Abstract
An experimental method of measuring mechanical impedance, which is used to ascertain the effective mass of vibrating discs and conical diaphragms, is described. It is shown that the effective mass of a circular aluminium disc vibrating in air is zero at the centre-stationary and centre-moving modes. At a centre-stationary mode the effective mass attains a positive maximum before the zero value and a negative maximum thereafter. From the shape of the curves for a disc it is possible to interpret those obtained for conical diaphragms. In the latter case the curves depend upon the apical angle of the cone. Three types are illustrated: (a) a large cone having ψ equal to 160°; (b) a loud-speaker cone having ψ equal to 90°, with reinforced edge; (c) a loud-speaker cone having ψ equal to 90°, mounted on a rubber annulus. In case (a) the disc characteristics are clear, whilst in (b) and (c) the behaviour is modified owing to the greater degree of conicality. The rubber surround acts as an auxiliary resonant diaphragm, introducing an abrupt change in the effective mass. Finally, the effective mass of a rigid disc vibrating in a finite and in an infinite baffle is considered.
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