Can we talk diffusion, absorption, and reflection - AVS Forum | Home Theater Discussions And Reviews
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post #1 of 20 Old 08-22-2016, 11:54 PM - Thread Starter
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Can we talk diffusion, absorption, and reflection

Since we are entering the era of multichannel 3D formats with Top speakers and more surround speakers shouldn't we start changing our thinking of so much absorption? I know absorption is easy and fast but there is no way it's the optimal choice for multi channel rooms as all it would do is shrink the room sound, lower the dynamics, and create a somewhat dead room.

I've really been looking at all sorts of diffusors but would rather have a broadband option that is DIY, obviously. I've been researching this for the past couple months and from what I can gather most true pro designers would rather diffuse even the first reflection points in a lot of scenarios. I'm not going to name any names but I honestly believe there are way more benefits from doing so and so did the pros I conversed with.

We are humans and are surrounded by diffusion for starters so it's more natural for our brains. Absorption can kill some sounds that would be pleasant not to mention shrink the sound stage.

The downside to diffusors are the depth needed but we have to start somewhere. We started allocating 2-4" for panels over time so why not allocate 7-9" or more for diffusion especially if it will give the room a better sonic signature? We have chairs and carpet that absorb all kinds of waves so by adding even 20% wall coverage would seem like over doing things to me.

The pros and cons are almost split.

Absorption for broadband is really thick.
Absorption is way easier to put up but it's not needed and can hurt more than help in some cases.
Absorption is cheap

Diffusion is complicated to most
Diffusion has many different styles to choose from for WAF
Diffusion is expensive for some types but cheap for others
Diffusion will create a more spacial environment even in small rooms
Diffusion could be cheap if have the right tools.

That is just a very small list but I truly believe we will be preparing for its placement in the not so distant future.

Anyone interested in sharing designs of their own diffusion panels that were DIY and further the conversation about over dampening our rooms instead of keeping the spls up but dissipating the reflections before they reach us? There are a bunch of companies overseas that sell all sorts of diffusion panels rather cheap with great specs but not so much here in the US so I've been searching the DIY designs.

Im just starting to figure out how each design actually works and where they would need to be placed but like the idea so much more than absorption in my brief research that I would like to learn a lot more and incorporate as much as possible in my HT2.0 build.

I just feel that we've grown accustomed to the standard absorption at first reflection points and even in other areas where it might not necessarily be the optimal acoustic model.

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post #2 of 20 Old 08-23-2016, 02:21 AM
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Well according to @Nyal Mellor you should never use absorption at first reflection points but yet we see it rampant on AVS like cancer. And the other day I seen yet another buying a giant package from GIK acoustics that consisted of pallets of panels and it was all 100% absorption and they had ATMOS :/ Sigh...But hey to each his own...And shame on GIK for selling these 100% absorption packages, but I guess Merica is still driven by the 'sale.'

I mean in the Savoy theater Nyal did slat style diffusion at first reflection with absorption behind it, Can't remember the % of slat coverage though but it was obviously calculated.

Also in Home Theater last year Geeks Anthony Grimani said you should never have more than 20% of wall coverage that is pure absorption.

Let the discussion begin

Why do you fellow AVS-ite think it is wise to buy 100% absorption?
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post #3 of 20 Old 08-23-2016, 08:51 AM
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the science of "popular" room acoustics for home theaters has evolved over the years. When I first became interested the one size fits all solution was a mixture of absorption and nothing. It was known that you could overdue it so there were guidelines where to put it. Later strategies to add diffusion and reflection entered into the mix.

It really doesn't need to be expensive/complicated as you can convert a simple 2 inch OC705 fiberglass panel to a combination diffuser/absorber by mounting alternating vertical stripes of 3 inch metal FSK tape to the surface. You can also put metal/paper scrim on the surface so it acts more like a reflector of high frequencies. You can add perforated or slat panels in front of the OC705 but then it is a little bit more complicated as someone at a higher pay grade needs to determine hole size and spacing or slat size. Skyline diffusers are also a DIY friendly project and the data is out there for anyone to use.

