Measuring the effect of acoustic treatments in the room

[ToTo Man]

This is a continuation of Upgrading my acoustical room treatments but deserves a new thread given that I've decided to start from scratch by rotating the room 90 degrees. My speakers are now on the short (3.81m) wall firing down the 4.13m length of the room. I also persuaded my dad to remove all the acoustic treatments from the room (apart from the Earthwool that he stuffed into the cupboard), so it's a great opportunity to measure different treatments in different locations in the room and show their effects. This is therefore going to be a very long and dense thread with LOTS of graphs!…

However, I'm only going through this process ONCE, so I want to make sure my plan of action is logical! This is what I propose doing:

Remove all treatments from room, take measurements in 20cm increments from back wall to find listening position with smoothest bass response when the speakers are hard against front wall, I will then use this as the preferred mic position to measure the effect of adding the treatments. (This is hopefully NOT going to be the final position of the speakers because I'd like to get them away from the front wall and regain some soundstage depth, but I've got to start with them somewhere. I don't know if this is wise but optimising speaker position is a process that could take days in itself and also might depend on the treatments in the room). The measurements I plan to take are as follows:

1) Untreated walls and corners.

2) Early reflection side wall absorption (nearside speaker only).

3) Early reflection side wall absorption (nearside and offside speakers).

4) Early reflection side wall absorption, early reflection back wall absorption.

5) Early reflection side wall absorption, early reflection back wall absorption, early reflection front wall absorption.

6) Early reflection side wall absorption, early reflection back wall absorption, early reflection front wall absorption, range-limited bass traps on back wall.

7) Early reflection side wall absorption, early reflection back wall absorption, early reflection front wall absorption, range-limited bass traps on back wall, late reflection side wall Q7D diffusion.

8) Early reflection side wall absorption, early reflection back wall Q7D diffusion, early reflection front wall absorption, range-limited bass traps on back wall, late reflection side wall Q7D diffusion.

9) Early reflection side wall absorption, early reflection back wall Q7D diffusion, early reflection front wall polystyrene diffusion, range-limited bass traps on back wall, late reflection side wall Q7D diffusion.

10) Early reflection side wall absorption, early reflection back wall Q7D diffusion, early reflection front wall polystyrene diffusion, range-limited bass traps on back wall, late reflection side wall Q7D diffusion, 1x corner absorber in each corner.

11) Early reflection side wall absorption, early reflection back wall Q7D diffusion, early reflection front wall polystyrene diffusion, range-limited bass traps on back wall, late reflection side wall Q7D diffusion, 2x corner absorbers in each corner.

12) Early reflection side wall absorption, early reflection back wall Q7D diffusion, early reflection front wall polystyrene diffusion, range-limited bass traps on back wall, late reflection side wall Q7D diffusion, 3x corner absorbers in each corner.

I've intentionally left adding the corner traps until the end so that I can see how effective the rest of the treatment is. Also I'm not including the 40Hz tuned membrane Scopus bass traps in any of these measurements as these are a solid box trap that can bugger up other areas of the frequency response depending on where they're positioned.

So... am I good to go?!

 

[Purité Audio]

The very first step is to measure the speaker’s on and off axis dispersion incrementally, placing the speaker on a turntable would be convenient, then you can determine which treatment might be beneficial.
Keith

 

[ToTo Man]

The very first step is to measure the speaker’s on and off axis dispersion incrementally, placing the speaker on a turntable would be convenient, then you can determine which treatment might be beneficial.
Keith

My speakers' off-axis horizontal response is smooth (see here). My speakers' off-axis vertical response is poor (broad cancellation at 5kHz as you move above tweeter axis), but treating my 3.25m high ceiling isn't currently on the cards.

Unless I change from monopole box speakers to dipoles I don't see why it's crucial to treat the room based on what my particular speaker requires. All monopole box speakers with wide-dispersion drivers are going to excite my room in a similar way and the goal is to make my room a good sounding room, regardless of whether I've got Celestion Ditton 66s or Jim Rogers JR149 (or another of my numerous wide-dispersion speakers) in the room.

I am aiming for even decay times of around 400ms across the board and minimising the strength of early reflections arriving within the first 6ms, achieving the latter is considered to be important from an imaging/soundstaging POV.

 

[Purité Audio]

I would look at almost any of Amir’s loudspeaker measurements, he measures the various floor/ceiling/wall reflections and suggests if and how specific treatment will improve the SQ.
Keith

 

[ToTo Man]

A quick teaser...

This is how my room measures with all treatments removed and the listening position 39% from the back wall. I can confirm that it sounds as bad as it looks! The room is definitely 'singing along with the music'! On the plus side, I've gained around 2dB efficiency from my speakers, which doesn't sound like a lot on paper but my 1wpc typical average listening level is sounding almost uncomfortably loud now!...

