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Author Topic: Test Mic placement for room tuning  (Read 7144 times)

Steve Richard-Preston

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Test Mic placement for room tuning
« on: November 17, 2010, 07:12:13 pm »

Background- I work for a University AV workshop, we build, install and maintain all the AV from large theatres to small classrooms.

Up until now we've taken a pretty unscientific approach to tuning the room; by ear, just letting the system ring out and notching out the resonant freqs of the room. This has usually returned ok results, but we're looking to step it up a little.

We've just purchased Sys Tune as we've got some big theatre (300 seat, high rake) refurbs coming up and want to take a more scientific approach to tuning the room.

My question is, how to go about mic placement (test mic). What's the best approach taking into account room modes? Given that you could expect a variance in frequency response across the room.

The context is mostly with an aim to getting maximum gain before feedback in rooms that are usally not acoustically treated (although these new refurbs will be), using a fixed position lectern mic with a couple of roaming wireless mics.

Cheers in advance for your thoughts.
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Ivan Beaver

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Re: Test Mic placement for room tuning
« Reply #1 on: November 17, 2010, 08:39:34 pm »

The "best" position is lots of them.  To get an idea of what is happening around the various seating positions you have to measure in them.

Now what you can do about it, depends on the particular system you have.

In some cases you take a bunch of measurements and then just a
"average" them or determine what seats are more important than others.

Regarding room modes, a bull dozer is usually the best tool for the job. Laughing   No seriously, I'm not kidding.  And I have told more than one facility that the best thing they can do is to tear down teh building and build a proper one for what they are tying to do in there.  They don't like that. Rolling Eyes

Room modes are another reason for multiple mic positions.  To see if what you are reading in one position is the same all around.  If it is then you can do something about it.  but if-for example-you see a peak (or a dip) at some positions and the opposite at others, there is generally nothing you can do-unless those are caused by interactions of the loudspeakers themselves.

And then you can only do something about it-if you have the control (DSP-different amp channels etc) and the knowledge to realize what may be causing that dip in the response-overlap-time offsets-phase errors or the loudspeaker itself.

The first thing to do is to get the system aligned as good as possible-paying particular attention to the phase response.

Then you can do a feedback tuning on the fixed position.

The roaming mics are a totally different animal.  See if you have the same feedback freq in different places in the room.  If you do, then use the console eq to notch that out-NOT the house eq.

But get the system as good as you can, BEFORE using eq to try and get more gain before feedback.  Don't jsut start whacking away at the eq
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For every complicated question-there is a simple- easy to understand WRONG answer.

Can I have some more talent in the monitors--PLEASE?

Ivan Beaver
dB Audio & Video Inc.
Danley Sound Labs

Tom Young

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Re: Test Mic placement for room tuning
« Reply #2 on: November 17, 2010, 09:08:24 pm »

Ivan 'scratched the surface' very well.

But here's the deal: you will not be able to effectively use SysTune (or any other "complex" measurement system) by reading any amount of advice you can get here via a thread.

Your university/employer should invest in sending you (and your colleagues) to SynAudCon, first of all. This will help plant some of the seeds of the groundwork in comprehending all this stuff. It will also give you access to their Tech Topics (aka: newsletters) which have had at least a few articles (and threads, on their listserv) on measurement, mic positions, etc. This is all available when you become a member.

www.synaudcon.com

Here at ProSoundWeb you will find (if you search, and also look through Study Hall) at east a few articles on such measurement includng some discussion on where to put the mic(s).

You should also pick up Bob McCarthy's book on System Design and Optimization (2nd edition) and plan on slowly working your way through that.

Then you shoud take a training course in/by Smaart, SIM or SysTune measurement. Each of these companies/products offers very good, hands-on training that will get you started. They generally do them in different parts of the country several times a year.

This may seem daunting but very few folks can get their heads around this stuff without lots of reading, training and hands-on experimentation.

But it will be well worth it.
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Tom Young
Electroacoustic Design Services
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Steve Richard-Preston

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Re: Test Mic placement for room tuning
« Reply #3 on: November 17, 2010, 10:14:36 pm »

Tom, thanks for that. The good news is that we've had an acoustician model the room and spec speakers and placement with EASE modelling already. So I'm confident that side of things will be OK.

Thanks for the tip on multiple mic placements, I kind of figured that this might be the case but the practicality of such testing will be pretty time consuming  Sad

So I guess the process would be something like this?

1. Test in various positions
2. Calculate average
3. Make EQ adjustments
4. Retest in same various positions
5. Calculate average (hopefully not have to repeat the process toom many times)  Laughing  

Love the bulldozer idea, but yeah no luck with that, at least we've had a good analysis of the room done and are having good quality materials used in the refurb to address some of the issues.


Ivan, Thanks for the tip on training, would love to do it sometime, but that's up to the powers that be, as usual...  Am working my way through the maunual, seems ok so far, and will have some time to play around and get the hands dirty with it before setting up the room.

Awesome forum, it's been a while since I've lurked around here, but have to say always found the advice great Smile
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Brad Weber

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Re: Test Mic placement for room tuning
« Reply #4 on: November 17, 2010, 10:45:30 pm »

You might think a bit beyond just calculating the average, there's often a lot of 'fuzzy math' involved that is more subjective.   Making a change to improve the results for one location that negatively affects other locations may be more than looking just at the average, it may include assessing which locations are more important or whether the potential improvement to the one justifies the impact on the others.

There can also be some issues that may inherently vary throughout the space.  For example, you may be able to align devices for a measurement location, but move a bit and the relationship to the devices changes.  Adjust everything for that location and now the first one is off.  So it's often determining the 'best fit' solution for the specific application.

Keep in mind that SysTune, Smaart, Praxis, etc. can help with a lot more than just equalization.  When you start utilizing phase measurements and windowing as well as frequency and amplitude responses you can really start to see how the interactions of devices with other devices and the room affect what arrives at a listener.  The measurements can help indicate that the results you're getting are a result of the speaker itself or of a reflection off a room surface or maybe due to interactions between two speakers reproducing the same signal.  Being able to look beyond just equalization and frequency response is the big step in really getting the most from these tools.

I've been active in SynAudCon for over 20 years, read Bob's book among many others, used a TEF and have been to Smaart and SysTune training and I'm still constantly learning new things (and remembering ones I forgot) related to system tuning and measurements.  You never really know it all, you just keep knowing more than you did before.
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Brad Weber
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Ivan Beaver

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Re: Test Mic placement for room tuning
« Reply #5 on: November 18, 2010, 07:28:09 am »

Tom Young wrote on Wed, 17 November 2010 21:08

Ivan 'scratched the surface' very well.

But here's the deal: you will not be able to effectively use SysTune (or any other "complex" measurement system) by reading any amount of advice you can get here via a thread.

Your university/employer should invest in sending you (and your colleagues) to SynAudCon, first of all. This will help plant some of the seeds of the groundwork in comprehending all this stuff. It will also give you access to their Tech Topics (aka: newsletters) which have had at least a few articles (and threads, on their listserv) on measurement, mic positions, etc. This is all available when you become a member.

www.synaudcon.com

Here at ProSoundWeb you will find (if you search, and also look through Study Hall) at east a few articles on such measurement includng some discussion on where to put the mic(s).

You should also pick up Bob McCarthy's book on System Design and Optimization (2nd edition) and plan on slowly working your way through that.

Then you shoud take a training course in/by Smaart, SIM or SysTune measurement. Each of these companies/products offers very good, hands-on training that will get you started. They generally do them in different parts of the country several times a year.

This may seem daunting but very few folks can get their heads around this stuff without lots of reading, training and hands-on experimentation.

But it will be well worth it.

You are totally right about the training.  I was "assuming" knowledge of measuring a system and just talking about the mic placement.

There are 3 basic areas of expertise involved in measuring/aligning a sound system (but not limited to that few).

First is measurement knowledge-how to know if you have a good/accurate measurement.  That translates into all measurement platforms.

Next is how to analize the measurement and realize what may be causing the problem that shows up on the screen.  They may look the same-but the causes could be very different.  This translates into all measurement platforms.

Hence the need for basic training-like you suggest-Synaudcon.

Lastly is the knowledge of how to use the individual platforms.  This knowledge can be gained from the users manual-but the first two can't.

Then then you add things like-actual listening test-how good is your ear? perception of localization-knowing how far to push one element of the alignment-without interfering with another and so forth.

As with any skilled profession-it may look easy when you watch somebody do it (like playing a musical instrument), but doing it yourself and taking into account all the "little things" that go on during an alignment, is not so easy.
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For every complicated question-there is a simple- easy to understand WRONG answer.

Can I have some more talent in the monitors--PLEASE?

Ivan Beaver
dB Audio & Video Inc.
Danley Sound Labs

Ivan Beaver

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Re: Test Mic placement for room tuning
« Reply #6 on: November 18, 2010, 07:37:54 am »

Steve Richard-Preston wrote on Wed, 17 November 2010 22:14

So I guess the process would be something like this?

1. Test in various positions
2. Calculate average
3. Make EQ adjustments
4. Retest in same various positions
5. Calculate average (hopefully not have to repeat the process toom many times)  Laughing  



I start with individual alignments-then look at the overlap areas and at the end look at averages (if I need to).

I am a huge fan of multi mic measurements and as a result, I typically do "hundreds" of measurements during a single alignment-depending on the size of the room of course and the complexity of the loudspeaker system.

You cannot start doing averages on a multispeaker system.  That would simply be a waste of time.

It is a process-of which there are no real defined steps-because every job is different.

But here are my 3 steps (of course each can be broken down into many smaller steps).

1: Set out my mics and do an alignment and get all the seats as close as possible in both time and amplitude/freq response.

2: Walk the room and listen to pulses through the system.  This gives a good idea of reflections-where they are coming from-signal alignments/localization and in some cases even tonality.  Make adjustments as needed to improve the "bad" areas-but also rechecking to see if those adjustments have affected the good areas as well.  "Average" if needed.

3: Listen to a variety of speech/music tracks and make subtle adjustments as needed.

Go back to step 1 and recheck all the measurement locations to see if I have screwed anything up (to bad Laughing ).  

Repeat if needed.  

This can be an involved process that takes from a couple of hours to a couple of days-depending on the room and system in question.
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For every complicated question-there is a simple- easy to understand WRONG answer.

Can I have some more talent in the monitors--PLEASE?

Ivan Beaver
dB Audio & Video Inc.
Danley Sound Labs

Steve Richard-Preston

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Re: Test Mic placement for room tuning
« Reply #7 on: November 18, 2010, 04:21:04 pm »

OK, great.

If I take mulitple measurements (we only have one test mic) then I'll be moving the mic around to many different locations and then attempting to re-locate it back to exactly the same locations to take the next round of readings (after adjustments are made).

In the case of freq/amp, my question to follow that is how do you guys go about ensuring that the mic finds itself back in exactly the same position for each round of measurements?  Obviously make markings of where the mic stand is located. Is using a fixed position mic stand and orientating the mic the same way (Eg not adjusting the mic on the stand at all once it is setup) enough?

I'd expect that even a small amount off axis from a previous position will return a different response, and if multiple erors are made re-locating the mic the resulting average could be considerably off if all those errors happen to skew the same way?  Am I nit-picking now?  How far do you guys take this?  

Cheers!
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Ivan Beaver

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Re: Test Mic placement for room tuning
« Reply #8 on: November 18, 2010, 06:42:22 pm »

Steve Richard-Preston wrote on Thu, 18 November 2010 16:21

OK, great.

If I take mulitple measurements (we only have one test mic) then I'll be moving the mic around to many different locations and then attempting to re-locate it back to exactly the same locations to take the next round of readings (after adjustments are made).

In the case of freq/amp, my question to follow that is how do you guys go about ensuring that the mic finds itself back in exactly the same position for each round of measurements?  Obviously make markings of where the mic stand is located. Is using a fixed position mic stand and orientating the mic the same way (Eg not adjusting the mic on the stand at all once it is setup) enough?

I'd expect that even a small amount off axis from a previous position will return a different response, and if multiple erors are made re-locating the mic the resulting average could be considerably off if all those errors happen to skew the same way?  Am I nit-picking now?  How far do you guys take this?  

