The Science of EVP

A staple of any paranormal investigation, and some of the most compelling evidence that researchers can produce, has to be the EVP. If you’re unfamiliar with Electronic Voice Phenomenon, I’m not sure what rock you have been hiding under. Even outside of paranormal research, EVP’s have become popular in modern cinema with movies like White Noise, The Sixth Sense, and The Conjuring movies.

However, if you really aren’t familiar, an EVP is the phenomenon in which either an investigator can ask questions; or a recorder just be left indefinitely running, and during the empty space of the recording voices may be captured. Supposedly, if the investigator has done their due diligence they have somehow ruled out and proven that the voice did not originate from any known source. It is theorized that these recorded sounds are the voices of the dead.

Let’s for a moment put aside that most of these recordings have very simple explanations. Many are simply the subconscious ramblings of the investigator while others are a simple case of unidentified contamination of people or things going on in another room. But, I’m not talking about those instances. I’m referring to the cases in which a deliberate amount of control was used, and voices were still recorded. In those instances, the question is still posed: What created these messages? Before we get into the fantastic let’s discuss the logical. Outside of standard human contamination there are some very plausible possibilities that need to be ruled out first before we turn to the paranormal.

First and foremost, is the possibility of pareidolia. The fact is that our brains absolutely hate silence. So much so that it sometimes feels the need to fill in the gaps with whatever it can. Have you ever been on a date with a new person and shared an uncomfortable silence? Or, maybe a job interview in which the interviewer just stops talking? You may find that most people have a compulsory need to fill that silence with talking. Even if we can fight the physical urge to verbalize it, our brains will sometimes internalize that habit. Small patterns in the frequency of the silence often trick our brains into hearing words that aren’t there. In the earlier days of EVP recordings, it was thought that EVP’s would only show up if white noise was provided for spirits to use to make their messages. While some people use this school of thought, it’s more commonly known now that the white noise is just an anchor that our brain uses to trick us into hearing things that aren’t there and cause random patterns to sound anything but. Typically, the best EVP’s will happen in a more controlled environment and will be found in silence rather than buried in static. The phenomenon of pareidolia can be cut down through the practice of letting multiple people listen to EVP recording unbiased. Cutting the recording a few questions before and after the suspected section of audio, and without prompting the listener, see if they get the same result.

Another interesting thing that happens with modern technology is that they are known to receive stray radio signals. In fact, most devices will come with a warning telling the user that it is designed to allow signals per FCC guidelines. This can be anything from a baby monitor to your digital recorder. Sometimes your recordings are simply polluted by the stray broadcast of a nearby trucker on a walkie. These transmissions can be hard to spot as interference since they are literally people talking where people should not be talking. A big clue would be that there is usually no relevance to whatever the researcher is asking and typically the voice will be at a broadcast frequency.

With all the possibilities that exist for contamination, how can someone tell if they have a legitimate EVP? Do they even exist? The good news is that I honestly think they do. I have been doing EVP research for a long time and I was an audio engineer long before I turned my interest to the paranormal. While the answer is simple, the process of truly identifying and EVP is not as straight forward.

Getting a good EVP that will withstand the scrutiny of skeptics and scientists is an arduous task that starts during the recording process. The biggest thing that anyone worth their research will point out is the lack of control that is commonly present during audio experiments. If you’re doing EVPs outside or in any unsecure location, you are not displaying a level of control that will satisfy the scientific community. Sound waves are known to travel for miles and bounce around unpredictably. If you want your evidence to withstand scrutiny you will have to find a way to control the environment.

As of this writing I do not know of any other group who goes to the same lengths that our group does to capture their EVP’s. Typically, it’s just a few people with recorders running around in the dark. While some amazing EVPs can be captured this way; they simply cannot withstand scrutiny. To combat this, I came up with an experiment in 2014 that I feel is a step in the right direction; Knowing that I could not control the environment 100% I decided to reduce the size of the environments that I was trying to control. Eventually what I came up with was a sound box.

The sound box is a 12”x12”x12” wooden box that has several layers of insulation inside and out designed for absorbing sound. The concept is the same as a vocal booth or a sound room. Sounds outside the box will not contaminate the environment inside the box. The lid is designed to be air tight and the whole thing is lined with plastic so that air cannot move through the unit. After extensive testing I was able to get the box just right so that even when shouting very loudly mere inches away from the box, the recorder inside would not pick up my voice. The box is also lined with a metallic mesh that can be grounded via a cable. This is a built in Faraday cage that will block unwanted radio transmitions that might interfere with the recording.

Using this experiment, we place a recorder inside the box, and another synced one outside the box. We then perform our EVP experiments as normal by asking a series of questions throughout the investigation to be reviewed later. We usually keep a third recorder going so that we can do live reviews, but always leave the first two going and synced. After the investigation we will place the two recordings side by side to compare their waveforms. As you guess, the recording from inside the sound box will for the most part be a flat line except in instances where we move the box to a new room and bump the recorder a little. What you may be surprised to find is that in some rare instances we will capture EVP recordings on the control recorder. In one case we found that an EVP was captured on both devices in sync; however, on the control recorder due to there being no outside interference the EVP was much clearer being the only sound in that section of audio.

However, even with that level of control false positive can still arise. (I know there’s just no winning.) But, there is still a very vital step that must be considered, and it takes place during the validation of an EVP. Before I get into that, I we need to understand a few things about sound and the scientific principles of how sound waves work.

Sounds are vibrations that travel through the air or another medium of matter and can be heard when they reach a person's or animal's ear. These vibration waves can also be intercepted, absorbed, and reflected by a variety of surfaces. Most importantly sound travels in waves and are at their core just another form of kinetic energy that is generated by a source, travels over distance, and is intercepted by your ear to be perceived as sound.

Sound waves, when viewed in their wave format, are measured by two factors: Amplitude, which represents the loudness or Decibels (dBA) of the sound, and wavelength, which is measured in metres (m). By using a simple formula of dividing the amplitude by the wavelength, the frequency of the sound can be calculated into a unit of measurement called Hertz, which signifies the number of oscillations per second of a wave.

Frequency = Hz (1Hz = 1 Wave per second)

(f = V / λ)

λ = c / f = wave speed c (m/s) / frequency f (Hz)

This is important to our research because this is the standard by which all sounds are measured. This can tell an experienced sound engineer or investigator a lot of information about those sounds, such as origin, distance from the listener, and the acoustics of the environment. The human brain can properly perceive sounds within the range of 20 Hz to 20 kHz, meaning sounds between that range falls within the normal spectrum of hearing. Machines, voices, and all naturally occurring everyday sounds within that range sound like the normal everyday things you are used to hearing.


Sounds, which are above that scale, are called ultrasound and are probably familiar concept to most people. These sounds vibrate at such a high frequency that you cannot hear them through normal means. These sounds are produced by certain animals that use them for echolocation, and by machines that can use them in the same way in radar as well as medical equipment. Ultra sound can be used to map solid objects in three dimensions and can drive your pet’s nuts at the right frequencies. Ultrasound is any sonic vibration that occurs above 20 kHz (2000 Hz).


Infrasound refers to sound vibrations that are at a frequency too low to be heard by the human ear. Levels below 20 Hz are described as infrasonic (infrasound) and are the opposite of ultrasound (above 20,000 Hz).

While we may experience discomfort at sounds we can hear at volumes of around 80 decibels upwards, it is believed exposure to low-frequency sound vibrations which we cannot detect may also have a considerable impact on humans. In much the same way many find the audible bass of a high volume car stereo annoying, sounds at even lower frequencies may interfere with our emotions and perceptions. It is known that military forces have examined the effects of infrasound and even looked into its use as a weapon.

Exposure to infrasound has been demonstrated to affect recipients with symptoms including fear, sorrow, depression, anxiety, nausea, chest pressure, and hallucination. It can cause objects to move through vibration and some believe the body’s internal organs can be affected. It is suggested that levels above 80 decibels at frequencies between 0.5 to 10Hz may start to affect the vestibular of the inner ear thus causing disorientation. Any high volume sound can trigger the body to react by increasing respiration, heart rate, and blood pressure, but when they cannot actually hear the sound recipients are left with no explanation for the sudden onset of these symptoms. This may then lead to further effects caused by the minds possible reaction to the unknown, as outlined below.

