A sealed box that came out smaller than the software said, or a 15-inch sub whose box is never getting past the living room door: on the forums, the usual answer is to stuff it with polyfill because it makes the box act bigger. My Patreon supporters asked me to find out if that is actually true, so I built two boxes and measured it.
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Watch the polyfill test · 9 min
See the polyfill measurements
Jump to: The two test boxes · Where I would use this
Warning: Fiberglass insulation and power tools can be dangerous. If you're not experienced, get qualified help.
The claim I wanted to test
Polyfill is just sound absorption material that goes inside a speaker box. The claim goes like this: if a sealed enclosure is built a little too small, adding polyfill makes the speaker think it's in a much larger box. That should change the response, ideally to look like the response of the bigger box.
That's a big deal if it works, so I didn't want to guess. I wanted two boxes, one sub and a microphone.
How I set up the test
I built two sealed boxes that look identical from the front. Turned sideways, one is clearly much shallower. The deeper one is about the right size for the sub I used, the Tang Band W5-1138SMF, which is one of my favorite small subwoofers. The shallow one is about half that size.
To measure, I used the Dayton OmniMic with the OmniMic software. I put the microphone right up close to the cone. Normally I'd measure a speaker from about three feet out, but I was in a room, and I didn't want room reflections showing up in the results. Close-miking the cone gets rid of the first reflections. I also moved each box to the edge of the table so the sound wasn't bouncing off the tabletop. The first reflection was down at the floor, where it wouldn't interfere with the way I was testing.
First came the controls: the right-size box with no polyfill, and the half-size box with no polyfill. Then I would fill up the small box and see if its response moved toward the big one.
What happened as I added polyfill
The two empty boxes were clearly different. There was about a 10 Hz difference between the small box and the full-size box. That gap is what the polyfill had to close.
- About a quarter full, very loosely stuffed. It wasn't much polyfill at all, and I wasn't sure there would be a difference. There was a little one.
- About half full, lightly stuffed. Another small change, about 1 to 2 Hz. Minimal, but it was there.
- Stuffed heavily. At the rate I was going, light stuffing wasn't getting me much, so I stuffed the heck out of the box to see if it could reach the full-size box's curve.
The results were honestly pretty astonishing. The more polyfill I put in, the flatter the response got and the more extension I got on the low end. So does polyfill make the box bigger? Based on what I measured, it does.
Why polyfill changes the response
What the polyfill is really doing is changing the Q of the box. For a linear response in a sealed box, the usual target is a Q of about 0.7. Anything above that gives a hump in the frequency response and loses the low extension I'm after. Adding polyfill brought that hump down and let the bass extend lower, which is how the small box started behaving more like the bigger one.
Where this actually helps
This is the cool part. Here are two situations where I think it matters a lot.
A big sealed sub that has to fit in the house. Picture a 15 or 18 inch subwoofer in a sealed cabinet, and the builder is super excited. Then comes the news that the huge box is not going in the house. A smaller box stuffed with polyfill is a compromise everybody can live with.
A sealed two-way speaker. Think of a two-way with a 6.5 inch woofer and a tweeter, where WinISD or another program calls for a bigger box than I'd want for maximum extension. A smaller box with polyfill can get closer to the target response without the speaker growing.
One note on scope: everything in this test was a sealed box. I didn't test a ported box here, so I'm not going to claim what polyfill does in one based on this video.
The catch: measuring
There is one issue with this. For me, this only works with a measurement microphone. Without one, it's just guessing how much polyfill a box needs. Honestly, I can't say how much a given build needs, and nobody else can either unless they've already built that exact speaker themselves.
In this video I measured the box, added some polyfill, and measured again each time to see what changed. The UMIK-1 is a fantastic microphone for this, and the Dayton OmniMic is my favorite, especially for doing a lot of measurements.
Stuffing a box vs lining it
Filling a sealed box to change its Q isn't the same job as lining a cabinet. Lining the walls with foam, felt or polyfill is about absorbing internal reflections and standing waves, and in my mistakes video I talk about how the thickness of that damping material matters, with thicker material useful in larger boxes where space allows.
It doesn't even have to be polyfill for breaking up standing waves. In a short about car audio speaker pods, I mention that anything fibrous helps, like polyfill, jute or regular fiberglass insulation. I don't recommend ripping it out of your attic, though.
Prefer to watch it on YouTube? Here's Does Polyfill make a Box BIGGER? Tested and Revealed on the Toid channel.
Where I'd take it next
My speaker plans include projects where box size really matters. The DINAS plans pair a Tang Band bass driver with a midrange and tweeter in a compact cabinet. Box shape is another piece I covered in how box shape changes the sound, and the budget side shows up in the ISO-100 subwoofer post.
Keep building: The Best Subwoofer for Under $100? · The Best Shape for a Speaker · The Dinas: Do I Need a Subwoofer? · I Designed the Epic HiFi… Then I Built It · all my DIY projects
Parts Used and Mentioned
These are the parts from the polyfill test. Prices change, check the current price.
Some links below are affiliate links. Toid may earn a commission at no extra cost to you. As an Amazon Associate I earn from qualifying purchases.
| Item | Used or mentioned | Link |
|---|---|---|
| Tang Band W5-1138SMF 5.25" subwoofer | Used: the sub in both test boxes. | Parts Express |
| Poly-Fil polyester fiber fill | Recommended alternative: the Parts Express polyfill bag from the video description is no longer listed; this is polyester fiber fill. | Amazon |
| miniDSP UMIK-1 measurement microphone | Recommended: the mic I point people to for measuring their own box. | Amazon · Parts Express |
| Dayton Audio OmniMic V2 | Used: the measurement system for every curve in this test. Amazon currently shows it as unavailable. | Amazon |
Prices change, check the current price.
