How Much Poly-Fill Do I Need For A Sub Box

Norvan Martin
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If you are designing a sealed subwoofer unit, then you likely know the importance of filling the enclosure with some sort of material to improve the value of sound you get from it.

If properly used, the filling material can help to improve the quality of bass the sub delivers and its general performance. 

However, before you do that, how much poly-fill do you need for a sealed subwoofer enclosure?

You will need 0.8 lb to 1lb or (12.8oz to 16 oz) of crimped poly-fill per cubic foot of space in the sealed enclosure. As an alternative, you can add 1/2 lb at a time up to 1-1/2 pounds.

At this point, you should have a nicely filled box that’s not packed too tightly or too loosely. 

Let’s get into more details about poly-fill and how much of it you will need for your subwoofer enclosure. 

How Much Poly-Fill Do I Need For A Sealed Subwoofer?

Before we get into too much detail, follow the two criteria below to establish the amount of fill to use for your subwoofer box for improved sound quality. 

  1. For enclosures that are less than 2.5 – 3.0 cubic feet in size, use about one and a half-pound of poly-fill per cubic foot of your enclosure. Do not exceed this measure as it might lead to overfilling. 
  2. For enclosures greater than 2.5 – 3.0 cubic feet in size, use about one pound of poly-fill per cubic foot of your enclosure. Also, do not exceed this measure to avoid excess fill in your subwoofer box. 

Adding poly-fill to an enclosure causes the air spring within the enclosure to change from being “adiabatic” to “isothermal”. Stuffing a speaker box with poly-fill, therefore, makes the box appear larger to the sub-driver. The “larger” volume produces deeper bass.Poly-Fill Do I Need For A Sealed Subwoofer

Moreover, a small internal volume can sometimes be the main cause of subwoofer port noise

The term “adiabatic” is used to illustrate the lack of heat transfer in the media. However, once the poly-fill is added, an isothermal process occurs. As air passes through the poly-fill material, its fibers wiggle and result in some form of heat dissipation.

This occurs when some of the energy which the air spring creates is converted into heat. This way, surrounding air molecules are heated up, and the air becomes less dense. 

Because sound travels faster in denser media, the subwoofer relates with the system enclosure as though it is much larger than it is. Effectively, poly-fill increases the speaker enclosure size by over 40%!

With this level of increase in subwoofer enclosure size, you are more likely to enjoy the many benefits of expanded speaker volume or increased bass level. Some of the benefits of increased speaker enclosure include:

  • The expansion increases the efficiency of the speaker
  • Reduces SPL frequency and thus offers an extended bottom end. This lowers the effective damping of speakers and makes them more likely to bottom out

On the other hand, too much poly-fill may cause a few challenges. For instance, over-intensification of poly-fill causes the density of the material to increase so highly, that the fibers are no longer in a position to wiggle.

This way, the material loses its “size benefits, and the effective poly-fill volume is reduced. Also, it is important to note that poly-fill is not very effective in supporting bass extension in speakers with large enclosures. 

Is Poly-Fill Good For My Subwoofer?

Ever finished an enclosure that just sounded too boxy? You can hear unwanted reverberations, but you want to avoid the process of rebuilding the enclosure from scratch or you just do not want to.Is Poly-Fill Good For My Subwoofer

You tuck in some poly-fill and the improvement seems extraordinary.  You may not understand how it works or what it did, but you know the sound has improved. Maybe it’s the right time to learn more about it!

The white fluffy material, whether polyfill or fiberglass increases the interior volume of the subwoofer and makes it ‘think’ that it is in a much bigger box.

If you are looking to make a difference with improved bass, adding some poly-fill to your sealed subwoofer box will fix the issue.

Poly-fill is therefore a good choice for your subwoofer, especially if you intend to produce deeper low bass. Adding more poly-fill thus makes the bass deeper.

However, the quality of sound may reduce especially if the material is added in excess, but not too tight or loose. However, before you can make a selection, let’s look at the various types of fill. 

Types Of Fill

The following are the main types of fillings used for subwoofer enclosures:

  1. Polyester fiberfill
  2. Fiberglass insulation
  3. Long-fiber wool

Users with much more poly-fill experience recommend fiberfill as the best type of fill of the three.

It is easily available in local fabrics and crafts stores and is affordable. Besides, the fill is much easier to apply to the interior of your speaker enclosure.

Myths About Poly-Fill

Several myths about the use of poly-fill exist in the speaker industry. For instance, some audiophiles believe that poly-fill blocks standing waves within an enclosure.Myths About Poly-Fill

However, regarding a subwoofer enclosure, and when you are playing a fundamental frequency that falls within the range of standing waves, this belief is incorrect. 

This is because it is expected that a standing wave within the said range of frequencies should be very long – up to several feet. Therefore, there is little chance that standing waves will occur.

It is however possible that higher-order harmonic distortion will occur, and is much more likely that it will color the audio.

Now because we expect that these higher-order harmonics will have shorter wavelengths, poly-fill will be very effective. Music clarity however changes and can be extremely low at high SPL.

Poly-fill and Harmonic Distortion

If you intend to use poly-fill as a remedy especially to absorb sound distortion or standing waves, it is important to note that it is only very effective in large enclosures for midrange speakers, and is not for a subwoofer.

Does Polyfill Make Bass Deeper?

No. Polyfill does not make bass “deeper” by itself, it changes the tuning of a sealed system to allow for more low-end frequencies.

This is because bass frequencies require larger amounts of air, and a sealed box does not allow for the amount of air that is required.

To achieve more low-end bass without changing or upgrading your enclosure you need to add polyfill.

Can a Sealed Sub Box Be Too Big?

This can be answered with a simple ‘Yes’ or ‘No’. The answer depends on what you are doing.

If your goal is to make the subwoofer as loud as possible, then having a big box will do this.

A bigger sealed enclosure equals more air volume. This means that the subwoofer has more space to move air back and forth creating louder sound waves.

Therefore, this will end up causing a greater intensity of bass energy for listeners, making it seem like the sub sounds much better than smaller subs because there’s just so much “boom” being produced by them!

Does Sealed Subwoofer Box Size Matter?

Yes, the sealed subwoofer box size tends to matter. To find out how much polyfill you should use for your sealed subwoofer box, all you need to do is measure from the inside of the speaker location.

After that, add that amount of fill material before installing your speaker.

This will make sure that your sound doesn’t become too boomy or wonky while giving off an accurate level response curve around the entire frequency spectrum.

Bottom Line

Poly-fill can be applied to both sealed and ported subwoofer boxes. However, there is no standard way of fixing the stuffing material as every audiophile will have their preferences.

The right amount will always be the amount that sounds best, so take some time to add and subtract and ensure that you don’t pack it in too tightly and make sure it fills the entire space.

It is therefore more advisable that you apply a quantity that is within the recommended ranges but always make your sound preferences a priority. 

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Norvan Martin is the founder of He is a professional Electronics Engineer and is passionate about home theater systems and AV electronics. BoomSpeaker was created as an online hub to share his knowledge and experiences as it relates to home theaters and home audio electronics. My email: [email protected]  Connect on Pinterest and Linkedin