Once the mind receives information it considers unusual it may enter into “search mode” to try and explain what is being experienced, calling on all senses to assist - sight, sound, touch, smell etc... The longer the search goes on without an answer, the more intense the scrutiny. In the extreme, the body may react in “survival instinct” - fear sets in, pulse races etc... This is the body’s natural reaction to the unknown, preparing it for possible “fight or flight” from danger. At such times, because the senses are so heightened and “in tune” for experiencing something, the brain may begin to misinterpret what the senses are picking up. Much akin to sitting alone in the dark after watching a horror movie although to a much greater extent.

This is all a natural reaction of the brain and very real to the witness. Possible triggers (either alone or in combination) are anything that may suggest something strange is occurring including high EMF, infrasound, low atmospheric pressure, carbon monoxide exposure, darkness, isolation and any stimulus that may create suggestion such as watching a spooky movie, being in a spooky location, or Ouija board use. Ocean waves are known to sometimes generate infrasound and it has been suggested to have been a possible "trigger" causing ships crews to abandon their craft in fear, only to have the ship later found mysteriously drifting about unmanned.

The range of infrasound is generally accepted to be between 0-20 hertz with a specific area of interest between 17 and 19 hertz. Tests by NASA have revealed that the human eyeball resonates at around 18Hz, to which infrasound exposure may cause a reaction and lead to hallucinations.

Infrasound occurs quite naturally at some locations and possible causes include storms, earthquakes, waterfalls, volcanoes, ocean waves and wind reacting with structures such as chimneys. Some buildings or natural features can act as Helmholtz resonators and create infrasound at high levels. Ancient places of worship or ceremonial burial such as the Maeshowe mound in Orkney, have been shown to act in this way. Some animals are sensitive to these low-frequency vibrations and may appear to "foresee" approaching storms and earthquakes. Elephants are known to use infrasound as a form of communication over long distances.

It is possible that any room with an open doorway or window can operate like a Helmholtz resonator, similar to blowing a column of air across an empty bottle. Subsonic sound can travel long distances, pass through walls and may be amplified in tunnel like structures. Standard hearing protection is of little use for subsonic sound as it often can pass straight through and may even be amplified. There have been links reported between supposedly haunted locations and the presence of infrasound, which is the reason paranormal investigators may monitor infrasound levels whenever possible.

The following text gives some insight into how sound levels including infrasound are represented (usually in pascals, micro pascals or decibels) which may be of some assistance in interpreting the results of monitoring.

There is a huge variance in sound pressure ranging from the minimum that can be heard by the human ear, 20 micro pascals, to the threshold of pain, 20 Pa (pascals). Because of this huge range, a logarithmic scale is used to represent the sound pressure level (SPL). A reference of 20 micro pascals is commonly used, is the lowest level that can be heard by the human ear at a frequency of 1000 Hz. This is equal to .02 MPa (millipascals) or 0.00002 Pa (pascals). The unknown level is compared to the 20 micro Pascal threshold which is given a value of 0 dB (decibels) and the resulting level is expressed in decibels (dB). Because the human ear perceives sound intensity differently depending on its frequency, weightings may also be applied in attempt to match what the human ear experiences. "A-weighted” levels are the most commonly used, although a “G-weighting” is perhaps more suitable for infrasound.

For comparison, dB levels for some audible sounds are given below.

0-10dB Threshold of human hearing. 10-20dB Normal breathing, rustling leaves. 20-30dB Whispering at about 1.5 meters. 40-50dB Coffee maker, library, quiet office, quiet residential area. 50-60dB Dishwasher, electric shaver, office, rainfall, refrigerator, sewing machine. 60-70dB Air conditioner, alarm clock, background music, normal conversation, television. 70-80dB Coffee grinder, toilet flush, freeway traffic, hair dryer, vacuum cleaner. 80-90dB Blender, heavy traffic, hand saw, lawn mower, ringing telephone, whistling kettle.

85dB Lower limit recommended for the wearing of hearing protection.

90-100dB Electric drill, shouted conversation, tractor, truck. 100-110dB Baby crying, boom box, factory machinery, motorcycle, subway train. 110-120dB Ambulance siren, car horn, leaf blower, walkman on high, shouting in the ear. 120-130dB Auto stereo, rock concert, chain saw, pneumatic drills, stock car races, power drill. 130-140dB Threshold of pain, air raid siren, jet airplane taking off, jackhammer. 150-160dB Artillery fire at 500 feet, balloon pop, cap gun. 160-170dB Fireworks, handgun, rifle. 170 -180dB Shotgun. 180 - 190dB Rocket launch, volcanic eruption.

The vibration of the sound alters the pressure of the medium it is traveling in - be it air, water or living cells. If the sound level is very high, the entire organism may vibrate. For instance, the pressure of artillery with a few meters can exceed 200dB which is enough to cause blood vessels to tear and could even prove fatal. A level of 140dB is enough is to damage nerves of the inner ear which could lead to permanent deafness.

The sound we can hear (20-20,000Hz) gives us fair warning, but what of the sound frequencies we cannot hear? Such high levels of infrasound can easily pass through the skin and cause organs to vibrate which can lead to symptoms commonly associated with high infrasound exposure (see above). As we cannot hear the sound the cause of the symptoms often remains unidentified - but may be just as intense and harmful as any audible sound exceeding 120dB.

Such sound, although inaudible, is still subject to the laws and principles of pressure waves and may be amplified naturally through resonance etc.

So, while the human ear can only hear a finite range of sounds that are being produced in the world, as it turns out, the human vocal cords can produce an even more finite range of sounds. The voiced speech of a typical adult male will have a fundamental frequency from 85 to 180 Hz, and that of a typical adult female from 165 to 255 Hz. This boundary can be pushed either direction during activities such as shouting or singing. Ultimately this means, with some rare exceptions to the rule, sounds falling outside of that frequency will not be produced by a human.

So why is all this important? It’s important because it shows that there are scientifically known instances of sound that are beyond our sense, but can still affect us in very profound ways. Infrasound in particular can cause hallucinations, medical issues, and even paranoia. It can case a false sense of paranormal events happening in a situation that can be explained by science. But the implications are much grander. Even though our sense cannot properly interpret the data, sounds in these frequencies can still be recorded and analyzed. It also shows us that sound is dynamic and not straight forward leaving some wiggle room for questions about specific events.

No Explanation

Sometimes we will capture EVP’s that contain what sounds like human speech outside the range of what human vocal cords are typically known to produce. Situations like this are what make for excellent evidence that electronic voice phenomenon is a real occurrence that lacks a quick, easy answer.

With this information and the right software EVPs can be analyzed using spectral analysis to see what frequency and decibel range that they fall into. By using this method, we can still identify some great candidates for paranormal voices. Many, or most, of the EVPs that are presented fall within the 80 Hz – 660 Hz range. While this doesn’t mean that they couldn’t be paranormal, it does mean that they could be made by humans or any number of contaminating factors. However, with our knowledge of sound frequency ranges and how the human vocal cords operate we are left with a range of sound in which humans could hear, but human speech should not be possible. It’s this range that EVP’s sometimes fall into that we are most interested in, because anything that is meeting this criteria can be found lacking any scientific explanation.


  1. Has recognizable elements of human speech by more than one observer.

  2. Was recorded within a reasonably controlled environment (sound box).

  3. Falls outside the range of what human vocal cords are known to produce.

Diagram of Sound Ranges

To many the conditions by which we validate our EVP’s may seem ridiculous or impossible. Admittedly it is very hard to find any recording which meet all three criteria. Amazingly enough though it has happened more than once which should give us pause. Even under a mountain of scrutiny and seemingly impossible standards there is still evidence that stands. This should give it much more weight than anything presented before.

I have no doubt that many of the standard EVPs that we have captured in the standard ranged and outside our controls are real. But at the end of the day, outside communities are looking for validity. They are looking for proof. If we are going to provide and claim that we have found proof of the paranormal, we must think critically and take drastic measure to change how we are researching this phenomenon. We have to be willing to play by the rules. While I feel the methods above certainly add many levels of credibility to the research, we still have a long way to go for the data to be considered proof. Only through critical thinking and evolving will we get there. When we step out of the shadows of pseudoscience and into the realm of controlled and legitimate experimentation, even the paranormal field can thrive and move forward.

About the Author:

Ashton Rogers is the lead investigator of NTParanormal Investigations, a scientific research team based out of Fort Worth, Texas. He has been researching the paranormal with his team of experts since 2013 and worked and consulted on over a hundred cases. Ashton is an engineer with a background in electronics and media development. When he’s not investigating cases, Ashton is a writer. Author of the book: After Dark Paranormal Investigations, a syndicated columnist, and several fictional works as well. In addition to investigation and writing, he is the executive producer of his team’s web series and monthly podcast.

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