 

[ToTo Man]

A few years ago I stated that I didn't think GIK TriTraps did much below 50Hz but I didn't have conclusive proof. The before and after measurements I originally did in 2013 when I was learning how to use REW turned out to be erroneous, and because these corner traps have never been out of my room since they were first installed, I did not have the opportunity to repeat the measurements... until now!

This is how the room measures with the corner treatments still in place (8 GIK TriTraps plus 8 DIY wedges I cut to fill in the gaps above and below the TriTraps):

This is how the room measures after removing these corner treatments:

It is evident from the above measurements that the TriTraps work all the way down to my 41Hz axial mode but they are clearly most effective at absorbing frequencies above 50Hz.

In terms of frequency response, the measurements show that the frequency response in the mids and highs is smoother WITHOUT the treatments. I documented similar behaviour when I experimented with side wall and rear wall absorption in my previous thread, i.e. the more absorption I add to the walls the smoother the decay times become but the more choppy the frequency response becomes above 150Hz, so there's a trade-off between the two it seems. This could, however, be because I have left my ceiling untreated. By treating the walls and not treating the ceiling I am effectively skewing my speakers' power response towards its vertical off-axis response which is less smooth than its horizontal off-axis response. It will, however, be difficult for me to test this theory because experimenting with large, heavy absorption or diffusion panels when you have a 3.25m high ceiling is not easy!

 

[bottleneck]

Quick look - do you listen to your speakers where they are? - they appear to be inches from the back wall.
You also have a huge bass peak at 50/60hz.

If this is normal for you, I'd move them further into the room and re-measure.

 

[Richard Lines]

@ToTo Man - reading with great interest. You certainly seem to be covering most bases with your proposed plan. I will be following this with interest as I already have 9 GIK panels installed and am close to ordering another 8........

I found shooting from the long wall across the width of the room worked best for me.

I know what you mean by accessing high ceilings but if I were you I'd find a way of accessing this if possible.

Regards

Richard

 

[AndyU]

Maybe it’s down to the lens you used to take the photo, but it doesn’t look like you have the speakers set up to form the corners of an equilateral triangle with the listening position. If that is so, why? And are the speakers at the right height/angle to put their acoustic axis at ear level?

 

[ToTo Man]

Quick look - do you listen to your speakers where they are? - they appear to be inches from the back wall.
You also have a huge bass peak at 50/60hz.

If this is normal for you, I'd move them further into the room and re-measure.

Yes, the geometry of my room means the dip at 75Hz becomes a huge sink hole if I pull the speakers out from the wall. Room treatment helps a little with this but not enough. However, if it's a toss up between hearing 75Hz and getting a better soundstage then I'll probably prioritise the latter and see if I can find a way to shoe-horn my two XXLS400 subwoofers into a position in my room that will fill in the 75Hz chasm! The bass peaks aren't as much of a problem for me (as long as the ringing isn't too bad post-treatment) as I intend to continue to use parametric EQ to flatten these out.

Before I begin re-introducing treatments I'm going to try the rule of thirds to see what that offers me in terms of soundstaging. This will put my speakers 1.36m away from the front wall and my listening position 1.36m away from the back wall. I'm not sure how far this will put the speakers from my ears but I'm guessing it won't be much more than 1.36m and at such a close distance I'm not sure how well the rather wide vertically-spaced drivers will integrate. The joys of using a big, multi-way speaker in a small room!

 

[ToTo Man]

Maybe it’s down to the lens you used to take the photo, but it doesn’t look like you have the speakers set up to form the corners of an equilateral triangle with the listening position. If that is so, why? And are the speakers at the right height/angle to put their acoustic axis at ear level?

Good question! The speakers are currently 198cm apart (tweeter to tweeter) and the listening position is 235cm from the tweeters. I chose this as a starting point for the listening position as it's 39% from the back wall which is how far away I was from the back wall in my previous orientation. You are, however, correct that it's further outside of the equilateral triangle than my previous arrangement. I have not yet started my measurements to find the sweet spot listening position for bass as it's blowing a gale outside and my window is howling like a Husky (the sooner I get the damn thing replaced with triple-glazed acoustic glass the better! ), but I will make sure to include the equilateral apex in my mic positions. The speakers are tilted backward for maximum output at the MF/HF crossover point (5kHz) at the listening seat.

 

[ToTo Man]

@AndyU, making a 1.98m equilateral triangle with my speakers is going to put the listening seat 208cm from the back wall which, given that my room is 413cm long, means I'm effectively sitting in my axial length mode null and am presumably going to hear very little bass at 41Hz (I'll run a measurement later to confirm but you can see the effect in the REW Simulator below). Do you suggest I enlarge or reduce the size of the equilateral triangle so that I'm able to move my listening seat a few inches in front or behind of the room's mid point? I'm thinking reducing would be better because enlarging it would put the speakers even closer to the side walls (the tweeters are currently 95cm from the side walls).

