ICF Acoustics....

[Bret]

Prologue: When my wife and I purchased our last home, her requirement was a pool. My requirement was a theatre. We compromised and she got a pool...fast forward to now...

I am in the process of building a home here in northern Florida and I was able to fit a 16' x 25' dedicated home theatre (officially called the "safe" room) in the basement. All 4 walls are 6" of steel reinforced concrete sandwiched between two pieces of fairly dense foam approx 2.25" thick on each side. The 9' ceiling is approx 4" concrete. The ICF foam forms have built in furring (sp?) strips and are effectively isolated from the concrete inside. Has anyone done anything similar with this material? I am trying to decide if I will really need to add the typical absorbers up to eye level or simply cover the whole wall with some sort of fabric. I understand the need to add a couple bass absorbers to the mix but after that I am really not sure where to start. Hate to ask such an open ended question...I have done a ton of research but have not seen much info on this material. (For what it's worth, I am planning a 7.1 system using 7 ea. Heresy II's. Think that's a bit of overkill for this size theatre?)

Thanks all!

Bret

[mas]

If I may simplify things, there are 3 primary aspects of a small room acoustics.<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

1. Sound transmission from the room into surrounding areas.

2. LF modes within the room

3. MF-HF reflections within the room.

What it sounds like is that the sound transmission aspects of the walls and ceiling have already been addressed. What you will still want to remain cognizant of, is that the critical flanking pathways such as doorway seals and HVAC duct and AC outlets must still be addressed.

Regarding the last two aspects:

LF modes are addressed by traps and MF-HF are addressed by wall surface treatments, either via diffusion, absorption, or a combination of the two. Be aware that even wall materials that are terrific LF absorbers will still reflect the majority of MF-HF from their surface. Thus, LF absorptive material will not act as an effective MF-HF diffusive/absorptive surface treatment.

And now the MF-HF reflections:

Generally speaking, you only want to absorb the first order reflections that arrive within what is referred to as the Initial Delay Gap (ITD). These are reflections that arrive within an approximately 20-35ms period after the arrival of the direct signal. These signals trigger the Haas (Henry Precedence Effect) and result in a fusing and an inability to resolve the location of the individual signals location point of origin. Thus these are damped. This is done surgically with absorption.

All other signals ideally become part of a well behaved diffuse sound field. This diffuse sound field begins after the initial time delay gap and is defined by a Haas kicker that is ~6dB lower in intensity than the direct signal and of greater intensity than the remaining diffuse sound field. Ideally, the remaining diffuse sound field will decay without exhibiting and anomalous reflections (often sums of various order reflections) which are most easily damped by additional diffusion of the component signal paths.

Sound confusing? With the proper measuring equipment and measurements such as the ETC (Energy time curve) it is not only easily represented, but the needs are easily identified and addressed.

The included diagrams provide a graphical representation of some of the component relationships. (Please note one exception. In the second figure illustrating the relationship of acoustic levels versus distance and time in an idealized ETC (energy time curve), the reverberant field Lr does not exist in a small acoustic space. This diagram is intended to cover both small and large acoustic spaces; the large acoustical space reverberant field component is included.)

I suspect that you are still going to need to apply appropriate bass traps for room modes/standing waves as well as diffusion and selective absorption on the wall surfaces. Avoid overdoing absorption! Overuse is as bad, if not worse, than under use.

ConcatETCDiagrams.pdf

[Bret]

Thanks! It "sounds" like I am just going to have to wait until everything is built up and then go from there. I am concerned that the foam which will already be in place may act as an absorber of some form or another and was hoping that someone may have built something similar.

I am spinning in circles at this early point trying to decide if the room will be too dead with the foam as my wall surface. If so, how much of it should I cover with drywall to liven it up? On the other hand, maybe the foam does nothing acoustically and I will need to treat the walls as bare concrete. Trying to make decisions before sinking un-needed time and money into installing something I don't need. After doing all my research, I am surprised there is so little info on ICF used in this manner. Not even the ICF manufacturer could give me any idea of the sound absorbancy other that the room will be very, very quiet...

Thanks again,

Bret

[mas]

Bret,<?xml:namespace prefix = o ns = "urn:schemas-microsoft-com:office:office" />

The STC (sound transmission coefficient) value for ICF is >= 55, indicating that it is excellent for soundproofing - that is minimizing the sound transmission through the wall.

From what I have been able to gather after a few phone calls, it is treated about like drywall with regards to its reflective characteristics. I will try over the weekend and again Monday morning (depending upon who I can catch at home) to try to get more info.

