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Influence of curtains on room acoustics

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abdo123

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Here is a rough simulation with the frequency of contention highlighted.

1661249324551.png
 

Kvalsvoll

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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 not a useful way to evaluate acoustic performance of a room. Why and how this works has been repeatedly explained by many. Though you may get an indication of the liveliness of a room by looking at RT60 numbers evaluated from the slope of the decay, and if you see numbers in 400-500ms range, that is way off (not enough absorbtion) for typical smaller rooms.
 

ernestcarl

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While this would differ between homes, my own acoustically untreated open plan living room hovers around mid 300 ms. The bass, though fine, is quite lumpy and never even at any one spot anywhere. If I were more "enthusiastic" about putting in adequate acoustic room treatment, it would likely plunge that number closer down to ~200 ms or even possibly lower.
 

thorvat

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This is not a useful way to evaluate acoustic performance of a room. Why and how this works has been repeatedly explained by many. Though you may get an indication of the liveliness of a room by looking at RT60 numbers evaluated from the slope of the decay, and if you see numbers in 400-500ms range, that is way off (not enough absorbtion) for typical smaller rooms.

Topt in REW has been optimised for small rooms. I suggest you familiarize yourself with the way how it works before commenting.


RT60 Calculation​

The RT60 values are estimated by calculating the slope of the Schroeder curve, which is a plot of the energy (squared values) of the impulse response that is backwards integrated (summed starting from the end and moving backwards). The vertical axis of the plot is in dB. The different RT60 measures (e.g. T20, T30, REW's Topt) are derived by calculating the slope of a best fit line to the Schroeder curve over different ranges (detailed below). In a diffuse field the curve with the dB vertical scale is quite linear until it reaches the noise floor.

The start point for the classical T20 and T30 measures of RT60 is where the Schroeder curve has dropped 5 dB below its peak. That works well in the large spaces for which RT60 is most applicable, particularly if the source used for the measurement is omnidirectional. In domestically sized rooms using normal, directional loudspeakers as sources the initial drop of the Schroeder curve is quite sharp (the Early Decay Time is quite short) meaning the -5 dB point lies within the early decay region rather than the diffuse field region. That in turn means the T20 and T30 figures underestimate the RT60 time. Where the EDT is much shorter than the T30 RT60 figure REW's Topt RT60 calculation uses a start point based on the intersection of the EDT and T30 regression lines, to determine a point that lies within the diffuse field region. It then tests each possible end point in 1 dB steps and picks the one that gives a regression line with the best linear fit. That produces a more reliable RT60 figure.

The results are presented within octave or 1/3 octave bands, providing a view of how the rate at which sound decays changes with frequency. The lowest centre frequency for the RT60 plot is 50 Hz at 1/3 octave and 63 Hz at 1 octave, per ISO3382. For domestic listening rooms and recording studios with volumes of less than 50 cubic metres (1,800 cubic feet) the recommended RT60 value is 0.3 s. For larger rooms, up to 200 cubic metres (7,000 cubic feet) the recommendation is 0.4 to 0.6 s. In both cases the value should be fairly uniform across the frequency range, though it will typically tend to increase at lower frequencies.
 

thorvat

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The way i do measurements is as accurate as possible a hobbyist can provide,

I can easilly think of more than a few hobbyists on this forum that do measurements far better than you. But if you want to improve your measurement technique you should defintiely change your attitude, because as long as you think you're the best of the best you cannot possible learn and evolve.
 

thorvat

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While this would differ between homes, my own acoustically untreated open plan living room hovers around mid 300 ms. The bass, though fine, is quite lumpy and never even at any one spot anywhere. If I were more "enthusiastic" about putting in adequate acoustic room treatment, it would likely plunge that number closer down to ~200 ms or even possibly lower.

If you choose to try to treat your room you will quickly learn that reducing decay time below 200Hz is far more complcaited and expensive than reducing it above 200Hz.
 
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abdo123

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I can easilly think of more than a few hobbyists on this forum that do measurements far better than you. But if you want to improve your measurement technique you should defintiely change your attitude, because as long as you think you're the best of the best you cannot possible learn and evolve.
Well I’m yet to hear a constructive comment from you, so i think it’s hardly my attitude.
 

thorvat

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Well I’m yet to hear a constructive comment from you, so i think it’s hardly my attitude.

So, acording to you it's constructive to claim that your measurements are "the best they can be" and that your magical curtains are siginificantly affecting the response down to 150Hz, but when you are told that physics teaches us that curtains can't do that which implies that your measurement technique needs improvement (and I suggested which!) - that is according to you a non-constructive comment?

Sure M8.. nothing wrong with your attitude. :facepalm:
 
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abdo123

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So, acording to you it's constructive to claim that your measurements are "the best they can be" and that your magical curtains are siginificantly affecting the response down to 150Hz, but when you are told that physics teaches us that curtains can't do that which implies that your measurement technique needs improvement (and I suggested which!) - that is according to you a non-constructive comment?

