In the past we made do with light and easily portable dictaphones and many investigators still do. They were not capable of producing high quality results that could later be played back and easily interpreted.
These days we use two main types of sound recorder, the analogue tape recorder and the digital recorder , using either tape, disc or solid state memory to store the recorded audio.
Analogue types are the standard Cassette and micro cassette recorders that just about every investigator has in their toolkit. Simple and cheap to use they allow sounds to be recorded with varying degrees of quality. Many recorders have their own built in microphones that are not at all bad at recording something like a personal memo when the speakers mouth is a couple of inches away from the microphone. The quality of the recorded audio can often be greatly improved by fitting an external microphone if you can. This has the added benefit of reducing the amount of mechanical and electrical noise that is picked up as the microphone can be positioned well away from the recorder. Specific types of microphones can also be used to best suit differing situations - Omni-directional microphones are great for general coverage and 'zoom', shotgun or cardoid types are best when one is trying to record sounds from a particular area without picking up too much from other sources.
Analogue machines do have some serious drawbacks - they are very prone to 'noise and hiss' on the recording that may mask what you are trying to record, particularly if it is a quiet sound. Using a higher quality machine improves this 'Signal to Noise' (S/N) ratio but like so many things the better the quality, the higher the price.
Many machines make use of specialized electronic circuits to remove this hiss and noise. Systems, such as those developed by the Dolby Labs are the most well known. Unfortunately use of these systems cannot be recommended for paranormal investigation work as they do alter the nature of the sounds being recorded and that could affect the quality of the evidence gathered.
Cassette tapes, whilst cheap have a very limited life and may not properly erase earlier recordings made, so it is possible to hear previous recordings underneath a newer one - this could easily fool an unwary investigator into thinking they have recorded some great EVP. To prevent that, the investigator should use each cassette only once on any investigation. However, once used a tape may be re-used for witness interviews a few more times. Cassette tapes are often available in bulk packs from some supermarkets and can work out less than 30p per tape.
Analogue machines have some advantages too; most digital recorders need to heavily compress the amount of information that is recorded in order to make effective use of the space available on the disc or memory. Most digital devices do this by chopping off the parts of the audio spectrum above and below the range we humans can hear. That may be fine for listening to music and speech but not much use if you want to investigate the broadest part of the audio spectrum or use the recording later with some form of computer analysis software. If the audio information has been removed by the compression process in the recorder then it's gone forever. Analogue machines do not use audio data compression so the information recorded onto the tape is full-spectrum (within the already mentioned limits of noise) and may be a better choice in some investigation situations.
There is a final group of audio recorders that are commonly used by paranormal investigators - these are the Dictation machines. These use the 'Micro-Cassette' tape format which means the machines themselves are often small and highly portable and most have a built-in microphone. Such machines have their uses in paranormal investigations but they are specifically designed to be used to record speech from a nearby source - they are held close to the speakers mouth and spoken into. One of the main problems with such machines is that they are frequently misused and deployed for location audio recording. The microphone is not designed to be sensitive enough to hear quieter sounds and so they may end up not being recorded properly or else buried in the general tape noise.
The small tape format also means that the signal to noise ratio is high, the smaller recording head size can also create further noise issues - any physical distortion or misalignment of the tape as it passes the head will cause additional audio problems that may fool some investigators into thinking they are hearing an anomalous event. If you plan to use such machines, then if possible use an external microphone and the best quality tapes you can find ~ in fact that applies to all analogue recorders but is especially important with these dictation machines.
Digital recorders use digital sampling of the audio which is then recorded to one of a number of media - Tapes, MiniDiscs, CDs or Solid State memory chip, the latest even using small Hard Drives similar to those fitted to laptop PCs. The current trend is increasingly for using small digital recorders that use the MP3 (or similar) file format although many investigators make extensive use of the MiniDisc or MD recording format which uses small re-writable discs - not unlike mini CDs.
