History:

There are not many more famous inventors than Thomas Alva Edison and Nikola Tesla that tinkered with talking to those already in the grave.  On Dec. 22 1877, Thomas Edison made his first foil recording of "Mary had a Little Lamb".  He was astonished because it worked on the first try!  Edison would go on to invent the light bulb the following year.

Starting in 1886, Edison and Tesla worked in the power industry together.  For about a year, Nikola worked for Edison.  A financial disagreement over Tesla's invention of A/C current, left them with a severed friendship and parting ways.  Tesla went to work for Edison's competition, but both remained fascinated by communicating with those dearly departed. 

Each inventor was brilliant beyond their years and focused on different areas of achieving conversation with the dead.  Edison focused on the recording of sound, leading to inventions of sound in silent movies.  Tesla concentrated on the EMF spectrum and the various levels of intensity and magnetism within the full spectrum, including X-Radiation in 1891.  Wilhelm Roentgen would go on to use this knowledge to produce the first X-Ray image.  In July 1934, Tesla announced the feasibility of his death ray as a reality.  Unfortunately, his attempts to construct is ended in flames.  He moved into a New York hotel room, trusting nobody.  He died suddenly and his work was confiscated by US government officials.  What he achieved in his final days may never be known

In the following years many people continued to experiment with various ways of communicating with the dead.  Some used radio signals with Frank's Box, other tried reel to reel recording.  To date the trial and error of EVP recording continues.  Some add "white noise", while others try to give energy to disembodied voices with small Tesla coil models.

Application:  Whatever set up you go with, here are some basic guidelines for digital or tape recorders.

• Check the batteries and have extra batteries readily available to swap out if they die quickly.
• Keep your recording short enough to be able to listen too once completed.  Don't just let it run for hours on end.  As with anything, there is a time and place to let it run.  Just prepare for long review sessions.  And check on it to be sure it's still recording, and/or place back ups.
• Set the recorder down, don't hold it in your hand.  You will pick up your own heartbeat.
• Consider three recorders in a triangular, equidistant set up in order to locate the source of an EVP.
• Tell anybody present in spirit to speak into the recorder.
• Let the group know what you are going to do, and what you need of them if this is new to them.
• Begin the session with one lead.  State the place, date, time, and any noises heard.
• Have any person in attendance clearly state their name for post analysis comparisons if needed.
• Never whisper during the session.  If you accidentally do so, state your name.
• Have the lead state any noises heard during the session.
• Remember, post analysis is far more simple if that person doesn't have to guess at sounds easily explained during the session.
• Try and limit the session to 5 minutes.  Most EVP's will be in the first 3 minutes.  It takes energy to produce and EVP.  After 5 minutes, there usually isn't much more energy for quality EVP's.
• When you ask questions, keep it short and simple.  Then pause for your answer.  Most people do not wait or listen long enough for a response.  Answering is often hard because of the energy required.

Analysis:

• Always use noise canceling headphones.
• Sit in a quiet area with minimal noise.
• Turn up the volume during your first pass.
• Watch the audio recording on a dB graph to look for sound that may be below the human range. 
• Remember, some EVP's occur during or at the same time as the audible question is asked.  This is because people think the question before asking it out loud.  The paranormal will sometimes answer that thought, and not wait for the audible question.
• By understanding our hearing basics below, you can better analyze any EVP.  Some EVP's are well below human range, while others are very much above human speaking range.  The why is because a spirit is not speaking with vocal cords, but another energy source.  This is why different animals can hear spirits that humans cannot.  This is also why you need to have a very good audio recorder capable of recording a high range of frequencies.
• Learn how to use the different enhancement properties of your software.  As with post processing images, sound can become more distorted depending on how you enhance the clarity.  Before you do anything to your audio, save the EVP clip with no enhancements.

Hearing Basics:

Loudness is measured in (dB) decibels

• The normal hearing threshold is 0-20 dB
• A forced whisper loudness is about 40 dB
• Normal speech is around 60 dB
• A loud playground is 80 db
• At 90-115 dB, prolonged exposure, more than 8 hours, can cause hearing loss.
• Above 115 dB for 1-2 hours can cause irreversible hearing loss without proper protection.

Frequency or pitch is measured in (Hz) Hertz

• Some Animals can hear both infra-sound and ultrasound; Moles, Rats, and Elephants for example
• Normal human hearing ranges from 20 Hz to 20,000 Hz
• Cats and Dogs hear up to 40,000 Hz into the ultrasound range
• The lower the frequency the easier it is to hear with less loudness. That means a water facet dripping is easier to hear than a squeaking bird for a person with no hearing deficits.

Phonics:

Another area of EVP analysis is an understand of basic phonics.  If you are a speech therapist, you are miles ahead of understanding the EVP already.  EVP's take a lot of energy, so they are usually short.  They also don't use the human vocal cords, but another energy source.  Sometimes we can use a miniature Tesla coil for an energy source.  Usually the environment;  temperature, EMF, lights, or our body energy are used by the spirit or ghost trying to communicate.

If you are not tuned in on a physic level, you need to use the tools you do have to understand the message.  By breaking down speech in to phonics, you can better hear the actual message recorded.  Beware of your brain's desire to manipulate what is heard.  Just like matrixing in pictures by making a face out of random things in nature.  We also matrix in an auditory fashion.  We tend to hear; "Help", "Get Out", and other negative phrases before actual things said.

You can learn basic phonics just by going back to the basics of learning to speak.  Break down the sounds heard in to vowels or contestants.  Build from there.  And as always, when sharing an EVP never tell the person listening what you think is said.  Allow the person to listen without contamination.  Then talk about what you each think is said.

References:

http://www.cochlea.org/en/hear/human-auditory-range

https://www.widex.com/en-us/blog/human-hearing-range-what-can-you-hear

https://www.caohc.org/

https://www.soundcityreading.net/basic-phonics-patterns-lesson-outline.html