Shell testing

Transmission loss measurement of a Repercussion shell (a top closing surface is not shown)

Getting the right parameters

In order to achieve your unique voice, each Repercussion shell gets tested on a dedicated bench: A sound wave is applied in the inside of the shell and the sound flowing through the outside is measured. The difference between the two is called transmission loss. it is driven mainly by surface mass, which means thickness times material density. But other characteristics such as the size of the shell and the damping of the material will come into play.

Why ?

As shown above, the natural frequencies of the shells are directly related to the rigidity (modulus) to density ratio, for the 4 ply shells being of the same exact dimensions. For the stave mahogany shell, the thickness is really different, and the pitch is lower : the mass wins over rigidity.

Transmission loss :

is representative of the behavior of the shell under real life operation : all the surfacer is submitted to a pressure field as opposed to impact testing.

Repercussion shells

Are LIBRE (Low Impedance at Boundary for Resonance Enhancement) meaning that the shell once tested goes directly in the drum with no drilling and not any part attached.

Impact testing :

will give you some different sounds, but the boundary condition nor the excitation are representative of real life operation.

See the video above where Maple / Mahogany / Ash and Wenge then stave mahogany Repercussion shells are impact tested : different sounds indeed linked to different rigidity over density ratios, but this is not representative of what happens when the shell is in the drum.

Let’s build your sound together

%d bloggers like this: