The trend in investigation these days seems to be toward using phones or an app for everything. The result of this has been more and more evidence that is unsuited for analysis or study. The problem is that phones and most low-end audio devices lack sufficient bandwidth to obtain evidence that can hold up to the level of analysis needed to validate what was obtained. The purpose of this article is to provide the requirements of recorders used for capturing EVP or other audio evidence while doing an investigation. I will not specify any individual make or model because these constantly are changing as new equipment is released and older models become obsolete. Instead, I will provide a list of specifications that apply across the board to any recorder you wish to use. It means that any recorder meeting these requirements should provide the quality of audio needed to do an evaluation of the audio obtained.

Voice Recorders

Let’s begin by clearing the field regarding voice recorders. Most of these will not meet the specifications needed for EVP / AVP analysis. They do meet what is needed for doing interviews and keeping an audio log of your investigation. Because of this limitation, I do not recommend using any voice recorder for EVP or AVP evidence collection. The reasons will become apparent as we continue with the requirements a recorder must meet to be suited for this purpose.

EVP / AVP Evidence Recorders

There are five specifications a recorder must meet to do satisfactory EVP study.

1 – The recorder must use an uncompressed digital format.
2 – The recorder must record in stereo or multiple track simultaneously.
3 – The recorder must be capable of using external microphones that can be placed 30 inches apart.
4 – The Sample rate must be 96 KBPS or greater.
5 – The recorder must use a 24 Bit A to D conversion process.

A final requirement involves shielding the recorder against stray EM Fields. A recorder should have internal shielding for this purpose, but if it fails to adequately block offending signals it is permissible to use a Faraday cage around the recorder to obtain the same protection. These details of each of these requirements follows:

Requirement 1 – Uncompressed Digital Format

Compression is a technique used to shrink the size of audio files. It comes at a cost though. When compressed,certain parts of the audio stream may be removed. That may work for regular voice, but since we don’t know exactly what comprises an EVP how can we say what can be safely removed? The solution is don’t remove anything.

There are two areas that compression may take place. First is the digitizing process where the original analog signal is converted to its digital representation. The preferred method is using a form of Pulse Code Modulation (PCM). PCM is considered lossless and provides a true representation of the original audio. This is one point where voice recorders fail. In order to save file size most voice recorders employ a method known as Code Excited Linear Prediction.(CELP). With CELP, certain parts of the original audio stream are either omitted or in some cases averaged together to make estimates of what might be present in the audio signal. It does shrink file size, but also may introduce artifacts that lead to pareidolia or false positives.

The second area where compression may take place is in the final encoding of the digital signal. The method recommended is WAV format which is lossless. It also creates very large files. To shrink the file size many recorders provide MP3 as a file format. It is lossy and should be avoided for evidence recordings. I would point out that some high-end recorders can do both formats, as well as additional methods that have varied degrees of compression. As a result, it is up to the user to know their recorder and choose the proper setting to create lossless recordings.

Requirement 2 – Record in Stereo

This is almost a no-brainer, we have two ears, why not use both of them? Our brains are designed to process sound in stereo, our recorder should also. Consider, one of the recommendations when it comes to pictures is to take multiple shots to provide redundancy. Likewise, a stereo recording is done using two amplifiers, two microphones, two channels for the entire process, providing an output to a stereo headset or two speakers.. Like two pictures, this method provides audio redundancy.

A second advantage has to do with depth of field. If you record using two channels you can view the resulting signal on an oscilloscope and determine phase shift. This may be helpful when analyzing the audio to determine direction and distance between the source of the sound and the microphones.

Requirement 3 – External Microphones

This is related to recording in stereo. If external microphones are placed about 30 inches apart it enhances the separation between channels. This can be helpful in the analysis process when it comes to comparing the degree of phase shift between the channels. Sound travels at a known speed,. 1125 feet per second. Thus direction to the origin of the sound can be calculated if we measure the difference between when the sound is detected by each microphone. An oscilloscope will easily display this difference down to milliseconds. If the sound originates from a point at 90 degrees with respect to the microphones, directly in front of them, there will be no difference. Anything else will cause the sound to arrive earlier at the microphone closest to the source. Since the distance between the microphones is known, it becomes possible to calculate the angle with respect to the microphones and the source.

Requirement 4 – The Sample Rate

This is critical to obtain a recording with minimal distortion and artifacts. Lab tests have been done which demonstrate that 96 Kilobytes per second is the lowest sample rate that can assure a quality recording. Here is how we arrive at the 96 KBPS figure.

