It is all to do with physics and the way which vibrations in a solid object are transferred to sound waves in the air. If you put a vibration into a piece of balsa wood, it is rapidly damped down and produces very little volume. A more dense wood such as maple will transmit the vibrations more efficiently - try holding a tuning fork against a maple chair and observe the result. Part of what makes a banjo sound the way it does is the type of wood it is made from, but it also affects the volume produced. The second factor is if there is a tone ring, usually made from some form of "bell metal". This is the very heavy bit of the equation - it also produces a greater transmission of sound than just a dense wood - try holding a tuning fork against a bell - remember your ear plugs if it is in a belfry. NOW hold your tuning fork against the contents of a bucket 0f hogs livers - what do you hear? Not much! I hope that this answers your question! Quack.