[John Casler]

As I explained to Nathan, all room acoustic problems are created by reflections, and so it is with both low and high frequencies. --Ethan

Never were truer words spoken, on the subject of room acoustics "FOR" 2 channel audio.

[8thnerve]

8th Nerve,
It seems like you are making the assumption that the only way to get a "proper" amount of HF info is by boundary reinforcement? Are speakers designed in this way, to anticipate lively 1st reflection points (dunno)?

No, rooms without any treatment at all exhibit rolled-off high frequency response due to the comb filtering that Ethan mentions below.  We disagree as to what causes that comb filtering however.

Is is true (or how often is it true?) that if the first reflection points are damped, that this will by default result in rolled-off high frequency response?

All absorptive material absorbs more high frequency information than low frequency information.  The addition of any asorptive material in your room will result in a rolled-off high frequency response.

[8thnerve]

I am also confused.  I don't want to hear early reflections.  Even if there are "more effective ways to reduce standing waves," isn't the treatment of first reflections also desirable?

I'll say this even though I know it will make everyone crazy, because I believe that it is true.  You can't hear early reflections.  You can only hear amplified reflections caused by the corner.  I can say this with confidence as there is an obvious and measureable reduction in echo (reflections) when using the Eighth Nerve products even though they use NO exposed absorption and are not placed anywhere near the supposed first reflection points.  Give me another reason why this would be.

[8thnerve]

Personally I think most smaller rooms do need to "roll off" the highs slightly.  The long low bass wavelengths can readily pass thru walls whereas the highs don't to such a degree.  My feeling is that small rooms are unnaturally live because of this.

Listen to any small room with an adequate amount of Eighth Nerve treatments and you'll change your mind.  Everyone wants a flat frequency response.  Unfortunately, without addressing the corners with proper treatment, the only way to get a level high-frequency response is to jack up the high-frequencies that are distorted due to the lack of treatment.  I don't disagree with your feeling that most rooms sound better with a rolled off response because until you've heard a flat room with much less distortion, that would be your only conclusion.

[bpape]

All absorptive material absorbs more high frequency information than low frequency information.  The addition of any asorptive material in your room will result in a rolled-off high frequency response.

I would respectfully disagree.  There are many absorbtive treatments that yield less HF absorbtion that at other frequencies - though I guess that would depend on your definition of high frequencies.

Examples

OC 703 4" with FSK face exposed - mounted flat on the wall

0.88 0.90 0.84 0.71 0.49 0.23  (almost 4x as much at 125 as at 4kHz)

Any membrane type absorber.

Any Helmholz tuned absorber.

Now, for full range reflection control type panels like unfaced 703 or cotton, that's true.  It's also true that many of the great acoustical minds that have done exhaustive research, been published in AES, etc. have almost all come to the conclusion that one must treat the spectrum as a whole.  Absorbing any part of the spectrum out of balance with the others will have a change on the timbre of the perceived sound.

I guess what I'm saying is if you do more bass than mids and highs, it's bad.  If you do more highs than bass and mids, that's bad too. 

I will agree that it can be very easy to overabsorb the highs in a room.  I'll also say that how much you need to deal with early side wall reflections is directly related to the usage of the room and the off axis response of the speakers.  If the speakers are pretty uniform from 0 degrees to say 30 degrees off axis, and are somewhat directional, then the need for early reflection treatment is less important.  When you have speakers (as most are) that show 10-12db changes in off axis response, then it becomes more important.

Bryan

[8thnerve]

All room acoustic problems are caused by reflections. So absorbing the reflections solves the problems. This is true for excess ambiance and reverb, slap echo and flutter echo, "standing" waves, modal ringing - all of it. If a wave reflects off a wall, adding absorption to that wall stops the reflections and solves the problem.

No absorptive material in existence stops all reflections.  All absorptive material (including the Owens Corning 705 Fiberglas you use in your products, and I in mine) absorbs more in the higher frequencies than they do in the lower frequencies.  So when a wave enters into an absorber, reflects off the wall and returns, the exiting wave has a lower amplitude overall, but much more so in the higher than the lower frequencies.  This wave still returns to your ears and is able to set up more standing waves with other returning waves in the room.  The only difference between this wave now that it has bounced through some absorptive material and what it would be had it just reflected off the wall, is that the wave now is slightly lower in amplitude on average, with more attenuation at the upper end of the spectrum.

Meanwhile, the wave that bounced into the corner underwent a severe comb filter by collapsing into the 90 degree angle, and now returns and is amplified by the horn shape of the corner.  This wave is of much higher amplitude than the unabsorbed wave at the first reflection point would be and is also much more distorted.  Since the human ear can distinguish signals only so far below the sound of highest amplitude, this first reflection is for all intents and purposes, inaudible.

> that's why measurements that show a flattening of the frequency response in a room treated with no absorption lead me to believe there is more to echo and standing waves than is currently known. <

If not treated with absorption, then what "shows a flattening?"

The Eighth Nerve products.

[8thnerve]

All absorptive material absorbs more high frequency information than low frequency information.  The addition of any absorptive material in your room will result in a rolled-off high frequency response.

I would respectfully disagree.  There are many absorptive treatments that yield less HF absorption that at other frequencies - though I guess that would depend on your definition of high frequencies.

Examples

OC 703 4" with FSK face exposed - mounted flat on the wall

0.88 0.90 0.84 0.71 0.49 0.23  (almost 4x as much at 125 as at 4kHz)

Any membrane type absorber.

Any Helmholz tuned absorber.

Well of course, you have the foil facing outward which reflects the high frequencies.  Do you sell your products for first reflection with the foil on and recommend that they place the foil side out?  Foil is not an absorptive material, the fiberglass is and it is certainly more absorptive at the high frequencies.  At any rate, I don't specifically have an issue with absorbing more high frequencies than low or more low than high.  I have issue with any non-linear absorption at any frequency.

I don't understand why you consider Membranes to be an absorber.  It is my understanding that they work more like an an acoustical transformer.  They don't actually burn off any soundwaves even if that is the end result.  Please correct me if I am wrong.  A Helmholtz resonator attenuates based on a cavity and a port that excites a particular frequency.  It is not absorbing by the definition we use for an absorptive material any more than a room node is absorbing the bass frequencies.

Now, for full range reflection control type panels like unfaced 703 or cotton, that's true. 
I will agree that it can be very easy to overabsorb the highs in a room.  I'll also say that how much you need to deal with early side wall reflections is directly related to the usage of the room and the off axis response of the speakers.  If the speakers are pretty uniform from 0 degrees to say 30 degrees off axis, and are somewhat directional, then the need for early reflection treatment is less important.  When you have speakers (as most are) that show 10-12db changes in off axis response, then it becomes more important.

I don't understand this logic.  No matter what the frequency linearity of the wave that is hitting the first reflection point, it will still arrive delayed which is the reason everyone gives for trying to stop it.  Are you saying that if the tweeter is very directional then there won't be much high frequency energy in the reflected wave and therefore it is less important to treat that?  If so, I would disagree for two reasons.  One, just because it is easier to pick out higher frequency sounds when identifying echo, doesn't mean that the midrange frequencies are affected any less.  Two, most people can't pick out a 10KHz sound.  Most of what we perceive as echo occurs in the 4000-8000Hz range, and even extremely directional speakers are wide dispersion in that range.

My point is, as you can see from your NRC values of OC 703, there is no such thing as a linear absorber.  The only way to not affect the overall balance of the frequency response in your room is to not use any absorption.

[Rob Babcock]

Listen to any small room with an adequate amount of Eighth Nerve treatments and you'll change your mind. 

Hey, maybe I would if you'd ever check your PMs!   Any of those summer deals left?

[bpape]

No.  I don't recommend foil out for first reflections.  However there are many other uses in a room where that kind of treatment is called for - specifically to balance out the absorbtion curve in the room so you don't overdo any one area.  If one does broadband bass absrobers with a foil face and reflection panels with unfaced, the net result is a relatively linear absorbtion across the spectrum.