Thus far, the overwhelming suggestion is to simply shoot ETCs (energy time curves) of the room. These combined with PEQ software will perform the polar convolvement of each reflection peak into its 3 space polar coordinates as quickly can you reposition a cursor! Then, with a surveyor's transit and a laser pointer positioned at the same spot where the microphone was positioned, you simply point the laser in the direction of the coordinates and you have the exact spots requiring absorption or diffusion.

{Less precise alternatives are to use the reverse mirror trick where you sit in the listening position and have someone move a mirror along the walls and ceiling and mark all of the mirror positions where you can see the speakers in order to get an idea where absorption should be placed.. Unfortunately this will not help with diffusion.}

Additional diffusion treatment on the back wall and to a lesser degree on the side walls and ceiling will transform the discrete reflections of the sound field in to a diffused sound field.

Oftentimes the modeling process is much more complex then simply taking a series of measurements of the room. And with access to measurements, you can skip much of the calculation guesswork entirely! As, even with a model, you are still brought back to the same situation of measuring to determine what the real situation is in order to perform verification and proof of performance.

Feel free to PM me if you would like more info.

ICFsuperformproductspecification.pdf

[WMcD]

Bret,

It seems to me that most comments here are going in the same direction.

One is that you're going to have what you have. Try it out before worrying about changing it.

We got to hear speakers in one of Klipsch's rooms at Indy. The wall covering looks like fabric but I understand there is a layer of foam underneath. One of our guides said the room was 80 percent "dead". At what frequencies we don't know. Speakers sounded good in that room.

I do wonder about some of the approaches.

In 1) It seems to me that some start with the assumption we have a very live room with six surfaces. Like living inside a mirrored box, which makes for a house of mirrors and the barber shop effect of reflections of reflections. Then we have to add black surfaces to cut them down, or curved surfaces, and diffraction surfaces, to distort the regularity of the reflections.

In 2) (more like yours) we are living in a box of black surfaces. Then you might (might) want to add surfaces (I say curved surfaces) or diffractions to create some reflections.

It seems to me that 1. and 2. differ in that you're subtracting out in the first, and adding in, in the second. We've go the same tools in both.

Gil

[DrWho]

How into this projet are you looking to get Bret?

You can get yourself some decent measurement equipment for relatively cheap considering the benefits you can achieve.

http://www.etfacoustic.com/

Combined with this mic

http://www.musiciansfriend.com/product/Behringer-ECM8000-Microphone?sku=270400

And powered with this mixer (working as a pre-amp):

http://www.musiciansfriend.com/product/Behringer-Eurorack-UB802-Mixer?sku=631238

And then all you need is a computer near the room or preferably a laptop.

So software for $150 + $50 Mic + $40 mixer comes to under $250.

[Bret]

Not a bad investment if it saves me more than $250.00 in unnecessary time and materials as opposed to just winging it and throwing cash at it until it sounds right.....Thank you very much for the suggestion. As a plus, with this setup, I can at least perform some authoritive measurements and perhaps save someone else the aggro.

[rcarlton]

Much better investment is Room EQ Wizard. It is free.

[woodsman]

Bret, I might be able to shine some light for you here. We too built an ICF house, love it, would not want to live in another conventional built house again. Our basement had to be drywalled as part of code here in Wi. I don't think you want to leave it just exposed foam. We also have a home theater in the basement. It's 18 wide 29 deep. The sound is very nice down there. I would say the foam does help deaden the room. No echo's so I must have done something right.

As far as the main floor goes, that's where I found the ICF's to be a little trickier. One beauty of ICF's is it does not let noise in. It is the quietest house you ever can imagine. We get pissed off when we hear our mechanicals run now. On that point, it does not let sound out either. So the main 2 channel system in the greatroom played at a decent level, is heard throughout the house at a normal level. Which is good thing for us, as it is just my wife and myself. If you have small children napping, that's gonna be a problem. As with anything there are pluses and minuses. The pluses of the ICF's far outweigh the negatives. You are gonna love it when it's all done.

Jeff

[woodsman]

Bret, I forgot to add one thing. It will not be too dead. We even had to deaden it some more with some room treatments after we got the system going.

Jeff

[Bret]

That's exactly what I was looking for! Thank you very much for the info. For what it's worth, all 4 walls are ICF in the theatre. With a 4" concrete floor system over the top, I am hoping I should not have a problem with waking the children while watching "The Matrix"

I was planning on either covering the foam with a fabric and synthetic stucco so I guess I will still have to play it by ear....

