Help me understand the acoustic transparency of these different fabrics

[Snoochers]

Hi all I am looking at buying some fabric from guilfordofmaine.com to line a theatre room. I'll have speakers and acoustic panels behind this fabric. My hope is for this fabric to be as transparent as possible and to not do much absorbing of its own. I want the cloth "neutral" because I am going to have tons of it, and if it absorbs certain frequencies then those frequencies will get dead real quick.

I am looking at different fabrics from guilfordofmaine.com and thankfully they publish the acoustic properties of their fabrics. Here are 3 examples:

What am I looking at here?

Here are my questions:

• My impression is that I want the thick black line to be flat and right at the top, right? Meaning no absorption at any frequency.
• If this is true, then fabric number 2 appears to be best. Is this correct?
• If so, how much is the difference between fabric 2 and fabric 1? Is it significant enough to actually prefer fabric 2, or is it negligible?
• If they're different then why is the NRC seemingly the same?
• How does the NRC work in this case? I'm a bit confused.

EDIT: here might be an even better one, or maybe it is worse if I am misunderstanding everything!

 

[pozz]

Here are my questions:

1. My impression is that I want the thick black line to be flat and right at the top, right? Meaning no absorption at any frequency.
2. If this is true, then fabric number 2 appears to be best. Is this correct?
3. If so, how much is the difference between fabric 2 and fabric 1? Is it significant enough to actually prefer fabric 2, or is it negligible?
4. If they're different then why is the NRC seemingly the same?
5. How does the NRC work in this case? I'm a bit confused.

1. Yes.
2. Yes. The Whisper 1240 is best from the plots you've selected.
3. Hard to say because the data is somewhat smoothed. I don't have experience with working with various acoustical cloth, but I've spent extra to get fibreglass absorbers with covered with Guilford of Maine FR701 2100 (no others were available at the seller) because of the below.
4. This will be easier to address below.
5. See below.

On the left is an impedance tube. It's used to measure the effects of acoustic materials, and a rig like it produced the charts above. The "anechoic termination" is an attachment which is supposed to be totally absorptive for a certain bandwidth, though as you see in the Guilford charts there is some deviation from that. Fabric is placed inside over a rigid backing and the reflected sound is measured.

NRC stands for noise reduction coefficient. 1 is totally absorptive. It's a kind of average and depends on the bandwidth measured/calculated. Acoustics is kind of sketchy when it comes to exact absorption for actually built products, and the measurement techniques and setups haven't been totally figured out. NRC results depend highly on how you construct and mount the test fixture and material. While the actual data may not be super precise (although I think impedance tube measurements are considered generally consistent), the physical properties of absorbers are well understood, so it's not like your results will be something really unexpected.

What the plots show is that, for different fabrics, their permeability to air movement changes. FR701 2100 (fabric 1) is actually more reflective than the Whisper 1240 (fabric 2) as you get get to higher frequencies (see the falling coefficient).

In terms of a practical difference between fabrics 1 and 2? I don't know. Absorption/diffusion theory says that the effects of idea acoustic treatment should be linear, aka consistent across frequency. This is similar to the line of reasoning where the idea is to get the thickest absorbers you can, since they have decreasing absorption capacity as you get toward the bass, and the thinner the treatment, the less effective they are, and the less even the resulting absorption of your room will be. It's a kind of tone control in that sense, since absorption changes the spectrum of reflections coming from your speakers. Same goes for reflective fabrics, since they will cause the absorber to be more effective at the mids than the highs. From that perspective, the most linear treatments are the most welcome, since they will minimize the spectral difference between the direct sound and reflections. Again, in terms of acoustical theory, not personal experience, a deviation of 0.2 is considered audibly significant.

 

[Snoochers]

1. Yes.
2. Yes. The Whisper 1240 is best from the plots you've selected.
3. Hard to say because the data is somewhat smoothed. I don't have experience with working with various acoustical cloth, but I've spent extra to get fibreglass absorbers with covered with Guilford of Maine FR701 2100 (no others were available at the seller) because of the below.
4. This will be easier to address below.
5. See below.

