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Influence of curtains on room acoustics

thorvat

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A/B listening is important to me because what looks initially unacceptable in the presented steady state curve may not sound as "bad" as it looks. From time to time, what looks worse in a graph, may actually sound good to one's ears -- or, at least perceptually "neutral", or not even necessarily "worse" compared to the original (no EQ or "treatment") response.

A/B listening is indeed important, but trust me - when you shelve down from 300Hz upwards by 4dB you don't need A/B listening to notice the difference as that one can easilly be heard by your wife/girlfriend even from the kitchen. :)
 

TulseLuper

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I think the point is that the curtains do not have enough mass to alter the low frequencies. So the low frequency response should be about the same with open or closed drapes.

EDIT: Can we agree on that?

Just a thought - the curtains do not disappear when opened. They’re just pulled together in a corner, presumably. So it’s alternating between a narrow, thick area of material in a corner, and a wide, thin material. Any chance this explains a difference in bass response?
 

Kvalsvoll

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So in our living room we have a humble 2.2 setup with generally subpar acoustics. We installed relatively thick curtains (the thickest the shop can make them, usually for performance spaces where you want zero light entering) and this thread is basically some rough measurements showing before and after. The curtains cover around 80% of the surface area of an entire wall, this is why the decay times were dramatically influenced, even at frequencies lower than expected. Speakers are flat on-axis with smooth directivity.

The room changes all the time so don't pay too much attention the region below 300Hz. I tried to control the microphone location as best as I can.

Here is a picture showing how thick the fabric in the curtains is

View attachment 225798

and measurements

View attachment 225800

View attachment 225802

View attachment 225803

Listening impressions were very very positive, however, there are some caveats. Bass ringing is much more obvious now since the mid-range and treble experience that less. I'm probably going to install some bass traps in one of the corners next. Everything with regards to anything in the vocal region from 800Hz and up is incredibly clearer and more intelligible. Overall it's defintely a very positive improvement.
Those curtains work, as can be seen by looking at the decay in the waterfall graph shown here. And they work, because they are THICK. No, they do not work for bass/low freq, as the OP also points out.

The only problem with this sort of treatement is that it will give too much high frequency absorption, if applied on a high percentage of the rooms surface area.

This is a good alternative for getting some improvement, and it is quite easy to experiment with, as the curtain can be easily folded or unfolded.
 
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abdo123

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If the default natural voicing of your speakers is good enough for your ears, then there's probably no further need for that... leaving it be should be fine.

But, with adequate data -- i.e. multiple measurements, filtering and/or viewing methods -- one should still be able to safely apply more detailed full-range room and speaker EQ with little cause for concern.

To be honest i used to think so as well, but after i heard two subwoofers with with LR 24dB/oct @100Hz crossover make a pseudo center when the speakers were turned off I'm now of the opinion that people overestimate the frequency threshold at which ears perceive sound exactly the same as it is plotted in a steady state in-room measurement. For me flat anechoic on-axis above 100Hz sound the most natural to me.

However, i didn't have a peak of 5dB at 180Hz before, neither in my living room or main setup. So I might change my mind who knows.
 

thorvat

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The only problem with this sort of treatement is that it will give too much high frequency absorption, if applied on a high percentage of the rooms surface area.

This opens the question what ammount of decay is optimal for best sound, and I'm affraid that one heavilly depends not only on personal taste but also on usage type (for example: professional mixing vs casual listening).

Btw, if curtains were applied on a "high percentage of the room surface area" they would be called "wall drapery" not curtains. :D
 

Frgirard

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That is exactly my point.

Consequently, the only explanation for the existence of the diffferences in LF response is unprecise measurement, which makes impossible drawing any conclusions from it.
+1

With move the mic, he need to measure with and without. Without = the curtain should be retired from the window. This is not the case.

With rockwool Alpharock in 50 mm, I can measure an "absorption" around 100 Hz (Coincidence without direct link).
The reality is the rockwool is too denser to absorbe at 100 Hz and deviate (difracte) the wave outside the measurements field.

