to the AutoEQ cited in the OP.  Auto EQ works off of the RTA function (as I understand it) and suffers from the inherent weaknesses thereof.

I'm all for optimizing what you have before spending money on the problem.  The first thing that would pry a dollar out of my pocket would be hiring a pro to look over the situation, adjust it as well as possible and make recommendations where to go from there.

Structural changes (re-hanging speakers, etc) and/or room treatments would be well down the line for me.  The better you set up the system, the less room treatment you'll need.  That said, some room treatment is usually desirable......after system optimization.

Again, best wishes for your endeavors.

DR
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Brad Weber

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Re: Use of RTA
« Reply #4 on: June 29, 2010, 10:43:53 pm »

I won't go into the general RTA issues as Tom and Dick (need to get Harry in on this to complete the set) have already covered that.

Garrett Trask wrote on Tue, 29 June 2010 14:46

I only placed the microphone in one location.  It was suggested to me that I place a measurement mic in several locations.  I would then average these measurements to get an overall curve of which to apply.  This does make sense to me.

Analyzing the system at multiple locations in the listener area is good, although I'm not sure that would be applicable to the AutoEQ function.  I personally do not like simple averaging, if I measure in six locations and five are right one but one is off I may decide to not negatively impact the five just to help the one, but then that may also depend on where the one that is off is located.  What it comes down to is that there should be some subjective consideration and it can be more than just a simple averaging.

Garrett Trask wrote on Tue, 29 June 2010 14:46


Would you recommend this technique for stage monitors as well.  Our stage monitors are in the same location each week.  It would be nice to EQ my wedges so that they respond well with the stage.

Not really.  For one thing, you are usually most concerned with the monitor response over a more limited area, perhaps even at one location.  Much of the equalization applied to a monitor may be for gain before feedback rather than just for subjective quality.  For monitors you normally want things as close as possible to actual use.

Garrett Trask wrote on Tue, 29 June 2010 14:46

A concern that I am sure we all share is that a measurement microphone cannot necessarily tell the difference between the direct source of the audio and the reflections.  I want to minimize the amount of reflected energy that I measure...correct? Should I not place the microphone as close as possible to the speaker.

You've actually touched on some of the very issues that can be addressed with more advanced analysis systems like Smaart, SysTune, Praxis, etc.  On the one hand, all you can really adjust with equalization is the response of the speaker (although you may also be adjusting phase without realizing it), you can't change what the room does or interactions between speakers with an equalizer.  On the other hand, the room and interaction among devices does affect what the listeners hear.  So to use an RTA effectively you have to learn to differentiate what you can and cannot 'fix' with EQ.  The dual channel FFT type analysis systems allow you to 'window' the measurement in time so that you can more effectively see the direct response as well as the response with varying elements of the room and other devices.
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Arnold B. Krueger

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Re: Use of RTA
« Reply #5 on: June 30, 2010, 08:47:34 am »

Garrett Trask wrote on Tue, 29 June 2010 19:46

Hello everyone,

I recently installed a Driverack PX in a small church.  They meet in what is a more or less an acoustic nightmare of a room.  The room was never designed to support live audio.  I have been very interested in learning as much as I can about proper techniques in Equalizing the PA to the room.  The auto EQ function on the DBX unit really does not make any positive changes.  My best guess however is that my technique is bad, not the Auto EQ function.  I only placed the microphone in one location.  It was suggested to me that I place a measurement mic in several locations.  I would then average these measurements to get an overall curve of which to apply.  This does make sense to me.



I agree with the other posts that question the autoamtic use of a RTA for setting up a room. Depending on the room, setting up eq can be either difficult or impossible. The learning curve can be long and frustrating. Classes like the Syn-Aud-Con class that has been being discussed here are good prerequisites for attempting to eq a room.

Quote:


Would you recommend this technique for stage monitors as well.  Our stage monitors are in the same location each week.  It would be nice to EQ my wedges so that they respond well with the stage.



I question the idea of setting up monitors for flat response. Smooth yes, flat not so much. Furthermore, most of my monitors have some narrow anti-feedback notches in them. No RTA I know of would put them in.

Quote:


A concern that I am sure we all share is that a measurement microphone cannot necessarily tell the difference between the direct source of the audio and the reflections.



No concern there, mics just can't make that decision, especially the omnis that are customarily used.

Quote:


I want to minimize the amount of reflected energy that I measure...correct?



Ideally, your monitor speakers should not be generating a lot of reflections. You might want to address that problem first.

Quote:


Should I not place the microphone as close as possible to the speaker.



Get that mic too close to the speaker and your measurements are being unbalanced by being close to one driver, and off-axis from another.

Quote:


 I do not have experienced enough ears to do it by ear and we do not have the  money to pay someone to come in and balance our PA to our room.



Plan C: get some formal education. And/or find a Christian sound guy in the area who will take you on as a ministry.

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George S Dougherty

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Re: Use of RTA
« Reply #6 on: June 30, 2010, 12:47:37 pm »

I tend to skip the AutoEQ and tune things by hand, using the RTA as a guide to make sure I'm not missing anything with my ears.  I don't have a fancy tool like SMAART to help me see the direct vs reflected response so I have to work by alternate methods.