View attachment 108440

On the left is an impedance tube. It's used to measure the effects of acoustic materials, and a rig like it produced the charts above. The "anechoic termination" is an attachment which is supposed to be totally absorptive for a certain bandwidth, though as you see in the Guilford charts there is some deviation from that. Fabric is placed inside over a rigid backing and the reflected sound is measured.

NRC stands for noise reduction coefficient. 1 is totally absorptive. 0 is totally reflective. It's a kind of average and depends on the bandwidth measured/calculated. Acoustics is kind of sketchy when it comes to exact absorption for actually built products, and the measurement techniques and setups haven't been totally figured out. NRC results depend highly on how you construct and mount the test fixture and material. While the actual data may not be super precise (although I think impedance tube measurements are considered generally consistent), the physical properties of absorbers are well understood, so it's not like your results will be something really unexpected.

What the plots show is that, for different fabrics, their permeability to air movement changes. FR701 2100 (fabric 1) is actually more reflective than the Whisper 1240 (fabric 2) as you get get to higher frequencies (see the falling coefficient).

In terms of a practical difference between fabrics 1 and 2? I don't know. Absorption/diffusion theory says that the effects of idea acoustic treatment should be linear, aka consistent across frequency. This is similar to the line of reasoning where the idea is to get the thickest absorbers you can, since they have decreasing absorption capacity as you get toward the bass, and the thinner the treatment, the less effective they are, and the less even the resulting absorption of your room will be. It's a kind of tone control in that sense, since absorption changes the spectrum of reflections coming from your speakers. Same goes for reflective fabrics, since they will cause the absorber to be more effective at the mids than the highs. From that perspective, the most linear treatments are the most welcome, since they will minimize the spectral difference between the direct sound and reflections. Again, in terms of acoustical theory, not personal experience, a deviation of 0.2 is considered audibly significant.

Thank you for this and the photo of what looks like a hilariously small laptop! I think it is just my eyes being tricked, though.

The part that confuses me is the word "absorption". These fabrics seem to be very absorbing and not reflective, which sounds like a good thing. But I don't want either. I want "transparency"! The sound goes through the fabric and what I would like is 0% reduction in sound across the entire spectrum. This is aspirational of course and won't be achieved.

They have an NRC of .95-1 or whatever, which suggests to me they absorb all sound, but I suppose what it is really saying is that it reflects 0-.05 of sound, right? The microphone doesn't know whether the sound is being "absorbed" by the material or going right through it, the mic just knows that the sound isn't bouncing back. Is this a good understanding? My initial confusion likely comes from the word "absorption".

So if a material is 100% transparent then it would record an NRC of 1.0 in this test, right? Ok, so what if I have 12 inches of OC 703 absorption panels. What would I get there? If it is also 1.0, how can I tell the difference between a very transparent and very "absorbent" material?

 

[pozz]

Thank you for this and the photo of what looks like a hilariously small laptop! I think it is just my eyes being tricked, though.

The part that confuses me is the word "absorption". These fabrics seem to be very absorbing and not reflective, which sounds like a good thing. But I don't want either. I want "transparency"! The sound goes through the fabric and what I would like is 0% reduction in sound across the entire spectrum. This is aspirational of course and won't be achieved.

They have an NRC of .95-1 or whatever, which suggests to me they absorb all sound, but I suppose what it is really saying is that it reflects 0-.05 of sound, right? The microphone doesn't know whether the sound is being "absorbed" by the material or going right through it, the mic just knows that the sound isn't bouncing back. Is this a good understanding? My initial confusion likely comes from the word "absorption".

So if a material is 100% transparent then it would record an NRC of 1.0 in this test, right? Ok, so what if I have 12 inches of OC 703 absorption panels. What would I get there? If it is also 1.0, how can I tell the difference between a very transparent and very "absorbent" material?

So what's being measured is the permeability of the fabric, not its absorption.

