Speaker polyfill stuffing not working


I struggle with gaining any positive results from stuffing a speaker box with polyfill.
I went by a guide (which i also found somewhere here - i'm pretty sure we are all familiar with it) basically it says that 1.5lb/feet^3 is the best amount for boxes of 3 cubic feet and lower.

my speaker box is about 70L, so around 2.5 feet^3. so according to guide i can fit about 3.75lb of polyfill in a speaker.

in reality i can't even fit that much, most i managed to stuff is about 2.7 lb per speaker, and it seems too dense already (i practically have to shove it inside and hold it so it won't spit out).

measurements don't show much aside from sudden peaks in upper regions which i definitely don't want, but i definitely don't gain bass, by ear i can say that bass actually gets worse (more compressed, less resonating, and even lower in volume (i mean dB) - sorry for such non scientific description).

what am i missing here?

shouldn't stuffing make box seem larger, and more efficient for the driver?

maybe my fibber fill is not suitable? (idk, i bough the one i could find in abundance, basically from a store that sells to plush toys DIYers. compared to pillow stuffing is looks and feels exactly the same...)

Thank you,

Best regards,


2018-11-18 9:13 pm
Is the box closed or vented? What measurements did you take? FR and/or impedance?
When you have to force it in it is cetainly too much which also shows in what you describe as lower sound pressure.
As you seem to have measurement capabilities compare impedance plots with different amounts of stuffing. The resonance peak should get lower with more stuffing. When SPL also goes down you have overdone it.
Apart from that pure measurement approach listening is necessary.


2018-04-17 6:50 pm
Hi Vadim! I will presume it is a sealed enclosure.

My preferred method is to line the internal walls, particularly the rear wall, with dense absorbent (felt sheet) then fill the remaining volume loosely with a less dense absorbent (Polyfill) - leaving sufficient space at the front so that the rear of the driver magnet does not contact the absorbent.

The fine tuning I do by ear.
...measurements don't show much aside from sudden peaks in upper regions which i definitely don't want,

No, you don't. Assuming you're referring to peaks that were not there before, you've increased density to the point it is either reflective (probably not at the densities you're referring to) or is in sufficient proximity to the drive unit that the air is mass-loading the driver suspension & cone, altering its resonant profile.

but i definitely don't gain bass,

As a rule that won't happen; since you're adding damping material, the clue is in the name. ;)

by ear i can say that bass actually gets worse (more compressed, less resonating, and even lower in volume (i mean dB) - sorry for such non scientific description).

See above; sounds like you've over-damped the alignment, and potentially causing air-load issues on the back of the drive unit.

shouldn't stuffing make box seem larger, and more efficient for the driver?

To a very limited extent re the former, not really, re the latter.
Hi Vadim,

Can you let us know what driver you’re using since its Vas together with the box volume and cone mass define the low frequency resonance of the system. If the drivers Vas is very low compared to the box volume, then the system resonance will not change much. In my experience, the resonant frequency of the system after adding stuffing seldom shifts by more that 20%, which is not a very audible amount.
Even if the resonance doesn’t change by much, it is still important to stuff the enclosure in order to minimize standing waves.



2015-01-29 6:33 am
Vadim, for what it is worth I use under carpet felt (a layer is about 6 mm thick) on the walls of the cabinet. Use about 2 layers per side and 3 layers on the back of the cabinet. The front of the cabinet ( the baffle) is best left bare of damping but structurely well braced.
The baffle effectively influences the sonic output of the loudspeaker and if it is heavily damped with absorbent material so to shall the audio quality be represented. Polyfill is really only effective in the upper midrange and the high frequency areas of the response, much less effective with low bass frequencies. A much denser material like carpet felt is better in the lower ranges and takes up less space in the cabinet. A well braced cabinet makes all the difference to the quality of the output at the bass end.

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Hello everyone!

Thank you for all the replies

here's more details about the box
its 100x39x27cm (or about 39x15.35x10.6 inches).
it is sealed, but it has a passive radiator, so i'm not sure where the box stands, i only saw multiple times that box with PR are compared to ported boxes.

the speakers are all vintage Philips
AD0141T8 tweeter
AD5060SQ4 x 2 (series) midrange
AD12200W8 woofer
AD1200 passive radiator

I have attached screen shot of woofer tech sheet - what can you tell me about the speaker and the box combo (if anything)?

The speaker was not build by me, its a used speaker that i'm experimenting with.

the box has (i mean that's how it came originally) a sheet of fiberfill stretched from top to bottom, about 2'' thick, so i thought 'maybe i can make it better'.

I do measure FR, i've yet to measure impedance - so i guess i'll start looking into that. have to admit - i tried to avoid getting too much technical about it, but seems its unavoidable 🙂...

regarding the frequency in which stuffing is effective - what frequencies are we talking about, to get a perspective? like 250hz-500hz?

I kinda suspected that stuffing will not provide more SPL for the woofer, but the wording ''bigger box = more efficient'' mislead me.

also, regarding the peaks in upper region (i'm talking about ~1kh region, that where peaks started to appear (yes, they where not there before) but the mid drivers and twitter kinda have their own enclosure (although its just plastic cap), but should't they be isolated from the box?

Thank you



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2013-05-10 4:11 pm
You want the ideal box size. Not the largest box possible! :)

The size of the box affects the Q of the low end. Too small and the bass peaks very much. Too large and the bass rolls off too much.

The idea of filling a box with stuffing is that you get about 10%-15% more virtual volume. Meaning, you can use less wood to get the ideal size. This number varies a great deal based on how dense it is, blah blah blah.

Some makers eschew it altogether. They feel it softens bass impact.