The subject of Electronic Voice Phenomena or EVP has provided some of the most compelling evidence of paranormal activity ever captured. But the most intriguing characteristic of Electronic Voice Phenomena is that anyone with a quality recorder can capture it; the phenomena is available to any one willing to spend the time to investigate and carefully analyze their recordings.
There are many schools of thought regarding the physical nature of EVP, but few can be called theories in a scientific sense. Although most (if not all) spontaneous field investigations are conducted by non-scientists, it does not mean that these groups are incapable of gathering evidence according to the established principles of the scientific method. Wikipedia defines Scientific Method as follows: Scientific method refers to the body of techniques for investigating phenomena, acquiring new knowledge, or correcting and integrating previous knowledge. It is based on gathering observable, empirical and measurable evidence subject to specific principles of reasoning. A scientific method consists of the collection of data through observation and experimentation, and the formulation and testing of hypotheses. Most paranormal groups conduct investigations in a very methodical manner. They document personal experiences, isolate anomalous audio and video clips and catalog everything meticulously. But when it comes to testing the commonly accepted hypotheses in the paranormal field, many groups fail to take it to the next level.
From a practical standpoint, we know the simplest explantion is also the most likely explanation. To paraphrase Occam's Razor; "All other things being equal, the simplest explanation is usually the best." Occam's razor is not concerned with the simplicity or complexity of a good explanation; it only demands that the explanation be free of elements that have nothing to do with the phenomenon (and the explanation). If we apply this approach to the physical nature of EVP, we are able to conclude that all of the available evidence supports the hypothesis that EVP is a form of acoustic energy, not an unknown form of psychic imprinting. This hypothesis can be tested using the simplest audio recorder. We know that audio recorders are capable of recording acoustic energy. This can be repeated over and over again. You can record your voice and play it back and others can hear it. If you give the recorder to someone else they can record their voice and play it back and others can hear it. This proves that an audio recorder is capable of capturing acoustic energy within a certain frequency range. Does this mean that the recorder cannot record ultrasonic or infrasonic frequencies?
Not necessarily; the feasibility of recording sound above and/or below the human range of hearing is determined by the frequency response of the recorder's microphone. The mics on commercially available recorders are not designed to pick up sounds above or below the human range of hearing. This can be verified by reading the techincal specifications of your audio recorder. Playback in and of itself does not change the pitch of a recorded sound. Audio programs such as Audacity can alter the pitch of recorded sounds, making ultrasonic and infrasonic sounds audible, but recorders do not do this automatically. This fact alone disproves the theory that EVP are composed of infrasonic or ultrasonic energy. The same can be said for the theory that words can "linger" in a room centuries after they are spoken, or that words can be imprinted on recording media by some arcane psychic process. The former is just silly, but the latter has been attempted many times over the last 50 years in controlled settings without success. For these reasons the only logical assumtion we can reach is that EVP is a form of acoustic energy. Exactly how acoustic energy can be created from "nothingness", and how speech as we know it can be produced without a physical body remain a mystery, but working under the basic premise that EVP is a form of acoustic energy makes sense.