Early reflection panel thickness... - AVS Forum | Home Theater Discussions And Reviews
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post #1 of 70 Old 10-27-2011, 10:51 PM - Thread Starter
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So, my OCD kicks in and I end up doing SO much research on every aspect of my theater, it is overwhelming, to the point of finding contradicting evidence. What I have researched the most is how to treat early reflections and bass peaks/nulls. So much that I have found some ppl say 1" thick is plenty to calm down those reflections, others say 2" is a minimum and 4" is better, on top of that, space it away from the wall, yadda yadda.

Now, I am almost ready for drywall and have a few ideas on how to treat my early reflections. My first plan was to cover all walls with fabric panels, something like this... (IE wouldn't show the lines in between each panel correctly, try to picture 12 panels there)

====== ====== ====== ======

====== ====== ====== ======

====== ====== ====== ======

====== ====== ====== ======

Each wall will be covered with 1" fabric panels, and I will have OC703 or equivelent behind the "problem" areas (found using the mirror trick). This way, you won't be able to see the reflection panels. Now, since my room is small to begin with, I can't afford to loose too much more, if any, space.

I would like your opinion on whether or not 1" thick absorbtion will be "good enough" for the early reflections or if 2" or larger is DEFINITELY the recommended. If so, I will have to do some funky in-wall panels, but trying to keep this simple. I will also be treating the rear corners with superchunk style bass traps. I know the thicker the panel or more space behind the panel, the lower the frequency it will take care of, but, I just want to make sure I'm getting good imaging and front sound field without going completely overboard.

Also, I'm not trying to make the room a perfect sounding room, that would drive me CRAZY trying to figure that out. Instead, just the main problem areas (early reflection points and corner bass).

Thanks
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post #2 of 70 Old 10-28-2011, 05:43 AM
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why would you essentially 'EQ' the response by removing HF content of the specular region w/ thin absorption and allowing mid-lower specular content to persist? after all, the HF content has very little energy to begin with and needs little attention - it's the lower-mid specular region (extending down to ~300hz depending on room geometry) that contains the bulk of the energy content.

4" OC703 (or 4pcf mineral wool) w/ 2-4" air-gap.

porous insulation such as OC703 or mineral wool is a velocity-based absorber that converts kinetic energy into heat. by spacing the insulation away from the boundary, you are moving the insulation to places of high particle velocity for the lower (longer) wavelengths. 4" OC703 w/ 2" air-gap will perform much like that of 6" OC703 flat on the boundary. utilizing an air-gap is about as close as you'll get to getting a "free lunch".

remember, you're looking to attenuate the entire reflection with respect to gain such that the reflection is below the human detection threshold and not processed by the brain (~ -20dB).

most people recommending 1"/thin absorption are just the blind leading the blind and herd-mentality.

the reflection's gain (and arrival time, with respect to the original signal) can be measured with the Envelope Time Curve (ETC) response - part of the free measuring software suite: Room EQ Wizard.

the ETC will detail you how all specular energy impedes the listening position - gain with respect to time.

from there, you can identify what boundary is responsible for the reflection, and place absorption to attenuate. the ETC will then verify how effective your broadband absorber is at attenuating the reflection - such that it is below the human detection threshold.

you only want to apply broadband absorption to eliminate this 'early' specular energy. if you have a larger listening position (eg, a row of seats), then clearly the broadband absorber will need to be physically large enough to attenuate the reflection across the entire listening positions. but besides that, you do not want to place broadband absorption elsewhere in the room unless it is at an early reflection point. otherwise, you can quickly create a dead room *unless that is your goal..


the "mirror-trick" is not a trick at all. and it doesn't really tell you anything. the mirror trick does not tell you:
  • what the gain of the specular reflection is (if it is not above the human detection threshold, then you may not even need to treat that reflection point!
  • whether your broadband absorber is effective enough to attenuate the early reflection to your criteria (eg, -20dB from original signal) - and thus whether you need to increase thickness of insulation or provide larger air-gap
  • whether any early specular reflection is impeding the listening position from other 'not so obvious' boundaries (coffee table, edge diffraction from objects within the room, 2nd order reflections, etc)
  • whether your broadband absorber is effective enough to attenuate the early reflection for the entire listening positions.

due to herd-mentality, the "mirror-trick" has somehow become the de-facto standard in identifying first-order reflection points, but it details you nothing with respect to the energy content or arrival time of the reflection - which is exactly the criteria you need in order to know whether it needs to be treated.

