# Building Acoustic Panels at Low Cost

#### 617

##### Major Contributor
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This is a project I've wanted to do for a while.

I just ordered some wood and insulation for delivery from Home Depot here in the USA to make 8 acoustic panels, each 23" x 47.5" x 3 5/8" (~60 cm x 120 cm x 8.8cm)

It turns out that making eight panels is a sensible and economical number of panels, due to the fact that there is at least one type of mineral wool batting which comes in packs of 6. Each panel requires 1.5 pieces of the batting, so you get exactly 8 panels out of the insulation material.

In addition to the insulation, I am making the frame from 1/4" MDF sides glued to a 1/8" fiberboard back. I would normally use plywood, but plywood prices are still a bit silly, and I think the MDF will work just fine for this application.

See attached image for shopping list.

My total cost for this project will be about \$220 from the Depot, and \$80-150 for acoustic fabric. I haven't decided which to get, but the cheapest source in the USA seems to be ATS acoustics, who sells their own Burlap or Microsuede acoustic fabric. I'd expect my total cost per panel to be \$40. This is about 40% of the cost of panels made by ATS or some other supplier.

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#### 617

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I had some trouble locating NRC values for the specific product I ordered. Owens Corning provides something called an "R" value, I'm have no idea how this affects sound. However, using the Thermafiber NRC chart, I can infer the numbers I need to do RT60 calculations.

From the product description, the package weighs 24# and is 8.56 cubic feet of material. So we know the density is around 2.8 cubic feet.
The thickness is 3.5".

Owens Corning's NRC/STC data doesn't have either this density or this thickness, so we will have to interpolate values.

The values for this product are, by my calculations:
(125, 250, 500, 1000, 2000, 4000)
(.67, .8, 1.125, 1.06, 1.04, 1.03)

NRC values above 1 are a bit tricky; my understanding is that ASTM's testing assumed that the materials being measured were flat, but very thick materials have edge surfaces which absorb sound. Since my panels will have wood sides, I will use an NRC of 1 for frequencies where NRC is above 1.

#### BillH

##### Active Member
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I had some trouble locating NRC values for the specific product I ordered. Owens Corning provides something called an "R" value, I'm have no idea how this affects sound. However, using the Thermafiber NRC chart, I can infer the numbers I need to do RT60 calculations.

From the product description, the package weighs 24# and is 8.56 cubic feet of material. So we know the density is around 2.8 cubic feet.
The thickness is 3.5".

Owens Corning's NRC/STC data doesn't have either this density or this thickness, so we will have to interpolate values.

The values for this product are, by my calculations:
(125, 250, 500, 1000, 2000, 4000)
(.67, .8, 1.125, 1.06, 1.04, 1.03)

NRC values above 1 are a bit tricky; my understanding is that ASTM's testing assumed that the materials being measured were flat, but very thick materials have edge surfaces which absorb sound. Since my panels will have wood sides, I will use an NRC of 1 for frequencies where NRC is above 1.
This is the best info I know of
https://www.bobgolds.com/AbsorptionCoefficients.htm

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#### 617

##### Major Contributor
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I'm not actually making these panels for audiophile purposes. I am making them to cut down on reverberation at speech frequencies so that I can record and edit spoken word material with reasonable accuracy. The human voice doesn't do much below 80hz or so, and I find that in my home, the most noticeable spectrum of the reverb is in the mids/high, so I would expect this material to work pretty well for my purposes.

The efficacy of acoustic panels depends primarily on how much of your room they cover. Larger spaces will need more absorbtion to bring reverberation times down low. I would like an RT60 of below 1/2 second if possible.

To simulate the acoustics of a space, you can use this website:
https://www.10log.com/public/rt

This is a really sophisticated calculator and I wish I had it when I was in school. Doing RT60 calculations by hand is tedious.

One room is a bedroom which is 12.4' x 9.3' with a ceiling height of 8.2'.

