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"Tuned" LF Absorption vs Broadband Preferences
Old 24th May 2020
  #1
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"Tuned" LF Absorption vs Broadband Preferences

After reading many, many acoustic treatment threads, I've noticed what appears to be a general preference for broadband low frequency absorption (over against a more narrow, targeted LF application.

This seems at least partially contrary to the central treatment philosophy which strives to retain low midrange, midrange and high frequency content.

In much of the conversations/discussions, one would note that many professionals and others here seem to have a desired bass treatment outcome to include a high degree of effectiveness at, say up to even 5KHz (makes me wonder what happens to that absorbed original content).

What are the pros and cons of midrange and high range absorption in rooms, for example of approximately 2500 sqm? To put it another way, in terms of treatment philosophies, why the two distinctly opposing views?

My question(s) are not merely academic, as I am designing a treatment plan for a 2450 sqm, rectangular existing standard construction room. My somewhat studied opinion is that 800Hz and above can simply be diffused in most cases, using focused tuned perforated resonator bass traps/panels in the usual suspect locations.
Old 25th May 2020
  #2
Quote:
Originally Posted by glorycloud View Post
After reading many, many acoustic treatment threads, I've noticed what appears to be a general preference for broadband low frequency absorption (over against a more narrow, targeted LF application.

This seems at least partially contrary to the central treatment philosophy which strives to retain low midrange, midrange and high frequency content.

In much of the conversations/discussions, one would note that many professionals and others here seem to have a desired bass treatment outcome to include a high degree of effectiveness at, say up to even 5KHz (makes me wonder what happens to that absorbed original content).

What are the pros and cons of midrange and high range absorption in rooms, for example of approximately 2500 sqm? To put it another way, in terms of treatment philosophies, why the two distinctly opposing views?

My question(s) are not merely academic, as I am designing a treatment plan for a 2450 sqm, rectangular existing standard construction room. My somewhat studied opinion is that 800Hz and above can simply be diffused in most cases, using focused tuned perforated resonator bass traps/panels in the usual suspect locations.

Why 2 different opposing views?

Honestly? People are lazy, cheap and look for easy fixes, which when trying to do a full room treatment there really aren't any.

Its been what 20-25 years now that people on the internet have been pushing this idea of using 2' x 4' 4" inch broadband traps for treating low end issues, with uneven results. Over the years treating corners has been added and let's not forget the famous "38% rule".

The truth is when dealing with free standing monitors, there will always be a compromise somewhere and just to get a room to be close to EBu stereo room specifics takes alot of work and planning. And the requirements for an EBu standard stereo listening room aren't outrageous.

To get an idea, just go to Facebook and look at some of the late Boggy Petrovic rooms. Also its a great idea to read his AES paper on how he approached it.
The level of detail, planning and construction is far too difficult, time consuming plus expensive for someone that just wants to order a bunch OC 703 4" panels, wrap them in fabric and hang them on the wall.
Old 25th May 2020
  #3
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Look at the reverb time tolerance limits. The curve is flat full range absorption is required. Problems start when some absorbers are not due misguided specification interpretation and marketing speak. Most specs are for initial reflections above 500 Hz. So they think they only need absorption above 500 Hz ignoring the absorbers absorb sound in all directions ( mistake one) and use thin material (mistake two). The minimum for absorbers is 8" depth. That is just the start of mistakes.
Old 25th May 2020
  #4
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Thanks, Avare - - That's helpful. You've identified a couple of primary considerations, especially starting points.

