Apparent Equality of Loudspeaker Output at Various Frequencies

A type of alternation phonometer has been developed which permits rapid switching of power at two frequencies to the same loudspeaker without the distracting effect of the transients that would result from ordinary types of commutation. This result is obtained by the use of rotating condensers to provide variable capacitative reactance in the input circuit of the power amplifier that operates the speaker. The power consumed by the loudspeaker is measured with a specially constructed wattmeter of the electrodynamometer type and the output of the loudspeaker is measured by means of the torques produced on a Rayleigh disc. With the aid of the alternation phonometer and an additional capacitative reactance, the observer is able to adjust the power input to the loudspeaker until two tones of different frequencies appear to have the same intensity. The purpose of the research was to develop a method for the comparison of loudspeaker efficiency at various frequencies that could be used in ordinary laboratories with limited equipment.