Thursday, July 12, 2007

A simple test for comparing the acoustic properties of potential box materials

A common question that speaker DIYers have is: what materials should I use for building a speaker box? MDF (medium density fibre board) is one very popular answer. Plywood is another option. Then there's hardwood, glass, concrete – just about anything with a good combination of high stiffness, high density, and rapid absorption of high frequency kinetic energy.

I once experimented with polypropylene and that seemed very promising. I found some published mechanical properties of a variety of plastics and polypropylene was a bit unusual as it had a very low yield strength relative to its ultimate tensile strength. That suggested that it would readily absorb mechanical vibrations and convert the energy into heat, which is exactly what I want a speaker box to do. The classic "tap test" produced a dull thud with no discernable ringing, but that's hardly comprehensive.

So, what to use?

Well, how about a music box? It's perfect! It is:

  • Small and very portable,
  • Reproduces a variety of frequencies upwards of around 1kHz, with lots of harmonic content,
  • Does not require electricity.

Testing a material is as simple as pressing the music box against it and seeing how much of a difference it makes to the sound. Usually the difference is huge – the sound becomes dramatically louder. The apparent tone also varies depending on the size and shape of say, a block of wood.

Generally speaking, a suitable material for building the "box" part of a loudspeaker should be as acoustically inert as possible. We only want the speaker to produce sound, without various box panels vibrating and rattling in consonance.

If you'd like to recommend a good material(s) for building loudspeaker boxes, feel free to share your ideas here!