So, reverb time tolerance limits must be maintained. A flat curve is an objective. I think I'm picking up what you're putting down. Let me then rephrase the question (re: post topic) since you've refined the focus of my concern (and correct me if I my assumptions are mistaken here): Since my concern is to mitigate LF issues and preserve mid and high freq content while observing reverbing time tolerance limits and achieving within limits a flat curve, it is a given that LF issues "merely" a matter of proper application of LF treatment, such as perforated/Helmholtz-type absorption - most troubling from 30Hz-150Hz. If porous absorption is used such that those frequencies are addressed, but due to the design of the panels and traps also absorb well into upper midrange and beyond (which I am referring to as broadband for the purposes of this discussion, would it not be true that content material one would desire to preserve are "lost" or sacrificed - an unwanted trade-off resulting? Or, am I incorrect to assume that above a certain frequency range (800Hz and above) diffusion preserves this content and maintains desired signal information while also successfully addressing reverb time and the flat curve goal? I realize that here I've not dealt with 150Hz or 200Hz to 800Hz. Not that this concern is a separate issue, and is germane to the philosophy of approach to treatment, but my focus is to lean why "broadband" absorption is employed when perhaps it should not be where other effective means can be utilized. Broadband, when I see it advocated or made reference to, is represented by astute professionals (Jens and others) offering data and graphs indicating close to 80%-100% absorption all the way up into the 5KHz range. So again, would not the goal be to preserve mids and highs content, using, say, 1-D diffusion and still achieve the reverb time and flat curve objective? Am I missing something here?
Old 25th May 2020
  #5
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I will rephrase it. What most people think full range is really mid and high absorber. Use true full range to begin with and the need for LF absorbers drops significantly.
Old 27th May 2020
  #6
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OK, I see where folks are heading with the responses so far, and some of the comments/replies are informative. But the majority of the replies assume (or presuppose) with the qualifier that the LF treatment is less than effective or professionally applied. I should have been a little more clear: Let's say the LF treatment is very well done, okay? What remains is treating low mids, and mids, and let's assume the higher freqs are treated with diffusion. And let's further assume one wants to retain all the musical information possible - while keeping an eye on reverb time and the freq response curve. Now, insert the "broadband" element, where broadband is understood to address a very broad frequency range, including LF and somewhat high frequencies. Why would one want to use broadband LF treatments when other effective options exist? Not saying there isn't a good reason - I just don't know it. Additionally, going the broadband route necessitates massively deep/thick units (e.g. BBC, others mentioned here on GS). And, what happened to all that content that has been absorbed - together with the sorts of "liveliness" characteristics being sacrificed?
Old 27th May 2020
  #7
Quote:
Originally Posted by glorycloud View Post
OK, I see where folks are heading with the responses so far, and some of the comments/replies are informative. But the majority of the replies assume (or presuppose) with the qualifier that the LF treatment is less than effective or professionally applied. I should have been a little more clear: Let's say the LF treatment is very well done, okay? What remains is treating low mids, and mids, and let's assume the higher freqs are treated with diffusion. And let's further assume one wants to retain all the musical information possible - while keeping an eye on reverb time and the freq response curve. Now, insert the "broadband" element, where broadband is understood to address a very broad frequency range, including LF and somewhat high frequencies. Why would one want to use broadband LF treatments when other effective options exist? Not saying there isn't a good reason - I just don't know it. Additionally, going the broadband route necessitates massively deep/thick units (e.g. BBC, others mentioned here on GS). And, what happened to all that content that has been absorbed - together with the sorts of "liveliness" characteristics being sacrificed?
Because a lot of people here in this forum are trying to setup control rooms in small rooms and to get "well done" LF effective treatment takes some depth plus area.
This means your small room will start to get small pretty quick. Also designing "well done" LF effective treatment takes some knowledge, experience and time. Sometimes you have to build in "situ" because even though the theory is correct depending on the what the walls are made of and where its located you may not get the effect you thought you would.

Now tell that to someone that doesn't have the time, patience or budget and you have your answer.
Old 27th May 2020
  #8
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Quote:
Originally Posted by glorycloud View Post
OK, I see where folks are heading with the responses so far, and some of the comments/replies are informative. But the majority of the replies assume (or presuppose) with the qualifier that the LF treatment is less than effective or professionally applied. I should have been a little more clear: Let's say the LF treatment is very well done, okay? What remains is treating low mids, and mids, and let's assume the higher freqs are treated with diffusion. And let's further assume one wants to retain all the musical information possible - while keeping an eye on reverb time and the freq response curve. Now, insert the "broadband" element, where broadband is understood to address a very broad frequency range, including LF and somewhat high frequencies. Why would one want to use broadband LF treatments when other effective options exist? Not saying there isn't a good reason - I just don't know it. Additionally, going the broadband route necessitates massively deep/thick units (e.g. BBC, others mentioned here on GS). And, what happened to all that content that has been absorbed - together with the sorts of "liveliness" characteristics being sacrificed?
Broadband absorption is cheaper than most thinner absorbers. So you can spend more money on mid/high and then spend more on LF. What is "massively deep?" The recommendation of many studio designers is 8" minimum. Is that "massively deep?"

BTW what does "the higher freqs are treated with diffusion" mean?
Old 27th May 2020
  #9
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Avare - - Good points. Thanks much for the interaction - I appreciate your time.

In several places, I had read 12" for porous LF treatment if achieving effectiveness at below 50Hz-60Hz and to also be effective broadband. IDK if that's accurate. So I was just using that information. RE: Expense/cost...I am planning to DIY; it looks like any affordable products on the market aren't sufficiently effective or they lack testing data...or both. But it appears that in terms of cost, even if DIY, your point is well-taken in regard to cost.