Assuming that the walls and ceiling are plaster, and the floor is thick carpet, I can use the above calculator to get the following RT60 graph:

The blue area is my desired RT60, in this case I am shooting for .2 seconds. That's a very small and rapidly decaying reverberation. The other lines represent different RT60 values for this room, based on different calculation methods. Notice that they seem to agree below around 300hz, and then diverge from there, with some estimating an RT60 above 1.2s and some being around .4s.

I've seen .3-.7 seconds cited for recording studio RT60. For a vocal booth I would assume you want something towards the lower end of that.

Adding the panels changes the graph:

Now each of the estimates puts my RT60 below .4 seconds at all frequencies. However, getting an RT60 below .2 is not going to be realistic in this room with this kind of material.

If I want a super low RT60 in this room, it would require something like 20 panels of this composition. I don't have space for that! This is what the graph looks like if I did:

#### DonH56

##### Master Contributor
Technical Expert
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Covering Corning OC-703 panels with cloth and gluing them to a backing for hanging (or not, they can be hung without) is fairly cheap. A frame around them looks nice but also adds reflections and reduces their effective absorption area. Unless needed for looks, I'd skip the frames.

#### BillH

##### Active Member
Forum Donor
I'm not actually making these panels for audiophile purposes. I am making them to cut down on reverberation at speech frequencies so that I can record and edit spoken word material with reasonable accuracy. The human voice doesn't do much below 80hz or so, and I find that in my home, the most noticeable spectrum of the reverb is in the mids/high, so I would expect this material to work pretty well for my purposes.

The efficacy of acoustic panels depends primarily on how much of your room they cover. Larger spaces will need more absorbtion to bring reverberation times down low. I would like an RT60 of below 1/2 second if possible.

To simulate the acoustics of a space, you can use this website:
https://www.10log.com/public/rt

This is a really sophisticated calculator and I wish I had it when I was in school. Doing RT60 calculations by hand is tedious.

One room is a bedroom which is 12.4' x 9.3' with a ceiling height of 8.2'.

Assuming that the walls and ceiling are plaster, and the floor is thick carpet, I can use the above calculator to get the following RT60 graph:
View attachment 148315
The blue area is my desired RT60, in this case I am shooting for .2 seconds. That's a very small and rapidly decaying reverberation. The other lines represent different RT60 values for this room, based on different calculation methods. Notice that they seem to agree below around 300hz, and then diverge from there, with some estimating an RT60 above 1.2s and some being around .4s.

I've seen .3-.7 seconds cited for recording studio RT60. For a vocal booth I would assume you want something towards the lower end of that.

Adding the panels changes the graph:
View attachment 148316
Now each of the estimates puts my RT60 below .4 seconds at all frequencies. However, getting an RT60 below .2 is not going to be realistic in this room with this kind of material.

If I want a super low RT60 in this room, it would require something like 20 panels of this composition. I don't have space for that! This is what the graph looks like if I did:
View attachment 148317
Your objective sounds difficult but I’m not an expert. I would think you would want to look at free standing absorption baffles near the speaker in addition to treatments on the wall. I’m just concerned with achieving 60 db in that time with parasitic reflections. It seems like a modest amount of reflection times a dozen round trip times might still be above your objective.

OP

#### 617

##### Major Contributor
Forum Donor
So let's say I wanted to make a voice recording booth. Both isolation and absorbance are needed in this application, but let's focus on the former. I have a room in my basement which is the size of a large closet, with angled walls. It is approximately 36 square feet, with a ceiling height of 7 feet or so. Maybe a bit less actually.

A space this small is a totally different kettle of fish:

Due to the ratio of reflection to absorbance, getting a super low RT60 is very easy in a smaller space. Incidentally, this is what happens if all room surfaces are treated with the mineral wool:

I also have a small guest bedroom that I use for playing pedal steel and storing clothes:

The dotted line shows the existing RT60 and the lines below indicate I can get the value down well below .5s with these treatments.

