AVS Forum banner
Status
Not open for further replies.
1 - 14 of 14 Posts

·
Registered
Joined
·
6,697 Posts
Discussion Starter · #1 ·
I just starting reading about room acoustics and treatments. The recommendations of Acoustisoft, the designer of EFT5 , the popular acoustic measurement software program, got my attention. They stated that imaging could be improved by using an absorber to deal with early reflections. They were quite specific on the need for a very thick 6†absorber. I was wondering whether some of the forum experts might share their opinions on this subject. Is it really necessary to have such a thick absorber when placing acoustic foam on the ceiling?


Specifically I’ve been considering Sonex Classic Urethane panels of various thicknesses, 2â€, 3†and 4†on portions of my ceiling.


Here’s three excerpts extracted from Acoustisoft's tutorials. In fairness to them, since I extracted the statements from different portions of their material, it is possible that I may have unwittingly taken them out of context, although I don’t think that’s the case.

Quote:
Accurate multichannel sound depends on the direction and timing of sound coming from the loudspeakers. Interfering reflections will upset the stereo or multichannel auditory image that is presented by the system. For this reason it is advantageous to remove these harmful reflections. Excessive use of absorption may lead to a loss of ambience leading to a very "dry" sounding environment. For this reason, only specific reflections should be absorbed. Taking ETF measurements before and after absorber placement with trial and error will lead to the optimum placement with very little loss of natural room ambience.
Quote:
Absorbers should be very thick, in the order of 6 inches or greater. The absorber should be at least 8 times its thickness in width and height. Do not be fooled by advertisers peddling 2 inch thick absorbers as having great effectiveness at low frequencies. The methods use to derive this specification are in no way similar to this application. The lowest frequency of absorption is dependent apon absorber thickness rather than "exotic high end technologies". A 6 inch thick absorber can be very effective down to approximately 600 Hz - 800 Hz, a 3 inch thick absorber can be effective to frequencies as low as 1200 Hz - 1600 Hz.
Quote:
The use of absorbers will not greatly affect the perceived sound quality in a room, but they will improve imaging qualities, particularly when used on the ceiling. Human hearing did [not]evolve to hear sounds from above. We were never attacked from above, nor did we hunt above so hearing does not process sounds that come from above as well as it does from at our level & below.
NOTE: The bracketed “not†is my correction. I assumed its omission was in error. I have not corrected their spelling errors. ;)


Since the purpose of the absorbers in this application is to deal with imaging, and presumably higher frequencies, and not room modes, how low should we be going? How does one go about using the absorption coefficients of a given absorber to determine its effectiveness at a particular frequency?


Acoustisoft suggests using their software to measure the "before" and "after, but I'd obviously like to have a pretty good idea before I invest the time, money and effort.



Thanks.


Larry
 

·
Registered
Joined
·
7,656 Posts
First, let me say that I am not a professional but have done enough work in enough rooms to hear real world results in many different situations. I also worked for a short time in high school in a recording studio. So here is my 2 cents worth.


Sounds to me like they are recommending the brute force method of pure abosrption rather than trying to provide some absorption and some time domain spread and diffusion.


While I agree that 2" foam is NOT going to do much for bass frequencies, I think 6" is a bit of overkill. To me, a mix of different materials would be the best. In addition, if we are dealing with imaging here, let's be realistic. The lower the frequency, the less directional it is and therefore, the less it effects the perceived "image." How much of the "image" is really generated from 250 Hz up? Very little, and even less ambience.


In the 2 channel world, a Live End/Dead End arrangement with first reflection points damped on the two side walls and ceiling (2" is fine...) and the WHOLE front wall in 4" with some big chunks or helmholz absorbers in the corners works great. If you want to do a little more or you are doing surround, put another chunk of 4" in the center of the rear wall and a some more 2" at the first reflection points of the surrounds. Just be careful. You don't want the room to be TOO dead or "dry." Then you start losing lower level ambience information.


