When I first dug my heels into acoustic treatment, I had no intention of starting a business making acoustic panels. I was an audio engineer (probably much like yourself), doing mostly portable recording gigs. Fresh out of RMIT (Technical Production) and NMIT (Music Business), started to think about acoustic treatment. Anyway, long story short, I started making and selling panels. You can read more about it here.
My research on Gear Nuts and other forums led me to the infamous Owens Corning 703 - a 48kg/m3 density glass wool product, used for industrial applications. Its highly effective at broadband absorption at about 100mm, perfect for your average acoustic panel, and relatively cheap.
So this was great, I found a manufacture in Melbourne - Fletcher Insulation, their website is simply wwww.insulation.com.au - who made this equivalent product and it worked a treat. The products are FI32 (32kg/m3 density) and FI48 (48kg/m3 density).
For years we used this product, the only issue I had was handling the product during the build process. They were fine once sealed in a frame with fabric on the front and back, but being a traditional glass wool product, the glass fibers were somewhat nasty.
Polyester
Around 2022/23 I discovered a polyester acoustic product that I had no idea existed. Martini HD and XHD (high density) and Autex Reverb Blankets became widely used for direct application for studios and similar spaces. I eventually reached out to Autex to enquire more. To my surprise they could actually make a custom density and size that I needed (as the Reverb Blankets were 2.4x1.2m - too large for an average panel).
I first did my research... was this going to work just as well as the glass wool? There wasn't much on the forums about. Of course there was a lot of talk about gas flow and density, so I decided to chase up these stats, along with the regular absorption coefficients on data sheets.
Gas Flow
As many of you may know, there is a sweet spot for gas flow to match a certain depth of an acoustic panel. Now the 2 main types of materials that are used with acoustics is glass wool (made from glass) and rock wool (made from stone). They both perform (acoustically) much the same at the same gas flow, but not at the same densities. This is why gas flow is often the number to look for.
So for example:
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FI48 (glass wool, 48 kg/m³) = ~30,000 rayls/m
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Rockwool (60 kg/m³) = ~25,000 rayls/m
Despite Rockwool being denser, it’s less resistive to air flow — likely due to its coarser, more open fiber structure.
Glass wool and rockwool are both mineral-based insulation materials made by melting raw materials and spinning them into fibers. Glass wool is made from recycled glass and sand, resulting in finer, softer fibers, while rockwool is made from molten basalt or volcanic rock, producing coarser, denser, and stiffer fibers.
I'm going to leave rock wool out for the rest of these comparisons purely because it's about twice the price of glass wool and is less available - or at least 60kg/m3 batts which most suits a 100mm panel. Most are 60kg plus making them too dense for thicker panels.
Here is some more information regarding glass wool.
| Panel Thickness | Ideal GFR (rayls/m) | Typical Density Range (kg/m³) | Real-World Top Performer |
|---|---|---|---|
| 100 mm | 14,000 – 30,000 | 28 – 48 | FI32, FI48, OC703, OC705 |
| 200 mm | 6,000 – 14,000 | 20 – 28 | SoundShield, SoundScreen, SoundBreak |
| 300 mm | 3,000 – 6,000 | 12 – 20 | Loose-fill, light batts |
Key Takeaway
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Thin panels → Higher density / higher GFR
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Thick panels → Lower density / lower GFR
Matching material properties to panel thickness is the key to maximizing broadband absorption.
🔍 What Happens at the Extremes
⚠️ If GFR is too low:
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Air flows too freely — sound passes through with little resistance.
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Poor absorption overall, especially in mids/highs.
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Common in very low-density polyester or too-open glass wool.
⚠️ If GFR is too high:
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Air can't get in — sound reflects off the surface.
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You get a high-frequency bounce and poor bass performance.
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Especially problematic in thicker traps using high-density boards (e.g., 200 mm with FI48).
