Room response

[gfinlayson]

I've been thinking of adding some room treatments to dampen some of my listening room's resonances. The room is pretty bare with fairly reflective walls, ceiling and floor. Out of curiosity, I spent a couple of hours with a test CD downloaded from real traps, and a SPL meter to get a handle on the level of the problem.

The results were, err, interesting:



Measurements are 'C' weighted relative to a 75 dB reference level set from the pink noise track on the CD.

Looks like I've got some work to do.

[satterfi]

You'll find lots of information here:

http://www.johnlsayers.com/phpBB2/viewtopic.php?t=6037

and at the bottom of this link:

http://www.bobgolds.com/AbsorptionCoefficients.htm

I'm building several of the Jon Risch designed stuff. It's pretty easy to do.

[gfinlayson]

Thanks satterfi.

I already had the absorption coefficients link bookmarked - I came across it last week whilst hunting for acoustic treatments. An invaluable source of information.

I got myself some 50 mm Rockwool RW3 slab from a local supplier at a bargain price - 16 slabs for £40 ($ 75 US).

I've hung six fabric covered rock wool slabs from the ceiling - the 'echoey' sound in the room has now gone. I'm also going to install two fair sized wall panels (a la John Risch)- one behind the speakers, and the other behind the listening position, and a bass trap in the most problematic corner.

The fabric I'm using probably isn't acoustically ideal, but I've had to use fabric colours which coordinate with the room colour scheme to maintain a satisfactory WAF One of the wall panels will also have a stencilled picture on it.

Once I'm done, I'll repeat the sound measurements, and post the results, with some subjective listening impressions.

[gfinlayson]

The two wall panels are up now - the one behind the speakers is about 1m x 1m in a 4" deep frame with 2" of RW3 up front. The one behind the listening position is the same make-up but 2m x 1m. I've also made a 1 m x 0.5m bass trap from 2 layers of 2" RW3 in a 4" deep frame which sits in the corner behind and to the left of the LH speaker.

Measurement-wise, here's the before and after. Post treatmet is a little smoother with less overall spread in SPL. Not quite as good as I expected, BUT, and here's the biggy - Sonically it's night and day compared to the untreated room. Walking into the room is a strange sensation - it's like walking into a recording studio. Almost no noticeable echo or reverberation at all - it'll take a little getting used to.

Musically, the sound is so much more natural. Details which were previously unheard are now very apparent. Soundstage and imaging have vastly improved. That sensation of the soundstage starting right at the end of your nose is definitely there. Basslines are much crisper and clearer - no more of that 'waffly' bass that Al (Pinkmouse) described. The amount of energy being soaked up by the insulation is obvious. To reach the 75 dB reference level on the pink noise track took about an extra 1/8 of a turn on the volume knob and normal listening level is now at about eleven o'clock on the volume knob where previously it was just a bit over nine o'clock.

Overall cost for insulation, timber and fabric has probably been about £150. The improvement in sound is probably worth at least twenty times that amount.

The 20 dB of gain from 30 to 60 Hz is still a bit of a nuisance - I need to think about that a bit more, and try to find a way of taming it. On most recordings it's OK. On Holly Cole's 'Temptation' album the acoustic bass on 'Train Song' is positively brutal - it sounds like there's an 18" high power sub at work rather than a pair of 6 1/2" Scan-Speak 8531s.

[satterfi]

You might need to put a bass trap in each corner.

I've got two built but haven't done any testing.

Here's one before the covering material is on.


tube trap

[ShinOBIWAN]

You've got some comb filtering from 150-300hz

You could try different placements or rearrange the furniture. Do you have a coffee table between you and the speakers by any chance?

I absolutely love room treatments both physical and digital. The difference it makes in my room is pretty astounding and I wouldn't be without them now.

Start off with the cheap option first and thats EQ. Bass traps are huge and expensive for the effective ones. Trying to tackle 50hz-100hz requires the better traps and below 50hz is virtually impractical with physical treatments because the wavelengths involved are pretty large. Take a look at the Auralex Venus bass trap - its humongus and it only works effectively to 100hz. No, the best way to go about tackling the bass issues is placement first, then EQ unless you can accomodate imposing bass traps and don't have any problems below 80hz.

For the mid and high frequencies, your better off using carefully placed(usually were the first reflections occur) absorbers and maybe diffusors if you've got echo and flutter issues with your room. I also use the DEQX loudspeaker DSP which offers phase and amplitude correction but its a bit overpriced IMO, perhaps if it was half the price it is now then this would be more mainstream since it works very well, I actually have an in-room response of +/-0.5dB and +/-30 degrees phase from 200hz up.

There's a chap I know called Matt who works for RPG(acoustic treatments manufacturer). He really knows his stuff, I can put you onto him if you like?

[gfinlayson]

Satterfi,

Nice bass traps, but Mrs. Finlayson wouldn't go for those, I'm afraid

ShinOBIWAN,

The only thing between me and the speakers is the floor. There is a rug on the floor, but I suspect it could do with being thicker.

I'm going to play around with speaker positioning over the next few days to see if I can improve the comb filtering situation.

The DEQX is definitely out - 2 grand plus another pair of power amps, ouch.

[ShinOBIWAN]

Originally posted by gfinlayson
Satterfi,

Nice bass traps, but Mrs. Finlayson wouldn't go for those, I'm afraid

ShinOBIWAN,

The only thing between me and the speakers is the floor. There is a rug on the floor, but I suspect it could do with being thicker.

I'm going to play around with speaker positioning over the next few days to see if I can improve the comb filtering situation.

The DEQX is definitely out - 2 grand plus another pair of power amps, ouch.

Sounds like a plan, let us know how you get on.

I completely agree about the DEQX, no way is it worth £2k. If they dropped the price to £1k then that's a more realistic value. Its a real shame because the unit is very good at what it does but the price tag limits the appeal.

[darkfenriz]

A cure for this comb effect (caused by parallel walls probably) may be a diffusor. If this is caused by floor and ceiling than thick carpet seems only reasonable cure. What are the dimensions of room?

regards

[gfinlayson]

Darkfenriz,

Sorry it's taken me some time to reply - I've attached 2 .jpgs which show the room layout and the layout of the existing absorbers. To have just given the dimensions of the room would have been meaningless as it's such an odd shape. The ceiling is 2.35 m high. The French doors have heavy curtains which I normally close when llistening to music.