also, if you're using the mirror trick to quickly identify first-reflection points on large boundaries, many incorrectly tell you to look for the 'tweeter' in the mirror... the tweeter radiates HF/UHF content - of which as little energy to begin with. the bulk of the energy content is in the lower-mid specular region (extending down to ~300 depending on room dimensions) and as such, you need to look for the 'acoustic center' of the speaker in the mirror - and your broadband absorber needs to be effective enough to attenuate the energy within these longer (lower) wavelengths.

ideally, you should be using the ETC response to truly measure, identify, and re-measure once treatment has been installed to verify the treatment is successfully at fully attenuating the early reflection. the "mirror-trick" is guesswork, and details you nothing with regards to the signal. don't let herd-mentality and blind leading the blind restrict you from doing things properly.
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post #3 of 70 Old 10-28-2011, 03:50 PM - Thread Starter
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Thanks for the info, although way to overwhelming and something I won't be able to accomplish anyway. Like I said, I have a small room with not nearly enough real estate for 4" panels with a 2"-4" gap behind, just not going to happen. So, with that mentioned, I guess I have no way of controlling down to 300hz like you recommend. And, if it takes that much space to control down to the recommended frequency, you're saying that 1"-2" panels directly on the wall will do almost nothing? Even though I've read that 2" panels by themselves will control somewhere down to around 500hz, if I'm not mistaken.

Maybe I can add a second question, has anyone gone from no wall treatments at all to 1"-2" panels at the 1st reflection points and noticed a difference?
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post #4 of 70 Old 10-28-2011, 04:12 PM
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Quote:
Originally Posted by Digital_Chris View Post

you're saying that 1"-2" panels directly on the wall will do almost nothing?

no -

Quote:
Originally Posted by Digital_Chris View Post

Maybe I can add a second question, has anyone gone from no wall treatments at all to 1"-2" panels at the 1st reflection points and noticed a difference?

do as thick as you can (or with largest air-gap) - just understand the constraints.
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post #5 of 70 Old 11-21-2011, 02:33 AM - Thread Starter
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Hey Local, I know you have your hands tied up in another thread right now but if I could grab your attention for another question, that would be great.

I haven't looked into REW a lot yet, but I absolutely will. When I do measurements with the ETC part of REW, how do I know where to start placing absorbtion, if any, around the room? I'm guessing REW will tell me what frequencies are problems but not where they're coming from. How do I go about finding out where?

Thanks Local.
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post #6 of 70 Old 11-21-2011, 06:50 AM
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I think REQ has a functions where you can find distance and there is another thread about string trick....

You gonna have to get busy!
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post #7 of 70 Old 11-21-2011, 07:22 AM
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Quote:
Originally Posted by Digital_Chris View Post

When I do measurements with the ETC part of REW, how do I know where to start placing absorbtion, if any, around the room? I'm guessing REW will tell me what frequencies are problems but not where they're coming from. How do I go about finding out where?

Thanks Local.

the ETC (measured one speaker at a time) will display the arrival of specular energy and the time-delta difference with respect to the original signal.

since the speed of sound will be a constant in the medium in your room, then the time difference of the arriving energy can be used to determine the total distance/flight path that the energy took (speaker -> reflection off boundary -> listening position).

Quote:
Originally Posted by sac View Post

** there are a number of ways to identify the paths, ranging from very basic to more sophisticated. With each increment, the amount of physical work is diminished as well as the time required. With a bit of practice, pattern recognition, ans in some cases an investment in a more sophisticated measurement platform, the rate and precision can increase (until you are dealing with a 3D polar ETC where you place your cursor on each point displayed on the display graph and the 3space X,Y,&Z coordinates are displayed allowing you to simply replace the measurement mic with a laser pointer and aligning the laser at the generated coordinates and having the laser point precisely to the optimal incident point on the boundary.)

Until then, here are two easy techniques that are both very precise and utilize the time and distance information provided by the ETC itself.

1.) - the most basic mechanical method is the string method.

Quite frankly, I don't expect anyone to do this more than a handful of times until you can more easily visualize the process, but it illustrates the concept very simply and effectively - if a bit awkwardly!

