In order to provide a more mellow and rich tone to some voices, concert accordions are fitted with a tone chamber.
This chamber extends the air path within the instrument, bringing a more mellow sound to the voices that are placed within it.
The inclusion of a tone chamber in an accordion makes its right-hand mechanism more complicated.
Instead of being fitted with a single soundboard, a cassotto accordion has two, each equipped with its own set of pallets. The second soundboard is positioned almost perpendicular to the first, and is also fitted with reed ranks. The space in which the reed ranks are placed is the tone chamber.
Right-hand mechanism on a 3-voice concert chromatic button accordion.
In the tone chamber instrument represented on the above figure, two of its three voices are out of tone chamber, while one is inside (with smaller pallets).
Tone chamber accordion model examples: