There are many aspects to consider under this topic.
First off it should be stated that 20,000hz or above is consider above human hearing level and 300hz and below are considered below human hearing. (most Evp will be captured above 22,000hz or between 150hz-200hz.
EVP is not generally detected by the human ear while it is being recorded, but is often able to be heard by the naked ear upon playback. More often a good set of headphones enhances the ability to hear EVP.
It is said that spirit takes the ambient noise in the surrounding area (electronic, mechanical, human voice and even residual sounds of the same.) Some say that spirit does not need external noise to create EVP. The end result is a word, phrase or even a sentence of intelligible communication.
It is thought that energy, such as mentioned above, is easily manipulated by spirit to form the communication we call EVP. This could be the reason why some EVP sounds mechanical or robotic vs. fairly human like.
It sounds like me!
Some EVP has been known to sound like the voice of those present in the room when it was captured. Only no one present during capture remembers the words being spoken by the individual. Depending on the surrounding area the audio properties of actual spoken words might be collected by spirit from residual deposits at the location (i.e. Limestone, running water etc..) and manipulated from there.
How do they do that?
There are also instance where by two recording devices are used in the same room (it does not necessarily matter how far apart they are) and each recorder captures a distinctly different audio at the exact same moment. The theories for this are still being researched.
Some EVP is heard only when played from the playback device in reverse. It is not clear how this takes place but distinctly apparent that it does happen.
How does the EVP actually get onto the recording device? Hmm.. It is thought that spirit somehow imprints the information. Of course that is still being determined.
A few question arise concerning EVP, among them are….
Does the sound level or clarity of the EVP relate to the age or the strength of the spirit?
How much does the surrounding area help or hinder EVP?