From the total time of travel, you calculate the distance of travel (TOF X 1.13'/ms or .344m/ms). You might want to leave a few inches on each end to hang on to, but mark the precise endpoints corresponding to the distance. Find a few friends who you will not mind being a bit worn out by the process and have then each hold one end of the string, one end with the point marked on the string precisely located where the measurement mic capsule would be (don't move the mic!!!) and the other end placed in the center of the source speaker. Forget the tweeter stuff, as the ETC measures the total energy, and there is much more energy content in the low-mids and mods than in the tweeter. Besides, if you want to determine the actual acoustic origin of the speaker, the ETC can be used to identify this as well! But that's another exercise for some other time!. Now, with the endpoints firmly located, at one point in the body of the string, extend the loop body out and see what boundaries/surfaces you can tangentially touch with the string being stretched taut' The point you can touch, is the point of incidence indicated by that particular spike in the ETC. Note the incident spot on the boundary. Repeat for the other energy returns.


Easy, but I suspect you will quickly tire of this - as will your friends who will want to be doing something a bit more exciting!


2.) - Alternative Methods:.

Although it's often easy to hear where a reflection is coming from by playing a pulsed signal and cupping your hand to your ears, sometimes it's necessary to employ measurement systems to do it. Looking at an ETC or a log-squared impulse response, we can see a reflection as a spike that sticks out above the reverberation decay.

Place the cursor on the spike and note the arrival time, or set that time as a reference for difference measurements. Now move the mic a small distance, say 6 inches, in the direction you think the reflection is coming from. Take another measurement. If the reflection is earlier in time, you've moved towards it. If it is later in time, you've moved away from it. If it didn't change much, you've moved sideways across it.

Try another direction and see which way the reflection moves. Also try moving the mic up or down. A lateral reflection from a side wall won't change much, but a ceiling reflection will change a lot. Also be sure you are looking at absolute time units, not time relative to the direct arrival. The direct arrival will be moving around as well.

[i]Another way to determine arrival direction is to block the microphone's view in a certain direction with a sound absorbent barrier, such as a sheet of Sonex. When you block the offending reflection its spike will go away (See blocking technique illustration below.)

*** The acoustical model is the acoustical response of the room. Examples are the Non-Environment room, and the LEDE room. These are defined most completely in terms of their ETC response. If one can interpret the ETC in terms of arrival time, gain, and the spatial temporal energy density indicated by the spacing and gain of the energy arrivals, it is easy to use the ETC as a template for the creation/recreation of the room model.

Quote:
Originally Posted by hannes View Post

As you will probably have noticed the ETC measurement shows you how much reflections you have, how strong they are and which time they take to arrive at the listening position. What you want is to have the first 20 ms free for your ear to digest the direct sound which means that all reflections within that time should be lower than - 20 dB compared to the direct signal.

In order to do that:
  1. You set the zero of the timeline to the direct signal (REW does that for you ususally)
  2. You pick the strongest ETC peak that you want to defeat
  3. You place the cursor on it and read the according time. Example: 5.34 ms
  4. Now that you know the time gap between the direct signal and the reflection you calculate the room distance:
    Metric: 0.344 m x time
    American: 1.13 ft. x time
    Result of this example: 1.83 m or 6.03 ft. This is the distance that the reflected sound travels longer than the direct signal.
  5. Now you add your speaker-listening position distance. For example 1.12 m + 1.83 m = 2.95 m or 9.68 ft. Now you know how long the sound travels from your speaker to the reflection point and then to the listening position.
  6. Prepare a string with knots each foot. Do this halfway properly. I use bricklayers cord that is used for lots since it does not tangle easily. I use 7 m (21 ft.).
  7. Fix one end near the tip of your microphone, determine the distance (2.95 m or 9.68 ft.) on the cord and fix that part in front of the big driver of your speaker with a slip knot you can resolve later (see attached photo).
  8. Now try to find the spot that causes your reflections by stretching the cord to a triangle between speaker, reflection point and microphone. Good candidates are bare walls and edges, however even edges of absorbers can reflect.
Treat the spot or/and move your existing treatment there, repeat the measurement in order to make sure it helped. Make sure you are looking at the same spot in the ETC by moving your cursor there (often there pops up another spot but at a different time, so be exact there).

Repeat with the next spike.

I learned this string method from SAC but instead of searching thousand posts I described it here.

http://www.gearslutz.com/board/6133764-post8.html

http://www.gearslutz.com/board/6397239-post80.html
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post #8 of 70 Old 11-21-2011, 09:49 PM - Thread Starter
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Thank you for the links Local, I appreciate it.

It makes pretty good sense to me, I will have to play around with REW / ETC a bit more to see it more clearly but I get the idea behind it.

