True EVP's are classified under 300Hz, with the lowest being recorded at 80Hz. 

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Human Voice: Useful from 300Hz - 3,500Hz

Human Hearing: 20Hz - 20,000Hz (This for the average person, some people can hear lower and higher)

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Electronic Signals:

AM radio: 535,000Hz - 1,705,000Hz

FM radio: 88,000,000Hz - 108,000,000Hz

Cordless Phones: 40,000,000Hz - 50,000,000Hz

Baby Monitors: 49,000,000Hz

Cell Phones: 824,000,000Hz - 849,000,000Hz

Garage Doors and Alarm Systems, etc: ~ 40,000,000Hz

 

Some EVP's are recorded at less than 300 Hertz with some going down to 80Hz. And some EVP's are recorded above 20,000 Hz up to 22,500Hz.

 

20 Hz is considered the normal low frequency limit of human hearing. When pure sine waves are reproduced under ideal conditions and at very high volume, a human listener will be able to identify tones as low as 12 Hz. Olson, Harry F. (1967). Music, Physics and Engineering. Dover Publications. p. 249. ISBN 0486217698.

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The human voice:

The Human voice range is from 300 Hz to 3000 Hz.

Most spirits have been recorded well below 300 Hz, ruling out someone else in the background making noise or whispering off camera. 3000, and 300 are the extreme ends. No one human alive has yet to hit either end, yet, we get EVP's every day that are in the 200 Hz range.