• Anechoic termination: NRC 1.0 (full absorption)
• Adding a fabric to it: NRC 0.95 (a small proportion of the sound gets reflected; actually another thing to note is that the resolution of the NRC calculation is 0.05. There is no smaller step.)
• Replacing the fabric with a latex sheet: 0.5 (massive reflection; I'm just eyeballing this value from the charts)

but I suppose what it is really saying is that it reflects 0-.05 of sound, right?

Yup.

So if a material is 100% transparent then it would record an NRC of 1.0 in this test, right?

Yup.

Ok, so what if I have 12 inches of OC 703 absorption panels. What would I get there? If it is also 1.0, how can I tell the difference between a very transparent and very "absorbent" material?

Also 1.0 for a certain bandwidth. In this case you're making this analogy:

• Impedance tube=your room
• {insert fabric here}
• Anechoic termination=12" of fibreglass on the walls

So your final question is somewhat mixing things. The way to think about it is that the fabric, because it is in the way of the fibreglass, reflect some sound no matter what. It will make the absorbers less effective. What you're choosing here is how much a loss you can tolerate.

In your situation, the Whisper 1240 will be best because it will have the lease effect on your absorbers' efficiency and will be the most acoustically transparent for your speakers.

 

[Inner Space]

The usual rule of thumb is to pick something you're totally comfortable breathing through, if you were to hold it over your nose and mouth. Linen, maybe, with a fairly open weave. Anything will give very mild HF loss, but that's unlikely to be noticeable.

 

[Snoochers]

So what's being measured is the permeability of the fabric, not its absorption.

• Anechoic termination: NRC 1.0 (full absorption)
• Adding a fabric to it: NRC 0.95 (a small proportion of the sound gets reflected; actually another thing to note is that the resolution of the NRC calculation is 0.05. There is no smaller step.)
• Replacing the fabric with a latex sheet: 0.5 (massive reflection; I'm just eyeballing this value from the charts)

Yup.

Yup.

Also 1.0 for a certain bandwidth. In this case you're making this analogy:

• Impedance tube=your room
• {insert fabric here}
• Anechoic termination=12" of fibreglass on the walls

So your final question is somewhat mixing things. The way to think about it is that the fabric, because it is in the way of the fibreglass, reflect some sound no matter what. It will make the absorbers less effective. What you're choosing here is how much a loss you can tolerate.

In your situation, the Whisper 1240 will be best because it will have the lease effect on your absorbers' efficiency and will be the most acoustically transparent for your speakers.

Sorry I wasn't clear. I was wondering what the NRC would be of those 12" of insulation in front of the anechoic material, without any fabric involved. Would the NRC be 1.0? If so, how do I tell the difference between NRC 1.0 because it has a ton of absorption, or NRC 1.0 because it is completely transparent? Does this make sense? Maybe I am misunderstanding things.

Here is a felt product with an NRC of 1.0

It is hard to believe that felt would be so much more acoustically transparent compared to other materials, but I could be wrong!

 

[pozz]

Here is a felt product with an NRC of 1.0

View attachment 108455

It is hard to believe that felt would be so much more acoustically transparent compared to other materials, but I could be wrong!

Felt 9900 is 100% polyester. I don't know much about fabric other than the general fact that the thickness, weave, spacing and composition of the individual fibers is what's responsible for the acoustical effects.

Sorry I wasn't clear. I was wondering what the NRC would be of those 12" of insulation in front of the anechoic material, without any fabric involved. Would the NRC be 1.0? If so, how do I tell the difference between NRC 1.0 because it has a ton of absorption, or NRC 1.0 because it is completely transparent?

Oh I see. It's a frequency-dependent thing. 12" of fibreglass will absorb everything in the audible spectrum pretty well but will show signs of droop below about 200Hz. It will break the test since it's acting the same way as the anechoic termination. The resulting data wouldn't be useful.

 

[Snoochers]

Felt 9900 is 100% polyester. I don't know much about fabric other than the general fact that the thickness, weave, spacing and composition of the individual fibers is what's responsible for the acoustical effects.