 

[tuga]

@AndyU, making a 1.98m equilateral triangle with my speakers is going to put the listening seat 208cm from the back wall which, given that my room is 413cm long, means I'm effectively sitting in my axial length mode null and am presumably going to hear very little bass at 41Hz (I'll run a measurement later to confirm but you can see the effect in the REW Simulator below). Do you suggest I enlarge or reduce the size of the equilateral triangle so that I'm able to move my listening seat a few inches in front or behind of the room's mid point? I'm thinking reducing would be better because enlarging it would put the speakers even closer to the side walls (the tweeters are currently 95cm from the side walls).

(Easier if you have a laptop) you can use pink noise + RTA and a tape measure to try different the listener / speaker position (move only one speaker, don't worry about toe-in) taking screenshots and noting down the position of mic and speakers.
I have found this an effective way to optimise positioning but you will need someone to help you.

 

[tuga]

@ToTo Man have you seen this (alternative) 38% calculator?

http://noaudiophile.com/speakercalc/

 

[sls4321]

This is a continuation of Upgrading my acoustical room treatments but deserves a new thread given that I've decided to start from scratch by rotating the room 90 degrees. My speakers are now on the short (3.81m) wall firing down the 4.13m length of the room. I also persuaded my dad to remove all the acoustic treatments from the room (apart from the Earthwool that he stuffed into the cupboard), so it's a great opportunity to measure different treatments in different locations in the room and show their effects. This is therefore going to be a very long and dense thread with LOTS of graphs!…

However, I'm only going through this process ONCE, so I want to make sure my plan of action is logical! This is what I propose doing:

Remove all treatments from room, take measurements in 20cm increments from back wall to find listening position with smoothest bass response when the speakers are hard against front wall, I will then use this as the preferred mic position to measure the effect of adding the treatments. (This is hopefully NOT going to be the final position of the speakers because I'd like to get them away from the front wall and regain some soundstage depth, but I've got to start with them somewhere. I don't know if this is wise but optimising speaker position is a process that could take days in itself and also might depend on the treatments in the room). The measurements I plan to take are as follows:

1) Untreated walls and corners.

2) Early reflection side wall absorption (nearside speaker only).

3) Early reflection side wall absorption (nearside and offside speakers).

4) Early reflection side wall absorption, early reflection back wall absorption.

5) Early reflection side wall absorption, early reflection back wall absorption, early reflection front wall absorption.

6) Early reflection side wall absorption, early reflection back wall absorption, early reflection front wall absorption, range-limited bass traps on back wall.

7) Early reflection side wall absorption, early reflection back wall absorption, early reflection front wall absorption, range-limited bass traps on back wall, late reflection side wall Q7D diffusion.

8) Early reflection side wall absorption, early reflection back wall Q7D diffusion, early reflection front wall absorption, range-limited bass traps on back wall, late reflection side wall Q7D diffusion.

9) Early reflection side wall absorption, early reflection back wall Q7D diffusion, early reflection front wall polystyrene diffusion, range-limited bass traps on back wall, late reflection side wall Q7D diffusion.

10) Early reflection side wall absorption, early reflection back wall Q7D diffusion, early reflection front wall polystyrene diffusion, range-limited bass traps on back wall, late reflection side wall Q7D diffusion, 1x corner absorber in each corner.

11) Early reflection side wall absorption, early reflection back wall Q7D diffusion, early reflection front wall polystyrene diffusion, range-limited bass traps on back wall, late reflection side wall Q7D diffusion, 2x corner absorbers in each corner.

12) Early reflection side wall absorption, early reflection back wall Q7D diffusion, early reflection front wall polystyrene diffusion, range-limited bass traps on back wall, late reflection side wall Q7D diffusion, 3x corner absorbers in each corner.

I've intentionally left adding the corner traps until the end so that I can see how effective the rest of the treatment is. Also I'm not including the 40Hz tuned membrane Scopus bass traps in any of these measurements as these are a solid box trap that can bugger up other areas of the frequency response depending on where they're positioned.

So... am I good to go?!

Unbelievable planning!

I did a new room last year and measured it from bare walls and floor to finished product. The initial measurements were done with my ears because the sound was so terrible that I couldn't wait to get the furnishings and absorption in.

When I was finished, and a step or two before, I tested with REW and there was the inevitable bass node around 70-75Hz or so. I previously also had a BK400 subwoofer, but I wasn't going to put one in (too ugly) and I happily live with the node. As I use Wilson speakers, when I was finished my dealer came round and did the Wilson set-up thing and the result is splendid. He achieved in an hour imaging and soundstage that I'd not been able to achieve with a couple of months of random adjustments.

