Loudspeakers

If a drive unit is to behave pistonically, and not sacrifice sensitivity achieving it, then its diaphragm material must have a high ratio of stiffness to mass (more precisely, of tensile modulus to density). This determines the speed of both longitudinal and bending waves, and must be sufficiently high to force bending resonances – the breakup modes revealed in our CSD plots every month – above the driver's passband. Setting aside exotic composite materials, the best way to achieve this is to use a low-density metal. Aluminium is the favourite because it's relatively inexpensive and easy to work, and can be made stiffer by suitable anodising, as in the DCC mid and bass drivers here. Magnesium and titanium are less widely used but perform similarly since all three metals have a speed of sound of about 5000m/s. To do significantly better requires the use of beryllium which, with a speed of sound of 12,000m/s, is far and away the best metal for the job. But it's very expensive and, as a vapour or in finely particulate form, highly toxic. KH