# Using 2,3,4 microphones concurrently for measurements



## Jargon (Oct 26, 2012)

Hello,

Although new to Acoustic Measurements I am reading up to get as much information as I can as fast as I can. 

My goal is to find out my room modes/problems and then starting with acoustic treatments move to Digital Room Correction for the last bits of performance. It is my understanding, that since everything is based on accurate measurements one has to ensure that his measuring gear is at least calibrated yet there might be advantages for example with going for a better microphone / pre-amp and digital converter.

I already ordered an Earthworks M30 and also have a Focusrite Scarlet to start testing the waters and eventually I will move to a Metric Halo to be used both as a mic pre-amp and a DAC in a 7.1 system (with 2 subwoofers). 


Now, I guess many of you are aware of the Trinnov Room Correction system and although I am not planning to buy it (too expensive) I was intrigued by its microphone.










They are using 4 microphones as we can see. 


I was wondering then, how could one use 4 Earthworks M30s for example to achieve something similar (they would have to be matched I guess)? 

What are the advantages of using multiple microphones and should they be placed like the Trinnov microphone? 

I understand that averaging their response measurement data would have to take place then apply weighting per position and finally apply EQ but if someone could give a proper explanation or some links for reading I would be obliged.

Is anyone here using 2-4 microphones for the production of Room Correction curves?


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## JohnM (Apr 11, 2006)

To take advantage of multiple microphones per the Trinnov arrangement the signals from them must all be captured simultaneously, this allows the direction of reflections to be determined through analysis of relative timings of the signals. For most acoustic measurement it is a good idea to make measurements across the likely listening positions, but not necessary to go to the lengths of the Trinnov system. 

It is also not necessary to use such high quality microphones and soundcards as those you mentioned, equally good results (from a measurement perspective) are obtained using much more modest (but calibrated) equipment, such as calibrated mics from Cross-Spectrum Labs.


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## Jargon (Oct 26, 2012)

Even with the Trinnov you still take measurements in multiple positions though, so its not a matter of averaging positions but a matter of identifying the directions of the reflections then as I understand from your reply.


I am just wondering if this can be made using 2 or more matched microphones like the M30 and how to set it up ensuring proper microphone placement. Isn't the Trinnov still getting a one channel signal or is it getting 4 simultaneously and then deduces the result through a special algorithm. If it is the latter then I guess yo need special software too to do something similar.

If you only want to measure and EQ only one row of seats for example is there any advantage of using a 2-4 microphones as per trinnov or even at a different arrangement. I understand that it is not necessary but if the determination of the directions of the sound waves serves a scope then I would like to know.


Yet, apart from the matched response of the microphones, I guess we need matched timing of the signals through the preamp and the AD converter but this is hardware's job.


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## Jargon (Oct 26, 2012)

Here is another example of such microphone systems that I am sure you know better than me but a picture is a thousand words.


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## JohnM (Apr 11, 2006)

Jargon said:


> Isn't the Trinnov still getting a one channel signal or is it getting 4 simultaneously and then deduces the result through a special algorithm. If it is the latter then I guess yo need special software too to do something similar.


It is the latter.



> If you only want to measure and EQ only one row of seats for example is there any advantage of using a 2-4 microphones as per trinnov or even at a different arrangement. I understand that it is not necessary but if the determination of the directions of the sound waves serves a scope then I would like to know.


No. Special software is required to make any use of multiple microphones and they have nothing to contribute to domestic acoustic measurement and room setup. The biggest effects by far will come from optimising speaker/subwoofer positions, listening positions and room treatments, money spent on elaborate measurement systems will not improve the end result - far better to spend the money on the room and the sound system, not the measurement kit.


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## Jargon (Oct 26, 2012)

I see and I intend to act as you suggest. I will use the M30 and REW to take measurements and then play around with optimally positioning the speakers. The next step will be DIY room treatments since it is a dedicated room and I can do what I want. The final step will be Digital Room Correction through the PC and Audiolense or Acourate.

Even the M30 was chosen for very low frequency measurements of the subwoofers and the fact that it has less distortion at high SPL measurements. 

The use of multiple microphones is just interesting and I am trying to identify the advantages since both Trinnov and JBL employ this method for measurements although JBL's is only for subwoofer EQ.


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## RayJr (Jan 14, 2007)

Hello John..
How about using 4 mics spread apart thou a multiplexer? or better yet 4 MiniDSP mics and a 4 port hub.
For doing spatial averaging of a given seating area.

Thanks
RayJr


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## JohnM (Apr 11, 2006)

REW can only measure from one mic at a time, so all it would save is moving the mic from place to place as you measure.


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## Jargon (Oct 26, 2012)

So I guess it is not only REW but many other packages that are not specifically designed to measure from multiple mics simultaneously.

Even Dolby's method with their multiplexer used RTA for analysis. 

It would be interesting thought if in the future you would create and averaging algorithm in Trinnov style. 

But what are the advantages there? I mean multiple mics could be used either spread out (1) or in one point close together (2).

1) This then compromises the "money seat" I guess which is more important as far as I am concerned but can be sorted out using multiple curves.

2) Do you need to specifically calculate mic positioning and distances? Then is the determination of speaker placement achieved through that (vertical * horizontal planes considered) of specific use and how much better is the use of such data in the final correction curve.


Food for thought.


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## sdurani (Oct 28, 2010)

Jargon said:


> But what are the advantages there?


Spatial averaging for multiple seats (especially in the low frequencies) and being able to confirm corrections by re-measuring from the exact same locations (hard to re-trace your steps with single mic).


Jargon said:


> This then compromises the "money seat" I guess which is more important as far as I am concerned but can be sorted out using multiple curves.


Place the multiple mics in a tighter pattern. I know plenty of people that, for example, do all their 8 Audyssey measurements within a 2-3 foot circle.


Jargon said:


> Do you need to specifically calculate mic positioning and distances?


Can be done with the first measurement by placing one mic in the sweet spot or bundling all your mics at that location before spreading them out for the remaining measurements.


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