# SPL room mapping



## rewjack (Aug 24, 2011)

Hi
By this intervention, I want to share some results of the SPL mapping that I have done in my dedicated music room. It shows why I was not able to correct some frequency dips by simple EQ-ing. Some readers might have suggestions or solutions regarding possible changes that can be made for better compromises in speaker positioning or frequency settings.
I have been working on hundreds of hours to fix the right *digital crossovers*, delays/phase matching and some EQ corrections using REW for all 7 channels used. At the end, everything looked all right but some narrow dips and bumps. With time, I have deleted most of the EQ's corrections because there was that feeling of killing some life in sound. Meanwhile, I worked a lot on room acoustics, adding Schroeder diffusers and other reflectors of different patterns on walls, ceiling and even floor, in addition to corner traps. They were the best thing ever done for clearing sound in my room, avoiding most of the absorbent material.

At the end, this mapping idea came out. It permits to view the real distribution of sound level around the room. So I would greatly appreciate any comments and suggestions for the next step.

You will find attached 3 PDF files, 65hz, 100hz and 1000hz. The readings were done every 33,5cm (X-Y) at 90 cm in height (listening and tweeter levels) for the plan map. I did a vertical section on the A-B line (on top of the figures) with 3 measurement sets, 60, 90 and 124cm, hopping to understand the vertical distribution of the SPL. Note that the measurements of the section were located a little on the right side of the room.

65hz:
Very clear situation. This frequency was the one who gave the widest and deepest dip (60-70hz) at my chair location and it was getting worst moving toward the back wall. It was getting better in the speaker direction. This map clearly shows this feature. In the more stable SPL region (yellow-red) this dip was disappearing ok but I don't like close listening, less soundstage. The vertical section shows a stable distribution too. 
100hz:
Great interesting picture of the SPL. I understand why I was happy for this range of frequency. The chair was almost in the best area. But you can see in the vertical section that further back, forward or down, the SPL would have been lower.
1000hz:
This higher frequency has 34,3cm in length, so the SPL responses at every 33,5cm are more irregular. It is quite amazing to see the distribution around the speakers and on the side. The vertical section is significant too.

The question remainded. Wath can I do to influence these distributions regarding my favorite chair location?
If you are curious, I can provide some free limited data processing and maps if anyone is interested for a try. 

So this is it for the moment and I hope some will get interested in SPL mapping.
thanks


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## vann_d (Apr 7, 2009)

WOW! Extremely cool graphical plots and, I imagine, extremely labor intensive to generate! I wish I could offer you some insight or advise but, alas, I think your knowledge exceeds mine on the subject.


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## rewjack (Aug 24, 2011)

vann_d said:


> WOW! Extremely cool graphical plots and, I imagine, extremely labor intensive to generate! I wish I could offer you some insight or advise but, alas, I think your knowledge exceeds mine on the subject.


I'm happy that you liked my graphs. It's data gridding using a GIS system (geographic information system). For a better understanding it is good to closely look at the db numbers on the plots.

I'm lookin for a way to move the yellow-red area further back in the room (65hz). I can't explain the exact additions/cancellations phenomena occuring here, but I hope there is a solution. My goal is to keep the listening location at distance from spks and keeping the space behind them and the wall. Changing delays won't do it, I think? I tried to move them ahead but the 65hz dip location did not changed. Is it the room proportion?

So thank you for your reply. Maybe I should transfer this post elswhere? You know well this forum, can you tell me something about it.


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## Peter Loeser (Aug 11, 2012)

Very interesting experiment - thanks for sharing your results.

I'm curious, what kind of test tones did you use? Sine wave or sweep at each location? Something else? Also, how did you choose which frequencies to map in your room?

Have you calculated the theoretical modes in your room and compared them to your measurements?


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## rewjack (Aug 24, 2011)

Peter Loeser said:


> Very interesting experiment - thanks for sharing your results.
> 
> I'm curious, what kind of test tones did you use? Since wave or sweep at each location? Something else? Also, how did you choose which frequencies to map in your room?
> 
> Have you calculated the theoretical modes in your room and compared them to your measurements?


Hi Peter
I've used a simple 65hz sine wave for this experiment. Since It took around 45min for each frequency a sweep tone would have been to much long. Have a look at a sweep response (20-20kz) attached. You'll see the hudge dip around 65hz. This is what I was looking at. So this frequency was targeted first.
The 100hz is for me a kind of reference frequency. I usually EQ the rest relative to this one. It is a good stable base without much variations in all my tests. The 1kz is just a curiosity for higher spectrum.