I've been in many rooms but so far the best sounding room had the old formula of a combination of 1 inch Linacoustic on sections of the wall combined with Dacron polyfill in others. One quote that stuck with me with from Dr Toole's audio science class is that as the number of speakers in a sound system increases the importance of the room treatment decreases. The % of sound you hear directly from the speakers increases relative to the reflected sound. Kind of why headphones often sound really good.
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post #4 of 20 Old 08-23-2016, 10:58 PM - Thread Starter
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Can we get some kind of design links for the best broadband diffusers? Is there a way to plug the numbers in and make them no matter how deep it takes? I have aprox 12" that would like to make broadband diffusers for on the side walls and rear wall possibly.

I'm literally ignorant as to how they need to go together like as far as in pairs or trios etc...but know when there is enough depth we can defuse pretty low.

I want skyline styled for ceiling since they can be light and cheap but for the sides I want as deep as can go in respect to the seat distance.

The last thing I want is to tame the 125-500hz range with absorption since that's what 90% of the soundtrack is.

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post #5 of 20 Old 08-24-2016, 01:23 AM
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knock yourself out, all kinds of calculators assembled here:

http://www.mh-audio.nl/spk_calc.asp
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post #6 of 20 Old 08-24-2016, 07:52 AM
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Perhaps you'll find something of value here:
http://www.lydogakustikk.no/?p=2281&lang=en
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post #7 of 20 Old 08-24-2016, 11:50 AM - Thread Starter
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My issue is I can't seem to find a design that I can make broadband.

Like I can make one effective from 280hz-1400hz but that's about as far as I can stretch one which might not be a bad thing. Maybe I just need to concentrate on putting different designs in different places. And that one is deep I tell ya!
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post #8 of 20 Old 08-24-2016, 07:40 PM
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As a general rule of thumb, you should not exceed 25% absorption for the room. Slat diffusion is the first order beginnings of abfusion. In other words, it's where it begins in its most simplistic form...and where many companies stop due to complexities with calculations and patents already in place by those who figured it out first. Slat diffusion is actually pretty darn easy and economical to employ. The questions begin when you start to break the slats down further into sub parts. Similar to calculus, what happens to the value under the curve as we integrate to zero? Similar approach, and now we begin to understand how abfusion is calculated. It gets into some serious mathematics as we begin to determine the necessary binary placement of holes, their diameters, and depth. We need to take into account individual frequency behavior at that point, and testing needs to be conducted.
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post #9 of 20 Old 08-25-2016, 10:58 AM - Thread Starter
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Originally Posted by SierraMikeBravo View Post
As a general rule of thumb, you should not exceed 25% absorption for the room. Slat diffusion is the first order beginnings of abfusion. In other words, it's where it begins in its most simplistic form...and where many companies stop due to complexities with calculations and patents already in place by those who figured it out first. Slat diffusion is actually pretty darn easy and economical to employ. The questions begin when you start to break the slats down further into sub parts. Similar to calculus, what happens to the value under the curve as we integrate to zero? Similar approach, and now we begin to understand how abfusion is calculated. It gets into some serious mathematics as we begin to determine the necessary binary placement of holes, their diameters, and depth. We need to take into account individual frequency behavior at that point, and testing needs to be conducted.
Thanks Mike. That makes sense and haven't really tried modeling slats but have a ton of QRD and skyline styles designed and just need to figure out when and where to use them and when to flank them by others or other materials.

Like how full is a full bandwith diffuser? I have designed some like 250-13,000hz but don't know if that's truly full bandwith. Or if that will work as I extend it to across the left and right walls.

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post #10 of 20 Old 08-25-2016, 11:47 AM
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I think some of the trouble you are having developing a comprehensive approach and understanding could be remedied by a change in tack. You are looking at it from the perspective of what tools you have and can make work for you, but you would perhaps be better served by looking at what psychoacoustic principles are weighing on your perception and what you can do to influence those particular aspects.