Sorry for the ambiguous language regarding high frequency treatment. I have read as much as I can so far - a boatload of forum material and applicable chapters from Toole, but I'm still a little uninitiated and still learning with respect to the science and method of treatment. But, I had meant to communicate that in the frequency range from around 1,000 Hz to 5,000Hz, I assumed absorption would not be necessary to achieve a relatively flat curve, providing 1-D diffusion (such as Opptiffusers ?) was used in the appropriate areas. I'm probably wrong again.

My intention was to use this thread to help clear up my confusion regarding the pros and cons of the treatment philosophies (as above), including (as per Toole) retaining mid and high freq signal content and "life." But also to make inquiry as what negative aspects exist in an RFZ/LEDE room when the listening position is at the rear 38% of the room, 7'-8' from the rear wall, with speakers out from front wall at approximately 38%. I can measure the untreated room and consider treatment approach, as I've been doing, but I'm carefully thinking through addressing the reverb time and the curve, plus setup options. So that's where I'm coming from in the post. And again, I'm thankful for the interest and feedback I've gotten.
Old 27th May 2020
  #10
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What a mouth full. To get it right, 38% is not a rule, it is a possible depth. Where did it come from? The goal is to minimize deep nulls. Values that are inverse integers are used. E.g. 1/7, 2/7, 3/7, 3/8, 5/8. 38% is 3/8 rounded of to 2 significant digits.
Old 27th May 2020
  #11
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One not-uncommon scenario where diffusers get used is for killing flutter echoes (between opposite parallel walls), where the amount of absorption needed to do the same would deaden the room too much.

If you're reading Toole's book, be mindful of reference to or discussion of the somewhat/sometimes differing goals for recreational two-channel vs multi-channel (surround) listening, and both as distinct from critical listening like for studio work. But getting rid of ringing flutter-echoes is one pretty common goal whatever the scenario.
Old 27th May 2020
  #12
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Thanks everyone for the replies/feedback which helped to sort out the several issues at play and give me enough information to move forward.
Old 7th June 2020
  #13
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Hi, guys, sorry to hijack this thread. Avare, I have a few questions regarding this same topic, It would help me reduce a bit of confusion if you answered some of them and I'd be really grateful for it if you did.

Quote:
Originally Posted by avare View Post
Broadband absorption is cheaper than most thinner absorbers. So you can spend more money on mid/high and then spend more on LF. What is "massively deep?" The recommendation of many studio designers is 8" minimum. Is that "massively deep?"
I'm assuming that 8" size is just regarding insulation thickness, right? I've seen you say that like 4 times today let me explain: This morning, my goal was set on deciding what panels to build, I've spent like 8-12 hours reading GS just today and I'm even more lost about the real use of 8+" with 4 inch airspace broadband absorption on rfz AND trapping vs 2" rfz and trapping (assuming you do the same panels, just using different thickness).

I found good stuff tho, I even ended up reading that NASA paper about 703 and rockwool that concluded that PF 3350 had the best performance. (Which is very confusing because i looked for that material online and I only found that same nasa study! what on earth is PF3350? i'm probably missing something really obvious here and will feel like an idiot when i realize it.) And I eventually found that there are several materials with certain airspace that have 1 as absorption coefficient at 125Hz, is there a way to know lower than that?

http://johnlsayers.com/Stuff/AbsorptionCoefficients.htm

In simpler terms: How would a full range absorber look like? I really couldn't find a decisive full answer anywhere, most discussions went into broader themes, I even found a thread with someone with my exact same mode problems that derived into a 12 page long fight with danish acousticians I think my room is cursed.

When I started researching I got the impression that broadband absorption was the way to go (as thethrillfactor mentions, I was guilty of some of that) and as time goes further I start to feel like it's not even worth it, especially in a room like mine with a most prominent 37Hz axial mode. I wanted to make my panels broadband just to try to make the most out of them, even considered superchunk or making a lot of panels instead of proper bass trapping... but after some people from GS (thank you thrillfactor) kindly pointed me to the right direction I realized it was probably a bad idea. I'm going to do limp mass and tube trapping but now i'm confused regarding what i thought was gonna be the easiest part, designing my RFZ (because i've done it before).

I'm sure there must be a thread about the definitive full range panel for max RFZ efficiency in bassy room use but I just haven't found it, but my brain is fried, it's almost 3 am and I still don't know how much insulation i should buy

My respect for you guys has only gone up ever since I started this odyssey, acoustics are so complex, i wish i could just be confident enough on what i've learned to just make a starting design to do the first treating of this nightmarish room i have without fear of doing things wrong...