Frankly, I'm not very good with theorical calculations so I prefer emperical studies. There is so much variables to consider. But I can imagine that a 65hz wave of 5,28m could be easily calculated for my case. I'm trying to figure summations and cancelations for this wave form and room length (5,9m) to match what I measured. If you can help?

I made a test this morning moving the spks by 38cm foreward the chair and did a new mapping. This one is compared with the first one (attached PDF). The major changes are close to the spks but the dip at the rear is still at the same location.

Effects from walls near spks relative the position of reflectors may be significant locally. Interesting anyway. 

It's now easyer to visualized the whole picture of my room in my mind. It may take 45 minutes to map it but I've got something to work on without wrong intuitioning.

Keep asking questions. If you want to try, I'll do some graphics for you.


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## Owen Bartley (Oct 18, 2006)

Really interesting project and methodology. I'd like to see what comes of this. The only thing I can think of to suggest if you want to keep the speaker distance the same would be to see if moving them closer together (farther away from the outside reflection points) would help to move your "sweet spot" back... although it might destroy your imaging. Either that or some heavy duty panels at the reflection points, but I assume that was something you've already tried.


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## Peter Loeser (Aug 11, 2012)

I'm afraid I don't have much to offer in terms of advice here either. If subtle changes in speaker position don't give the improvement you're looking for then try more drastic ones. I have a similar dip just below 50Hz and have found that speaker/subwoofer placement only does so much to minimize the problem. Additional acoustic treatment is on my short list of things to do, although I see you have already done quite a bit in your room. I may have to try your mapping method at some point in my HT as well.


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## Owen Bartley (Oct 18, 2006)

Just had another thought while daydreaming on the subway. I wonder how toe-in and toe-out would affect the sweet spot. Again, it would probably throw off the imaging.


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## rewjack (Aug 24, 2011)

Hi guys
I'm glad that you still Wonder and interested.
I am a traitor for you because since I was looking for some answers, I've posted a different version on "elswhere" and the discussion keeps going on so far. 



Owen Bartley said:


> Just had another thought while daydreaming on the subway. I wonder how toe-in and toe-out would affect the sweet spot. Again, it would probably throw off the imaging.


Nice thoughts for subway.. 
Yes I tried it and every possible position changes for all parts. It ended at the same location than I found several months ago.



Peter Loeser said:


> I'm afraid I don't have much to offer in terms of advice here either. If subtle changes in speaker position don't give the improvement you're looking for then try more drastic ones. I have a similar dip just below 50Hz and have found that speaker/subwoofer placement only does so much to minimize the problem. Additional acoustic treatment is on my short list of things to do, although I see you have already done quite a bit in your room. I may have to try your mapping method at some point in my HT as well.


You can do it within an hour or two for measurements. Plot your numbers on a plan view and draw iso-contours at every 2 or 3 db. For color maps, give me an excell file x-y-value and I'll do it.



Owen Bartley said:


> Really interesting project and methodology. I'd like to see what comes of this. The only thing I can think of to suggest if you want to keep the speaker distance the same would be to see if moving them closer together (farther away from the outside reflection points) would help to move your "sweet spot" back... although it might destroy your imaging. Either that or some heavy duty panels at the reflection points, but I assume that was something you've already tried.


In fact you may be right, fiberglass pannels or modifying back wall location...
I'll be back if you're not angry about me.

cheers
jacques


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## rewjack (Aug 24, 2011)

Owen Bartley said:


> Just had another thought while daydreaming on the subway. I wonder how toe-in and toe-out would affect the sweet spot. Again, it would probably throw off the imaging.





Owen Bartley said:


> Really interesting project and methodology. I'd like to see what comes of this. The only thing I can think of to suggest if you want to keep the speaker distance the same would be to see if moving them closer together (farther away from the outside reflection points) would help to move your "sweet spot" back... although it might destroy your imaging. Either that or some heavy duty panels at the reflection points, but I assume that was something you've already tried.





Peter Loeser said:


> I'm afraid I don't have much to offer in terms of advice here either. If subtle changes in speaker position don't give the improvement you're looking for then try more drastic ones. I have a similar dip just below 50Hz and have found that speaker/subwoofer placement only does so much to minimize the problem. Additional acoustic treatment is on my short list of things to do, although I see you have already done quite a bit in your room. I may have to try your mapping method at some point in my HT as well.


Well I'm late and I'm having strange expériences with "multi-quoting"


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