Talk to the "tunee's." They will tell you that they get tremendous balance and imaging with NO foam. They use controlled absorption through the use of devices designed to produce sympathetic vibrations which "absorb" the sound wave. Their rooms are basically all wooden surfaces if you do the whole thing.


Just like anything else, there are a ton of ways to tackle this. However, ALL OF THEM deal with points of first reflection and also deal with proper speaker position in relation to room boundaries and listener position.


I just shudder to think about the muffled mess for a video system with a top mounted center channel firing into a 6" blob of foam. While it might push the apparent sound source "down" to the screen by absorbing ceiling reflections, a thinner piece would do the same and not interfere with projector angles, screen heights, etc.


Just my opinion.
 

·
Registered
Joined
·
235 Posts
A lot depends on the speakers, their placement, and the room. For instance, planar drivers have little vertical dispersion, so generally don't benefit as much from ceiling treatments. Assuming you do indeed need ceiling treatment, 6" is overkill, IMO. Here's some data from Auralex:

http://www.auralex.com/acoustic_stud...ofoam_2w.html#
http://www.auralex.com/acoustic_stud...iofoam_4w.html


Note that the 2" is strong down to 500 Hz. If you really need it down to 250 Hz (which may be overkill) the 4" will handle that. Personally, I'd go with the 2", which you can get for $2.00/sq. ft. The author is correct that too much absorption will make the room too dry.


Seth
 

·
Registered
Joined
·
6,697 Posts
Discussion Starter · #4 ·
Thanks for the responses.
Quote:
Originally posted by bpape

While I agree that 2" foam is NOT going to do much for bass frequencies, I think 6" is a bit of overkill. To me, a mix of different materials would be the best. In addition, if we are dealing with imaging here, let's be realistic. The lower the frequency, the less directional it is and therefore, the less it effects the perceived "image." How much of the "image" is really generated from 250 Hz up? Very little, and even less ambience.
Hi bpape:


I assume you meant "How much of the "image" is really generated from 250 Hz down?"


I agree the 6†recommendation seems like overkill and that’s what prompted me to start this thread. 6 inches seems contrary to what is generally available in the marketplace. The only product I’ve seen so far that thick is the RPG Skyline foam which can get as deep as 7 inches, but isn’t a solid 7 inches thick.

Quote:
Originally posted by sbradley02

Note that the 2" is strong down to 500 Hz. If you really need it down to 250 Hz (which may be overkill) the 4" will handle that. Personally, I'd go with the 2", which you can get for $2.00/sq. ft. The author is correct that too much absorption will make the room too dry.
Hi Seth:


You make an excellent observation.


I don’t know if there’s a “scientific†way to calculate the effective frequency by using the absorption coefficients charts, but I can see what you mean that there’s usually a frequency where the knee of the curve starts and where the coefficient of absorption drops off. You'll notice that the Acoustisoft folks claim that a 3 inch absorber will only go down to 1,200 Hz at the best, and they say it takes a 6 inch absorber to go down to 600 Hz. This informal inspection method apparently contradicts their observation.


Thanks.
 

·
Registered
Joined
·
6,697 Posts
Discussion Starter · #5 ·
Regarding a little bit of knowledge being a dangerous thing, it would seem that, as usual, the average do-it-yourselfer runs the risk of creating a problem (dryness) where none existed, through the over zealous application of acoustic treatments. I suppose to minimize the risk one could buy a bunch of acoustic foam, temporarily attach it to the ceiling, listening to the results (and perhaps scientifically measure them) and finally permanently affix it. Unfortunately this labor-intensive approach is not very attractive to me and, pardon the pun, it sounds like a lot of extra work in which you may not hear the difference, and if you do hear the difference it may not be a good thing.


However, I did run across some RPG nestable foam. ProFoam . With this stuff, assuming you were pretty sure where to locate it, you could permanently install one layer. Then check the results. If they were insufficient you could install another layer and check results, and so on.