Digital recorders sample the sounds via the microphone thousands of times per second and many offer the user the option of changing the sample rate - the lower the sample rate, the more data can be stored on the media and so this is often simply expressed in terms of available recording time or in terms of a quality mode - HQ (High Quality), SP (Standard Play) and LP (Long Play). Before being written to the media, the data needs to be compressed as the amount of data is still huge, the compression rate is also something that the user can adjust in some models. Sony offer LP2 and LP4 recording modes on some models with claimed recording times of more than 10 hours for a single disc. This is done by progressively lowering the sample rate or increasing the data compression levels - the results are always a reduction in the quality of the audio and an increase in the noise levels often rendering the audio almost useless for later analysis (this may not really be initially noticed when one is simply listening to the playback through the supplied 'walkman' headphones).
The standard audio sample rate for CD audio is 44.1kHz (i.e. 44,100 samples per second) MiniDisc recorders use this same rate as their standard recording rate. MP3 audio recorders can exceed this - sample rates of more than 200kHz are possible on some machines fitted with small hard drives. Such rates can be used to make very high quality recordings although at the expense of much reduced recording times.
There are several different makers producing digital recorders and at the higher end of the market some are fitted with microphone sockets. Most digital recorders are small and light and can run on batteries for many hours, if a longer recording time is required then they may be plugged into the mains via an adaptor. Digital recording media can often re-used many times too, meaning that it is a cost effective method. MiniDiscs for example can be re-used thousands of times so they are very economical to use - in fact, the disc makers claim they can be used up to 2,000 times and still record as cleanly and clearly as on day one.
Sadly, the MiniDisc format is quickly disappearing from the high street but it is still possible to find hi-Spec machines available in some stores. Sony launched the Hi-MD format in 2004, which uses larger capacity discs (1Gb) allowing the user a variety of very useful options. The Hi-MD recorders allow the user to select a very high quality audio mode known as PCM (Pulse Code Modulation), this mode is similar to that employed by Professional recording studios and has a lower compression rate that the standard methods employed in previous MiniDisc models. Coupled with a high quality microphone the quality of the recording from a Minidisc is excellent and there is virtually no hiss or noise to mask the quietest sounds and as stores clear their stock at much reduced prices some excellent audio kit can be acquired.
There are drawbacks to these digital recorders though, they are fairly expensive - even the cheapest are approaching £100 and for a machine with a good build quality and good battery life you need to spend more than £250. Some shops sell small MP3 players that offer a 'Voice Recording' option usually via a small built-in microphone. Prices vary but can be as low as £20. They may seem to be an attractive option until you hear the awful sound quality - it is very heavily compressed and the tiny microphone is not very sensitive. Such models should be avoided and have no place in the serious investigators kit. Currently, there are no solid state or hard drive digital recorders that offer a good quality built-in microphone (Sony do offer a MiniDisc dictation machine that has an excellent microphone built-in). Most of these recorders also do not have a microphone socket, recording only via a 'Line level' socket, so choose carefully and check out the features before you buy.
Another potential drawback is that the recording quality is so good that unless you have a very quality microphone your recordings will show up the weaknesses of the microphone. A good budget microphone can be found for around £40 and rise in price with specification to many hundreds or even thousands of pounds. It is best to avoid 'Dynamic' types of microphone as they need high input levels to work efficiently and are not great for recording quieter sounds. Electret - sometimes called condenser microphones are better for 'area' recordings and offer better sound quality, they do require some form of power, either from an internal battery or from the recorder - this is not an issue as most devices support such microphones.
A key consideration with all forms of digital audio recorder - except a few very high spec and outrageously expensive studio types is sample compression. Compression is needed to 'squeeze' the vast amounts of data onto the recording media to allow for reasonable recording times. Most compression methods remove unnecessary parts of the data such as all the audio frequencies above and below the normal range of human hearing i.e. 20Hz~20kHz - that's not a major problem if your recording something you can hear. Anything the witnesses report hearing so can the Minidisc. However, recent advances in paranormal research mean that we now want to listen for sounds that are outside this range (see below) and for that the Minidisc is useless.
