ultralightweight speaker cabinets from XPS foam

[Bob Hammond]

Hi, I'm busy building a foamie with PMF skin & plywood interior. I expect to have some XPF foam left over, and I think I'll try to make some ultra-lightweight speaker cabinets for the foamie, because XPF is acoustically inert. Here's a video about a guy who built a cello using foam for the body. It sounds pretty good!

https://www.youtube.com/watch?v=JUcPhIMQC5I

[aggie79]

Hi, I'm busy building a foamie with PMF skin & plywood interior. I expect to have some XPF foam left over, and I think I'll try to make some ultra-lightweight speaker cabinets for the foamie, because XPF is acoustically inert. Here's a video about a guy who built a cello using foam for the body. It sounds pretty good!

https://www.youtube.com/watch?v=JUcPhIMQC5I

I believe the build only used foam for a portion of the top, and the backs, sides and central portion of the front is wood veneer. The video said that the foam enhances the sound levels and amplifies the bass end. I would imagine a similarly built speaker box would have the same characteristics too.

[Bob Hammond]

We'll see.

There is a difference between a stringed instrument whose parts are under tension, and a speaker that only needs a support plate for attachment to a box of a certain internal volume and configuration. The additional weight of MDF or plywood does little or nothing to enhance sound. Indeed, many speaker cabinets have layers of acoustic fiberglass insulation to dampen the resonance of the material.

[Tomterrific]

The fiberglass or absorbent material in a speaker cabinet damps the reflected sound waves inside the box. Increasing the amount of the material gives the effect of increasing box volume. The cello example is not good. The instrument is the sound but a speaker attempts to reproduce a sound while not adding anything. A light weight cabinet can be made but it must be stiff. Heavy cabinet walls would be much easier. As an amatuer audio engineer I've had the tough job of listening to many speakers that sounded horrible but they all made sound.

T

[Bob Hammond]

Interesting. But since materials are cheap I'll give it a go. Something that is readily available today for experimenters is the smartphone with apps that can do an FFT, to do at least a rudimentary comparison of speakers. Besides that, placing any speaker inside a foamie camper will have some effects as well- maybe it will be an 'inside out' speaker with the listener inside the cabinet.

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[Bob Hammond]

Hmm, i'll need two smartphones. One to generate a sine wave sweep from 20-20kHz (although i hear nothing above 11kHz now), and the other to record the sound for the FFT.

[DrewsBrews]

My thinking on the cello is the foam starts acting like a passive radiator, actually flexing at resonant frequencies based on the shape to produce higher spl while deadening other "unwanted" frequencies. This flexing is not something generally wanted in a speaker box... However, If you have a material that is consistent you can begin to play with the resonance and tune to appropriate bass response instead of using ports, or other such devices. Though there is definitely some math to be had in order to determine the necessary thickness and size/shape of the pieces used for a given material density at the target frequency. Someone experienced in modal dynamics may be best to consult.