Quote:
Originally Posted by Hartley118
I guess the cone is a rather clever means of creating a diaphragm stiff enough to move as a whole (at low frequencies) when driven only from the centre by the voice coil. Otherwise it would flap around. I wonder who first thought of using a cone. It was certainly used in the balanced armature speakers of the 1920s before the moving coil came along, and I think in some telephone receivers.
The flat plate is feasible in an electrostatic speaker because the drive force is distributed right across the diaphragm, so there is very little bending moment involved. This tight drive control across the diaphragm helps the electrostatic speaker to avoid much of the characteristic 'loudspeakeriness' of most moving coil units.
Speakers such as the KEF B139 and Leak Sandwich increase flat diaphragm stiffness using an expanded polystyrene sandwich. Again, I wonder who did it first. Their performance suggests that the idea has proved successful.
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I agree with Martin. I also thought I'd add that acoustically the cone shape behaves as a flat piston - both in terms of its radiation impedance, and in terms of the agreement between near- and far-field radiation models based on a circular piston assumption, and what you get when you measure in practice. That is, until the cone starts to 'break up' (vibrate in its higher resonant modes) - which is (as Martin notes) what the cone shape is helping to push to higher frequencies than would otherwise the the case, by virtue of increasing the geometric stiffness of the structure (the material stiffness and inherent damping is, of course, what it is).
The radiation impedance of circular pistons has been appearing in acoustics books almost since Rice and Kellog did their thing (20s?), so these are ideas which go a long way back. A book from the 50s, Hunt 'Electroacoustics' Chap 1 gives a nice readable historical review (starting with Benjamin Franklin!) with no maths, if anyone is interested.
Sorry -