Room treatment is a complex subject, and there is a lot of conflicting information about it. Part of the reason for this is that people have a very poor intuition about how sound works, but part of the reason is that people treat rooms for different reasons.

For music listening in a small room, I think that room treatments should only be used to reduce early reflections. If you look at all these graphs, you can see that they disproportionately absorb higher frequencies, which will change the tonal balance of your music dramatically. The appropriate amount of room reverb is a different subject, but room treatments like this will change tonality - they are wall mounted low pass filters.

For my purposes, I am trying to create voice recordings which are relatively free from room effects, because such recordings are more intelligible, less fatiguing to listen to, and sound better on headphones than voice recordings made with lots of reverb - especially small room reverb.

I also want to treat the area near my monitors, so that my playback can be very dry as well, but I believe overall tonality may be affected as well. Still, for assessing intelligibility, I think room treatments make a lot of sense.

The human voice has content down to around 80hz. My resonant frequency is around 120hz, and I can cut my voice above 11K without much noticeable change. 120hz is around where these treatments start becoming reasonably effective, so my hope is that I will be able to make and assess really natural sounding recordings.

OP

#### 617

##### Major Contributor
Forum Donor
Your objective sounds difficult but I’m not an expert. I would think you would want to look at free standing absorption baffles near the speaker in addition to treatments on the wall. I’m just concerned with achieving 60 db in that time with parasitic reflections. It seems like a modest amount of reflection times a dozen round trip times might still be above your objective.

If I understand you correctly, I think I agree. My goal isn't necessarily to just have a super low RT60, but to reduce early reflections in both the mic and speakers. Since the distribution of the panels on the wall doesn't really make a difference for the overall RT60 (from what I have read) it makes sense to place them near transducers and sound sources.

#### DonH56

##### Master Contributor
Technical Expert
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Isolation is a different thing than absorbance. At mid to upper frequencies, 2"~4" panels or so will do quite nicely to absorb sounds (bass is tougher, requiring thicker panels). But isolation, particularly for recording, generally requires floating surfaces all the way around, isolation from the home HVAC, some sort of door treatment (my cheap solution is a heavy solid-core exterior door with full weather sealing), and so forth. Two different problems.

Everest's handbook has some charts and tips: https://www.amazon.com/Master-Handbook-Acoustics-Sixth-Everest/dp/0071841040

HTH - Don

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#### 617

##### Major Contributor
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Covering Corning OC-703 panels with cloth and gluing them to a backing for hanging (or not, they can be hung without) is fairly cheap. A frame around them looks nice but also adds reflections and reduces their effective absorption area. Unless needed for looks, I'd skip the frames.

I favor the mineral wool for a few reasons. If I didn't have a tablesaw and other tools, using OC 703 would be a great choice, however it is much more costly and would need to be shipped. I'd be looking at almost \$400 just for the acoustic material, before the fabric.

I've never worked with OC703 but I doubt it it strong enough to hold screws, and I like the idea of making a wood frame I can screw a french cleat onto. Also, I hate working with fiberglass. I'm sure 703 is a dream to work with but I really wanted to try the mineral wool.

#### DonH56

##### Master Contributor
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I favor the mineral wool for a few reasons. If I didn't have a tablesaw and other tools, using OC 703 would be a great choice, however it is much more costly and would need to be shipped. I'd be looking at almost \$400 just for the acoustic material, before the fabric.

I've never worked with OC703 but I doubt it it strong enough to hold screws, and I like the idea of making a wood frame I can screw a french cleat onto. Also, I hate working with fiberglass. I'm sure 703 is a dream to work with but I really wanted to try the mineral wool.

I used mineral wool and OC-703 panels in my media room build. I find mineral wool just as much a pain if not more but they are pretty comparable IME. OC-703 you should be able to get at a building supply store (Home Depot, Lowes. etc.)

As for hanging, I just hung the panels on the hooks like these: https://smile.amazon.com/Stainless-...id=1629337924&s=hardware&sr=1-16&ts_id=511352 I used some 2"x2" soft foam blocks to space them off the wall to increase their efficacy (they are velocity absorbers, and velocity is zero at the wall, so a couple of inches greatly improves their ability to absorb sounds). But, I covered them in cloth first, mainly for looks and to keep the Rockwool or fiber shreds in, but also to provide a better support for the hangers

OP

#### 617

##### Major Contributor
Forum Donor
Isolation is a different thing than absorbance. At mid to upper frequencies, 2"~4" panels or so will do quite nicely to absorb sounds (bass is tougher, requiring thicker panels). But isolation, particularly for recording, generally requires floating surfaces all the way around, isolation from the home HVAC, some sort of door treatment (my cheap solution is a heavy solid-core exterior door with full weather sealing), and so forth. Two different problems.

Everest's handbook has some charts and tips: https://www.amazon.com/Master-Handbook-Acoustics-Sixth-Everest/dp/0071841040

HTH - Don

I studied this stuff a bit in grad school. My favorite resource for STC is the catalog of STC and IIC Ratings for Wall and Floor/Ceiling Assemblies. You can find a pdf of it online.

Unfortunately, you are quite right that STC is just as important for my purposes, and I plan on addressing that. STC is a lot harder to achieve without modifying walls, so I thought that the NRC information would be more useful to normal audio people.

For actually blocking sound, it's a combination of mass, airtight-ness and de-coupling. Staggered studs with elastically fixed drywall and insulation for example. You probably know this stuff but it's rare to see new construction really do this stuff right.

When building material costs come down to sane levels, I think I will build out the room in my basement with a room inside the room, with double layers of gyp, fiberglass in the cavities, and so on. Inside this room, with the treatments I am building now, I think sound ingress will be fairly reasonable.

One of the things which makes voice recording a bit easier is that you filter everything below 80hz (or higher) which tend to be the frequencies that travel through everything. Also, unlike music, which can be spoiled by a helicopter or dump truck, if you hear a noise, you just need to repeat a sentence.

OP

#### 617

##### Major Contributor
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I used mineral wool and OC-703 panels in my media room build. I find mineral wool just as much a pain if not more but they are pretty comparable IME. OC-703 you should be able to get at a building supply store (Home Depot, Lowes. etc.)

As for hanging, I just hung the panels on the hooks like these: https://smile.amazon.com/Stainless-...id=1629337924&s=hardware&sr=1-16&ts_id=511352 I used some 2"x2" soft foam blocks to space them off the wall to increase their efficacy (they are velocity absorbers, and velocity is zero at the wall, so a couple of inches greatly improves their ability to absorb sounds).

I can't find 703 anywhere! Where I am the only rigid insulation is styrofoam or polyiso, neither of which are acoustically useful. If I had access to 703 I would probably use it.

Thanks for pointing me to those hooks, I've never seen those. I have plaster/lathe where I am so wall anchors are a complicated business.

The air space issue is very interesting and I would love to see some data on that. I've certainly seen a lot of credible sources suggest using an airspace.

I plan on mounting the panels using a french cleat made of 1/2" wood. This will bring the panel off the wall by 1/2", so it might be wise to cut some holes in the back of my panel in order to get the added performance.

#### DonH56

##### Master Contributor
Technical Expert
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I studied this stuff a bit in grad school. My favorite resource for STC is the catalog of STC and IIC Ratings for Wall and Floor/Ceiling Assemblies. You can find a pdf of it online.

Unfortunately, you are quite right that STC is just as important for my purposes, and I plan on addressing that. STC is a lot harder to achieve without modifying walls, so I thought that the NRC information would be more useful to normal audio people.

