Acoustical Treatments Master Thread - Page 79

[quote=hardax]Thanks drunkpenguin and Glenn. Looks like we are all thinking on the same level.

One other question Glenn (or if Bryan is reading). Do I really have to worry about the material I use to cover the 703 in this application? I have some basic black fabric (old curtains) that I could use but wonder if I would be messing anyhing up acoustically?

QUOTE]

The thing to do is to use open celled fabric. So if you can blow through it you should be fine. If you are only trying to focus on the low end that it really is not going to matter to much. Bass will go right through it.

Glenn

Question:

http://www.bobgolds.com/AbsorptionCoefficients.htm From Bob's site, it would appear that UltraTouch Cotton batts(Bonded Logic, near the end) would make a great insulation to fill theater walls vs. fluffy pink fiberglass. It is a great absorber, all the way down to 125Hz. I wonder how well it does even lower? Is it worth using? All opinions welcome. An R-13 batt is not as expensive as I thought.

Fatawan

re Utratouch cotton:

R-13 3.5" (mm) A 0.95 1.30 1.19 1.08 1.02 1.00 1.15

It's often mentioned when people start talking about avoiding man-made fibers.

But I was watching Holmes On Homes Pasadena part 2 and heard a couple of things:

1) They used to dilute boric acid in water, and use it to wash newborn babies eyes out in hospitals with it
2) The boric acid is added to make it a class A fire retardant material
3) As a side benefit it repells all the insects you might encounter. Termites will not live in it. Repells mice and roof rats.
4) The boric acid, because of the acidic environment mold will not grow in it. If you get this wet and dry and wet and dry 50 times, it'll dry out because cotton breaths and gives off the water, but you'll never have mold or mildew.
5) environmentally friendly (recycling blue jeans)

Because I was told by folks on this forum that for absorption (such as panels at the first reflection points), your fabric didn't need to be "acoustically transparent" and that was more for covering your speakers, etc. However, once I changed to felt, it appears I lost some absorption I had when I used Muslin. I wanted to check in again to see if Felt was 'OK' to use to cover Rigid Fiberglass at the First Reflection points. GOM is expensive for me, and when I found out Felt would be ok, that was good for me as it was less expensive. I wanted to find out if someone can tell me "Technical Reasons" why felt would be good or bad to cover broadband absorption panels.

It won't help or hurt as wall cavity insulation. Any low-density porous absorber will work as well. There really isn't any way to get improved performance for this function.

Regards,
Terry

Ok--thanks Terry---fluffy pink fiberglass it is then! I am saving money left and right!

What's the best way to attach 4'X8' fabric wrapped panels to the ceiling? Can't screw or nail them as this will ruin the fabric I have chosen (which is black speaker grill cloth).
The panels are only 1/2" thick - so Rotofast Snap-on anchors won't work. Glue? Liquid nails?
And what spray on adhesive works best to ensure the fabric doesn't come loose?
Thanks for your help.

I'd be interested in being pointed to a post where someone said broadband absorbers didn't need to be covered with acoustically transparent material. As your ears have told you, you've lost absorption because the felt is reflecting some frequencies instead of allowing them to pass and be absorbed. GOM certainly is expensive and I understand your desire to find a lower cost alternative that meets your aesthetic sensibilities. As for technical reasons that give the thumbs up or down on felt, see Glenn's post above.

As to fabric adheavsive Super 77. As to nails or screws, You certanly CAN screw through it. you simply punch a hole first. there are various caps you can put on screws as well.

I have seen similar things posted, but you have to be careful. I can see three categories of cloth:

1) Acoustically Transparent:
All (most) sound gets through, any slight absorption is even across frequencies so it won't color your sound.

Uses: covering speakers, covering broadband or mid-high frequency absorbing wall treatments.

Examples: speaker cloth, good AT screen material, some types of GOM.

2) Acoustically non-reflective:
Very little sound is reflected from the cloth. However, some sound may be absorbed by the cloth and not make it out the other side. This would color your sound if put in front of speakers.

Uses: fine for covering treatments.

3) Acoustically reflective:
Reflects significant sound energy back into the room at some frequencies (typically mid to high frequencies are reflected, bass gets through). Sometimes this is what you want (to cover a bass trap if you have enough high frequency absorption.)

So, while it may be true that speaker-cloth quality acoustically transparent cloth isn't needed on your first reflection points, anything that reflects mid-high frequencies defeats the purpose of the panels. Your ears are telling you that the felt is doing that. Trust your ears and switch to GOM or some less expensive non-reflective fabric (muslin, etc.)

Where do you think 100% cotton would fall in this list of cloth categories?

I'll do some searches and post the links. I was told by a few of the "Acoustic Gurus" on the forum that Felt would be perfectly acceptable for broadband absorption. But, as you pointed out, it does sound different to me in comparison to the ugly muslin I was using. For the price I spent for the Muslin, the dye, and now the felt, I could have purchased the GOM. Price lesson learned I guess.

As requested, here is the thread regarding the use of Felt:

http://www.avsforum.com/avs-vb/showt...highlight=felt

There is conflicting information there and, IMO, Bryan's post is at conflict internally. For an absorber to be broadband, it must receive the broadest band of frequencies. Cloth that is not acoustically transparent, and does not outright absorb what it does not pass, will reflect. Therefore, the underlying absorptive material will not do it's thing on those frequencies. Perhaps the different posters in that thread are not on the same page. Anyway, if felt is not appropriate to use in front of speakers, it is not appropriate to use to cover broadband absorbers.

