Does Polyfill make a Box BIGGER? Tested and Revealed 

My listening room was a little small acoustically, I filled it with polyfill foam and it now a perfect size... if a little bit difficult to move around.

Stuff that box and make your wife happy!

You were right! My wife rejected my massive dual 18' subwoofer project as designed. However, I told her about the virtues of fiber-fill. Then, when I assured her that a 9.5% reduction in net internal box volume would make the monstrous subwoofer "look" much smaller, she was overjoyed and promptly gave me the cash to move forward with the project. Thank God for fiber-fill.

We were doing this in the late 80's early 90's. I went with the rule of thumb 1lb per cf. I only would reduce the box by 1/3 but I'm sure 1/2 would have been fine. I had 8 kicker 8" comps in my t-bird in a 3 cf box and it went low for 8". Awesome to see you demonstrate it on here because we just played it by ear. Awesome video

I always assumed it made a difference simply because OEs use it in their boxes and we all know how much they skimp on materials to squeeze as much profit as they can. Great video!!

The first enclosure I built was back in '84, when most drivers were in the 8 ohm variety and the only crossovers out there were passive 100-150 Mh coils with caps. Finding a driver with the T/S parameters were almost non-existent. SpeakerLab was a good choice. Any-who... Enclosure stuffing was one of two ways of killing off the unwanted 100Hz and up resonances that would pile inside of the enclosure and it would always yield better low end. I even used fill in mid applications for the same reasoning.

I use fiber-fill or foam to absorb reflections and standing waves in 2 way or 3 way speakers and also because it dampen some of the midrange nasty peaks.

Excellent video! I appreciate the effort that goes into this kind of testing and love seeing the results.

Probably a bit late to the party, but BassBox Pro has for years taken into consideration and can plot out the effects of box "damping." You can choose no, light, med, or heavy damping. It's been a long time since I've used it, but it appears to assign a percentage increase of apparent volume to the box based on the amount of material.

I've used insulation inside of speakers and subwoofers to "color" the sound. The larger the box, the more the insulation, or lack of, effects the sound, but it also does effect the sound of small speakers. Very interesting, I'll have to do a similar test and measure it myself.

It's beyond me how anyone can build a speaker without polyfill and/or other absorbent material. I always stuff my boxes full - you get more low-end extension and a flatter curve (ie less booming), as described in the video. You don't really need testing gear to know this, if you've played around enough with speakers there is no question about it.But of course it's nice that a video like this has been posted for people to see and...be convinced! And it does not have to be a small box. My main speakers are huge, and they are still stuffed full.

It changes the relation of the front sine wave to the rear wave, but it will also have a direct correlation on other aspects as well (like power handling/requirements, excursion, phase, etc.)

Thanks for the vid. This is the best answer I've found so far on the poly-fil issue.

What the polyfill or any fine filament material does is change the compression of the gas inside the box from an adiabatic compression to a isothermal compression so the volume of the gas acts as a softer spring. For Isothermal compression the gas has time to exchange energy with it's environment (polyfill) so the minute increase in momentary temperature when the gas is compressed without the polyfill is changed to a constant temperature environment. There is a diminishing return because the polyfil is actually also replacing the air in the box; so stuffing it with too much polyfil will reduce the volume of the box more than the gain in softer spring constant for that air.

That's why I pack polyfill in my underwear.

Good to see the results.Very interesting.

I always hear that scenerio and think: if I'm not allowed to have large equipment in the house this isn't going to work out. But luckily I don't have that problem. Cool to see that stuffing will do more than just kill resonances anyway, cause I'm trying to fit a 15" on a bicycle just for fun

Professional audio engineer here. Love your stuff. Two things on this video; 1) the frequency response of microphones when placed close to a sound source are impacted by proximity effect which causes a significant bump in the low end response of the mic. All things being equal that doesn't invalidate your comparison as long as the mic placement is exactly the same between both measurements, but it is worth noting that the low end output of the source as measured at the microphone is inaccurate compared to the actual acoustic low end output measured at 1m. 2) When doing these kinds of measurements it is much more accurate to use sine sweeps and/or pure sinewave test tones at various specific frequencies to give precise comparisons of output level at various frequencies.

i just love the sound of a heavy stuffed box. always. just love what it does.

Interesting video. I have always thought that polyfill tends to slow down the speed of the sound wave but have heard other opinions saying that it does not significantly change the speed of sound. I think most would agree that it does dampen the wave motion (particularly the higher frequencies) which can help reduce diffraction effects in the enclosure. I designed and built a transmission line speaker and experimented with polyfil to try and tune the transmission line to optimize the base response and it seemed to work for me. They do caution that too much fill can reduce the base response. As well as improving the base extension of the speaker it helped reduce the harmonic distortion inherent in transmission line speakers.

PolyFill slows down the air inside the box resulting in the sub thinking (performing) like it's in a bigger box. A good rule to start, 1lb of PolyFill per .75cf.

I always add polyfill to my sub enclosures. But for SPL competitions, I think it’s not recommended. I think it’s because even though it drops the frequency response, it also reduces the SPL slightly. I could be wrong though since I’m not into SPL competitions.

Thanks for the informative video.

It makes sense that it works. When the air in a sealed box gets compressed it also gets hotter giving it evwn more pressure and the same thing happens in the exact opposite way when it gets depresurized. The polyfill absorbs that heat making the preassure in the box decrease during compression and increase during expansion. This would also simulate a slightly bigger box that would have less preassure overall in both circumstances due to the fact that there is more air compress and decompress.

I wish you had done this with a ported box instead, but this is really nice work

Good video. I encounter this challenge when I want to build a speaker inside of an existing object (e.g., vintage radio case). Sometimes the ideal space just isn't there. I'd love for you to address the question of whether or not you get same impact from poly-fill in a ported enclosure. Also, the "weight" of speaker poly-fill you get from places like Parts Express is much heavier and or denser than the typical pillow stuffing. What about the different impact of type of fill? what about fiberglass wall insulation? I see people using that stuff as well. You can do more on this subject that would be of great interest. Thx!

A software to calculate the polyfill amount does exist! AJHorn can calculate boxes with a roughly given amount of polyfill. You can enter a value in it for a parameter called beta (β) at different places, where zero means no polyfill and 1000 means fully stuffed. Just try out the demo version where the horn lenght parameter is limited to the values of zero and 50 centimetres. With it you can at least calculate closed boxes if you set the horn legth to zero and the areas at beginning and end of the horn to the size of piston area of the driver. You can verify the amount of polyfill by comparing the measured impedance response to the simulated one. I personally think it's totally worth the 120 euros if you use it frequently because its calculations are very accurate and it's capable of calculating closed boxes, bass reflex, transmission lines, frontloaded horns, backloaded horns and band pass boxes including the influences of a crossover network. Just above ~200 Hz its accuracy decreases because it doesn't take baffle step and measured frequency responses into account. But there are other programs for that.

I have a pair of old speakers I'm refurbishing. I couldn't find a woofer in a 10" that would work correctly with the small cabinet, so I installed a pair of varivents in the boxes. One for each box. Will that help? I'm still waiting on crossover parts so I can put these back together, but I don't have an A/B speaker to test it for myself. I'm also putting some dpstuffing in it along the walls, but not impeding the flow of air in and out of the vent.

I have a BOSE acoustic wave cd unit. Why is there poly fill in the small speakers and some by the inner speaker used in the waveguide?

You should have ran some impedance plots to see where the Fsc was before stuffing and after stuffing. That would allow you to see how much the Q of the system changed. Another thing worth noting that was not in the video is the mechanical power handling difference between the two enclosures.

Can you do same test but use the polyfill that's for quilts attached to each side of the box?

Read "The Loudspeaker Cookbook". It tells you that polyfill simulates a 20% increase in box volume - not 50%. Also, it depends on the material used for the fill. 100% wool offers the best response. It's a tough (ie dry) read, but has LOADS of information regarding box building.

what if its a prefab 7 cub ft ported box that already has it in it came from skar audio. Dont make sense to have poly if its 7 cubs and ported idk.

