Room Acoustics - Successes and Failures

Posted by: Halloween Man on 03 May 2017

I've spend the best part of ten months on and off measuring and fine tuning my listening room acoustic response. I thought it might be useful to share my experiences with forum members.

It all started when my new SCM40A speakers arrived. They sounded wonderful but I had the impression they perhaps had a little too much bass for my modest room (4.3m x 4m). I thought, not to worry, I can always re-position them or treat the room so I decided to accept and keep the speakers. Unfortunately, as the speakers were breaking in over the first weeks, the bass gradually got bigger until eventually the bass was overwhelming the music and destroying the balance and therefore spoiling my enjoyment of the music.

I contacted ATC and asked what the problem could be. They stated that the frequency response of the speakers was pretty much flat so the problem likely rests with the room's acoustic response. After some research, I decided to measure the room using Room EQ Wizard (free software) and UMIK-1 measurement microphone. To my horror, I had a large +12dB low frequency peak between 40-50Hz at the listening position. This verified what I had been hearing.

First step was to find the optimal speaker position and listening position, at the time time considering room practicalities and limitations. Speakers sounded best at least 2ft from side wall to side of speaker cabinet, and at least 2ft from front of speaker driver to rear wall. Listening chair about 18 inches gap from rear wall.

The next step was how to treat the low frequency peak. After some consideration, I decided to purchase four Gik Acoustics Soffit broadband bass traps, two in each corner behind the speakers. Result? Barely touched the low frequency peak. They also looked a bit unsightly in the room. They were returned.

Back to the drawing board and after more research I discovered RPG Modex Plates (manufactured by German company Renz). The only scientifically tested product on the market shown to be effective sub 50Hz that is also thin in depth and just about visually acceptable to put on your wall. RPG Europe were a nightmare to deal with, like pulling teeth, communications were very poor. We got there in the end and I placed an order for six type 2 modex plates, costing £3K. Many weeks later the plates arrived on a pallet. They must have weighed at least 40kg each and it was a mammoth task and a two-man job to get them fixed to the walls. This was exciting, they are now in position. Result? Barely touched the low frequency peak. They also looked unsightlier in the room than I thought they would. After much to and fro between myself, RPG, and Renz, I was informed that Renz had changed the specification on the plates (substituted melamine foam for glass mineral wool) and this had rendered them ineffective, faulty, and useless. At this point I'd had enough and decided to return the panels for refund. I had to repair my walls caused by the damage of drilling 24 holes required for the plate brackets. RPG took two weeks to issue a refund and decided only a partial refund was appropriate and didn't refund postage costs.

I was now out of options. Not a single aesthetically acceptable product exists on the market to treat low end peaks. I decided to accept defeat and downsize speakers from SCM40A to SCM19A.

After weeks of waiting the new SCM19A speakers arrive. Result? Even with no treatment room acoustic response is vastly improved. Peak is now an acceptable +6dB.

Final step, fine tuning. Now that the basics are right I decide to add a Gik tri-trap broadband bass traps in each corner behind the speakers, Gik 242 panels to the side walls and behind the listening position to counter mid/high frequency reflections. Result? Much improved sound quality and imaging, as good as possible room response across the spectrum. As good if not better than any kit/hardware upgrade. Music is balanced from top to bottom. Treatment is unobtrusive and visually acceptable. Very happy.

I consider room acoustics to be the most neglected aspect of hifi. Matching speakers to room and treating room brings massive gains in sound quality, you could argue more important than any hardware/kit upgrade and should be a crucial aspect, together with making sure your amp is capable of driving your speakers correctly and with enough headroom (in my case not an issue as ATC actives have amp built in).

Hope this helps!

Posted on: 03 May 2017 by Innocent Bystander

When I looked into mine, seeking assistance remotely from GIK, the amount of treatment recommended was impractical for a living room and would in no way meet WAF, though after I factored in the expensive but impressively described RPG Modex plates in one particular location I could see a compromise that might work. Then I started asking questions as to how much evening out of significant peaks and troughs I might expect given the REW data, and GIK were readily forthcoming (surprisingly little at the bass end) - RPG needed a lot of pushing before I got something vague out of them (surprinsingly little came the reluctant answer).

I shelved the idea, intending to come back to it at a later date and then probably only deal with early reflections, meanwhile identifying with REW more radical repositioning that made the bass peaks and troughs only a few dB, which is where I position things when wanting to listen critically a chair right in front of the sofa, and sitting surprisingly close to the speakers ((about 7ft) despite the big room (about 24 ft wide and long, though an odd shape not a simple square). Most of the time I find I don't bother!

But you are right, room acoustics can be very significant to the sound you hear, unless you are very lucky indeed.

Posted on: 03 May 2017 by Halloween Man

Yes, Gik customer service is excellent, highly recommend them.

I did find that moving the sofa to the middle of the room, closer to the speakers, helped eliminate the bass peak at the listening position, only problem in my modestly sized room was that you ended up way too close to speakers to be workable. In a large room this would be an ideal solution.

Posted on: 03 May 2017 by Huge

My story is documented on the forum here.

And my experiments tailoring a sub to the room are documented on the forum here.

Posted on: 04 May 2017 by stuart

Thanks for the post. Some useful information. I had similar problems with bass response recently with a home demo of motive sx1. They have been returned which is a shame as I really enjoyed most of what they did and in a larger room I'm sure they would have worked.

I haven't got round to using Rew yet as I have been embralled in lots of other jobs recently but really should go ahead with this. I now have some pmc twenty 23 on home demo which seem to have more bass control. Need to do more listening however.

Do you have any pics of your set up as this would be good to see?

Posted on: 04 May 2017 by heihei

Great write-up and glad you're in a good place!

Any advice on how to use REW for hifi? I've just bought the USB microphone but haven't started to play yet. Most of the docs on line talk about home cinema set-up rather than 2-channel.

Posted on: 04 May 2017 by Innocent Bystander

heihei posted:
Great write-up and glad you're in a good place!

Any advice on how to use REW for hifi? I've just bought the USB microphone but haven't started to play yet. Most of the docs on line talk about home cinema set-up rather than 2-channel.

Three recent threads have info that may be of use to save repeating here: Acoustic Treatment, Room Treatment and Bass-how much is too much. Do they help?

Posted on: 04 May 2017 by Huge

There are four plots in REW that are particularly important

1 Frequency Response
With 1/3 octave smoothing, this should be reasonably flat (not raised below 100Hz as for home cinema). With smaller smoothing windows, the effects of comb filters (caused by reflections) can be seen; it's not necessary to eliminate all these comb filter effects unless you're talking about a very sophisticated room treatment scheme, but see the comment about impulse response below, for reducing these.

2 Group Delay
This not show any discontinuities around at the crossover of the speaker drive units or the crossover to a sub (re-position the sub to change the main to sub time difference to compensate for the sub's greater group delay).

3 Excess Phase
Again this should be reasonably flat, and particularly not showing any discontinuities.

4 Impulse response
This should show a single sharp rise and a smooth decay. Any addition rising pulses indicate reflections. The time delay of the reflected sound in relation to the direct sound can be used to calculate the path difference and this can be used as a guide to help find the location of the reflecting surfaces.