Sure M8.. nothing wrong with your attitude. :facepalm:

I have never made any of those claims though, you're arriving to these conclusions yourself and you're putting me in a position where i have to defend a position that i never held to begin with and if i don't join this petty discussion I'm some sort of fraud.

Can't you see that these measurements I shared are not favoring the curtains in general and their effect seems to be negatively effecting the response linearity? Why would I defend measurements that doesn't paint my personal decisions in a good light unless I'm very confident that they're true?

I would love for these measurements to be wrong, I don't want them to be true, you might think i'm on some sort of ego trip or something but this is not the case. I'm just sharing what happened.
 

holbob

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I don't think I've seen as much hostile straw man behaviour in any thread on ASR before. Bizarre. The message gets lost if you present it in such an obnoxious manner.
 

ernestcarl

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If you choose to try to treat your room you will quickly learn that reducing decay time below 200Hz is far more complcaited and expensive than reducing it above 200Hz.

Not sure what the point of your platitude about room treatment below 200 Hz is all about…

It seems as if you think that the topt curve is not something anyone who’s a regular user of REW knows about — moreover, that just because it’s more reliable for the kind of smaller rooms we often measure that it’s somehow exempt from the same weakness of drawing generalized conclusions from rest of the other RTx curves. I’m pretty sure that’s no big revelation to Kvalsvoll as well.

Being helpful is one thing, but unnecessarily condescending to other members… esp. to the OP who has actually bothered with the effort of sharing these measurements to begin with?

While I won’t stop anyone from discussing whatever it is they want, it’s worth pointing out — something that should already be obvious from previous posts which you seem to continue to ignore — you’ve overstayed your welcome.
 

thorvat

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Not sure what the point of your platitude about room treatment below 200 Hz is all about…

If you have any practical experience with acoustic treatment below 200Hz you would know what I'm talking about. As you don't, well.. we'll discuss it if and when you gonna have it. LF treatment requiers special materials which is rarely suitable for rooms unless they are dedicated for listening/Home theater. I have seen many enthusisats trying that ending up only with acoustically overdumped MF and HF while LF remains boomy.


It seems as if you think that the topt curve is not something anyone who’s a regular user of REW knows about — moreover, that just because it’s more reliable for the kind of smaller rooms we often measure that it’s somehow exempt from the same weakness of drawing generalized conclusions from rest of the other RTx curves. I’m pretty sure that’s no big revelation to Kvalsvoll as well.

I strongly suggest you dig into the way how actually Topt curve is calculated and why exactly it is more reliable for small rooms then RT60 curve before stating any conclusions about it's usefullness. As for the Kvalsvoll, I believe he doesn't need an advocate, he's an adult person who will speak for himself, so let him do that.

Being helpful is one thing, but unnecessarily condescending to other members… esp. to the OP who has actually bothered with the effort of sharing these measurements to begin with?

You may as well say that he's stubbornly sticking to his ways of measurement while I'm bothering with the effort to explain how to improve his measurements so he and the others reading this thread would learn to do it better.
I dont' advise using the word "condescending" whenever you encounter a person with more knowledge and experience than yourself as that can only prevent you from learning new stuff.

While I won’t stop anyone from discussing whatever it is they want, it’s worth pointing out — something that should already be obvious from previous posts which you seem to continue to ignore — you’ve overstayed your welcome.

Let me remind you again that OP is not the owner of this thread, and neither are you. So it is not his privilege, nor it is yours, to judge who is entitled to post into this thread as long as forum rules are obeyed as for this we have admins. I absolutely see no need for OP to be offended neither I see the need for you to "defend" him as nobody "attacked" him at the first place. I simply disagreed with some of his opinions providing arguments in a civilised manner while sticking to the forum rules. I also suggested some ways of improving his measurement technique which may lead him to discover why were the differences in LF which shouldn't be there if he chooses to accept those suggestions. As a response he got offended and claimed that his measurements are "the best they can possibly be", which frankly looks quite childish to me - but ok, his life and his choice.

So, regarding your last comment - what exactly gives you the right to play admin sending message that someone's "welcome has been overstayed"? If you have an internal urge to act like that either become an admin or open yourself your own forum. In the mean time I suggest you keep yourself inline with the rest of us "commoners". ;)
 
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Absolute

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Since this dude is destroying a fun experiment, I decided it would be constructive to buy some curtains myself just to see what happens. They're not proper acoustic curtains, but some rather heavy, thick and soft velour curtains I found on sale. Those will cover the whole wall of windows on the closest sidewall to one speaker, so I expect the effect to be a best-case scenario.

Stay tuned for more drama later.

*Edited because moderator has a point.
 
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AdamG

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We kindly ask that some of the participants dial back their personal insults and slights. This forum is open to everyone who contributes in a constructive and respectful manner. If for some reason you feel bothered by a certain member? You have 2 options. Report them if they violate the rules. Or simply place them on ignore and carry on. It’s just a forum conversation and not worth getting emotional about. Life presents plenty of real problems to get stressed about. If this Forum is causing you stress maybe take a break and go do something else and decompress.

Nothing here is worth high blood pressure and stress. We are here to share and learn. Focus on that. Thank you for your understanding and cooperation.
 