My thinking on this is, just as you don't want a listening room full of hard reflective surfaces, and to that end we put carpets on the floor, and some other sound absorbing material in the room, so one would want to eliminate that in a speaker box. For me it's not about "making the box seem bigger", but eliminating the hollow booming sound of an empty box with an opening in it.

That's not how it works. Woofer/enclosure systems are best modeled using Newtons Second law of motion applied to forced oscillation. Three factors control the frequency response, the moving mass, the damping factor, and the spring constant. In all but one design the damping factor and spring constant are frequency variable. For example in a ported design at some frequencies the speaker pushes air through the port easily, at others it meets strong resistance. In the acoustic suspension design the springiness force depends on the difference in air pressure between the inside and outside of the box. The relationship is linear and independent of frequency. An added bonus is the restoring force is applied uniformly over the surface of the cone. Where does the polyfill come in. The speaker is forced to push and pull air between the fibers. Properly designed the aggregate surface area of the fibers is enormous. This creates an aerodynamic drag proportional to the velocity of air. It is also independent of frequency. Therefore this design can have any desired frequency response just by adjusting these 3 variables mass, spring constant and damping. In the solution as you fill the box with polyfill you are displacing air so as you increase drag you are also increasing springiness. A suitable woofer for an acoustic suspension design will have a low Fs 20 hz or lower, a low Qms around 2 or 3 and a low Vas of around 3 o4 cubic feet. A rule of thumb is optimal enclosure size is about 1/2 Vas and about 1 pound of polyfill per cubic foot. Parts express technical department will help you optimize your design using their computer modeling program and the Theil Small parameters for your woofer.

great video! thank you. I've had a question about polyfil for a while if anyone can shine some light! I always get nervous stuffing the enclosure completely because I have a nightmare of things overheating and igniting after prolonged use if its stuffed completely and the polyfil is covering the amp board or any other electronics. Has this ever been an issue? should the components be separated from the polyfil via enclosure design, or am I overthinking things? Thanks in advance!

I have a question. I have some 30inch deep canadian scoop but I'm not getting the chest pounding bass that im expecting. Can you help me out with how far the baffle board should be from the woofer and what length the actual baffle board should be. Or can you provide me with the correct specs please, I'd appreciate some help

When you say hertz are you meaning to say decibels?

Standard fiberglass insulation works great in large cabs 15 and upwards but don't use in a ported enclosure unless you like breathing glass dust, but the Eco wool loft insulation is made of recycled plastic bottles and is much cheaper staple gun it to all internal walls sorted, to tell the truth what I notice most is a tighter thump and a less hollow flimsy output in the low-end.

I would definitely want to see more videos like this where you test bigger drivers/enclosures and ported enclosures as well. I actually thought this was a myth until I saw this.

You forgot to mention how Polyfill also dampens standing wave resonance. Given the choice, I think you'll get a flatter response curve, by using the option that works well with Polyfill. A flat frequency response, with no ringing is going to give the best sound. You want to listen through the speaker, not to the speaker.

How do the results compare in ported enclosures? Will poly fill make a small ported enclosure act like a larger one as it does for sealed enclosures?

My girlfriend said, she doesn't want my 18" ugly box in living room so I returned it. Now I have an audiophile girlfriend.

Would it actually reduce the bass output if you stuffed a box that uses a passive radiator?

PF doesn't make the box "bigger". PF slows down the sound pressure waves inside the enclosure making them act as if they were in a larger enclosure.

So if I just bought the 15" HO driver from Parts Express and the Denovo sealed enclosure, to get the best performance, I should use polyfil and a lot of it right?

I'm just curious, but have you considered eliminating the potential room reflections by simply using a vocal isolation stand positioned behind the mic? (when getting measurements)

How do i fix the opposite problem, i made a big ported box for my 13w7 tuned down to like 18 or 19 hz. After hearing this, it rsttles the damn walls but plays so low that I now want to tune it higher to maybe 26hz. Any ideas?

