A mass-produced harmonica has metal covers. They are often in Nickel Silver.
The Systeme BALLE harmonicas have covers made of wood.
The type of wood chosen will produce a rounder or brighter sound.
In addition to the aesthetic aspect, gilding with gold leaf makes possible to trigger unique harmonics.
The musician will then choose the materials of his instrument according to the music he wishes to play. They may tend to prefer a slightly hushed sound for playing jazz or classical music, or a clearer sound for blues.
Here a link to a youtube video made by Alexandre Thollon
In addition, the woods used during manufacturing promote bone conduction. The opening gills in the covers are then of little use for the musician to have a return of sound. By removing the gills from the covers, it is then possible to play by air blocking (= wrapping the instrument tightly in the hands, and tongue blocking using both cheeks to play a blown note and a drawn note at same time).
Testing has been conducted in partnership with ITEMM (Institut Technologique Européen des Métiers de la Musique) and the LAUM (Laboratoire d’Acoustique de l’Université du Mans).
Below is the spectrogram of the sound of a Systeme BALLE harmonica (on the left) and of an Hohner Marineband Deluxe harmonica (on the right):