As far as membranes and Helmholz absorbers not being absorbers, that's not true - they're just not velocity absorbers - they're pressure absorbers.  No - they don't technically directly convert soundwaves to heat via resistance.  What they do is work over a specified range and only react to that range.  In the case of the membrane, it's basically a drum in reverse.  Think of a bass drum with and without a pillow in it.  You're damping the membrane when it's excited.

As for the Helmholz, yes, it excites a column of air much like blowing across a coke bottle.  Everything else get's reflected except the specific frequencies that the Helmholz is tuned to.  That is absorbed inside the treatment by the absorbtion.

As for not being able to hear early reflections, that's not exactly true either.  You certainly can hear VERY early reflections - things such as early bounce off a console, early bounce off a TV between 2 speakers, early reflections from speakers placed close to a wall.  Those things that are within say 5ms of the original signal.  You can also hear late reflections - that which we call echo.  It's the ones in between that are up for debate.  There is some research to suggest that those are in fact beneficial - for speech intelligibility - not necessarily for music.

Also, the 'reflections' of a spherical bass radiation from a pattern reflect off of the near boundaries and cause SBIR effects where some frequencies are reinforced and others are cancelled.  While not the type of reflections in the mids and highs that one normally thinks of in terms of treating the side walls, it is an effect based on reflecions that occur in time based directly on the distance from the driver to the boundary behind it and beside it.

Bryan

[PhilNYC]

Meanwhile, the wave that bounced into the corner underwent a severe comb filter by collapsing into the 90 degree angle, and now returns and is amplified by the horn shape of the corner.  This wave is of much higher amplitude than the unabsorbed wave at the first reflection point would be and is also much more distorted.  Since the human ear can distinguish signals only so far below the sound of highest amplitude, this first reflection is for all intents and purposes, inaudible.

Nathan...based on this, could you conclude that if you treat the corner (a al a product like an Adapt Rectangle/Triangle) and reduce the amplitude of the corner amplification/distortion, then proportionally, a reflection at the first reflection point then becomes more audible?  From your comment, perhaps now the highest amplitude sound gets reduced by the corner treatment, and the first reflection suddenly becomes proportionally significant?

[Ethan Winer]

Nathan,

> All absorptive material (including the Owens Corning 705 Fiberglas you use in your products, and I in mine) absorbs more in the higher frequencies than they do in the lower frequencies. <

It's not quite that simple. There is a midrange frequency band where, for a given panel thickness, 100 percent of the sound is absorbed. As you go higher in frequency the surface becomes more reflective, especially for sound arriving at an angle. At some point the waves tend to bounce off the fiberglass, like a rock skipping over water on a lake. And at lower frequencies the material absorbs less as you describe. But it's not a continuous curve.

> So when a wave enters into an absorber, reflects off the wall and returns, the exiting wave has a lower amplitude overall, but much more so in the higher than the lower frequencies. <

Ignoring the bouncing effect I described above, it's more like a one-pole shelving curve than anything else, where the transition frequency depends on the material thickness.

> The Eighth Nerve products. <

That doesn't explain anything. According to what you just said above you use 705 rigid fiberglass. So how is that not absorption?

Also, in the graph you posted, missing is how many of each type of product was used, and also how large the room is. Got some stats?

> You can't hear early reflections. <

Actually, I partly agree with that. I believe what we hear most is the skewed frequency response caused by comb filtering. The reason early reflections affect "imaging" is because the response is skewed differently at each ear. So it's probably not due to phase shift and "time smear" (whatever that is) as so many people claim, but simple frequency response differences.

> The addition of any asorptive material in your room will result in a rolled-off high frequency response ... The only way to not affect the overall balance of the frequency response in your room is to not use any absorption. <

Technically speaking, adding absorption into a room reduces the extra energy that lingers if it were not absorbed. The flattest response is in an anechoic chamber. Everything else is skewed compared to that.

Good discussion guys!