Thanks again
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post #9 of 70 Old 03-01-2012, 09:00 PM - Thread Starter
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I know it's an older thread but I'm about to finish my acoustic panels and was curious about something that I can't recall with the search feature...

Will 1" fiberglass panels spaced 1" away from the wall absorb roughly the same frequencies as 2" panels placed directly on the wall?

I ask because it would be wise for the pocket to buy 1" panels and space them away vs 2" panels directly on the wall. Then again, I might be able to give up even ANOTHER inch and do 2" spaced 1" away. Either way, I'm curious
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post #10 of 70 Old 03-03-2012, 02:22 PM
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Placing a plastic membrane (3 mil) between 2 layers of 1" OC703 will help absorb some lower frequencies than just a 2" thick piece alone. The plastic acts as a membrane and turns the rear piece of OC into a pressure based absorber. Where the OC703, by itself, is just a velocity absorber. Some are putting this combination on their front walls. It has been recommended and applied to front screen walls and for duct runs into the room (by DE). I have not seen data on how much it helps, but with your limitations it is something you can do without much additional cost.

Search for "plastic", "mil" etc to find people's applications.

One option for the plastic film is a self adherent plastic film used to protect carpet during construction. Similar to what car dealers put in the foot wells of cars. I found it in the carpeting department at Menards (regional building supply store). http://www.plasticover.com/
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post #11 of 70 Old 03-04-2012, 11:47 PM - Thread Starter
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Thanks for the interesting bit of info, if that's the case, I will be spending more money up front but if it will dig deeper in sound absorbtion, I think it will be worth it. Have you done this type of reflection panel? I don't think I've read about anyone doing something like this, not here anyway.

If anyone that has done this can chime in, it will be nice to hear about someone having good results
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post #12 of 70 Old 03-05-2012, 05:30 PM
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post #13 of 70 Old 03-06-2012, 01:21 AM - Thread Starter
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Thanks and sorry, should have searched harder :-P

I'll see what I can do but still worried because I'm limited in space that I can give up :-/

This stuff is too overwhelming, worrying myself sick again that my room is going to sound like trash because I can't properly treat the room :-(
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post #14 of 70 Old 03-06-2012, 09:38 AM
 
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Don't worry about it. Use the room without any treatment to get a baseline. Put a folded up blanket against the wall (held in place by a 2x4 or something) at the first reflection points found by the mirror trick. Listen to the room again, being critical about the sound. If it sounds better, put absoption there. If it sounds worse, do not put it there. It there is no change, putting it there or not is up to you based on how it looks.

It does not have to get full on complicated...you are not building a recording studio.
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post #15 of 70 Old 03-06-2012, 10:39 AM
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In my last HT, I had excellent result with a combination of 2'x4'x2" RPG absorber panels, and RPG BAD diffuser panels. Generally, the diffusers were on the sides opposite the seats, and absorbers were used on front, back and remainder of the side walls. Also, the ceiling had a makeshift diffuser using slanted 2x6 boards. Also bass traps in all 4 corners and one along the side wall.

This was a small difficult room ( about 12' x 19' x 7'), and the acoustic design was by Rives Audio.
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post #16 of 70 Old 03-06-2012, 08:57 PM
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One more

BAD panels will not absorb more broadband, they will add diffusion and not suck so mush of the HF out of the response. In essence it could help to give a more even absorption across a broader band at the expense of no as much total absorption.

Depending on your speaker's polar response, you may or may no want to add diffusion.
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post #17 of 70 Old 03-15-2012, 11:59 PM - Thread Starter
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Thanks for the info guys. I do have to ask another question, well, similar question just asked differently.

In 2003, when the "Acoustical Treatments Master Thread" was started, I noticed a few of the big wigs (Dennis, bpape, etc) mentioning 1" insulsheild/fiberglass for first reflections and front wall absorbtion to the OP and no one was really recommending the recent treatments (e.g. 4" with 4" gap).

I understand that we now have the technology to do more testing and what not so the data is there to prove why the thicker stuff is better on paper, but to the partially untrained ear, what frequencies are less important/more important to absorb with respect to first reflections? If they recommended 1" for first reflections, did they not see the need for the thicker absorbtion because we really couldn't/wouldn't hear the difference between that and the 1" fiberglass?

As for my front wall, no treatment has been placed yet because I have read many do's and don't's on that subject but I do have space for 1" insulshield for not the complete wall, but for behind the screen/around the LCR speakers only. Is this something I should think about?