I would look at almost any of Amir’s loudspeaker measurements, he measures the various floor/ceiling/wall reflections and suggests if and how specific treatment will improve the SQ.
Keith

In practical terms, that speaker stuff ASR does seems to be nonsense. Once you've been through what I did and @ToTo Man is doing, some theoretical in-room response is about as useful as ice cream in the Arctic, because in-room response changes so dramatically when domestic furnishings and any room treatments are installed. Room simulation is not much use either with a bay window and split level ceiling.

 

[AndyU]

@ToTo Man re “the 38% rule” - imo there is no such thing, it’s just a bit of pseudo-science hogwash invented without evidence by a guy to hype the sale of his room treatments. If you check out the pro experts on gearspace like pentagon and northward audio you will see they give the concept short shrift indeed. I think the only “rule” of room treatments is that you can’t win, unless you rebuild the room with angled surfaces creating a reflection free zone for the listener, put your speakers in the wall, and use a huge about of absorption to soak up the room modes that that will excite. Given that you can’t do that, I think you just have to mess about. There are those (like Pentagon on GS) that say the best thing you can do in a normal room is put your speakers against the wall and avoid them and you being in nulls. As soon as you pull them out you will get cancellations at the frequency for which the quarter wavelength corresponds to the distance from the wall. This is unfixable with DSP. There are others who will advocate pulling speakers out to get less colouration, better stereo and less chance of setting off room modes, and live with the anomalies at the bottom end. My own personal strategy, seeing as how I am lucky enough to have a large listening room, is to listen close - around 1.8m, slightly inside the apex of an equilateral triangle with the speakers way out from walls. But really the only “rule” is to mess about till you can live with it. Play pink noise in mono. Do you get a rock solid, well defined, narrow central image? Play some speech, does it sound natural, or does it sound like someone trapped in a wardrobe with broken teeth? Does a solo acoustic guitar sound like it is right there in front of you? But the last thing I could compromise on would be the equilateral thing - the image just goes to pot.

 

[ampedup]

the only “rule” is to mess about till you can live with it.

So true: Measurements give you data. Messing about gives you a solution. The solution (one of many) can be scientifically non-intuitive. I love my first reflections off the side walls.

 

[ToTo Man]

Step 1 of my plan is complete, - frequency response measurements of my speakers in their current location (45cm from front wall, 95cm from side wall) at different listening positions with no treatments in the room.

I started at the back of the room (60cm from back wall) and then moved forward in 15cm increments until I was 345cm away from the rear wall (which is another way of saying 70cm from the front wall!). The apex of the equilateral triangle (give or take a couple of cm's) is labelled '210RW'.

To save on time and bandwidth I've included three mic locations in each graph. If this makes things too difficult to interpret then let me know and I'll post individual graphs of the distances that are of particular interest.

Observations:
- Output at 41Hz is almost completely cancelled out in the '210RW' measurement because it's the null zone of my axial length mode.
- The measurements in the front half of the room are better than the measurements in the back half.
- The '180RW' measurement (which is 30cm behind the midway point) looks a decent compromise between bass smoothness and extension.
- The '240RW', '255RW' and '270RW' measurements are all excellent but their feasibility depends on how close to the speakers I feel comfortable sitting.
- The other thing that stands out to me in all of these measurements is just how smooth the midrange is. If you look at 200Hz-2kHz in particular, some of the measurements are as good as +/-2dB (albeit with 1/12 oct smoothing) which is probably better than some control rooms!

Before adding any treatments I'm going to experiment with the rule of thirds and also the rules that @tuga linked to as I'm curious as to just how much of my room's sound I can eliminate prior to treating it.

 

[tuga]

Did you move the speakers+listener triangle or just the mic?
If the former, it might be interesting to pick a couple of RW distances and try moving the mic 10cm forward and 10cm backward. A lot of hassle, I know.

Also, it would be interesting to compare the (single speaker) decay below 300Hz of RW285 and RW240.

 

[ToTo Man]

Did you move the speakers+listener triangle or just the mic?
If the former, it might be interesting to pick a couple of RW distances and try moving the mic 10cm forward and 10cm backward. A lot of hassle, I know.

Also, it would be interesting to compare the (single speaker) decay below 300Hz of RW285 and RW240.

I only moved the mic for these measurements, not the speakers, and the measurements were made using Periodic Pink Noise with RTA so there is no time domain information. I can, however, repeat some measurements using impulse sweeps for the distances that are of interest.

I'll try listening at 180cm, 225cm, 240cm, 255cm, 270cm and 285cm to see what they sound like, however I think 285cm is going to push me too close to the speakers! Can I ask why 285cm is of particular interest to you?