Speaker Polyfill Vs. Foam

By David Lipscomb

Acoustic stuffing and foams enhance speaker performance.
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In the quest for better sound, something that should never be ignored is the space that speakers operate in. Whether that means the room the speaker plays in or the inside of the speaker's cabinet, Polyfill stuffing and acoustic foam both serve different purposes. Proper use and understanding of the materials will help you make that next speaker build sound better than before.


Polyfill is a polyester batting, typically sold in upholstery and fabric stores. Audio shops may sell an audio-branded form of the material, but it is effectively the same. Polyfill is used to make a speaker driver inside an enclosure operate as though it were in a slightly larger enclosure. This works by slowing down the rear wave of the speaker, which outputs in equal intensity as the forward wave. Users normally report tighter, deeper bass as a result.

Acoustic Foam

Typically called "egg crate" foam, this material is found lining the walls of professional studio recording environments and amateur theaters. Acoustic foam is designed to reduce a phenomenon called "slap echo," which is the reverberation of sound off of hard walls and surfaces. In a recording environment, this reduces the focus and clarity of the finished product. For serious listening spaces, sound is more defined and devoid of the harshness commonly associated with echo and ringing.

Specialized Foams

Many acoustic treatment companies offer a variety of specialized treatments, all designed to address specific problems. Bass traps are foam cylinders or pillars, placed in the corners of listening spaces. These prevent bass frequencies from collecting in the corners of a room, smoothing bass response across a wider listening space. Foam speaker isolation pads de-couple the speaker from the platform on which it sits, increasing clarity from reduced vibration. Other treatments diffuse the sound in a space, making it seem larger than it is. This is commonly used in commercial and residential home theater applications in the rear where surround speakers are normally placed.


With all the benefits of acoustic foams and fillers, over-use is detrimental to overall performance. Too much Polyfill can make an enclosure appear too large to a speaker driver, causing damage from moving beyond its forward-and-back excursion limits. Too much acoustic treatment placed on walls or in a listening space can make a room appear too "dead," removing the life and excitement of recordings. Extreme cases of this cause people to become dizzy and disoriented from the lack of normal environmental cues. Professional acoustic analysis using high-tech instruments allows a space to be analyzed properly, showing where and in what quantity treatments should be placed.