The first thing I do is setup a speaker outdoors and do my baseline EQ there.  This only works of course if you can move a speaker outside in the first place.  Pre-hung speakers will be a challenge.  I've had 80ft from any buildings or large reflective surfaces recommended to me as a good guideline.  The RTA mic typically needs to be a minimum of 10ft from the speaker to allow for proper integration of the drivers as Arnold alluded to, and the volume of the pink noise needs to be at least 20db above any background noise, preferably 30db.

At that point, you're doing a pretty decent job of EQ'ing the speaker itself and not compensating for a room.  I target something closer to the equal loudness curves for my EQ rather than shooting for flat 20-20.  I've found flat gets near intolerable at loud volumes.  It tends to be overly bright and piercing because of where our ears are more sensitive.  In the Driverack I'd do all that in the parametric EQ's and store those as a preset.

Once you move the speakers into a room you could again use the RTA if you don't trust your ears and look at several points around your coverage area.  You're not trying to EQ the room at this point, just identify broader room nodes that may need to be pulled back to improve clarity in that space.  These adjustments can be done with the graphic EQ.

All that said, if the room is an acoustic nightmare, that's your first problem.  Getting a certified acoustician to help address those issues will have the biggest impact on problem spaces.  Outside of coverage patterns and placement, you can't solve acoustic problems with electronics.  The issue there is physical and modifying your signal won't correct that.
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Dick Rees

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Re: Use of RTA
« Reply #7 on: June 30, 2010, 03:12:55 pm »

George.....

Personally, I would just deal with the speakers in place.  I would hope for the following:

1.  Proper placement for coverage.
2.  Proper speaker for coverage.
3.  A speaker of high enough quality that tweaks to the basic output would be unneeded.....or at least minimal.

If you happen to have speakers which require equalization to pass decent sound, I'd look at having them brought back up to specs or replaced with better speakers.  That said, most professional grade speakers will have information available regarding any DSP which should be applied.  You can simply open the PDF and look it up.

Best.

DR
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Brad Weber

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Re: Use of RTA
« Reply #8 on: June 30, 2010, 07:30:41 pm »

George S Dougherty wrote on Wed, 30 June 2010 12:47

I don't have a fancy tool like SMAART to help me see the direct vs reflected response so I have to work by alternate methods.

What, SAC doesn't do this?  Smile

George S Dougherty wrote on Wed, 30 June 2010 12:47

At that point, you're doing a pretty decent job of EQ'ing the speaker itself and not compensating for a room.  I target something closer to the equal loudness curves for my EQ rather than shooting for flat 20-20.  I've found flat gets near intolerable at loud volumes.  It tends to be overly bright and piercing because of where our ears are more sensitive.  In the Driverack I'd do all that in the parametric EQ's and store those as a preset.

I tend to go for flat response for this type of speaker/array tuning and to then address any subjective tuning when addressing the room and system.  I want to be sure that without the effects of the room, the rest of the system, etc. the speakers can be as transparent as possible before putting them into the space.

George S Dougherty wrote on Wed, 30 June 2010 12:47

Once you move the speakers into a room you could again use the RTA if you don't trust your ears and look at several points around your coverage area.  You're not trying to EQ the room at this point, just identify broader room nodes that may need to be pulled back to improve clarity in that space.  These adjustments can be done with the graphic EQ.

Oops, the age old issue that you cannot "EQ the room" and you cannot fix room modes with EQ.  You may be able to adjust the speaker response to try to compensate for the acoustical environment but as you say later, you can't solve acoustical problems with electronics (except maybe some of the electronic reverberation enhancement systems).
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Darin Brunet

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Re: Use of RTA
« Reply #9 on: June 30, 2010, 11:27:43 pm »

Agreed with the above statements. IMO, use the best CD (Constant Directivity or Controlled Directivity) loudspeakers you can find/afford and properly set them up to minimize reflections from side walls, floors and ceilings.

Room EQ is of little use in most instances. For instance, you have "corrected" the response with the use of an RTA and EQ. Yet, the correction is only valid at that particular location where the mic was placed.

In other words, even if the central or other axis has been corrected to be perfectly flat, in no way, shape, or form is the response problem “corrected”. In fact, it might even be worse. It should be understood that electronic EQ can correct for any linear problem found in a loudspeaker prior to the acoustic domain, but it does not correct for problems within the acoustic domain itself, except along a single direction, i.e. a single point.

This is independent of whether or not this problem comes from the speaker or the room itself. The correction is valid only at that single point in space and nowhere else. Also, some very important aspects of a speakers sound quality is not at all apparent from an on-axis curve or any "single direction" curve.

Point is, once you have a well designed loudspeaker, there is no need for electrical EQ and in fact it is likely to make things worse. Acoustic problems can only be solved acoustically, and once that is done, nothing more is possible.
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Re: Use of RTA
« Reply #9 on: June 30, 2010, 11:27:43 pm »


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