I thoight it helped with box vibration and resonance sound from a box sound lol Wouldnt you want the mic at you listing spot ? that's what matters right ? I didn't know their was a saying that it makes it louder but cleaner sound

When you keep saying Hertz are you referring to where the smaller box starts dropping in db / deviating from the proper size enclosure?

Surely poly fill dampens sound when sound hits it it changes how it bounces and deflects it and makes it bounce in multiple paths. It does not make it bigger, it dampens the higher frequencies and makes the lower frequencies resonate more efficiently.

I’ve always had sealed box’s in my car. Built a ported box and is much louder but the kick drums sound too boomy. I listen to speed metal. Have you done a experiment on ported box with the poly mat?

What happens if you stuff the right size box?

If you stuff your sub box it will be less peaky and be able to play lower notes better, however, it will be quieter than the same box empty. While a properly sized large box will be louder and play lower. Basically, if you want SQ lower tones and have no space stuff but if you want loud SPL and low end, you need the right space for proper box.

What if the box is a smidge on the large side already? My sub calls for 1.25cf and the box is supposed to be 1.5cf? Plus its a labyrinth "SPL" box because has a extra divider in the slot port at least doubling the length of usual prefab slot ports. (By the way it's a killer $50 prefab)

you can increase the liters with about 10% by adding poly. or any other non branded cheapers stuf like rockwool or whatever

Not only dos it change the inside dimensions of the box but it also works off of hot cold cycle’s from the air being compressed and uncompressed rapidly threw the movement of the speaker moving the air back and forth

Is a box on 44 liter good to two 8 inch speakers? Or is too small so I have to add poly-fil

Great test... I was sure that it would work, but not to such an extent.

Can rockwool do the same & can you put too much fill?

Thought experiment. The additional stuffing increases viscous damping. Viscous damping implies velocity damping. Velocity damping lengthens rise time. Lengthening rise time limits transient response. So does the addition of a large amount of stuffing end up degrading the transient response?

I always fill boxes, always had good results both audibly & on measurements.

My speakers use 15" woofers in a 5.5 cu. ft. enclosure and 10" midrange in a 0.5 cu. ft. subchamber. The woofer portion has two 4" ports and the midrange is sealed. The ports are quire long and include a 90° turn. They only really come into play where frequencies are below the driver resonance. The woofer portion of the cabinet has some poly batting to reduce internal reflections on some surfaces. The midrange poly was determined empirically. I added and removed batting and listened to a lot of music. I think you can get in the ballpark with measurements, but leave it to your ears to get it right. If you alter one speaker at a time, you can compare sounds by switching back and forth. Nothing was indicated in the video about how batting works in an enclosure. As the pressure changes, the batting provides a much larger surface area for energy exchange. The pressure is converted to heat at the fiber surface. When pressure is decreased, the fiber gives up heat as pressure. Adding surface area in this way only works to a certain extent. Adding more batting reduces the energy transfer efficiency. The sound does change. As an absurd analogy, consider singing with a mouth full of cotton candy.

What happens if you keep increasing the box size. Will the low end keep rising ?

I remember doing this! You can also do a similar trick with a vented box. Build the box a little smaller and run a longer vent. The longer vent length should be able to get to the correct tuned frequency.

Very cool video! You are very darn smart with your speaker and speaker vox knowlegde! I love it! Can u plz refer me to a book or website that tells u how to tune a speaker box for every size speaker and every kind of box.

What would the differences be in a ported/vented enclosure?

does it only work for a sealed enclosure?

So what happens when you stuff the full size box?

I was just thinking about my cousin had some old 10in zues in a slim box whooping shit and always thought that's just how old subs were

Dont you guys remember when kicker created the first solo barics...I had the 15 inch sub and the box was small I didn't think it would hit being the sub looked bigger then little cube box,hooked a punch 100 with the low end chip in it and that's the deepest bass I've ever had,..poly fill was in the box and it was a sealed..

How much cu.ft. do You gain per 12 oz. standard bag of Poly-Fill ?? I have always estimated that You gain something like .375 to .45 cu.ft. with Half the bag and about .75 cu.ft. with the whole Bag.

