Horn speakers materials overview
When I started to construct my first DIY horn loudspeakers, I had to decide what material to use for building the loudspeaker enclosures. It is well known that the materials a loudspeaker cabinet is made from can affect to the sound quality produced by the speaker.
A kind of "Coloured sound" is the result due to the resonance of the cabinet walls. This subject is often discussed among speaker manufactures, and there are lots of opinions if the material of the cabinet should contribute to the sound or not.
For my DIY horn loudspeakers I choose the FE206E driver by Fostex. Fostex recommended the use of plywood, that's why I decided to learn more about this material, and its sound insulation and damping characteristics.
Although I am not an expert in building loudspeakers I know that for building a well damped speaker cabinet there is a combination of strength, stiffness, surface stability and the sound insulation of the material.
What is plywood ?
Plywood is made up of thin multiple cross-banded veneers which gluing together to form a sheet. The nominal thickness of birch veneers is 1.4 mm and thick conifer veneers range from 2.0 - 3.2 mm thickness.
Mechanical characteristics of plywood.
Modulus of elasticity.
Modulus of elasticity, is the mathematical description of an object or substance's tendency to be deformed elastically when a force (in our case sound vibrations) is applied to it. In solid mechanics, Young's modulus (E) is a measure of the stiffness of an isotropic elastic material.
Talking about birch plywood measurements shows that as the thickness increased the material's stiffness increased also. So a large number of of thin plies will give a more satisfactory engineering material for cabinets construction.
Sound absorbent and insulation characteristics
Because sound insulation is dependent on the density of the insulating material, the Technical Research Centre of Finland performed tests and determined that plywood is a good insulating material in relation to its weight. For this reason is suitable for acoustic improvement solutions (opera halls, concert halls etc) . The average measured sound reduction index (for the frequency range 100-3200 Hz) for single panels of plywood is given in the next table
( measurements according to the Technical Research Centre of Finland )
Also, I made a graph, according to these measurements where we can see that as the thickness increase more sound reduction is achieved.
...Manufactured products such as plywood and chipboard are more consistent and nearly isotropic and are therefore potentially suitable for use in loudspeaker cabinets. Of the two , plywood is preferred because it is manufactured in a grater range of thicknesses and has less resonant modes for a given configuration than chipboard...
1977 H.D Harwood , R. Mathews "FACTORS IN THE DESIGN OF LOUDSPEAKER CABINETS"
In my DIY horn loudspeakers project I choose for building up the loudspeaker cabinets a type of Finnish birch plywood 21 mm thick ( 15 plies of hardwood veneered plywood layers ) for main section and side panels.
This type of birch plywood sheets has a sound reduction index more than 23.8 dB. The result is a well damped cabinet.
However the answer in the question if the speaker cabinet should contribute to the sound or not , is purely subjective. It all depends on the listeners preference.
Read a review of my DIY loudspeakers here