For instance: why would you want to diffuse 300Hz? (That's a rhetorical question, for now at least) Are you trying to make the frequency response more uniform around the room? Are you concerned about image shift or precision? Are you concerned about frequency response at your listening position? All of these problems are potentially important, but I think probably none of them are best addressed with diffusion at 300Hz.
(Also, have you looked into the size of the nearfield for a diffusor in that band? You might not be happy with the answer - much like the device, the nearfield response of the device is large.)

As a particular piece of advice, I suggest you take Shawn's (not Mike - SMB are his initials, but I know he gets that all the time) comment about the ease and effectiveness of slat diffusors to heart. You might not get perfect diffusion, but I think the performance to ease to cost to size ratios are all very good.

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post #11 of 20 Old 08-25-2016, 12:36 PM - Thread Starter
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Originally Posted by HopefulFred View Post
I think some of the trouble you are having developing a comprehensive approach and understanding could be remedied by a change in tack. You are looking at it from the perspective of what tools you have and can make work for you, but you would perhaps be better served by looking at what psychoacoustic principles are weighing on your perception and what you can do to influence those particular aspects.

For instance: why would you want to diffuse 300Hz? (That's a rhetorical question, for now at least) Are you trying to make the frequency response more uniform around the room? Are you concerned about image shift or precision? Are you concerned about frequency response at your listening position? All of these problems are potentially important, but I think probably none of them are best addressed with diffusion at 300Hz.
(Also, have you looked into the size of the nearfield for a diffusor in that band? You might not be happy with the answer - much like the device, the nearfield response of the device is large.)

As a particular piece of advice, I suggest you take Shawn's (not Mike - SMB are his initials, but I know he gets that all the time) comment about the ease and effectiveness of slat diffusors to heart. You might not get perfect diffusion, but I think the performance to ease to cost to size ratios are all very good.

Fred
What I'm wanting is pretty much all of what you said. Image shift for Atmos obviously and with speakers along every wall and ceiling I don't want to absorb them so want the sound dispersed like a glowing wall. I have a foot of depth to work with and then another 6 foot to the very outside seat so aprox 9 foot to center seat. I just figured that if was diffusing the vocal range it might be better to diffuse as much of that range as possible for better clarity.
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post #12 of 20 Old 08-25-2016, 12:45 PM - Thread Starter
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And SMB. I can calculate whatever I need to but the problem is setting up the calculation lol. Was a math major so any Calculus is just a matter of understanding the process of setting up the equation which they really don't get into acoustical engineering in a architectural and civil engineering program lol.

Why should or shouldn't I concentrate on diffusing the entire vocal bandwith if am diffusing part of it. I'm doing a 30+ channel room so I'll be using less than 15% absorption most likely and none at first reflections since most of the height/top speakers will be absorbed by natural room materials. No sense in making the room less dynamic or using more power when can diffuse and deflect. I may be going about this wrong according to some but there's always more than one way to skin a cat.
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post #13 of 20 Old 08-25-2016, 01:19 PM
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Fred pretty much nailed it. You only want to diffuse high frequencies. What's there to model? It's an on off binary aspect. Only thing you need to keep in mind is the frequencies you intend to diffuse. Use the lowest frequency you intend to diffuse, or the highest frequency you intend to absorb and work from that.
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post #14 of 20 Old 08-25-2016, 01:22 PM
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You do not want to absorb the entire vocal bandwidth throughout the room. Only in certain locations, such as the sides adjacent to listening positions. Keep in mind direct sound, reflected sound and where the listeners are positioned when making calculations.
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post #15 of 20 Old 08-25-2016, 01:47 PM - Thread Starter
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Quote:
Originally Posted by SierraMikeBravo View Post
Fred pretty much nailed it. You only want to diffuse high frequencies. What's there to model? It's an on off binary aspect. Only thing you need to keep in mind is the frequencies you intend to diffuse. Use the lowest frequency you intend to diffuse, or the highest frequency you intend to absorb and work from that.
What frequencies are "high frequencies" in your meaning?
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post #16 of 20 Old 08-25-2016, 02:00 PM
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That depends on your speaker, but somewhere around 1000 Hz and above or so would be considered higher frequencies.

