Possible Explanations of EVP

  EVP, or Electronic Voice Phenomena, is, put briefly, the hearing of spoken words or other noises emanating from electronic equipment where no such sounds were expected.    Very often, paranormal researchers will carry a micro-cassette tape recorder or other portable audio recording device through an area of interest.  Sounds may or may not be heard by people present in the area, but of particular interest are sounds heard after the fact, when the recording is played back.

  The following list of causes is not meant to be exhaustive, but is intended to explain some of the more common and easily understood ways electronic equipment can "misbehave" to produce effects often interpreted as evidence of manipulation of the recording equipment by non-physical, autonomous entities.   Depending on interest, a future paper may go into more depth, as the author certainly has more to say on the topic.

 

RF Injection

  Sufficient RF, or radio frequency energy - particularly AM (amplitude modulated) sources, can enter a high-gain audio amplification circuit (e.g. the mic preamp or record head preamp), and cause one or more active components to enter a non-linear mode of operation, producing demodulation (often distorted!) of the AM signal, with the result of baseband audio being heard in the amplifier circuit.  Some common examples of AM sources are:

1. Nearby high-power AM broadcast transmitter (within a few miles)
2. Aircraft VHF or UHF voice transmitter - either in an aircraft or on the ground, e.g. at a nearby airport
3. CB radio transmitter, especially mobile "bootleggers" radiating at power levels as high as that of a small AM broadcast station
4. Amateur radio AM or SSB (similar to AM in some ways) phone operation

  Why is AM of particular interest?  It's because the instantaneous power produced by the AM transmitter is directly proportional to the amplitude of the original, or baseband audio signal.    The carrier frequency of an AM transmitter is only relevant in sharing the airwaves with other stations.   A near enough AM transmitter using any frequency can be heard in unexpected ways and places.   As explained above, there are a variety of ways electronic components - whether designed for the task or not - can function as a detector/demodulator of AM RF energy.   By contrast, FM (frequency modulated) reception is more complex and much less likely to be unintentionally demodulated.

Unintentional Audio Recorded

 The microphone picks up voices from an adjacent room or other source not apparent to the recording device operator.  On review of the recording the operator hears unexpected sounds and/or words.   Microphones, while modeled in ways after the human ear, are different.  What and how a mic "hears" is not how we hear.  The mic's directional and frequency response characteristics can differ markedly.  For instance, a sensitive omnidirectional microphone can make what are (correctly) perceived as faraway sounds by a person, sound much closer than they are.  Both the directional characteristics of our ears as well as the lack of binaural information from a single microphone are factors.

 

Incomplete Erasure of Magnetic Tape (analog tape devices only)

  Magnetic tape,widely used in EVP studies, has its own set of behaviors that are important to understand, the main two being erasure and print-through.   Unintentional voices may be heard on a tape in the following scenario:  The operator makes a recording over (reuses) a tape already containing voice, music or other sounds.  The recording equipment may incompletely erase the previous recording before making the new one, allowing some amount of sound from the previous recording to remain.   The reasons for this can include: head alignment differences between the recorder on which the original and new recordings were made, partial failure of the erase head or erase bias current amplifier, and poor - even momentarily - contact between the tape and erase head.   For this kind of malfunction, usually the residual sound heard is at a very low level - perhaps enough to discern words, but not clear or loud enough to readily identify as a previous recording.   Also, due to interaction between the rotation of the tape reels and misalignment between the erase and record/playback heads (a common cause of this), an incompletely erased recording may be heard as fading in and out, in a regular cycle.