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Sound absorption - what's audible?

JTC

JTC

PFM Villager...
I'm curious (but learning fast): I've got my head around the concept of sabins, but I'm not clear on roughly how many sabins of absorption at a given frequency would be required for (let's say) a 3dB reduction at that frequency.

I'd imagine that it varies depending on the frequency, but beyond that I'm a bit lost.... anyone?
 
I'd say if you need all this bollox , you've got the wrong system. Not that the unit you speak of is bollox, but if you have to mod the room then, well, wrong system, IMO
 
Sabins are all a bit theoretical as you can't buy say 10 sabins of absorption. Your question doesn't really help you.

What are you trying to do?

A 10 square metre room with no roof has 10 sabins of absorption......
 
Even out the bass response in the room somewhat. In theory. In practise, I've not done anything yet apart from think about it and put out some feelers...
 
Sorry, flippant answer. I'm trying to get my head around just what level of absorption (in sabins) is likely to be audible. Using that rough figure, I can then establish the type of treatments likely to provide at least that level of absorption to see if it's worth going after. The room is pretty even, but I just have a hunch that I can eek a bit more out of the room:speaker thing with some careful treatments...
 
The usual way to go about the problem is to measure the room and then fit solutions. As I said your question doesn't help you at all.
 
Martin D is in fact talking bollox. All rooms have problems some large and some small. The only room with no effect on your stereo is one with no walls floors or ceilngs. An anechoic room.
 
It would probably be wise to first check on what you actually *can* do to the room before deciding what needs to happen.
If you discover you need big corner bass traps (likely) but they will cause domestic unrest then it's a slightly wasted exercise.
I guess what I'm trying to say is - this is a dedicated listening room or a shared living space? If a shared living space then you are going to have to compromise, and equalizing bass response does not usually = compromise. Equalising bass response with traps (sabins?) is usually a fairly big ask.
 
If you have a window in your room, you could try opening it, just to experience the effect.
1 square meter open window = 1 sabin.
 
As mudlark says , you got to know what to treat before you treat.. so hunches will not really get you the most effective medicine. You really need to diagnose whats wrong before embarking on specific solutions.

However , just using some bass trapping in corners and absorption/diffuser panels at 1st reflections points will NEVER do any harm and only lead to benefits
 
JTC said:
I'm curious (but learning fast): I've got my head around the concept of sabins, but I'm not clear on roughly how many sabins of absorption at a given frequency would be required for (let's say) a 3dB reduction at that frequency.

I'd imagine that it varies depending on the frequency, but beyond that I'm a bit lost.... anyone?
Click to expand...

The absorption of a sabin does not depend on frequency, but the underlying absorption coefficient of the material usually does - thus an attenuator only gives the specified X sabins at a specific frequency.

Also remember that absorption only affects reflected sound - so you would first have to know how many dB the reflected sound contributes (as compared to the direct sound that won't be affected).

This is a good, condensed reference.
 
mudlark said:
Martin D is in fact talking bollox. All rooms have problems some large and some small. The only room with no effect on your stereo is one with no walls floors or ceilngs. An anechoic room.
Click to expand...

This. Can't understand what Martin is thinking of here either.

You can minimise the room's negative contributions (we all have them to a degree) by careful set up and positioning of speakers and listening position. The room will make the biggest contribution to what you hear, even if it's not as much fun as box swapping.

After that, if any annoying aspects of the room acoustic persist they can be managed by some attention to physical means such as furnishings or treatments, room correction software or a bit of both. The less that can be left to correction the better, but it too can be very helpful and relatively inexpensive. Provided the kit is competent none of that needs changing to combat room effects, the road to Hell lies in that direction.
 
You could also use some nice stand mounted small speakers away from the edges of the room. The lack of low frequencies reduces the bass boom and the music sounds detailed. If you can cope with lower bass then enjoy it. I enjoy Ackroyd/Royd audio 7 litre speakers and part of their charm is a lack of poor low bass.
 
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.
 
PS. With more smoothing in the measurements, some of those anomalies above 150Hz would probably go away. I think they're artefacts resulting from things like the furniture and fixtures in the room.
 
Panels won't do a thing at 30Hz, unless they are diaphragmatic and tuned to that frequency.

A single corner trap is also not going to do a lot. Face it: for a trap to work the sound must hit it, and if only a minority of room surfaces is treated there is not a lot of sound going to hit it.

And is a bit of boom at 30Hz that bad, really? Above, say, 80Hz it is a different story, but down below???
 
I would have thought moving your head well away from the back wall would be the best move. Are you listening 18' away? Or have I misunderstood how you've laid out your room? Might be worth using the speaker/room response calculator at hunecke.de - here, to get an approxiamation to the effects of different speaker/listener locations if you haven't done so already. I had my 100s in a room about 24' long - but listened at about 8', with my head and the speakers well away from room boundaries. Another thing you can do is download a tone generator - eg here, use it to play tones at troublesome frequencies and move speakers and you about to see what the effects are. It is quite amazing to walk through peaks and troughs while a low note is playing - the difference can be incredible.

p.s. another possibility is that the room resonance you have is the same as the frequency the port is tuned to, so tje effect is being exaggerated. Ben Lillie at ATC once told me what the port frequency was, but I've forgotten - might be worth a phone call. You can damp the port if necessary - he'll tell you how.
 
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):

MeasuringRoom1.jpg


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