Some measurements on a loud-speaker in vacuo

The electrical impedance of a moving-coil loud-speaker is measured under three different conditions: (1) with the coil held fast; (2) with the coil free, but in vacuo; (3) under normal conditions. From the basic analysis of this type of instrument, which is given briefly, it follows that this series of measurements should allow a sufficient separation of its various characteristics to lead to the possibility of estimating its actual performance, and also to indicate whether any defects shown are electrical, mechanical or acoustic in origin. The measuring equipment is described briefly, and the results are given of a series of measurements between 40 and 6000 cycles per sec. From these the characteristics of the instrument are derived. The results give some indication as to why loud-speakers of this type can give a much better performance at high frequencies than would be expected from simple theory: the inductance of the coil and the mass of the diaphragm, which both cut down the output at high frequencies, are shown to fall off rapidly as the frequency rises, while the acoustic resistance does not fall off to the same extent and is, in fact, much in excess of the theoretical radiation resistance. The characteristics found are used to re-compute the input impedance, and give good agreement with the original observations. The computation is extrapolated to obtain estimates of the output and efficiency at frequencies for which the actual observations do not give opportunity.