I will agree that if felt reflects some sound energy back into the room, it is not appropriate for first reflection point treatment coverage.

I disagree that the material used on broadband absorbers must be appropriate for use in front of speakers. In speaker covering cloth there should be very little absorption. In broadband absorber covering cloth, absorption is just fine, as long as there is no reflection/diffusion of sound energy back into the room.

That said, it's definitely safer to go with AT material. You can get data on acoustic transparency. I haven't seen a lot of data on acoustic reflectivity across frequencies.

Ahh yes, I see the distinction now. Transmission good. Absorption good. Reflection bad.

Couldn't one derive reflectivity by frequency by looking at NRC and STC? If it's not absorbed and not transmitted, it is reflected?

To add to the complexity: if it reflects the energy back, but in a diffuse way (diffuse reflection as opposed to specular reflection), it might still be ok for first reflection points. The mid-high energy will still be in the room, but will not arrive at the listener's ears as a short-delay echo that would affect localization.

I'd stick with GOM, or go with the 'if you can breathe through it relatively easily it's ok' test.

I like that; everyone has the necessary gear to do that test.

Paul,


Just to be clear, nothing as thin as cloth can offer a useful amount of diffusion.

--Ethan

So,

Should I rip my felt off my first reflection absorption panels and use something else? I'm receiving conflicting information. I was told a while back that Felt would be fine, but am now reading that it probably isn't for the use of covering absorption panels at the first reflection. Does Felt reflect too much sound back into the room, making my panels ineffective at high frequencies? I respect everyone's replies and suggestions, but please note that I was told by two well respected folks in this thread that Felt was fine, so I ask that you aren't harsh to me. I just DON'T KNOW the answers and am willing to follow the suggestions of those that do.

Thanks. When I think about the depth requirements for the quadratic residue diffusors to get any useful frequency range, that makes sense.

Tony,

A few things are clear:

1) first reflection point treatment should not reflect mid-high sound.

2) When you switched from muslin to felt on your first reflection points you did not like the change in the sound.

Assuming nothing else changed, that could indicate that the felt was reflecting more high frequency sound than the muslin was. As muslin is a very open weave and felt is denser, this is plausible.

I don't think you are getting any conflicting advice at this point: if the felt sounds worse than the muslin, switch to a more open fabric. What sounds good to you in your room always trumps any theoretical advice you get.

The fact that previous advice indicated felt would be ok is irrelevant. Perhaps there are different thicknesses/densities/types of felts. Maybe some pass sound better than others. Maybe felt doesn't pass high freq sound and the advice was bad. It doesn't much matter. I'm sure the advice was given in good faith.

Bite the bullet and try again. Let us know what you end up with and whether it works better.

Felt reflects significantly above around 2 kHz. It should not be used to cover broadband absorbers. Sometimes free advice is worth exactly that.

Yes, I'm afraid you need to remove the felt and replace it with a much more acoustically transparent fabric.

Regards,
Terry

I wouldn't be so quick to have Tony go to all that trouble. Maybe some felt reflects, but the stuff I've seen does not.

Tony, try this simple test: Stand in front of one of your panels and "talk into" it. Does it sound like you're talking into a total void? If so, then the felt is probably fine. If you still have some raw fiberglass, use that as a comparison. If they both sound the same, then the felt is fine. If you hear more of your voice coming back on the panel with felt, then you must have bought some of that "reflective felt" Terry is referring to.

--Ethan

Exactly.

Felt, contact with wall, absorption coefficients
125hz 0.13
250hz 0.41
500hz 0.56
1000hz 0.69
2000hz 0.65
4000hz 0.49

At low frequencies, low absorption coefficients might mean that the sound is going right through it.
At high frequencies, low absorption coefficients mean that the sound is reflecting off it. In this case, at 4000hz, according to that test, more than half the sound energy is reflecting off the felt.


50% of the energy is 3dB
90% of the energy is 10dB

Thanks Ethan. I can definitely tell a difference when talking into my felt covered fiberglass than say, the wall. However, I haven't tried to test between the felt covered fiberglass and fiberglass in general. Not sure why I didn't think of it. I do have a few pieces of rigid boards left. I'll do this simple test today and see how it fairs. Thanks!!!

Bob,

> At high frequencies, low absorption coefficients mean that the sound is reflecting off it. In this case, at 4000hz, according to that test, more than half the sound energy is reflecting off the felt. <<br />
Not at all - it depends entirely on how thick the felt is! Most felt is 1/16th inch, so I can see some amount of high frequencies passing through and reflecting back off the wall behind. Now, if you test felt that's half an inch thick and it absorbs only 0.65 at 2 KHz, then I might agree that the rest is being reflected. I also agree that the lower absorption at 4 KHz than 2 KHz in your data implies some reflection. But without testing thicker material we're only guessing.

I imagine that all felt is not the same either. The 1/16th inch felt I've used is mostly acoustically transparent. I once held a large piece in front of my face to hear how it affected the sound from a speaker in front of me, and it made little if any change.

--Ethan

Get someone to snap your picture when you're talking to your acoustical treatments. This could really be helpful to others.

I have 2 windows on each side of my 118" Carada theater screen. What would be the easiest way or cost effective way to treat/cover this area. I know the search function is my friend but my father in-law just passed away unexpectantly and we have family coming in. I want to get it up and running as this is a new build and I don't have a lot of time. I am having a hard time thinking straight. I would like to be able to take their minds off of the situation. Thanks for any suggestions.