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post #17 of 20 Old 08-25-2016, 08:06 PM
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I'm by no means an expert in any of this, so please don't get bogged down in any of the details I might present here. The numbers are only approximate, and I may have ranges fairly wrong as well. Also, I'm sure I am leaving out some details that I have misunderstood in my own reading.

Here's a general presentation of some of the (from my perspective) most important psychoacoustic principles to understand when designing speaker layouts and acoustic treatments, in no particular order.

One of the key ways your ears and brain work together to localize sound is through the time gap and phase change between when your left and right ears hear a sound. For this to work, the sound waves have to be a fraction of the size of your head, or the SPL and phase will be effectively the same for both ears and your brain won't know the difference (a 10cm wave is 3.4kHz - pretty high treble) This only works for sounds side to side. Your brain uses different tricks for up and down and fore and aft (the hardest to discern).

Elevation of sound sources is largely based on the diffraction cancellations created by your outer ears themselves (the pinna). When a sound source is above you, the outer ear creates a cancellation of a range of high frequencies (somewhere around 8kHz, I think). When your brain hears a sound with that portion of the spectrum missing, you hear it as above you - simple as that. It's very reliable - just ask proponents of reflected Atmos speaker implementations.

Why high frequencies? Well, they are less perturbable, in a way. A small wave encounters most anything and is either reflected or absorbed - because the object it encounters is so much bigger than it. In contrast, large low frequency waves refract around most normal sized objects. So, when you want to know if the sound of someone's footsteps is coming from within your bedroom, your brain listens for the high frequency content of the sound to see if it's there - when it's not there, you know the sound came from around the corner.

Commonly, high frequency sounds are associated with the onset of a sound - the sound of the tongue against the teeth and lips when speaking; the fingers on a guitar string as they strum; the shoe leather touching down on the floor before the weight of the step causes the lower frequency boom. As your brain listens for those sounds, it determines the direction of the sound source. If an acoustically reflective surface near a speaker (or any other sound source) directs a reflection to your ears, your brain can be tricked into thinking the source of the sound has moved toward the reflection. This is the foundation of the (generally misguided) argument to absorb early lateral reflections in listening rooms. Especially for music (and most surround channel content) these early high frequency reflections will broaden the image, making them seem more ambient and less pin-point. That's found to be an advantage to most listeners, and why absorption is not the best tool in most cases. If those early reflections were slightly delayed through diffusion, all the better for those goals. I think this is one reason we see more and more diffusion to the sides and behind listening positions in recent treatment plans. But again, this is only relevant to the frequency ranges that your brain uses to localize sounds.

For midrange and low frequency sounds, your brain is much less sensitive to localization issues. Instead, the most pressing concerns are frequency response, frequency extension (bass), and ringing/decay. Frequency extension I mention just because people talk about it with speaker evaluation - lower bass extension is pretty universally preferred - end of discussion.

Frequency response is obviously a function of speaker design, but also and usually more strongly a function of speaker and listener placement. The mess of reflections that your brain is always sifting through make it less sensitive to the "comb filtering" or other frequency response anomalies associated with the mostly destructive interference caused by reflections. Think of these in terms of intervals (octaves and so on - on a logarithmic scale). When the bandwidth of a cancellation dip is 50Hz that's nothing in the treble ranges (a small portion of an octave), while below 400Hz or so that's a huge fraction of an octave - several notes on a scale all swallowed up by a cancellation dip. Where did that cancellation originate? Usually from a strong room boundary within a few feet of either the speaker or listener. This problem is simple, and best addressed through speaker positioning and/or the implementation of a baffle wall, or simply moving your seat away from the wall. Once you are away from the wall by more than a few feet, the frequency of the cancellation has shifted up the scale to where it is less audible.