Well, this whole thing’s got me scratching my head as whether to consider ceiling treatments or not. Does anyone have an opinion as to whether, in general, the risks and/or effort, outweigh the potential benefit?:(


Thanks.


Larry
 

·
Registered
Joined
·
6,697 Posts
Discussion Starter · #6 ·
I ran across this interesting piece of information from Auralex Acoustics.

What is an NRC?


Here's an excerpt that may be helpful (or not). :rolleyes:

Quote:
The thickness (normal and angular) of an absorber does limit its low frequency performance. However, the low frequency performance of panel absorbers might be better than other audio "experts" have led you to believe. Often, the limitations of low frequency performance is described as a function of the thickness of the material versus the ¼-wavelength of the lowest frequency that is affected by the material. In other words, if a material is 4" thick, that corresponds to a low frequency cut-off of:



f=c/4t


where:



f = low frequency cutoff (Hz)



c = speed of sound in air (usually about 1130 ft/sec)



t = thickness of absorber (ft)



This is misleading. This would be valid if the panel was only placed at normal incidence to a sound source. "Normal incidence" means at 0°. A loudspeaker facing parallel to a wall would be considered normal incidence. When was the last time you saw this in a control room? Usually, walls, and hence absorbers, are placed at different angles to the sound source. The angular incidence of the sound on the panel increases the absorbers effective depth. Click here for an illustration. Therefore, lower cutoff frequencies are achievable. To use the earlier example, this is the direct cause of absorption values for 4" materials below 850 Hz - the supposed low frequency cutoff for normal incidence. The bottom line: Absorbers such as 4" Studiofoam, Sunbursts, MAX-Wall, LENRDs, Venus Bass Traps, etc. can all absorb low frequency energy.
Interestingly, the absorption coefficient seems to have nothing to do with the low frequency cut off. The only thing that seems to matter is the thickness of the absorber and its geometry (i.e, the thickness that the material presents to the incident sound waves.) So the more oblique the absorber is relative to the sound wave, the more effective it is at absorbing lower frequencies.


Larry
 

·
Registered
Joined
·
523 Posts
Another thing to consider in these calculations is that the sound is going to pass through the absorber (at an angle no less), hit the wall, bounce back and travel through the absorber again, effectively doubling the thickness for the previous calculation.


Here is an interesting excerpt from the ProFoam page:

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

1. Conventional flat backed wedge and convoluted foams are applied directly to the mounting surface, where the particle velocity is zero. Since no absorption can occur when the particle velocity is zero, a significant portion of solid foam is wasted.

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


It seems that mounting your absorber away from the wall would be more effective that mounting it to the wall. The ProFoam accomplishes this automatically. It is also fairly reasonable at $2.50/sq. ft.


I am definitely going to look into these products when it comes time to "fine-tune" the room.
 

·
Registered
Joined
·
6,697 Posts
Discussion Starter · #8 ·
Quote:
Originally posted by cgblack
Another thing to consider in these calculations is that the sound is going to pass through the absorber (at an angle no less), hit the wall, bounce back and travel through the absorber again, effectively doubling the thickness for the previous calculation.
Hi Graeme:


Thanks for responding.


I don't believe that we'll see a doubling of effective thickness because the equations probably already considers the fact that the sound is traveling one thickness into the absorber, and one thickness out of the absorber. (They just happen to be the same route.) Obviously, sound traveling at an angle increases each of those effective in and out dimensions.


On shallow angles of incidence the effective thickness could be more than twice the normal (perpendicular thickness). I figure that at an angle of 30 degrees the effectiveness thickness would be twice the normal thickness and the cut off frequency would be half of that calculated by the formula. In real world situations we of course would rarely see such angle except in the situations where speakers and listener are positioned close to walls. Not a good place to be for good acoustics.


Larry
 

·
Registered
Joined
·
80 Posts
LarryChanin,


I personally have no acoustic material on the ceiling since my speakers have very controlled dispersion. If you believe you have a problem with first reflection I would purchase 8-16 sq ft of 2" acoustic foam and try it temporarily mounted at the first reflection (mirror) points. If you get a big improvement in imaging then I would consider permanent installation.


IMHO dry rooms are not bad, just different. Movies usually benefit from dryer rooms. Have you ever seen a 5.1 mastering room? 6" foam on every surface with bass traps and diffusers everywhere else. Is it really bad to eliminate room signature when it's all supposed to be recorded on the sound track?


Just a thought.
 

·
Registered
Joined
·
6,697 Posts
Discussion Starter · #10 ·
Hi LordHz:


Thanks for the response.


It's not that I am aware of a particular reflection problem, my real problem is that I read too much. :eek:


Everytime I read about some sort of tweak or improvement I naturally think of how that could apply to my situation. I currently have a home theater and I'm also thinking about what I would do if I were to move into a new house. So in the back of my mind I'm asking how I would do acoustic treatments now where things are relatively fixed, and later when I might be able to design them in.


On my current home theater I'm in what I call the "Last Tweak" phase. In my opinion the last tweak is equalization, but the experts say that before you apply electronic corrections you should first apply room treatments.


The attached files are rough ideas of what I'm thinking about for my current situation. You can click on my signature to below see some photos as well.


Thanks.


Larry
 

·
Registered
Joined
·
6,697 Posts
Discussion Starter · #11 ·
I had a little trouble posting the attachments in the previous posting.


Here's the first.
 

·
Registered
Joined
·
6,697 Posts
Discussion Starter · #12 ·
Here's the second picture.



Not shown in these concept drawings is the folding mirrored doors comprising almost the entire back wall of the room.


I ususally have them opened slightly so the back walls are not parallel to the front walls.


Larry
 

·
Registered
Joined
·
502 Posts
I agree with LordHz regarding the "dry room" effect. I too agree that the ambient material is on the original recording mix.

I am in the middle of treating my room (12.5'wX30'lX7'3"h), the ceiling is really low. The plain room (no treatment) suffers from terrible "slap echo" and ringing. As I was painting the ceiling black (CRT:D ) I figured that it might be easier just to attatch black 1-2" acoustic Sonex to the ceiling instead. Any ideas on this?

I consulted with a acoustic wall salesman yesterday and he suggests just a couple of wall panels 1--2" thick in a couple of spots along the walls and up high near the ceiling wall boundries to get rid of the echo, ringing problem. When I went to see him, I had plans to treat ALL the walls with custom paneling but he told me this was not necessary as this would create too dead of a room and suck up amplifier power.
 

·
Premium Member
Joined
·
8,710 Posts
I don't know what "too dry" means. I'd suspect he means a room that is "too dead", or overly absorptive. This can certainly be done if you don't know what you're doing when starting out on the project. However, what is "too dead" for two channel playback and "too dead" for multi-channel playback are two entirely different points..and, in relative terms, widely separated.


When using a multi-channel playback system (ie, L,C,R and various surround, or effects speakers) EVEN IF THE RECORDING IS TWO CHANNEL, you must have a much lower reverberation level (RT60) than what you need with only two speakers. A proper two speaker room will sound confused, muddy (and a bunch of other negative terms) when used for multi-channel playback. Conversely, using only the L/R pair in a proper multi-channel room, will result in an overly dead presentation.


Equally to the point, the placement of absorptive and reflective surfaces have entirely different requirements between the two playback methods.

Quote:
===========================

1. Conventional flat backed wedge and convoluted foams are applied directly to the mounting surface, where the particle velocity is zero. Since no absorption can occur when the particle velocity is zero, a significant portion of solid foam is wasted.

===========================
This is not entirely true and ignores a long, long list of basic physics.
 
1 - 14 of 14 Posts
Status
Not open for further replies.
Top