Speakers are also flush mounted into the front wall so there is no "space" between them and the wall.

Also, is it room size that plays a bigger part in the need for thicker acoustical treatment panels (eg smaller room requires thicker panels)?

Thanks
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post #18 of 70 Old 03-16-2012, 06:15 AM
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Quote:
Originally Posted by Digital_Chris View Post

and what not so the data is there to prove why the thicker stuff is better on paper,

porous insulation is a velocity-based absorber. to be effective, it needs to be placed (spaced away from rigid boundary) into areas of high particle velocity for a given frequency/wavelength. velocity is zero directly at the boundary. as frequency decreases (wavelength size increases), this means that the porous insulation must be spaced further and further away from the boundary for maximum effectiveness at attenuation. this is accomplished by utilizing a thicker panel and combination of spacing said panel away from the boundary (by utilizing an air-gap).


Quote:
Originally Posted by Digital_Chris View Post

but to the partially untrained ear, what frequencies are less important/more important to absorb with respect to first reflections?

if you are going to attenuate (absorb) a specular reflection, then you need to attenuate the ENTIRE broadband reflection. if you do not have sufficient treatment to fully attenuate the broadband indirect signal, then you are filtering/EQ'ing the reflection - which colors the sound. is this a design goal for you?

think of it this way:
for typical speakers, the mid-HF range of the specular region is more directional, thus there will be less "off-axis" energy dispersed to the sidewalls where your treatment is.

as frequency decreases, the wavelengths become longer and also contain MORE energy content - AND they will become LESS directional. thus, you have longer wavelengths with more energy content and more of this energy being dispersed off-axis to the sidewalls where your treatment is.

with this information, what do you think the priority is with respect to the side-wall absorbers? thin absorption to attenuate mid-HF specular reflections, or sufficiently thick porous absorption to attenuate the lowest frequencies of the specular region with the most energy content and the longest wavelengths?


Toole: the AES paper: Loudspeakers and rooms for sound reproduction" section 7.1.2 Attenuating, Reflecting, and Scattering Indirect Sounds
JAES vol.54No6 June 2006


Quote:
Originally Posted by Toole View Post

Although reflections appear not to be great problems, it
is reasonable to think that there must be a level above
which the good attributes are diminished and negative attributes
grow. Obviously an empty room is not a comfortable
listening environment, even for conversation. The
furnishings and paraphernalia of life tend to bring normal
living spaces into familiar acoustical territory. Custom listening
spaces need to be treated. In all rooms absorption,
scattering or diffusion, and reflection occur, and devices to
encourage each are commonly used by acousticians.

It appears that much of what we perceive in terms of
sound quality can be predicted by the anechoic characterization
of loudspeakers. Because most of these data pertain
to sounds that reach listeners by indirect paths, it is proper
to suggest that nothing in those indirect sound paths
should alter the spectral balance.
For example, a 1-inch
(25.4-mm) layer of fiberglass board at the point of a strong
first reflection is effective at removing sound energy
above about 1 kHz. From the perspective of the loudspeaker,
the off-axis response of the tweeter has just been
greatly attenuated—it will sound duller and less good.
Obviously if the purpose of the absorbing material is to
attenuate the reflection, the material should be equally effective at all frequencies.


Given the duplex nature of
sound fields in small rooms, it seems reasonable to expect
similar performance at all frequencies above the transition
region.
In their examination of the audibility of reflections, Olive
and Toole looked at detection thresholds as high frequencies
were progressively eliminated from the reflected
sounds, as they might be by frequency-selective absorbers.
They found that only small to moderate threshold elevations
occurred for low-pass filter cutoff frequencies down
to about 500 Hz, where the investigation ended. Removing
the high frequencies alone is not sufficient to prevent audible
effects [32].

Finally there are the indications that the precedence effect
is maximally effective when the spectra of the direct
and reflected sounds are similar [4], [18], [20]. If the spectrum
of a reflection is different from that of the direct
sound, the probability that it will be heard as a separate
spatial event is increased—not a good thing.