One of the more nasty (but common) problems with low frequency response anomalies is ringing, caused by uncontrolled standing waves or simply an unhappy set of reflections leading to constructive interference. The increased output leads to a variety of problems; obviously the uneven frequency response is in itself wrong for one, but for two, the high output at one frequency masks nearby (usually higher frequency) sounds - your brain doesn't perceive them at all, even when they are loud enough for you to hear them under other conditions; and three, depending on the reason for the increased output (ringing vs simple constructive interference) there are temporal problems where the bass lingers longer than it should.

So where does that leave us? Here's a couple relevant observations, I think. Frequency response is still king, but keep in mind that's frequency response at your listening position - which is not as simple as the anechoic response of the speaker, as the spectral content of the reflections play a role. So, if you are treating first reflections of the speaker, bandwidth is a concern (though not a huge one, IMO). Mostly, at this point I want to refer you to some well-thought-out best practices: http://www.acousticfrontiers.com/wp-..._standards.pdf <--- This should be the target. The psychoacoustics explains why and helps with how.

(Sorry for this rambling nonsense, I know this isn't an easy read. There's also a good chance you know most of this already - I hope you don't feel like I'm talking down to you or anything. I'm just trying to shift your perspective to finding solutions to problems. Acoustic treatment design is an engineering exercise in pursuit of a set of goals, so it's always a problem-solving endeavor.)
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post #18 of 20 Old 08-26-2016, 10:17 AM - Thread Starter
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Quote:
Originally Posted by HopefulFred View Post
I'm by no means an expert in any of this, so please don't get bogged down in any of the details I might present here. The numbers are only approximate, and I may have ranges fairly wrong as well. Also, I'm sure I am leaving out some details that I have misunderstood in my own reading.

Here's a general presentation of some of the (from my perspective) most important psychoacoustic principles to understand when designing speaker layouts and acoustic treatments, in no particular order.

One of the key ways your ears and brain work together to localize sound is through the time gap and phase change between when your left and right ears hear a sound. For this to work, the sound waves have to be a fraction of the size of your head, or the SPL and phase will be effectively the same for both ears and your brain won't know the difference (a 10cm wave is 3.4kHz - pretty high treble) This only works for sounds side to side. Your brain uses different tricks for up and down and fore and aft (the hardest to discern).

Elevation of sound sources is largely based on the diffraction cancellations created by your outer ears themselves (the pinna). When a sound source is above you, the outer ear creates a cancellation of a range of high frequencies (somewhere around 8kHz, I think). When your brain hears a sound with that portion of the spectrum missing, you hear it as above you - simple as that. It's very reliable - just ask proponents of reflected Atmos speaker implementations.

Why high frequencies? Well, they are less perturbable, in a way. A small wave encounters most anything and is either reflected or absorbed - because the object it encounters is so much bigger than it. In contrast, large low frequency waves refract around most normal sized objects. So, when you want to know if the sound of someone's footsteps is coming from within your bedroom, your brain listens for the high frequency content of the sound to see if it's there - when it's not there, you know the sound came from around the corner.

Commonly, high frequency sounds are associated with the onset of a sound - the sound of the tongue against the teeth and lips when speaking; the fingers on a guitar string as they strum; the shoe leather touching down on the floor before the weight of the step causes the lower frequency boom. As your brain listens for those sounds, it determines the direction of the sound source. If an acoustically reflective surface near a speaker (or any other sound source) directs a reflection to your ears, your brain can be tricked into thinking the source of the sound has moved toward the reflection. This is the foundation of the (generally misguided) argument to absorb early lateral reflections in listening rooms. Especially for music (and most surround channel content) these early high frequency reflections will broaden the image, making them seem more ambient and less pin-point. That's found to be an advantage to most listeners, and why absorption is not the best tool in most cases. If those early reflections were slightly delayed through diffusion, all the better for those goals. I think this is one reason we see more and more diffusion to the sides and behind listening positions in recent treatment plans. But again, this is only relevant to the frequency ranges that your brain uses to localize sounds.

For midrange and low frequency sounds, your brain is much less sensitive to localization issues. Instead, the most pressing concerns are frequency response, frequency extension (bass), and ringing/decay. Frequency extension I mention just because people talk about it with speaker evaluation - lower bass extension is pretty universally preferred - end of discussion.

