Posted on Oct 5, 2012

This article has two parts. Part I describes a way to determine the effective average impedance of a pair of mono headphones or a speaker using a device called a FILVORA. This is the optimum resistance with which to drive the headphones or speaker to obtain the maximum possible volume. Part II shows how to compare the sensitivity of two speakers or two mono headphones using a dual FILVORA, even if the headphones differ greatly in impedance. Both parts also discuss how the use a FILVORA to measure the impedance and sensitivity of hi-fi stereo headphones.

Measuring Headphone Impedance with a FILVORA To use the FILVORA, connect a source of audio voice or music to the input jack J1. (I use the output jack of a transistor radio for my source.) Connect the plug of the mono headphone set or speaker to be measured to the output jack J2 of the FILVORA. Adjust the switch for the loudest volume. The correct setting indicates the average impedance. It can be very broad and somewhat hard to determine. Call it P2. Rotate the switch in one direction from P2 for a small reduction in volume to position P1 (generally a two positions movement), then in the other direction from P2 by two positions to P3. If the volume at P1 and P3 are the same, P2 indicates the average impedance of the headset. If the volume at P1 and P3 is not the same, increment both the P1 and P3 settings ccw or cc by one position. When you obtain the same volume at the new P1 and P3 positions, you are done. As stated above, the average headphone impedance is the calibration indication of the switch at point P2. Sometimes equal volume settings cannot be obtained with switch settings five positions apart. If this is the case, try to get equal volume settings four positions apart. If this is done, the average impedance is equal to the geometric mean of the settings of P1 and P3. (Take the square root of the product of the calibration readings at P1 and P3.) Note: With magnetic elements, setting the switch to a...

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