Quote:
Originally Posted by Toole View Post

Thus from the perspectives of maintaining the excellence
in sound quality of good loudspeakers, rendering an
unwanted reflection inaudible, and preserving the effectiveness
of the precedence effect, there are reasons not to
alter the spectrum of reflected sounds. One is free to redirect
them with reflectors or diffusers, or to absorb them
with lossy acoustical devices, but in each case, the process
should not alter the spectrum of the sound above some
frequency toward the lower side of the transition region in
a small room.
It seems reasonable to propose, therefore,
that all acoustical devices used in listening rooms—
reflectors, diffusers, and absorbers—should be uniformly
effective above about 200 Hz. For resistive absorbers this
means thicknesses of 3 inches (76 mm) or more.

...and the 4" absorber with a 4" air-gap is based on updated gas-flow-resistivity figures calculated using the Delaney, Bazely, and Miki models (and subsequent modifications, as they satisfy all of the various mods to it) - based on the ACTUAL values for the recommended fiberglass and rockwool porous materials.
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post #19 of 70 Old 03-16-2012, 06:54 AM
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This all needs to be couched within the context of what actually needs to be accomplished in the room within the context of what is in the room already ... from drywall, to carpet, to seats and salt water bags (carbon units). Being glib and simply putting some 200Hz to 20kHz uniform absorption panels can unbalance the spectral content because other elements are not being considered (for example, what the gypsum board behind the fuzz is doing). There is no uniform, standard, works everytime approach for every room/speaker combination.

If this were that simple there would not be such a variety of approaches gleaned from both theoretical work as well as been there, done that experience.

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post #20 of 70 Old 03-16-2012, 08:10 AM
 
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Originally Posted by Dennis Erskine View Post

This all needs to be couched within the context of what actually needs to be accomplished in the room within the context of what is in the room already ... from drywall, to carpet, to seats and salt water bags (carbon units). Being glib and simply putting some 200Hz to 20kHz uniform absorption panels can unbalance the spectral content because other elements are not being considered (for example, what the gypsum board behind the fuzz is doing). There is no uniform, standard, works everytime approach for every room/speaker combination.

If this were that simple there would not be such a variety of approaches gleaned from both theoretical work as well as been there, done that experience.

The presence of absorptive seating or people has NOTHING to do with the modification and resulting coloration of the direct sound as a result of its superposition with indirect signals whose spectral content has been modified by treatment that results in the effective equalization of sidewall/lateral reflections.

And what the acoustical impedance of a surface is behind the treatment is moot if the incident energy is effectively absorbed, or in the case of redirection or diffusion, reflected.

You see, that is the purpose of treatment - to modify the behavior of the 'native' surface.

And pray tell, what is the effective result of your 'carpet' aside from aesthetics, dirt collection and high frequency absorption?

The real irony: Toole's observations are based upon the practical and empirical testing and validation based upon not only "experience", but upon the applied science of "acoustics".

In ALL cases, the incomplete equalization of lateral/sidewall reflections (as well as from any other surface - most of which Toole recommends absorbing) results in the coloration of the direct sound - thus changing the nature of the very thing one seeks to preserve.

Perhaps the smarter response would be to suggest that various different designs of treatment may be available that may effectively achieve equivalent results while overcoming a limiting aspect, such as thickness, based upon more sophisticated design - provided one is fully aware of the full array of actual behavioral ramifications.

But on the other hand, actual acoustical science aside, you can do whatever you want, provided you accept the role of your room as a special FX generator and jettison the notion of it reenforcing the accurate reproduction of the source.
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post #21 of 70 Old 03-17-2012, 06:52 AM
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Anytime direct sound reflects off of anything and finds the ears, you have indirect sound. That could be a dirt collector carpet, a salt water bag, or engineered fuzz on a wall. Acoustical impedance of the materials behind a treatment is not a moot point by any stretch of the imagination. The treatment materials are not 100% effective, their effect varies by incidence angle and by frequency. The entire environment must be considered (salt water bags are a big consideration in auditoria) for proper sound quality management.

We're all in the FX business ... from the manufacturer of the microphones, the lad at the mix console, the engineer designing the decoder (pre-pro), the designer of the speaker(s) ... everybody. Herbie Hancock life at Wolf Trap is different than Herbie Hancock in the Cobb Energy Center, Mechanics Hall, Meyerson Hall ... because they are all FX machines...as is the furniture, walls, pictures in Dr. Toole's living room.

