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Sound absorption - what's audible?

JTC said:
SWMBO has agreed to a corner trap, and I'm hoping I can sneak in at least a couple of wall-panels (something like the GIK 244 or Monster Bass Trap). I may measure the room again later today, as I've reorganised a few things and my previous room measurements (see below) are likely now invalid.

atc100-spl.jpg


As you can see, it's pretty flat +/- a few DB except for the big mountain at 32Hz; this is the axial room mode for length (i.e. ~18') which I hope to tame by a corner trap, moving the listening position slightly away from the back wall and maybe a couple of panels. I have to stress that the sound is pretty damned good already - this isn't a question of fixing something that's broken, more a matter of improving it further, if possible.
Click to expand...

Your in-room low-end response looks quite good TBH! :). In agreement with others, conventional-sized broadband panels or corner triangles won't touch a thing under 50Hz. I'd be more interested in viewing the waterfall and RT60 graphs to see what the decay times are like across the frequency range shown in the above graph, as IME long decay times are more unpleasantly audible and problematic than a peaky/dippy frequency response.
 
That 32 hz peak will be an issue in the RT60 etc, the nulls are too steep to be audible.
You will have to move the listening position out of the node or use Eq to flatten it.
Bass trapping will help , but not a +15d broadish band peak like that.
 
JTC said:
No, I was listening more like 10-12' away. I've moved the sofa forward - here's a pic of the 'before' (showing the mic position from which that sweep measurement was taken):

I've moved that sofa forward by around 1'. Will measure hopefully later this coming weekend.
Click to expand...

Interesting - those windows probably let a fair bit of bass out (can you hear it from outside?) - and opening them temporarily will definitely reduce the effect of that room dimension, so that's something else for you to play with. Deffo recommend checking out the port tuning frequency and the room simulators/tone generators, then bribe the mrs to go out for a day and drag stuff about to see where the root of the issue is. Messing about with a bit of digital eq - JRiver has a decent one - might also give you some useful insight.
 
OK, thanks, lots of ideas for me to try. My wife keeps telling me that table is too big anyway :) Mudlark, is the mic meant to be vertical in use? I assumed it had to be pointed at whatever it was measuring.
 
OK, but best done not on a school in-service day when number one son is watching Indy Jones through the ATCs ;)
 
Not in everyday musical replay although BR and DVDs often bring it into play. I'm intrigued to try some of these suggestions, but I'm technically working so ought to get back to work....
 
I have a similarly low peak upstairs, I use the Grimms low shelf filter just to reduce the subs output, it seems to be working, I might try actually limiting the subs output ie instead of 20Hz I can adjust upwards, at 35Hz the boom ime has disappeard.
Perhaps a combination of the two.
If your bass is only an infrequent problem perhaps you can just ignore it, to passively is going to be difficult, Helmholtz resonator perhaps?
Keith
 
Honestly, it's not a problem for most listening, although I am keen to see if a bit of tweaking and room treatment might result in a better sound.
 
If I may chip in here guys, as I feel your shooting off in the wrong direction.... I've went through the same experience, measured room, saw big bass peak at 48Hz thought I had to fix/trap it.
Having now completed my room it wasn't the case....
A few bass peaks are not an issue, you can't really hear them, what they could be heard as is say the bass guitar is a bit louder in the mix.... well we didn't hear the recording so we wouldn't know anyway....

What I think most people misunderstand ( I did) is the decay time is the key issue to fix. Measured as RT60.
So the addition of things like the GIK Monster Bass traps (you can make them easily ;) ) will shorten the decay time significantly, also only in the bass region (a good thing)

Having a short decay time in the low frequencies is a revelation.... don't over do it in the higher frequencies as you will deaden the room, better to scatter and diffuse the upper frequencies not absorb them.

So i still have a bass hump by measurement, but now does no more than give the impression of a bigger more powerful bass.... which if your speakers were lacking a little that would be a benefit and not an issue.....and position can be used to tune that.
What I have now though is bass notes that start and stop, have ends and nice decay... don't blurr/drone into the next. Music times so much better, ambience is bigger detail is much better, clarity and layers to the music is huge step forward..... Spending 20K on electronics wouldn't get close..... panel cost about a grand.... cheaper by DIY.

So first step treat the low end with broadband bass trap panels, get the RT60 short. Don't worry if it's not flat........ as for 32Hz, what plays down that low anyway ? :)
 
OK, on the subject of decay, here's the plot from before. I think it's clear that the decay in that 32Hz region is a bit slow. Seems okay throughout the rest of the measured range though.

waterfall-atc100.jpg


So, a tri-trap and a couple of monster bass traps to tweak the decay? Would that do the trick? Or at least make a useful improvement?
 
(this is assuming that 200-300ms is the ideal RT60 decay time for listening room, um, listening?)
 
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