Frequency response is obviously a function of speaker design, but also and usually more strongly a function of speaker and listener placement. The mess of reflections that your brain is always sifting through make it less sensitive to the "comb filtering" or other frequency response anomalies associated with the mostly destructive interference caused by reflections. Think of these in terms of intervals (octaves and so on - on a logarithmic scale). When the bandwidth of a cancellation dip is 50Hz that's nothing in the treble ranges (a small portion of an octave), while below 400Hz or so that's a huge fraction of an octave - several notes on a scale all swallowed up by a cancellation dip. Where did that cancellation originate? Usually from a strong room boundary within a few feet of either the speaker or listener. This problem is simple, and best addressed through speaker positioning and/or the implementation of a baffle wall, or simply moving your seat away from the wall. Once you are away from the wall by more than a few feet, the frequency of the cancellation has shifted up the scale to where it is less audible.

One of the more nasty (but common) problems with low frequency response anomalies is ringing, caused by uncontrolled standing waves or simply an unhappy set of reflections leading to constructive interference. The increased output leads to a variety of problems; obviously the uneven frequency response is in itself wrong for one, but for two, the high output at one frequency masks nearby (usually higher frequency) sounds - your brain doesn't perceive them at all, even when they are loud enough for you to hear them under other conditions; and three, depending on the reason for the increased output (ringing vs simple constructive interference) there are temporal problems where the bass lingers longer than it should.

So where does that leave us? Here's a couple relevant observations, I think. Frequency response is still king, but keep in mind that's frequency response at your listening position - which is not as simple as the anechoic response of the speaker, as the spectral content of the reflections play a role. So, if you are treating first reflections of the speaker, bandwidth is a concern (though not a huge one, IMO). Mostly, at this point I want to refer you to some well-thought-out best practices: http://www.acousticfrontiers.com/wp-..._standards.pdf <--- This should be the target. The psychoacoustics explains why and helps with how.

(Sorry for this rambling nonsense, I know this isn't an easy read. There's also a good chance you know most of this already - I hope you don't feel like I'm talking down to you or anything. I'm just trying to shift your perspective to finding solutions to problems. Acoustic treatment design is an engineering exercise in pursuit of a set of goals, so it's always a problem-solving endeavor.)
Thanks Fred!

Yea I've read much of that link and watched countless videos/lectures and it's still a mess of conflicted info since so many use different methods.
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post #19 of 20 Old 08-27-2016, 12:01 PM
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It's an art form. There are only a few hard and fast rules to adhere to in the physical side of things. I've heard stated that Erskine rooms are the reference to outstanding sounding rooms, and many try to emulate then. However, each room is differently treated based on experience and knowing precisely what sound we are trying to achieve. This is the part that cannot be conveyed in a paper.
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post #20 of 20 Old 08-29-2016, 09:17 AM
 
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There was a good thread I saw on another forum that looked in detail at doing the peg board style combo panel- they played with the size of holes, location, depth... etc... It gets pretty complicated quickly and it's a difficult thing to actually measure too.

If you start with absorption you can always add back in reflection or diffusion by putting other materials over the absorption. Sometimes designers will leave an adjustment path in the design, so that on final calibration or listen if the room needs to be adjusted there is a couple options to do so easily and nail it perfect.

Like someone explained above it's an art form, as much as science. There is an aspect of personal taste and experience that ties into the science of it. I've heard a lot of different rooms and it wasn't until I heard a variety of professional ones that I really started to understand the differences. I think another aspect that makes it hard is for a laymen they are quite limited on experiences so how would you know about something you never experienced? It's like trying to explain what it feels like to be launched into space in a space shuttle to a non astronaut, or explain what a cheeseburger tastes like to someone who never ate one.

The other part that was always complicated for me until I learned enough to really understand was that it's not about what treatments you use, or what they do. They are all good. But they are only tools not solutions. You need to understand what they do yes... for sure, but also you need to understand what you are trying to do, or what you should be doing and that is different thing.
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