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post #22 of 70 Old 03-18-2012, 04:44 PM
 
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Same goes with wireless coverage...if you are going to use wireless HDMI (for example), you have to remember that the ugly bags of mostly water (to steal the description of us from Star Trek The Next Generation), alter it considerably.
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post #23 of 70 Old 03-18-2012, 10:22 PM - Thread Starter
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If one needs ATLEAST 4" panels on the wall to effectivly absorb all unwanted sidewall reflections, it seems that 9 out of 10 rooms on this forum are "special FX generators", and I'm guessing that most of those rooms still sound fabulous (i.e. no one complaining about it sounding bad or "off")

Now, I don't understand some of the terminology in Toole's statements, but I did get the part that if any alteration of the reflective sound is made, it will inflict with the direct sound and make it sound unnatural or "less good". I get that, but if I don't want these reflections to hit my ear unaltered, or at all, my only options are either vury thick absorbtion panels or diffusors? Again, I got it, but why haven't more people heard this outcome in their room if either of the previous methods were not used? That is why I asked if it is heard as easy as seen on paper. Will 1" panels on the wall do more bad (I use the term bad lightly because I am referring to how ppl react to the altered sound, not to the data on paper) than no panels on the wall? I want to beileve not by the way many people have done thin absoprbtion on their walls and have actually liked the changes.

Or, does this go back to the whole BOSE debate... people like them because they think they sound good (when actually, it's just a different sound, an unnatural one at that). But then AGAIN, if it sounds good to our ears (smooth, linear, non distored), who cares, right? How do I know that the sound is "off" anyway, I don't know how the sound mixer intended for "that sound" to sound. So, in the end, after thinking about this while I write this response, I don't think I'll ever know nor will I ever care what "it" (movie, music, etc) is supposed to truely sound like unless I have the perfect room, and I don't, so I shouldn't stress over it. I just think that by filtering or EQ'ing some of the highs out, the harshest frequencies to my ear, will greatly help with my enjoyment of the soundtrack which is, by my guess, why a lot of ppl have had success with thin absorbtion

And to answer your question local, I guess it is my objective to color the sound because I have no other choice, I believe it will sound better than hearing the echos.

Another thing I just thought of, since lower frequencies have more energy along with a wider spread, that means it will take even MORE super thick absorbtion panels to treat the whole room/seating area and then that can lead to making the room sound too dead. Thougts?
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post #24 of 70 Old 03-19-2012, 04:21 AM
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Anytime sound reflects off anything, it is changed. The issues are: (1) is it audible?, (2) is it adverse to your listening enjoyment; (3) is it making a bad problem less bad or more bad.

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post #25 of 70 Old 03-19-2012, 08:22 AM
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Digital Chris

I've been on a multi-month long experiment, attempting to quantify those issues. It began with a disagreement with Dennis on early reflected energy, particularly early, first reflected sidewall energy, that returns back to the LP.

I cannot disagree with the findings of the acoustic gurus of the 70's, 80's, 90's etc, determining what's acceptable wrt the current design and acoustical sciences of the time. I can't disagree with the work of industry educators like Davis and Patronis, illustrating the hows and whys of acoustic design. What I could do was experiment in my room, because ultimately, that's what mattered most to me.

I'm still experimenting in my room, and I'm getting ready to rotate the entire seating/LCR orientation, so I can continue to experiment with a wide, yet short front to back environment. I don't plan on uncovering any groundbreaking science, however I just want to experience these issues and environments for myself. What is a common theme to my findings to date is, similar to Toole's statements, it amazing how adaptable we are to a variety of known problematic environments. I'm not referring to below the transition, those hurdles and solutions are quite well vetted. I'm referring to my subjective opinion on various approaches to the reflected energy off the boundaries at the speaker end of the room.

So Chris I believe I'm trying to become decently educated on the theoretical components of what to strive for and how to get there. However focusing on the subjective elements is made difficult because of how adaptable we seem to be. With repeated exposure to known issues, yes they're discernible. Assigning them a weighted amount of importance is very difficult.



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post #26 of 70 Old 03-19-2012, 07:32 PM
 
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Quote:
Originally Posted by localhost127 View Post

velocity is zero directly at the boundary.

While this is true, it is irrelevant. Sure, the velocity for a specific direction becomes zero, but the sound has not stopped...it simply has a new vector and therefor has a new velocity.

From what I understand, the sound hits the surface and reflects off in its new direction, losing energy in the process. This energy is seen in a reduced amplitude of the wave, not in a reduced frequency of the wave (though that can happen based on the material struck). Depending on how porous the surface is, it may lose a very small amount of energy or a lot (more porous means more energy lost). Depending on how smooth the surface is, it may scatter the sound less or more (smoother means less).