Speech is made up of two major components, vocalizations, and fricatives. Vocalizations are the sounds created by the larynx, fricatives are those sounds created by the positioning of the tongue and lips to create modifications to the sounds. For the purpose of this discussion, we need only be concerned with the higher frequency components. These are the fricatives associated with the “S” and “T” sounds. The “S” is the highest frequency generated by speech; the “T” has the fastest rise time (that explosive start to the sound as the tongue releases that puff of air.) These two sounds have two important characteristics we need to note. They are about 3,000 Hz and they are non-sinosodial waveforms. It is from these that we must set the high-frequency specification and sample rate on our recorders.

The Nyquist Point is established as the minimum number of samples needed to simulate a sine wave. Most agree that if the sample rate is seven times the highest frequency being sampled that is sufficient. In other words seven times the frequency of the highest fricative or 7 X 3,000 = 21,000 BPS (21 KBPS). However, that only applies to a sine wave; the “S” and “T” are non-sinosodial waves. For these what is known as a square wave must be employed. A square wave is a sine wave plus an infinite number of its harmonics. Since infinity is not practical another term, called a pseudo-square wave is used. This is the sine wave plus its first three harmonics. So since we are concerned with the Nyquist Point of a Pseudo-square wave we can  set our minimum sample rate as follows:

3,000 – (Frequency of “S”)
X 7 – (Nyquist Point)
21,000 BPS (Rate for Sine wave)
X 3 – (3rd Harmonic)
63,000 BPS (Actual Sample Rate Needed )

Thus it is evident the minimum sample rate for a good digital recorder is 63,000 BPS or 63 KiloBytes Per Second (KBPS). This fits with testing conducted here where no noticeable distortion is present at 96 KBPS, and minimal degradation detected at 44 KBPS. It should also be noted we really do not know what higher frequencies might be present in an EVP. For this reason, it would be helpful to go even higher when setting a sample rate. There really is no upper limit; the higher the better since the use of higher sample rates only improve the bandwidth of your recording. Of course, cost becomes a factor at some point, you will have to decide how much exceeding the minimum is worth for your research for yourself. This is also another reason why voice recorders or phones are not suited for EVP / AVP work. The sample rate for these devices is usually 8 or 16 KBPS, far below that which can provide a distortion-free audio sample.

Requirement 5 – 24 Bit A to D Conversion

All sound is analog in nature. We hear in analog, and it is essential that a digital recorder process analog as it records its digital equivalent values. The problem is by its nature analog can have an infinite number of values, while digital must have definitive numeric values assigned to each sample made. Thus some rounding up or down must occur as the conversion takes place. Each bit added to the count will double the number of possible combinations, thus giving improved resolution to the audio quality. More bits means the recorder will not need to round off to as great of a degree to assign a specific digital value to a particular sample.

Consider, if we have a 16 bit A to D converter, there are 64,000 possible numeric values which can be represented. This may be suitable for noncritical applications, but where data is involved and an analysis to be done, it is not sufficient. By going to the 24-bit system the number of possible combinations reaches 16 million. This allows for a much finer definition, and details can be refined much more accurately.

It becomes even more important when you consider that most EVPs are very low in volume. This means the recorder is limited by the low volume to only the lowest bits of its spectrum. You are going to have to amplify the audio, sometimes hundreds of times, in order to bring it up where you can hear it. Just like the digital picture begins to pixelate when you zoom in, quality begins to deteriorate as you amplify the audio. And just like the digital picture, if you start with more detail you can amplify more before deterioration becomes a real problem. So it is advantageous to use the best resolution you can when you make a digital recording.

This covers the basics of what you need to look for when choosing your recorder. Other factors also may be important to you. Battery life, ease of operation, as well as a simple means of sending your audio to an external source are also something to be considered. In some cases, these are a matter of personal preference based on your needs. But as long as the main requirements are met, you should be able to obtain audio that is well suited for analysis in the lab.

Jim Brown has been an investigator into claims of the paranormal since the mid-1970s. Before retiring in 2010, he worked over 40 years in the field of electronics design and technology. His background has allowed him to provide insight into the hows and whys of the methods used in paranormal investigations. It has served to support what works and what does not with regards to equipment investigators utilize in their work. Jim’s current interests cover anything that seems to have no obvious explanation. That ranges from UFOs and aliens to claims of hauntings and cryptid sightings as well as what are just considered strange events. He has found most, but not all, can be explained using scientific methods. It’s those last few that keep his interest in the paranormal active. For more information about Jim, please visit his website at: