Perceptual Effects of Room Reflections

[dasdoing]

There is such a thing as too much absorption

I think it depends. the problem is absorbing the deepest octaves. If you don't manage to hit those and absorb too much of the rest it obviously will sound terrible. the worst one can do is absorbing treble only (foam). they say stereo in an aechoic room would sound bad....personaly I don't believe in that. those are the same people that are against any absorbtion.

 

[krabapple]

'Absorbing' the 'deepest octaves' requires quite aggressive wall/corner treatment. Nowadays we have DSP to treat peaks in those octaves, at least.

For the rest of the spectrum, broadband absorption is achievable with 4 inches of fiberglass (e.g Owens Corning) or similar (e.g. Knauf 'glass mineral') product

 

[MRC01]

'Absorbing' the 'deepest octaves' requires quite aggressive wall/corner treatment. Nowadays we have DSP to treat peaks in those octaves, at least.
...

DSP is helpful but not a proper substitute for room treatment.
If you have a null, boosting that freq with DSP often won't fix it. It just pumps more energy into a black hole, and is counterproductive, increasing power, speaker displacement & distortion.
If you have a peak, reducing that freq with DSP helps, but the room is still resonating at that freq so it won't be as clean as room treatment, which kills the peak at its source by weakening the mode and resonance.

 

[srrxr71]

DSP is helpful but not a proper substitute for room treatment.
If you have a null, boosting that freq with DSP often won't fix it. It just pumps more energy into a black hole, and is counterproductive, increasing power, speaker displacement & distortion.
If you have a peak, reducing that freq with DSP helps, but the room is still resonating at that freq so it won't be as clean as room treatment, which kills the peak at its source by weakening the mode and resonance.

Absolutely. It is not magic.

The other issue with it is that it might correct the direct signal at your listening position. However, what about the decay and waterfall?

I have 500ms at 59Hz that cannot be controlled with DSP. Maybe it can be mitigated with a high Q cut at that exact frequency. That is the subject of my recent experimentation. No idea if it’s right or wrong. Of course only REW can tell me. I cannot use GLM to see the results.

However more and bigger corner traps are coming to try to mitigate that. At some point I have to stop throwing traps at the problem but I am far from that point.

All this not treating the side reflection is debatable I suppose. Only if one has tried both. However corner bass traps are hard to debate about in almost any room.

 

[tuga]

aggressive wall/corner treatment (...) broadband absorption

What about a pair of tibetan mastiffs?

 

[Duke]

What about a pair of tibetan mastiffs?

Best post I've seen in a long time!!

 

[dasdoing]

'Absorbing' the 'deepest octaves' requires quite aggressive wall/corner treatment

the traps need to be deeeep. corners wont help unless you straddle 2m.
I have a 70cm deep trap at the backwall and it took out the boominess pretty good.

 

[Killingbeans]

Or PSI Audio AVAA if you're made of money.

 

[dasdoing]

Or PSI Audio AVAA if you're made of money.

and you need a bunch of them as I heard

 

[srrxr71]

Or PSI Audio AVAA if you're made of money.

Wow. That’s insane.

From the marketing materials maybe 4 per wall on each corner will work the best.

 

[srrxr71]

the traps need to be deeeep. corners wont help unless you straddle 2m.
I have a 70cm deep trap at the backwall and it took out the boominess pretty good.

So basically all I can do is place 17” soffit bass traps on the corners. There is some fiberglass something inside. It can absorb down to 40Hz but with not much efficiency below 50Hz. I think my 59.3 Hz mode will be helped but nothing below that. I kind of don’t care about below 40Hz can’t afford to do much about it.

Maybe I can throw some traps around it. But finally in life one must accept some compromises and live with them. Or win the lottery I suppose.

 

[dasdoing]

So basically all I can do is place 17” soffit bass traps on the corners. There is some fiberglass something inside. It can absorb down to 40Hz but with not much efficiency below 50Hz. I think my 59.3 Hz mode will be helped but nothing below that. I kind of don’t care about below 40Hz can’t afford to do much about it.

Maybe I can throw some traps around it. But finally in life one must accept some compromises and live with them. Or win the lottery I suppose.

absorbtion is best at 1/4 quater wave-lenght, that's were velocity is the biggest. look it up.
for the first longitunal mode that means you would lose 1/4 of the room at the backwall.
it still helps to have 1/8 or so......but near the wall there will be no effect (minimal velocity).
if that trap is all filled, you now also have a very deep trap in the other axis; so the second mode also gets help.
if the room is big enough it might be enough to straddle 8 corners (vertical and ceiling);.
in a small room like mine a big deep trap at the backwall is the only solution.

 

[srrxr71]

absorbtion is best at 1/4 quater wave-lenght, that's were velocity is the biggest. look it up.
for the first longitunal mode that means you would lose 1/4 of the room at the backwall.
it still helps to have 1/8 or so......but near the wall there will be no effect (minimal velocity).
if that trap is all filled, you now also have a very deep trap in the other axis; so the second mode also gets help.
if the room is big enough it might be enough to straddle 8 corners (vertical and ceiling);.
in a small room like mine a big deep trap at the backwall is the only solution.

Wow that some incredible science. I calculated i would need to be about 4.7 feet out on these traps. 59.5 Hz and 60.3Hz.

The front wall I have 5 corners treated with 17” on the vertical and 8.5” on the bottom horizontal edge. The only next step is to get the top edge covered.

I guess I would have to mount the 42” height 8.5” traps vertically and get 9 of them to cover the 18ft back wall. That only gets me 21” height on a 45 45 90 triangle. Not even enough to hit the 1/8th wave. This would be at $2k+ cost for one edge.

Those PSI AVAA traps might be looking like the better option?

Or just let it go?

Or maybe cut the energy at the exact frequency with a high Q dip filter?

What’s the best option?



EDIT: also that 40.3 feet of the 28Hz mode correlates with the long oblique floor to ceiling along the long wall.

The 20Hz mode correlates with 56.6ft. I have no idea where that is coming from.

I would wager having that 20Hz mode is not a bad problem to have. Even the 28hz mode. Just the 60Hz mode needs some dealing with. I’m pretty excited to get those soffit bass traps in and cut that 60Hz down. It’s going to be some tight (audible) bass in here soon. While the inaudible feeling of bass power will be all over the place. Again not a bad problem to have for a listener.

Unless i’m wrong on that. Not much I can do about it. Installing PSI AVAA traps would cost more than my entire system cost of I hit the top and bottom corners of the walls I have. Some partial walls separating the living and kitchen spaces.

At that point it could order a pair of W371 for less and see if it helps (probably won’t) but it would be a better audible upgrade I would guess.

 

[MRC01]

the traps need to be deeeep. corners wont help unless you straddle 2m.
I have a 70cm deep trap at the backwall and it took out the boominess pretty good.

With tube traps (as opposed to bass traps), you don't need them 2m diameter, but you want them to run full height from floor to ceiling. My home made ones are 2' diameter (60 cm) and measured response shows a difference down to about 40 Hz.
My bass traps (MegaTraps from RealTraps) are equilateral triangles with 34" front face, 24" sides (thus 17" deep from the corner), and affect frequencies down to about 70 Hz.
PS: measured response difference of the 2' diameter tube traps in both of the room's rear corners, running floor to ceiling:

 

[Sal1950]

There is such a thing as too much absorption,

I'm not so sure about that? I do know that anechoic chambers can be somewhat unsettling to be in, but how much different can that be than listening to music on a good pair of fully sealing closed back or IE headphones? Just wondering out loud?

 

[srrxr71]

I wish I could run from floor to ceiling. I have a 13” gap at the ceiling. My acoustic consultant basically said that covering that last 13” is more cosmetic than useful. From my waterfall he’s probably right.

I had that idea that coverage must be complete. But instead he convinced me to hit another 2 corners with bass traps instead of chasing that last 13”.

 

[srrxr71]

I'm not so sure about that? I do know that anechoic chambers can be somewhat unsettling to be in, but how much different can that be than listening to music on a good pair of fully sealing closed back or IE headphones? Just wondering out loud?

It’s more the sound of your voice or breathing in the space. Same effect as having headphones I suppose especially IEMs. However the difference is that you know your ears are occluded and often painfully aware of that fact.

With this it’s just initially unsettling to hear it when nothing is physically occluding your ears.

It’s not a big deal it’s a weird feeling that goes away within the minute.

 

[MRC01]

The advantage of going floor to ceiling is not so much the total area, which you may be able to compensate for elsewhere, but treating the corners at both floor & ceiling. A given bass treatment (trap, tube, etc.) is usually most effective in the room corners where the lowest bass waves usually concentrate.

 

[srrxr71]

The advantage of going floor to ceiling is not so much the total area, which you may be able to compensate for elsewhere, but treating the corners at both floor & ceiling. A given bass treatment (trap, tube, etc.) is usually most effective in the room corners where the lowest bass waves usually concentrate.

That’s what I thought too. So I could in fact place a 13” riser in between the 2 tri traps and cover that corner and compare. I was surprised to hear what the acoustic consultant told me.

I think it’s time to take matters into my own hands and apply 13” of riser in between the 2 and run some sweeps.

 

[MRC01]

There is such a thing as too much absorption, but it's a matter of taste and the opposite problem is much more common.

I'm not so sure about that? I do know that anechoic chambers can be somewhat unsettling to be in, but how much different can that be than listening to music on a good pair of fully sealing closed back or IE headphones? Just wondering out loud?

This topic was debated a year or two ago, involving some of our technical professionals & experts. The debate got so heated that, sadly, it chased away one of our valuable professional contributing members. I don't want to rehash that, but acknowledge that the notion of "too much absorption" has proponents among technical professionals, most of whom say that the logical extreme, an anechoic chamber, doesn't make a good listening room.

My speculation is that while headphones have less/faster/better CSD than most listening rooms, they still do have some reflections and non-zero decay, which would make them more "live" than an anechoic chamber. And good headphones sound too dry for some people (not for me!).