For actually blocking sound, it's a combination of mass, airtight-ness and de-coupling. Staggered studs with elastically fixed drywall and insulation for example. You probably know this stuff but it's rare to see new construction really do this stuff right.

When building material costs come down to sane levels, I think I will build out the room in my basement with a room inside the room, with double layers of gyp, fiberglass in the cavities, and so on. Inside this room, with the treatments I am building now, I think sound ingress will be fairly reasonable.

One of the things which makes voice recording a bit easier is that you filter everything below 80hz (or higher) which tend to be the frequencies that travel through everything. Also, unlike music, which can be spoiled by a helicopter or dump truck, if you hear a noise, you just need to repeat a sentence.

Got it, sorry if I was talking down to you. I had a couple of grad courses in acoustics way back in college but my career took a different path.

I was lucky in that we were finishing our basement so I used Kinetics Iso Max clips to float the walls and ceiling (concrete floor). My builder liked them, and I helped out a bit when another client of his used the concepts on a small studio. We built a suspended inner room for the recording booth, which took away about 6" from every surface, but it was a larger room than it sounds (reads) like you have.

Edit: Lots of sites have data showing the additional absorption with an air space, so of course I don't have any of them bookmarked. I know the NRC (in Canada) has a bunch of data on their site. Basically the absorber converts sound energy to heat in proportion to the velocity, but right at the wall the velocity is zero as it hits and reflects back, so spacing them off the wall a little where there is higher ("normal") velocity helps.

Edit 2: Holes in the back, yes, but why not just use a couple of narrow strips on the back instead and hang them on those, leaving the back mostly open?

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#### 617

##### Major Contributor
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Got it, sorry if I was talking down to you. I had a couple of grad courses in acoustics way back in college but my career took a different path.

I was lucky in that we were finishing our basement so I used Kinetics Iso Max clips to float the walls and ceiling (concrete floor). My builder liked them, and I helped out a bit when another client of his used the concepts on a small studio. We built a suspended inner room for the recording booth, which took away about 6" from every surface, but it was a larger room than it sounds (reads) like you have.

No need to apologize, I like spelling out everything I know because it helps other people learn.

How did your room turn out? Do you find it's a bit bassy, or does the added intelligibility balance it out?

From what I understand about the Kinetics clips is that they are a way of mounting RC without just screwing it onto a stud.

If you look at their data here:
https://kineticsnoise.com/isomax/testing-results.html

Simply using the RC clips on one side of a conventional gyp/stud+insulation/gyp wall gives you an STC of 57. From my source, that wall would normally have an STC around 39. That's an 18db difference! Obviously this is A weighted or whatever, but still, that's no gimmick.

#### DonH56

##### Master Contributor
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No need to apologize, I like spelling out everything I know because it helps other people learn.

How did your room turn out? Do you find it's a bit bassy, or does the added intelligibility balance it out?

From what I understand about the Kinetics clips is that they are a way of mounting RC without just screwing it onto a stud.

If you look at their data here:
https://kineticsnoise.com/isomax/testing-results.html

Simply using the RC clips on one side of a conventional gyp/stud+insulation/gyp wall gives you an STC of 57. From my source, that wall would normally have an STC around 39. That's an 18db difference! Obviously this is A weighted or whatever, but still, that's no gimmick.

Thanks.

No, the room turned out well, at least IMO, but suffers from some doubled-up bass modes (nulls) due to the dimensions. I had nice prime ratios until we added a bedroom for my younger son and sucked about 1/3 of the volume out. Oh well, things we do for our kids.

It is very well isolated, at least for a home setup where I was limited in how far I could go. My son next door can't hear movies playing, and my wife upstairs no longer has to endure my trumpet when I practice.

The clips mount to a rail system so construction is very similar to normal wall construction, a boon to the builder (and drywaller). I used double panels staggered and glued together for walls and ceiling (also floating on the clips), and sealed all the seams with acoustic caulk. Typical "studio" construction. A minisplit HVAC unit is used so there is no ductwork to the rest of the house (ducts are a pain).

Kinetics and Mason have a variety of different isolation schemes but the clips were the easiest for a consumer-oriented project IME/IMO.

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#### 617

##### Major Contributor
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Thanks.

No, the room turned out well, at least IMO, but suffers from some doubled-up bass modes (nulls) due to the dimensions. I had nice prime ratios until we added a bedroom for my younger son and sucked about 1/3 of the volume out. Oh well, things we do for our kids.

It is very well isolated, at least for a home setup where I was limited in how far I could go. My son next door can't hear movies playing, and my wife upstairs no longer has to endure my trumpet when I practice.

The clips mount to a rail system so construction is very similar to normal wall construction, a boon to the builder (and drywaller). I used double panels staggered and glued together for walls and ceiling (also floating on the clips), and sealed all the seams with acoustic caulk. Typical "studio" construction. A minisplit HVAC unit is used so there is no ductwork to the rest of the house (ducts are a pain).

Kinetics and Mason have a variety of different isolation schemes but the clips were the easiest for a consumer-oriented project IME/IMO.

This is the type of resilient channel I can get right now:
https://www.clarkdietrich.com/products/rc-1-pro-resilient-channel-rcur#description

The hat channel is harder to come by in the quantities I'd need for a 36sf room. Notice that it isn't the hat shaped furring channel, and it's only screwed to the stud on one side. Sort of a shame, because those clips look cool.

However, Clark Dietrich advertises that you can use this product and get a 62 STC rating with the clip alone (I'd assume this is 2x4, batt and gyp both sides).

Thanks for reminding me of the RC. I've specified it and knew about it, but I didn't realize how effective it was for cheaply increasing STC. It's a no brainer, really.

#### localhost128

##### Member
One room is a bedroom which is 12.4' x 9.3' with a ceiling height of 8.2'.

RT60 is absoltely not valid/relevant in a bounded space of that size. no statically random-incidence, reverberant sound-field develops above the ambient noise floor. performing RT60 calcs here is a case of garbage in = garbage out.

if the primary requirement is speech, you'll want to isolate and place panels at locations that are incident of high-gain, sparse (focused) specular reflections that will be detrimental to speech intelligibility .

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#### 617

##### Major Contributor
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RT60 is absoltely not valid/relevant in a bounded space of that size. no statically random-incidence, reverberant sound-field develops above the ambient noise floor. performing RT60 calcs here is a case of garbage in = garbage out.

if the primary requirement is speech, you'll want to isolate and place panels at locations that are incident of high-gain, sparse (focused) specular reflections that will be detrimental to speech intelligibility .
That is an interesting point and I think it makes sense the more I think about it it. It seems that in smaller spaces, early reflections from harder surfaces are the biggest issues.

Do you have any references or data supporting the utility of RT60 as a metric for small spaces?

OP

#### 617

##### Major Contributor
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I just wrapped the first panel. Cutting the fabric accurately is difficult, it is something like 65" wide. I'm allowing 1 yard for each panel, which gives ample fabric to grab onto as I tighten it up and staple it in place with a pneumatic staple gun. I'm using 1/4" staples which sit flush with the fabric when driven. The folded corners are on top for a neat appearance.

I noticed when I was carrying the panel up some stairs that my ear felt something wrong with it! It was a very strange feeling. I was walking with the panel a few inches away from my right ear and it felt like there was something stuck in my right ear, but it was just the panel completely absorbing all the sound which would have gone into that ear.

I've designed a french cleat/z clip system I can use to mount these on walls:

It uses leftover scrap from the panels (masonite and fiberboard) and is designed so I can slide it side to side a few inches.
The current design leaves a c. 5/8" (16mm) gap between the wall and the panel. I think this will look good, but a closer or further mount is possible.

For ceiling mounting I haven't figured it out. It will depend on which ceiling I decide to mount it to.

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