On the Amplitude of Driven Loud Speaker Cones

Bragg´s method for measuring small amplitudes of vibration was developed technically for the measurement of amplitudes of driven loud speaker cones. It is shown, that amplitudes of 1 micron at 500 cycles may easily be measured within a few per cent. Nodes of symmetrical cones may be radial or circular. It is shown that radial nodes do not influence the effective sound radiation area and the effective mass although circular nodes do. A quantity η is calculated from experimental data, to which both effective mass and effective area are proportional. It is shown that circular nodes exist at as low as 500 cycles in most of the paper cones measured, except especially stiff ones, which up to 2200 cycles did not show any circular node. Effective mass and effective area of most cones diminish rapidly with increasing frequency, so as to become very small at, say, 1000 cycles. Here again, especially stiff cones made a favorable exception. Different loud speaker systems were tested as to proportionality of amplitude to a-c strength. A direct method for measuring the effective mass as a function of the frequency offered a check on these conclusions.