Over the years there has been a lot of debate regarding the nature of Electronic Voice Phenomena or EVP. One prevailing idea is that EVP is composed of either ultrasonic or infrasonic sound waves, and that is the reason why investigators are unable to hear EVP at the time of capture. Although this is an elegant idea, it is unlikely because it defies the basic understandings of acoustic energy.

Sound is very simply a physical vibration (acoustic energy) transmitted by either a gas, liquid or solid. Sound is characterized by the properties of sound waves, which are defined by frequency, wavelength, period, amplitude and speed. Although the human ear is capable of detecting sound between 20Hz and 20,000Hz, the ear is most sensitive to sound waves that register between 1000Hz and 3,500Hz. Most human speech occupies a range between 200Hz and 8000Hz. So if EVP is truly “speech” as we currently understand it, isn’t it logical to assume that EVP should occur only in that narrow frequency range?

As a commercial music student in college we were granted access to a "state of the art" synthesizer lab. These music synthesizers could produce both ultrasonic (above 20,000Hz) and infrasonic (below 20Hz) sounds, and we could see their waveforms on an oscilloscope. All of the equipment was patched into a huge 4 track reel-to-reel tape recorder. Yes, I'm dating myself here. This was back in the day when personal computers did not normally use hard drives. I know, it’s hard to believe such things existed, but they did…

Anyway, we recorded both infrasonic and ultrasonic sounds directly to tape from their source, because the frequency response of our microphones made recording such sounds impossible. When we played back the recorded infrasonic and ultrasonic sounds they were still inaudible. If we cranked up the volume all we could hear was the hiss of the speakers. Because the oscilloscope was patched in to the recording system, we could see the infrasonic and ultrasonic waveforms remained unchanged, and we knew the sound had been recorded but it remained inaudible. I distinctly remember the way people reacted when we played these sounds at high volume levels; almost everybody experienced an odd sense of uneasiness in the lab. This demonstrated quite simply that we could physically perceive the ultrasonic and infrasonic sounds although we could not hear them. However, if we increased or decreased the playback speed, which increased or decreasing the pitch, the sounds became audible.

So what did these simple experiments reveal? That ultrasonic and infrasonic sounds can be recorded, but not with everyday microphones and recording equipment. Exposure to ultasonic and infrasonic sound can adversely affect people, and cause them to experience uneasiness. Paranormal researchers in the UK have succesfully induced "paranormal" experiences in their control groups by exposing them to infrasonic and ultrasonic sounds. But most importantly, it revealed that inaudible sounds do not become audible on playback because playback in and of itself does not change the pitch of recorded sound waves. So if EVPs are a form of acoustic energy, it's clear they are not generated in the infrasonic or ultrasonic range.