Cheers!

Actually if the mic is a little bit off-then that would add more positions to the average.

If the system is well designed, and the first round of "alignments" is done properly, then the second round should go much quicker.
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For every complicated question-there is a simple- easy to understand WRONG answer.

Can I have some more talent in the monitors--PLEASE?

Ivan Beaver
dB Audio & Video Inc.
Danley Sound Labs

Steve Richard-Preston

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Re: Test Mic placement for room tuning
« Reply #9 on: November 18, 2010, 09:18:44 pm »

sure more mic positions per round is better for the average, but if you want to make a comparison between the first round of meaurements and then the second round of meaurements (after tweaking has bene done), then you wouldn't want any variance between the two readings of the same mic position? (as opposed to simply having more mic positiotns to average)

Cheers and thanks again for the info/discussion
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Ivan Beaver

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Re: Test Mic placement for room tuning
« Reply #10 on: November 19, 2010, 08:04:00 am »

Steve Richard-Preston wrote on Thu, 18 November 2010 21:18

sure more mic positions per round is better for the average, but if you want to make a comparison between the first round of meaurements and then the second round of meaurements (after tweaking has bene done), then you wouldn't want any variance between the two readings of the same mic position? (as opposed to simply having more mic positiotns to average)

Cheers and thanks again for the info/discussion

If you really want to push the "no variance" then you will get into things like-is the temp the same at both times of measurements?  What about the humidity?

The deeper you dig-the deeper the hole gets.

In a lot of room alignmnets-it is more about getting the same realative sound everywhere-rather than geting really picky aobut little things that you don't have control over.

That is part of the whole alignment process-knowing what you can and-often more importantly-what you CAN'T fix.  So you just leave those areas alone and ignore them.  You can't do anything about it anyway.  Don't let it bother you.

A question for thought-if you have a multispeaker setup-what tool would you rather have 1: eqs   2: delays
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For every complicated question-there is a simple- easy to understand WRONG answer.

Can I have some more talent in the monitors--PLEASE?

Ivan Beaver
dB Audio & Video Inc.
Danley Sound Labs

Steve Richard-Preston

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Re: Test Mic placement for room tuning
« Reply #11 on: November 21, 2010, 07:58:21 pm »

Hey Ivan, yeah wasn't going get into temperature etc  Laughing  Keep the aircon running the whole time anyway!!

Regarding your question, the point your getting at is ensuring phase between multiple speakers right?  Good question, so far I've been focussing more on a flat response. But thanks for the heads up. We've got Nexias going into the theatres, so I'm lucky that I don't have to choose! gotta love DSP.

What would you choose, if you had to?

We've got EV ZX5's going into the front corners of the room and then 2 x AT ALA arrays as a centre fill on the ceiling about 2 meters from the front wall... just FYI
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Ivan Beaver

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Re: Test Mic placement for room tuning
« Reply #12 on: November 21, 2010, 08:05:56 pm »

Steve Richard-Preston wrote on Sun, 21 November 2010 19:58

Hey Ivan, yeah wasn't going get into temperature etc  Laughing  Keep the aircon running the whole time anyway!!

Regarding your question, the point your getting at is ensuring phase between multiple speakers right?  Good question, so far I've been focussing more on a flat response. But thanks for the heads up. We've got Nexias going into the theatres, so I'm lucky that I don't have to choose! gotta love DSP.

What would you choose, if you had to?

We've got EV ZX5's going into the front corners of the room and then 2 x AT ALA arrays as a centre fill on the ceiling about 2 meters from the front wall... just FYI

If you have loudspeakers that are "out of time" you will have all sorts of notches in the response-that no amount of eq cold possibly fix.  But get them lined up with a delay, the they will either go away-or be greatly reduced.

Yes the amplitude may still be a bit "off" without eq, but the ear is more sensitive to phase response (at least that has been my experience).  OF course that is "assuming" that the natural response of the loudspeaker is not all that bad. Rolling Eyes
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For every complicated question-there is a simple- easy to understand WRONG answer.

Can I have some more talent in the monitors--PLEASE?

Ivan Beaver
dB Audio & Video Inc.
Danley Sound Labs
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