--Ethan

[bgewaudio]

Ethan or anyone else,

How effective would it be to build say, a 6lb OC diffusor for first order treatment?  Could this be the answer in reducing HF rolloff?  Or would this promote a suckout in the midrange.

[Rob Babcock]

This can be a contentious and controversial subject.  I want to commend all concerned for keeping it civil so far. Let's all continue to do so.

[Ethan Winer]

> How effective would it be to build say, a 6lb OC diffusor for first order treatment? <

What is "a 6lb OC diffusor?"

--Ethan

[bgewaudio]

Six pound Owens Corning ridgid fibre, is what I should have said...........Sorry!

 

[bpape]

I'm not sure that would work.  The only time you get much 'bounce' off of the denser material is when you are dealing with extremely shallow angles of incidence.  At all the other angles, it's still going to absorb.

Now, if you want to build a series of very narrow absorbers and frame them, then put them on the wall in an array with the same distance between frames as is inside the frame, you'll get some diffusive effect (not perfect polar plots but certainly better than a flat wall) and use the same amount of absorbtion over a much larger section of wall.  I do this all the time in rooms where people want to mix music and HT listening.

Bryan

[bgewaudio]

So you say build an absorber with protruding frames around them?

So then you would have an absorptive, diffusive, absorptive etc. type pattern?

Also sounds like a good idea! I can see the logic in that.

[bpape]

Actually, it's kind of simulating a well diffusor - but you only have one depth.  However, every other well has absorbtion in it.  It's still different physical pieces but hung as an array, it functions somewhat like a diffusor.  I generally spec inside dimensions of 6"x48" and use 1x4 or 1/2x4 for the framing.  That yields a 3.5" deep well, 6" of absorbtion, and 6" of spacing between frames.  That lets you cover approximately 3.5' of wall surface horizontally with no more absorbtion than a 2'x4' panel would - plus, you get a nice 1.5" air space behind the 2" absorber.

It yields a nice mix of absorbtion and dispersion.  Pretty much anything that doesn't get absorbed gets sent back on a different axis.  Also, even some of the stuff that does get somewhat absorbed get's sent back in multiple directions.

Also, when you have multiple edges like that, there is an effect that increases low frequency absorbtion over a single 2'x4' pc hung 1.5" off the wall.

Bryan

[bgewaudio]

Thanks a lot Brian, I see exactly what you mean, I was thinking of building an abfuser that consisted of staggered wood in behind and then facing the wood with 703 or similar to absorb, then when it hits the irregular surface in behind, anything that comes back out into the room will be on a different axis.  However, I feel if I went this root, then I would then be once again faced with the attenuated response in the top end (which I don't want).

Just to be perfectly clear, I wish to have a room that is more live than dead.  If my memory serves me correctly, I recall a quote that someone had made earlier in this discusson saying "I think some people actually prefer a rolled off response in the top end".  Having read this I would think the reason for this is people that prefer a warmer sound would desire this attenuated top end response as opposed to someone like me who prefers clarity and openness would not.

Regarding your recommendation I think it is also an excellent idea, still kinda on the fence about this one, there are so may possible solutions, the hard part is just trying to figure out what is the best one to suit your decor and your tastes.

Thanks Brian

 

[8thnerve]

Listen to any small room with an adequate amount of Eighth Nerve treatments and you'll change your mind. 

Hey, maybe I would if you'd ever check your PMs!   Any of those summer deals left?

Sorry Rob.  This new Audio Circle software doesn't pop up a box when I have new messages and so I never notice.  It's also much harder to see what is new and what I've already read.  The sale page indicates to email me and that is certainly the best way to go.  I don't get PMs nearly as often.  If you are having trouble reaching me via email (meaning you've sent me two emails already, and the second one said, "Hey Nathan, where are you?") then shoot me a PM (and include your email address so I can compare) to make sure your mail isn't getting lost, but that shouldn't happen, I go through ALL my spam mail to make sure I don't miss anything.

At any rate, you have a PM. 

Best,

Nathan Loyer
Eighth Nerve