The sound never actually stops moving. Yes, its current velocity changes due to the vector changes, but it is instantaneously replaced with a new velocity on a different vector.

Why would an air gap of a few inches matter for something whose speed is measures at over 1000 feet per second in air?
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post #27 of 70 Old 03-19-2012, 09:01 PM
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Sound in air does not travel as longitudinal waves but as alternating zones of compression and rarefaction. Particle velocity does indeed become zero at a reflective boundary and pressure reaches its maxima. Stop thinking of sound as equivalent to light waves.

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post #28 of 70 Old 03-19-2012, 09:36 PM
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Personally I think the job of the early reflection treatment is to properly mate the speaker manufacturers intended response with the response at the listening positions.

What is certain is that a lot of speakers of the conventional forward firing cone dome type vary quite significantly in their off axis response from their on axis response and do so in an uneven way, with an off axis dip as the midrange starts to beam which fills in as the tweeter comes in. Depending on the severity and nature of this trend, which will vary from speaker to speaker, a suitable treatment scheme can be designed...this may be fully or partially absorptive over the range above 250Hz. I agree with Dennis that's it is a form of EQ and nothing wrong with that if intelligently applied.

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post #29 of 70 Old 03-19-2012, 11:38 PM
 
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The irony to all of this is that as the power response of most speakers is Not uniform, the predominate specular energy that is incident in the early reflections nearest the speaker (as opposed to toed in speakers on the opposite side which WILL also tend to contribute a greater amount of mid and high frequency energy incident in the same region), will be LOW and LOW-MID frequency energy due to its more cardioid dispersion!

The significance of this is that there will be a much greater need for the treatment to address the low-mid frequencies! Which, if you are using simple porous absorbers leads to the requirement that they be thicker and spaced from the walls.

Bottomline: The polar dispersion of most speakers which results in greater amounts of low and low-mid energy being incident at the early arriving first order reflection point of incidence presents a condition where the actual energy that required mitigation is comprised not of an equally distributed spectra, but one that is 'bass heavy', and thus INCREASES the need, rather than reduces the need, for such absorbent panels (or diffusers where they are used) to address the lowest specular frequencies!

Thus the treatment capabilities need to be more capable of being broadband and being effective down to the lowest extents of the specular energy.
Thus, if we are dealing with simple porous absorption, that means larger and thicker panels exhibiting an equally thick gap.

And Toole has explicitly stated that this requires panels minimally 3" thick with a 3" gap (which reach to ~600 Hz); and measurements and more recent modelling tools based on the latest GFR porous modelling algorithms indicate 4" thick with a 4 " gap of 3lb/ft^3 Fiberglass or ~4 lb/ft^3 mineral wool to be effective to 300 Hz.

And if that is not acceptable, then you can ether pursue alternative types of absorbers that are thinner with the same broadband capabilities, or you settle for 'moving the problem around' and settling for change rather than the mitigation of the destructive behavior.
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post #30 of 70 Old 03-20-2012, 02:51 AM - Thread Starter
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Just to requote myself here...

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Originally Posted by Digital_Chris View Post

I'm not trying to make the room a perfect sounding room, that would drive me CRAZY trying to figure that out.

...well, it has.

And to be honest, yes, all of this technical mumjo jumbo IS incredibly informing, don't get me wrong, but it is also driving me crazy because I just won't be able to apply the recommended absorbtion, which has been pulled forth only because of theory and others (Toole, etc) testing, it seems. How come I haven't seen anybody post for me that they've tried the thin panels, then went to thicker panels and were,for a lack of a better phrase, blown away? Atleast, that's what I expected, instead of test data only from the extreme audiophile. To me, it's almost like saying, "The new Honda Civic get's 2mpg more than last years model" Will we honestly notice it, I doubt it, but the test data is there to prove it. Bad anology, I know, but it serves the point.

Please, to all of you (Local, FOH, dragon), I'm not getting aggrivated at the awesome knowledge that you guys are providing me with, but instead, I feel like you are missing the point that I have a small room and can't give up the real estate for thicker panels, and instead of trying to find alternatives or give me other recommendations, all I get are facts and statements that, in lamens terms, sound to me like "you must do it this way or else your room will sound terrible" or "the only way to make your room sound good is to follow this procedure for treatment".

Sigh.. I'm sorry

Maybe I can postpone this thread for now untill I do some measuring and see where and what my problems are exactly, maybe that will help you guys get a better idea on some alternatives for me
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