There is another form of digital audio recorder that offers the very best of both analogue and digital techniques. The Digital Audio Tape or DAT recorder uses digital recording techniques and utilizes a cassette tape similar to an 8mm video cassette. The use of tape and the method of digitizing the audio result in a recording that has the full range of frequencies available without the removal of parts of the audio like Minidisc. The use of a tape recording medium does mean that for the highest quality each tape can only be used a couple of times and must be properly erased between each use. These DAT recorders are unfortunately very expensive, retailing at more than £2,500. They also have a limited lifespan before the recording heads need to be replaced.
A final mention in this section must be for the video camcorders. These will be dealt with later on but many of the models use a digitally recorded audio track, the data being recorded to an audio track on the videotape.
We look at sound very carefully in all investigations as it is a frequently reported component of many cases. Witnesses report that the 'apparition' may speak for example. Doors may be heard to open and close when in reality no door moves. If a witness or an investigator hears a sound then in some circumstances it may be said that they are imagining it. If the same sound is recorded by a machine then it is clearly not the result of any form of hallucination or imagination but that does not mean it is paranormal, however unusual it may at first seem.
Locations can and do have unusual acoustic properties, sounds may appear to come from unexpected sources or perhaps change in nature so that they sound strange. Infrasound, that is sound considered to be below 20Hz and therefore below our hearing threshold is also a factor that needs to be considered. Aside from any direct affects on the body of the experient it may also cause parts of a location's structure of objects within it to vibrate and produce sounds that are within the hearing range - above 20Hz.
A recording of a sound event may appear to have good evidential qualities but in reality it may not be. As an example, a recording of footsteps in an empty corridor, the recording cannot demonstrate that the corridor was actually empty at the time of the recording so the value of the evidence is diminished to others when you present your evidence later.
Sound recorders can allow the investigator to do more than simply record the sounds at a location; it is also possible with computer assistance and accurate positioning of microphones to triangulate and 'map' the position that a sound or sounds was coming from.
The method is relatively easy; At least 3 Stereo microphones are placed at several carefully marked locations throughout a building. This knowledge of the microphone placement is crucial. When they are played back it is possible to measure the difference in the time that it takes the sound to reach each microphone using a computer. This minute difference - often milliseconds - can then be used to indicate the direction from which the sound was coming from. By then comparing the different directions indicated from the already known microphone locations it is easy to triangulate the source of the sound. In some locations, such as those with long corridors or locations with unusual acoustic properties this method may only give you a rough guide as to the location of the sound event.
Sound that is above and below the normal range for our hearing is also of extreme interest to paranormal investigators as laboratory studies have shown that very low frequency sounds can cause some subjects to have 'paranormal' type experiences. A frequency of just below 19Hz for example is the frequency at which normal human eyeballs will vibrate imperceptibly. In some people this may cause a disruption to the peripheral vision and the feeling that they are being watched or that there is someone else in the room with them. Just a bit lower ~ between 14Hz and 8Hz is the frequency at which the bowel and organs, including the brain will vibrate and this can cause the victim to experience real physical discomfort and in some severe mental states such as fear and anxiety and in others feelings that strongly resemble 'paranormal' phenomena.
Very high frequency audio has also been suggested to be a component of some paranormal cases. There are a number of cases where the alleged ghost seems to exist in a speeded up time frame. Sounds have also been recorded that are much higher in pitch or at frequencies that are above our normal human range. Many animals including rodents, dogs can hear these frequencies, some animals can even produce them - Bats use very high frequency (Ultrasound) to hunt prey in the dark. It is worthy of investigation to discover any anomalies in the part of the electro-magnetic spectrum that is in the audio ranges. It is these anomalies that may give us valuable clues that may lead to further developments in paranormal research.
Very low frequency sound is very difficult to measure and requires some specialized items of equipment. Ordinary microphones are useless, even specialized microphones are unlikely to perform well enough in this 'Infrasound' region. One approach is to use a large plate or diaphragm and an accelerometer - a transducer that measures the speed and changes in vibrations within the diaphragm plate. The signal from this transducer can them be taken to a spectrum analyzer to display a read out of the frequencies that are present.