It's mandatory to understand what is do
 

Kvalsvoll

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This opens the question what ammount of decay is optimal for best sound, and I'm affraid that one heavilly depends not only on personal taste but also on usage type (for example: professional mixing vs casual listening).
No need to worry about that, if curtains are the acoustic treatment, because you will never get close.

Here, as always. there are different opinions and taste, like we see in other things, such as.. tuning of frequency response for speakers, just to pick one. Many speakers have a very artistic impression of tonality and frequency response, presumably due to some idea that flat does not sound correct. But, no, flat actually does sound correct, one of few aspects of sound reproduction that has actually been verified in controlled experiments. The same situation appears to be valid for rooms - they need decay slopes within some certain range, and this decay needs to be reasonably consistent across the whole frequency range, for best sound. For all rooms intended for sound reproduction, studio or living room.

Why curtains can not fix everything? Because when you have covered all surfaces, there is still not enough absorption below say 500hz, and too much high frequency absorption. So the room is now sounding too dead, and still has too little low freq absorption.

Curtains can still be a useable tool, especially very heavy and thick ones, like those shown here. In combination with other treatment, for treatement on surfaces such as windows, in rooms where other treatment is not an option.
 

thorvat

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Just a thought - the curtains do not disappear when opened. They’re just pulled together in a corner, presumably. So it’s alternating between a narrow, thick area of material in a corner, and a wide, thin material. Any chance this explains a difference in bass response?

Well sure, there is a chance - it is hard to discard that as a possibility. But in order to prove or discard that hypothesis one should measure very carefully. If you want to measure with sweeps I would suggest using microphone stand to fix the mic position and removing your body from the acoustic picture by not being in the room while sweep is running. That is usually done by putting the REW in state where it waits for timing reference signal so you have time to go out of the room and start the sweep from there. Then you pull the curtain by say 10cm and measure again, and so on, until it is pulled together. Then you move the mic to several other positions around LP and repeat the procedure.

If all is done right you should have a series of measurements showing the trend what happens when curtain is pullled to the end.
 

thorvat

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No need to worry about that, if curtains are the acoustic treatment, because you will never get close.

Here, as always. there are different opinions and taste, like we see in other things, such as.. tuning of frequency response for speakers, just to pick one. Many speakers have a very artistic impression of tonality and frequency response, presumably due to some idea that flat does not sound correct. But, no, flat actually does sound correct, one of few aspects of sound reproduction that has actually been verified in controlled experiments. The same situation appears to be valid for rooms - they need decay slopes within some certain range, and this decay needs to be reasonably consistent across the whole frequency range, for best sound. For all rooms intended for sound reproduction, studio or living room.

Why curtains can not fix everything? Because when you have covered all surfaces, there is still not enough absorption below say 500hz, and too much high frequency absorption. So the room is now sounding too dead, and still has too little low freq absorption.

Curtains can still be a useable tool, especially very heavy and thick ones, like those shown here. In combination with other treatment, for treatement on surfaces such as windows, in rooms where other treatment is not an option.

It is not what I meant, so let me rephrase it:

IEC 60268-13, which relates to rooms for conducting listening tests on loudspeakers, specifies a maximum reverb time of around 600ms above 200Hz, rising slightly in the bass. ITU-R BS.1116–3, which relates to rooms for subjectively testing degradations introduced by perceptual encoders, specifies a maximum of just over 300ms above 200Hz, with a target of 250ms.

Taken together, these arguably set up a tolerable range for "critical" domestic listening of between 0.25 and 0.6ms (above 200Hz). Usually, lower limit is aimed by dedicated home cinema enthusiasts and recording & mixing studios while upper limit is widely accepted for music listening. Depending on materials used for building (which varies by countries) what you will get in typical rooms is 0.4-0.5ms, which is perfectly ok for music listening, yet your personal preference may of course vary.

So, a message to audio enthusiast is: if your Topt curve (under RT60 tab in REW) is relatively flat after 200Hz and sitting between 400 and 500ms - you're good. This is pretty much what we all have in our apartments/houses and it will do just fine, so no need to fight with your wife to put some ugly things on your walls and ceilings and expecting acoustic miracles from them as that will not happen.
 