Kvalsvoll

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Running out of girlfriend patience after installing the curtains so don't have the time to do a whole lot of measurements today, but here's some preliminary ones;

View attachment 226449View attachment 226450View attachment 226451View attachment 226452
View attachment 226456View attachment 226457View attachment 226458
And this shows a similar result compared to OP. What it also shows, in the pictures, is the visual impact on the room, where you now really have to question whether living in a dark cave is worth the improvement..

There is a point where the cost (visual/effort/price) becomes too large, compared to the improvements in sound. DSP is great, it can fix the tonality so you get a more neutral response with no need for more acoustic fixes, it is invisible. It will not be the same, it will not reduce decay times, but it may be a better compromise.
 

Kvalsvoll

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Topt in REW has been optimised for small rooms. I suggest you familiarize yourself with the way how it works before commenting.


RT60 Calculation​

The RT60 values are estimated by calculating the slope of the Schroeder curve, which is a plot of the energy (squared values) of the impulse response that is backwards integrated (summed starting from the end and moving backwards). The vertical axis of the plot is in dB. The different RT60 measures (e.g. T20, T30, REW's Topt) are derived by calculating the slope of a best fit line to the Schroeder curve over different ranges (detailed below). In a diffuse field the curve with the dB vertical scale is quite linear until it reaches the noise floor.

The start point for the classical T20 and T30 measures of RT60 is where the Schroeder curve has dropped 5 dB below its peak. That works well in the large spaces for which RT60 is most applicable, particularly if the source used for the measurement is omnidirectional. In domestically sized rooms using normal, directional loudspeakers as sources the initial drop of the Schroeder curve is quite sharp (the Early Decay Time is quite short) meaning the -5 dB point lies within the early decay region rather than the diffuse field region. That in turn means the T20 and T30 figures underestimate the RT60 time. Where the EDT is much shorter than the T30 RT60 figure REW's Topt RT60 calculation uses a start point based on the intersection of the EDT and T30 regression lines, to determine a point that lies within the diffuse field region. It then tests each possible end point in 1 dB steps and picks the one that gives a regression line with the best linear fit. That produces a more reliable RT60 figure.

The results are presented within octave or 1/3 octave bands, providing a view of how the rate at which sound decays changes with frequency. The lowest centre frequency for the RT60 plot is 50 Hz at 1/3 octave and 63 Hz at 1 octave, per ISO3382. For domestic listening rooms and recording studios with volumes of less than 50 cubic metres (1,800 cubic feet) the recommended RT60 value is 0.3 s. For larger rooms, up to 200 cubic metres (7,000 cubic feet) the recommendation is 0.4 to 0.6 s. In both cases the value should be fairly uniform across the frequency range, though it will typically tend to increase at lower frequencies.
You still need to verify that the number for RT60 that has been calculated matches the slope of the decay profile, because this slope is not necessarily linear, it certainly will have a initial drop, and then some rooms actually does not have a linear slope by nature.

Since RT60 is not valid for small rooms, it is a questionable metric for acoustic properties of a room. I believe there is some value, the number does represent the slope of the decay.

I have stated that even small rooms, when treated properly, tends toward a sound field that is more like diffuse. "More like" does not mean it is actually a true diffuse sound field. The direction of the sound shows a strong directional direct sound, and reflected sound comes from all over, with reduced overall velocity. But in a true diffuse field, there will be very low velocity. Also, all resonances are gone. All this can be measured and verified. A small non-treated space will show a sound field dominated by resonances and specular reflections, with higher velocities in all directions.

The easy way to assess performance of a room is to look at the decay graph, with some experience it is possible to learn how a room performs by looking at the graph.
 

Absolute

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And this shows a similar result compared to OP. What it also shows, in the pictures, is the visual impact on the room, where you now really have to question whether living in a dark cave is worth the improvement..

There is a point where the cost (visual/effort/price) becomes too large, compared to the improvements in sound. DSP is great, it can fix the tonality so you get a more neutral response with no need for more acoustic fixes, it is invisible. It will not be the same, it will not reduce decay times, but it may be a better compromise.
Indeed, for all things meant to improve sound quality there is a price. I don't yet know if I can say there's a subjective gain as I didn't have time to listen much, but we all know that curtains are no substitute for broadband absorbers. Maybe especially true for the side reflections?

In the quest for "science" I will compare the curtains with a 4 inch RPG Absorbor placed in the first reflection just to get a picture of how much/little the curtains do for specular reflections.
For room ambiance/flutter the curtains are probably effective enough.
Extremely effective of blocking out light, though.

Those are gorgeous speakers! Are they full DIY or did you enhance a complete pair of M2's?
Thank you! These are full DIY speakers made of Baltic Birch and oiled for a nice look. I wish I could take credit, but these were not made by me. They were made as replicas of M2's with original components, similar dampening and internal bracing, but have since upgraded tweeter and experimented a little with stuffing.

The M2's are a nice way of dipping the toes into the DIY pool because they are relatively simple, cheap and well-supported information-wise on the web, and they sound okay as playstation speakers. :)
 
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