Isnt dacron (or pillow stuffing) exactly the same?

Might wanna document box size and amount of poly fill stuffed next time for reference but great test

Does it have The same effect in ported enclosures? Or Can it allow shorter port lengths?

Better idea, add a 2nd woofer in clamshell isobaric arrangement. Then the box acts as if it was twice the physical size (6 cu ft. vs. 3 cu. ft for example). Adding polyfill AND doing the double isobaric woofer as described will make it behave more like a 7 cu ft. box (being 3 cu ft actual and assuming the 3 cu ft box was not stuffed with polyfill).

If a box is already perfect size for the sub/speaker and you fill it up, would that cause it to have a worse sound or would it still improve it ?

Or go outside in like a parking lot or something on an early Saturday. High school football stadiums at the 50 yard line are cool too. Mic placement idea still sound. Sound...

I saw this video within a few minutes of you releasing it. And loved it! But forgot to comment. I worked in car audio for many years and always 'knew' the benefits of poly fil... and 'heard' the benefits of it. But never really knew 100% if either was true. This, without a doubt eliminated that 1% of my thinking that it was more of a placebo effect than actual results. There's very little evidence-backed stuff on RU-vid regarding this topic... You've done everyone a great service. If you ever decide to revisit this topic? the comparison/difference in possible materials to use would be great. As an example? Vance Dickason said the pink fiberglass insulation was the best material to use for sealed boxes in one of his editions of the loudspeaker design cookbook. It would great to know how it stacks up to polyfil. Again, thank you so much for this!

In my experience for subs only it's better to have none in a ported box as it's dampening the port function wich can't be avoided in some enclosures and in a sealed box it can be helpful. In a fullrange 3way (or any other fullrange speaker) it should always be used as the frequencies are so high that polyfill is needed to reduce reflections in the enclosure. TBH every audio person should have a mic. Even if it's only for getting the placement of the speakers or listening spot dailed in.

There's a reason car audio guys have been doing this for years!

My Buick Regal 4 door speakers have a soft black thick cushion filling inside each door panel. I guess that's what it is...

Does it work with ported boxes as well

Room reflection would be equal if there sitting side by side would it not? It would still be an even test.

I started building an acoustic suspension subwoofer enclousure but had to stop due to my claustrophobia, that is, I felt boxed in.

Polyfill slows down the "apparant" speed of sound, so the sound wave slows, and stretches, so it makes a lower freq. Hard to type out the explanation in full.

You should use the Woffer Tester instead of the mic and just get the Qtc and Fc readings.

Does this work on port boxes?

So if you are completely whipped by your wife....polyfill.

Polyfill works great for under seat truck boxes that are shallow mount.

Polyfill, a powerful driver, a powerful amp, and bass eq,

Richard Vandersteen uses in his enclosures, thats good enough for me.

I've heard the polly fill can make subs run hot, especially in sealed enclosures, where the sub tends to run hot already....

Adding insulation is only good to add 10% to cabinet space. It also helps to prevent box resonances.

A good compromise starting with a single woofer non polyfill stuffed ideal size box (let's say 9 cu ft) as a reference would be to build a 2/3rds size (6 cu. ft) box and stuff it about 50% with polyfill. Taking it a step further, you could use an isobaric pair of those woofers in a clamshell arrangement (woofers face to face) with one woofer inside the box and the other outside the box, and that would only require about a 3 cu. ft box. So we went from 9 cu. ft down to 3 cu. ft withOUT losing any low bass extension. Happy wife = happy life.

A very interesting test, this could have been a really great video. But... The lack of clear explanation of the results, the difficulty and actually seeing what the curves were meaning, all this conspired to make the video very unsatisfying. Just an extra minute or two of the theory and the exact results and you had a winner.

hihi toid you farvourite. driver from tangband is the one with rather high distortion :( at least mine had

What would the result be if you put polly fill in the larger box

You may have different results if you tried this in a much larger, real-world subwoofer sized box. The results may be amplified in magnitude due to box size?

Can you poly fill a box with inverted subs??