The 2 cents I've learnt on the diffusion vs absorption debate

MartinC

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Yes they do. As we increase the volume. We load the room with more sound energy. So the decay times start increasing with volume. So it is imperative, that room measurements are made at the usual listening volume. As at any other volume, either less or more than this volume, the decay times across the frequency range will change.
Sorry, I hate to contradict, but this is incorrect. Decay times do not increase as volume increases.

The only significant volume related effects are those associated with how human hearing response varies (Fletcher-Munson curves).
 

newlash09

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Sorry, I hate to contradict, but this is incorrect. Decay times do not increase as volume increases.

The only significant volume related effects are those associated with how human hearing response varies (Fletcher-Munson curves).
I'd be happy to be corrected 😁. As already mentioned at the beginning of my post, iam only sharing what I've read on the subject. And am open to still learning more 😃👍
 
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MartinC

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In case it helps...

Decay times (e.g. RT60) are the time it takes for a certain change in relative level e.g. 80 dB to 20 dB. When I say that decay times don't vary with volume I'm saying the time to go from 80 dB to 20 dB is the same* as the time to go from 100 dB to 40 dB. The upshot being that you don't need different acoustic treatment depending on where you set the volume control.

*This won't be 100% exactly true but it's close enough to not worry about.
 
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newlash09

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In case it helps...

Decay times (e.g. RT60) are the time it takes for a certain change in relative level e.g. 80 dB to 20 dB. When I say that decay times don't vary with volume I'm saying the time to go from 80 dB to 20 dB is the same* as the time to go from 100 dB to 40 dB. The upshot being that you don't need different acoustic treatment depending on where you set the volume control.

*This won't be 100% exactly true but it's close enough to not worry about.
I've read similar that a rooms rate of decay is constant. And find your above example fully agreeable. But my doubt is that the whole point of trying to reduce decay times is to get the remaining decay after 60ms to below the ambient sound level in the room, so that it is inaudible. By the same metric, considering a room with a fixed rate of decay, and a fixed ambient noise level. Will a sound of larger volume, not linger in the room longer before it drops below the audible limit. And to reduce this, should not the rate of decay of the room be increased with more room treatment.
 

MartinC

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I've read similar that a rooms rate of decay is constant. And find your above example fully agreeable. But my doubt is that the whole point of trying to reduce decay times is to get the remaining decay after 60ms to below the ambient sound level in the room, so that it is inaudible. By the same metric, considering a room with a fixed rate of decay, and a fixed ambient noise level. Will a sound of larger volume, not linger in the room longer before it drops below the audible limit. And to reduce this, should not the rate of decay of the room be increased with more room treatment.
What we're most sensitive to is relative levels rather than absolutes. Something just above ambient noise levels will be much less audible when music is being played loud than quietly :) .

p.s. I'm sure you didn't mean 60 ms above. Possibly 600 ms?
 
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newlash09

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What we're most sensitive to is relative levels rather than absolutes. Something just above ambient noise levels will be much less audible when music is being played loud than quietly :) .

p.s. I'm sure you didn't mean 60 ms above. Possibly 600 ms?
Haha... I actually mistyped again 😅😅. I was referring to bringing down the volume of the original sound by 60db, so that the resultant volume is below the ambient noise level, and is thus inaudible.
 
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StingRay

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In case it helps...

Decay times (e.g. RT60) are the time it takes for a certain change in relative level e.g. 80 dB to 20 dB. When I say that decay times don't vary with volume I'm saying the time to go from 80 dB to 20 dB is the same* as the time to go from 100 dB to 40 dB. The upshot being that you don't need different acoustic treatment depending on where you set the volume control.

*This won't be 100% exactly true but it's close enough to not worry about.
But that is the problem, if you play at higher volumes then those sounds will be reflecting around at an audible level for longer. At higher volume the sound is different, thats why when mixing it is best to do it at a mid volume level, say 82db. Certainly bass in some rooms can be much more of an issue at higher volume.
 

MartinC

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But that is the problem, if you play at higher volumes then those sounds will be reflecting around at an audible level for longer. At higher volume the sound is different, thats why when mixing it is best to do it at a mid volume level, say 82db. Certainly bass in some rooms can be much more of an issue at higher volume.
Sorry but I really don't think that's correct. Other things affect optimal mixing levels, not this.
 
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MartinC

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rdale

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So you think bass is the same, no matter what the volume is?

Why do sound engineers say things like 85db is the sweet spot?
The discussion was about whether an RT60 value for a room changes with measuring level. That is the time for a mid range/treble sound to decay by 60 db. But RT60 doesn’t apply to the bass frequencies.
 

StingRay

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The discussion was about whether an RT60 value for a room changes with measuring level. That is the time for a mid range/treble sound to decay by 60 db. But RT60 doesn’t apply to the bass frequencies.
Well it is not just bass that is affected by volume. Mixing is done in treated studios usually but volume seems to affect it.
 

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That's interesting.
So what's the scientific basis for the carefully crafted panels if the effects aren't measurable?
Is there one?
It does all seem a bit adhoc with lots of different rules of thumb floating around.
The definitive guide was written by Prof Trevor Cox, who developed the first of these modern diffusers

Acoustic Absorbers and Diffusers: Theory, Design and Application https://amzn.eu/d/hduQTBS

The performance of diffusers and absorbers is measured using random incident sound environments in a reverberation suite.
 

sjs

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As it happens I plan to be progressing my own room treatment significantly over the coming week. As described above, a key balance is managing to use acoustic treatment without making the room too dead (to short RT60 times). The last time I experimented with absorbers at lots of early reflection points in combination with multiple corner absorbers to address low frequencies I did end up with an unpleasantly dead sounding result. My new plan therefore has two stages:

  1. Use hardboard covering to significantly reduce the higher frequency absorption of my bass traps.
  2. Make early reflection point panels that are no larger than then need to be (e.g. last time I had one 1200 mm x 600 mm gobo where a 600 mm square panel would have sufficed).
The idea is for step one to provide enough decay time 'headroom' for step two to hopefully bring benefits without making the overall result too dead.

I'll be posting results of how I get on in time...
Personally I would use diffusers at early reflection points rather than absorbers.

Ideally you preserve the energy but scatter the directionality and enhance the sense of being surrounded, rather than killing it.
 

MartinC

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Personally I would use diffusers at early reflection points rather than absorbers.

Ideally you preserve the energy but scatter the directionality and enhance the sense of being surrounded, rather than killing it.
Isn't the limited frequency range over which a diffuser will work an issue, as well as the proximity problem.
 

sjs

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Isn't the limited frequency range over which a diffuser will work an issue, as well as the proximity problem.
In most rooms the proximity isn't a real issue, and absorbers also have limited frequency ranges.

My suggestion is based on my experience of what I think is a good outcome for domestic listening.
 

MartinC

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In most rooms the proximity isn't a real issue, and absorbers also have limited frequency ranges.

My suggestion is based on my experience of what I think is a good outcome for domestic listening.

For context the three closest early reflection points in my room are about 1.3 m from my head.

What sort of diffusers do you have in mind and what frequency range are they effective over? Yes absorbers have limited frequency range too but from examples I've seen I believe diffusers are usually effective over a narrower bandwidth?
 
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MartinC

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The wall behind my head is about 50cm away. And I can't change that!
Yes and I think that's true for most. I have consciously got my listening position forward of the back wall for acoustic as well as practical reasons (storage behind) but two of the early reflections points I just referred to are the rear wall ones.
 

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