Frequency Selective Voltmeters

You are feeding hundreds or thousands of different frequencies into a device that is designed to display a single frequency. What do you expect to happen? At best the device will display just one of the many frequencies that are present in the music (and that number will be meaningless in the larger context). At worse, you will confuse the thing and it will give you a completely bogus number.

If you used an o-scope and used the auto-measure feature to tell you what the frequency is then you would likely get an equally bogus measurement. The only difference is that with an o-scope you could actually see that the number was bogus.

How do you know that the music does not contain frequencies in the 0-15 Hz range? It is entirely possible! Drums will often have some low-level noise in that range that is often not audible but is also not often filtered out of the final recording. (Note for the pedantic: frequencies under 10 Hz are often filtered out, but there could easily be stuff in the 10-15 Hz range). I also recently worked with a recording studio that had equipment putting noise in the 0-5 Hz range into their recordings.

Get an MP3 of a sine wave and redo your measurements using that, not music. If the multimeter reports the correct frequency then my answer is correct. If it reports some other frequency then the problem is something else.

Reciprocity calibration is the technique used to calibrate a microphone without the need for any calibrated sources. It relies on the basic physics of transducers that show that the ratio of the receiving response to transmitting response of most transducers (designated as reciprocal transducers) is independent of their construction and depends only on easily measured parameters such as frequency, air density and sound speed. General Radio used to make a reciprocity calibrator that included all of the necessary equipment including a cavity to hold the microphone under calibration. The process requires a reciprocal microphone. A series of electrical measurements are made while using the reciprocal microphone as both a transmitter and receiver. These measurements are combined to yield the sensitivity of the microphone under test. By varying the test frequency, a complete frequency response can be obtained. Once that is done, the test microphone is calibrated and can be used to calibrate other hydrophones by using comparison measurements. Details of reciprocity calibration can be found in any acoustic textbook as well as Wikipedia and other on-line sources. Accuracy of reciprocity calibrations is very high because all of the required measurements are electrical and, with the proper procedures, only ratios of measurements are required so a calibrated meter is not even needed (but it must be linear).