Thanks a lot for your help, as complex as it is in my eyes now, I can't imagine how hard it would be without this forum and you guys

BTW: i tried different calculator but i don't know what's the most efficient insulation in low frequency to try to get the right measurements, that's how I got to the NASA paper... Also I think some roxul materials don't provide pa*s/m^3. The link i pasted has some data but still not conclusive, some of it even missing. Every time i try to get a definitive answer of how to attempt efficiency maximization i go into another deeper rabbit whole than the last time
Old 3rd July 2020
  #14
Assuming your monitors are good, you're starting out with the sound that you want.

Then you add walls and the problems start.

The cause of the problems are the reflections off of the surface of the walls and that true at any frequency. If you eliminate the reflections in a balanced way, you'll end up with your initial good sound that your monitors put out.

If you eliminate the reflections in an unbalanced way, then you're not going to be happy.

If you choose a material that only absorbs in one frequency range, then you have to figure out how to address the the range or ranges you neglected.

If you use a material that can absorb true broadband, meaning minimum 8 inches, or in my opinion minimum 12 inches, you're done. You don't have to address any frequency ranges that you've neglected because you addressed the entire range.

Even better - get some R-38 and turn it sideways for either 16 inches or 24 inches of absorption.

There are a lot of really easy ways to hang it, and rather than build panels, just build a fabric wall in front.

Check out Tom Hidley's design of BOP studios. It's a very cheap way to do it because it involves far fewer materials.

Quote:
Originally Posted by glorycloud View Post
Thanks, Avare - - That's helpful. You've identified a couple of primary considerations, especially starting points.

So, reverb time tolerance limits must be maintained. A flat curve is an objective. I think I'm picking up what you're putting down. Let me then rephrase the question (re: post topic) since you've refined the focus of my concern (and correct me if I my assumptions are mistaken here): Since my concern is to mitigate LF issues and preserve mid and high freq content while observing reverbing time tolerance limits and achieving within limits a flat curve, it is a given that LF issues "merely" a matter of proper application of LF treatment, such as perforated/Helmholtz-type absorption - most troubling from 30Hz-150Hz. If porous absorption is used such that those frequencies are addressed, but due to the design of the panels and traps also absorb well into upper midrange and beyond (which I am referring to as broadband for the purposes of this discussion, would it not be true that content material one would desire to preserve are "lost" or sacrificed - an unwanted trade-off resulting? Or, am I incorrect to assume that above a certain frequency range (800Hz and above) diffusion preserves this content and maintains desired signal information while also successfully addressing reverb time and the flat curve goal? I realize that here I've not dealt with 150Hz or 200Hz to 800Hz. Not that this concern is a separate issue, and is germane to the philosophy of approach to treatment, but my focus is to lean why "broadband" absorption is employed when perhaps it should not be where other effective means can be utilized. Broadband, when I see it advocated or made reference to, is represented by astute professionals (Jens and others) offering data and graphs indicating close to 80%-100% absorption all the way up into the 5KHz range. So again, would not the goal be to preserve mids and highs content, using, say, 1-D diffusion and still achieve the reverb time and flat curve objective? Am I missing something here?
Old 3rd July 2020
  #15
It would look the same as a regular one, but much thicker.

I recently covered and entire room including the ceiling with R-38 and I loved it. I'm sure it was "too dead" but it didn't bother me at all.

I could stand anywhere in the room and clap and the echoes would come from the next room.

If the ceilings had been higher, I would have looked for some kind of titanium mesh for the floor and floated it above another 12 inches of R-38 and let everything pass through the mesh into the absorption.

Tom Hidley's designs include a little bit of reflection above the mix position so that it feels like your indoors as opposed to in a totally open environment. I wasn't bothered by that at all and I spent almost all of every day in there.
Old 5th July 2020
  #16
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Sonic Vitriol's Avatar
Thanks mike, i ended up having a similar philosophy to yours, i made some HUGE panels with 6 inches of OC rockwool (OC says their ab coef is around 1 at 125 in 6" of material so it's worth a try)i haven't measured yet and now im deciding wether to choose the window as a front or back wall but your comments really resonate and i'm glad I did it that way, thanks for the answer. After i sort out placement i'll ALSO do Tim's LMR.
Old 5th July 2020
  #17
For acoustic reasons, I'd make the window the back. For monitor viewing reasons, I'd make it the front.

I always end up covering them.
Old 15th July 2020
  #18
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Sonic Vitriol's Avatar
Alright! thanks for the tip, i have one of those shutters that kill all daylight from the window so I'll use it as a back wall. :p

cheers
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