Decisions
With this info in mind, I thought, great! I can just match the gas flow to the corresponding polyester product. However after seeing the technical data sheets of both the FI32 and FI48 alongside the AAB (Autex product), things didn't line up. The AAB's gas flow was incredibly low for the performance it was stating to claim.
Here are the AAB and FI products side by side...
| Product | Depth | Density | R value | 125hz | 250hz | 500hz | 1khz | 2khz | 4khz | NRC | Gas Flow |
| FI fi32 | 100mm | 32kg | 3 | 0.5 | 1.26 | 1.21 | 1.08 | 1.03 | 0.97 | 1.15 | 17k |
| FI fi48 | 100mm | 48kg | 3 | 0.68 | 1.14 | 1.05 | 1.05 | 1.07 | 1.09 | 1.1 | 30k |
| Autex AAB 32kg 100mm | 100mm | 32kg | 2.64 | 0.5 | 1 | 1.1 | 1.05 | 1.05 | 1.05 | 1.05 | 5k |
| Autex AAB 48kg 100mm | 100mm | 48kg | 2.94 | 0.88 | 1.14 | 1.1 | 1.03 | 1.03 | 0.98 | 1.1 | 10k |
As you can see, the densities and performance are much the same, but the gas flow is far lower in the polyester. This was some very new news to me and seems to be unknown in the audio community.
Why? Polyester insulation is different from mineral insulators (like glass wool and rockwool) because it is made from synthetic plastic fibers rather than natural mineral fibers. Polyester fibers are thicker, smoother, and bonded by heat without resin, making them softer and less irritating to handle. Mineral insulators are made by melting rock or glass and spinning it into fine, rigid fibers bound with resin. These structural and material differences cause polyester to have lower airflow resistance and slightly different acoustic absorption characteristics compared to mineral wool products.
SO... it seems that polyester (as long as its manufacture for acoustic purposes - likely not you regular polyester batts), perform just as well, if not, BETTER at the same density as glass wool.
In reality
NOW. With all that information, there are still many caveats to all this, such as varying types of glass wool and polyester (as mentioned), whether to use an air-gap or not, or even a combination of densities (dense at the front and less dense at the back). And of course all these figures can be somewhat 'ball-parky' in theory but quite different when wrapped in a timber frame and fabric. So we should really only use all this as a guide and if you really want to scoop out the finer details, do some real world testing yourself! We might even do this at some point so you don't have to.
Long story short, we used polyester for a long time because it works just as well as glass wool and it was a lot easier for us to build/didn't have to cover the back of the panels.
Now, the price for polyester has gone up so much it's just unaffordable for us to use. So we are considering our options. And we will update our decisions here when they are made.
UPDATE 21st June: We have found another polyester supplier with reasonable prices, so will likely use this moving forward. However they only offer 20kg and 28kg batts which isn't as good as the 48kg batts we were using before in our Premium panels. But it might be a good stop gap.
>>>>>
DIY
If you are looking to DIY your own panels and want to know what's readily available, this chart will help!
| Cheapest to buy | Product | Material | R value | Depth | Density | $ batt gst | Notes |
| Bunnings | Nhauf EW S-Sheild | New tech GW | 2.5 | 90mm | 24kg | $8.21 | Form. free /low itch |
| Bunnings | Nhauf EW S-Sheild | New tech GW | 2.7 | 90mm | 28kg | $12.90 | Form. free /low itch |
| Bowens | Bradford S-Screen | Traditional GW | 2.5 | 90mm | 24kg | $14 | More VOC and itch |
| Bowens | Pink SoundBreak | Traditional GW | 2.5 | 90mm | 24kg | $9.40 | More VOC and itch |
| Pricewise | Pink SoundBreak | Traditional GW | 2.7 | 90mm | 28kg | $12.80 | More VOC and itch |
| Bowens | PS Wallblock | Polyester | 2 | 90mm | 20kg | $8.31 | Depth is less |
| Bowens | PS Wallblock | Polyester | 2.5 | 90mm | 28kg | $11.90 | Depth is less |