Oh I see. It's a frequency-dependent thing. 12" of fibreglass will absorb everything in the audible spectrum pretty well but will show signs of drop below about 200Hz. It will break the test since it's acting the same way as the anechoic termination. The resulting data wouldn't be useful.

So the felt has a NRC of 1.0 but isn’t good in terms of transparency, right? And fibreglass would have high NRC but wouldn’t have good transparency right?

so how do I tell what a good transparent fabric is vs one that is absorbing too much?

 

[pozz]

So the felt has a NRC of 1.0 but isn’t good in terms of transparency, right? And fibreglass would have high NRC but wouldn’t have good transparency right?

so how do I tell what a good transparent fabric is vs one that is absorbing too much?

The fabrics themselves aren't doing any absorbing. What the test is measuring is their reflectivity, how much they are taking away from the absorbtive efficiency of the anechoic termination or acoustic material they are draped over.

 

[pozz]

the felt has a NRC of 1.0

That's not the rating of the fabric. The rating of the fabric is 0.01 NRC.

 

[Snoochers]

The fabrics themselves aren't doing any absorbing. What the test is measuring is their reflectivity, how much they are taking away from the absorbtive efficiency of the anechoic termination or acoustic material they are draped over.

So the felt is not taking away anything from the anechoic termination? Is it therefore almost perfectly transparent ? Can you believe that I’m actually an educated person? Lol

 

[pozz]

So the felt is not taking away anything from the anechoic termination? Is it therefore almost perfectly transparent ? Can you believe that I’m actually an educated person? Lol

You got it.

 

[Snoochers]

You got it.

For real ?? When you mentioned the qualities of the felt I thought you were trying to describe why it’s not transparent. I had never considered felt to be a good acoustically transparent material... hard to wrap my head around it.

 

[pozz]

For real ?? When you mentioned the qualities of the felt I thought you were trying to describe why it’s not transparent. I had never considered felt to be a good acoustically transparent material... hard to wrap my head around it.

Haha, you're not going to like this...

Felt 9900 is the name of the product, not the material used. Regular felt is usually made out of wool, while this stuff is a synthetic fiber

 

[Snoochers]

Haha, you're not going to like this...

Felt 9900 is the name of the product, not the material used. Regular felt is usually made out of wool, while this stuff is a synthetic fiber

it looks quite dense. Hard to believe it’s their most transparent material

 

[pozz]

it looks quite dense. Hard to believe it’s their most transparent material

It might have limitations or special use cases, or might look ugly. I think there are also limits to how tight you can stretch it over a certain surface area. Worth contacting Guilford about it.

 

[Snoochers]

It might have limitations or special use cases, or might look ugly. I think there are also limits to how tight you can stretch it over a certain surface area. Worth contacting Guilford about it.

I'm still confused and I asked a friend of mine in the field and he indicated that the charts above are measures of reflection and not of transparency so they're not really useful to me in the way I want. He's saying that the microphone only picks up sound that is reflected and cannot differentiate between sound that is absorbed by the anechoic material or absorbed by the fabric. This is what has been confusing me. Thoughts?

 

[pozz]

I'm still confused and I asked a friend of mine in the field and he indicated that the charts above are measures of reflection and not of transparency so they're not really useful to me in the way I want. He's saying that the microphone only picks up sound that is reflected and cannot differentiate between sound that is absorbed by the anechoic material or absorbed by the fabric. This is what has been confusing me. Thoughts?

He's half-right. The fabric will have almost no absorptive capacity on its own. It will, however, have a "blocking" capacity. The In this test, the closer the fabric is to the anechoic value, the less reflective it is (the less air movement it blocks), the more transparent it is.

Philosophically, you're comparing a mirror and a window. Certain fabric will become more opaque at high frequencies.

 

[Snoochers]

FYI I did land on some comments by Dr. Floyd Toole that suggest the GOM fabrics are not as acoustically transparent as they need to be. Looks like they can drop frequencies up to 10db above 4khz when you take into account the fact that sound crosses the fabrics at various angles.

https://www.avsforum.com/threads/covering-diffusors-with-gom-fr701.2936592/