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abdo123

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Well sure, there is a chance - it is hard to discard that as a possibility. But in order to prove or discard that hypothesis one should measure very carefully. If you want to measure with sweeps I would suggest using microphone stand to fix the mic position and removing your body from the acoustic picture by not being in the room while sweep is running. That is usually done by putting the REW in state where it waits for timing reference signal so you have time to go out of the room and start the sweep from there. Then you pull the curtain by say 10cm and measure again, and so on, until it is pulled together. Then you move the mic to several other positions around LP and repeat the procedure.

If all is done right you should have a series of measurements showing the trend what happens when curtain is pullled to the end.

The whole irony of this whole interaction is that this is actually how i usually do measurements, not just that but i also use lasers to make sure the microphone position is relatively similar on different days. You're so entitled that you think i should do measurements your way and present data your way otherwise anything i share is automatically invalid, not only that, you said they're invalid without even asking me any details about how i do or did things.

1661241238847.png
 

thorvat

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The whole irony of this whole interaction is that this is actually how i usually do measurements, not just that but i also use lasers to make sure the microphone position is relatively similar on different days. You're so entitled that you think i should do measurements your way and present data your way otherwise anything i share is automatically invalid, not only that, you said they're invalid without even asking me any details about how i do or did things.

View attachment 226121

Well, as I already said, if you want to stick to your "opinion" that you did everything right, although your results doesn't support that, it is perfectly fine by me.
 

Frgirard

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The whole irony of this whole interaction is that this is actually how i usually do measurements, not just that but i also use lasers to make sure the microphone position is relatively similar on different days. You're so entitled that you think i should do measurements your way and present data your way otherwise anything i share is automatically invalid, not only that, you said they're invalid without even asking me any details about how i do or did things.

View attachment 226121
Absorbing at 100 Hz with your curtain is impossible in the state of the art of current knowledge in the sector.
-Either you have found a miracle solution. Present it in an article for validation by the peers.
-Either you wrong.
Return to the objective side of the force
 
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abdo123

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Absorbing at 100 Hz with your curtain is impossible in the state of the art of current knowledge in the sector.
-Either you have found a miracle solution. Present it in an article for validation by the peers.
-Either you wrong.
Return to the objective side of the force
Please refer to the statement where I said curtains absorb to 100Hz?
 

Frgirard

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Please refer to the statement where I said curtains absorb to 100Hz?
Here the FR curve
 
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abdo123

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Here the FR curve
And what can I do about that? falsify the measurements?

The way i do measurements is as accurate as possible a hobbyist can provide, if that's no value for you then just ignore it.
 

jaakkopetteri

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Play around with porous absorber calculators and you'll find that a 15mm curtain can well absorb at 100Hz, especially with an air gap included. The absorption is obviously minimal, but it's not like the difference is large in the measurements, either.

Obviously these calculators are just approximations, but IMO way better than "curtains will not do anything below 400Hz". I find it a bit disrespectful to say that abdo is uninformed or not being objective just because his measurements don't agree with some of these rather reductionist, mechanistic guesses
 

Frgirard

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Play around with porous absorber calculators and you'll find that a 15mm curtain can well absorb at 100Hz, especially with an air gap included. The absorption is obviously minimal, but it's not like the difference is large in the measurements, either.

Obviously these calculators are just approximations, but IMO way better than "curtains will not do anything below 400Hz". I find it a bit disrespectful to say that abdo is uninformed or not being objective just because his measurements don't agree with some of these rather reductionist, mechanistic guesses
you are wrong
have an alpha of 0,5 at 100 Hz needs an absorber with a porosity 10000 rayles and a thickness of 200 mm.
have an alpha of 0,6 at 100 Hz needs an absorber with a porosity 10000 rayles and a thickness of 300 mm.
 

jaakkopetteri

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you are wrong
have an alpha of 0,5 at 100 Hz needs an absorber with a porosity 10000 rayles and a thickness of 200 mm.
have an alpha of 0,6 at 100 Hz needs an absorber with a porosity 10000 rayles and a thickness of 300 mm.
Where'd you pull those alphas?
 
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