[woodsman]

2 of our 4 walls are icf. The others are framed by myself and heavily insulated. I missed the part of you concrete ceiling. Is that the spancrete? You should be able to do something with that. At least attatch a suspended ceiling, good luck putting the anchors in. You will be in basically a tomb down there. Make sure your hvac guy runs ductwork for the ERV into that room. With all that you should have no problem disturbing the upstairs family members. We have floor trusses that I filled with insulation and a suspended ceiling. I can still hear it through the floor, but usually only with loud action scenes.

Jeff

[woodsman]

Bret, you should cover that foam with drywall. It's cheap and easy to hang on the icf's. Channels run every 8in and you have to use course drywall screws. It's the cheapest way to go. Besides you are going to want to paint it and hang some movie posters up. Just don't leave the exposed foam, you would hate it.

Jeff

[Bret]

The floor system above the theatre consists of steel floor trusses made by Deitrich, supporting a product called Vulpan (Looks like corrugated steel). To this add rebar and 4-4.5" of concrete. On my plans it is called a safe room but we all know what it really is....For what it is worth, they placed the forms on Friday and are pouring the footings on Monday! Woot!

[Bandy]

Hi Bret..

I have been searching all over the web for a situation similar to mine when I came across your post.

I built my house about 2 years ago, the Basement is ICf 8inches concrete and the rest of the house is wood. My Home theater is in the basement , measures 17.5' by 25' and 12 .5' ceilings. Three of the walls are icf and the rear end is 6 inch framing. My ceiling is also 4 inches of concrete with the speedfloor suspended system.

When I built the house, I had the theater designed professionally but now as i am about to actually complete my theater and i review his plans i'm starting to get the impression that my theater designer did not really understand the ICF sytem as he indicated that I had to furr out the walls with wood and then treat the room as you would a regular concrete walls which for soem reason does not seem right to me.

I am very interested in what you have learnt about ICF acoustics as I am about ready to contract my work out.

Woodman, could you give me more detail about how you treated your walls on all sides. Did you use absorbers and reflectors in the usual places.?

[mas]

I have talked to a few folks who confirm that it is standard for ICF systems to come either prefinished with attached furring stips, or for the strips to be applied locally for subsequent finishing with paneling or drywall.

Essentially you have a surface which is impervious to hurricanes and tornados and has a very high sound transmission resistance- thus effectively isolating the room from adjacent areas (assuming other more fundamental acoustic transmission vectors such as air gaps around doors, HVAC ducts, and other pathways are madequately addressed). But as regards internal reflections and absorption, it is a function of the material used to finish the interior surface. The ICF material will have little to no effect upon the room's internal performance.

[gaspr]

Would a room with very high sound transmission resistance have more

problems with LF modes than a room that at least allows some LF to pass

through? On the plus side one should be able to achieve a

very low sound floor.

Garth

[mas]

Room Modes are predominantly reinforced by room geometry, not wall material.

The noise floor will be dependent upon many things. Although in unusual circumstances it certainly could be, sound transmission is not normally a significant factor. Noise due to various factors in systems such as HVAC, etc. are more significant.

With the exception of contributing to a high level of isolation due to its high STC coefficient, may I suggest that the ICF construction (while offering many construction benefits unrelated to acoustics) thereafter becomes rather a moot point with regards to the room's internal acoustics.

Once built, you basically have a classic room whereby the best method of obtaining a snapshot of the real issues to be addressed are best ascertained by measuring the room with a reliable time based tool.

[DrWho]

Would a room with very high sound transmission resistance have more

problems with LF modes than a room that at least allows some LF to pass

through? On the plus side one should be able to achieve a

very low sound floor.

On a conceptual level I would say "yes" - but on a practical level I would say "no".

The only person I know of that had a room where it was a practical consideration was "Horn-Ed" who hasn't been around these forums for quite some time now. He had a room that was acoustically transparent out in the middle of nowhere. I remember him talking about building "false corners" for his subwoofers to help keep the bass at the listening position (yielding him much more output). That had to be one very interesting listening experience (and I've always wondered about the impact of rain and the rest of the environment in general).

But I think mas makes a good point...you could just measure the room and get rid of all the guess work. Ya, there's a steep learning curve - but that's what the forum is for []

[TheSoundBroker]

I have an ICF home and an ICF theater building. I find the acoustics are pretty darned close to drywall but with a bit harder sound untreated. Probably depends on what they use to surface the ICF. In my case, when I bought the theater building, the original builder had the inside stucco'd []. I ended up firring the walls out with 2 x 4's, put R19 and drywall in followed by acoustic treatments on the surface. I now have a fairly even room that tends more towards dead than live. As mentioned before, ICF is also extremely quiet...not to mention quite energy efficient.