A sound is produced by a source (S) in one room and heard next door (L). Where does the sound from S appear to come from to listener L? There is sound reflected from a wall (R), diffracted sound (which goes around the wall) through the doorway (D) and attenuated sound through the wall (T).
Groans and moans
Groaning noises are often caused by the wind. It can enter hollows or holes in buildings or natural features (like rock formations or trees) and sometimes produce odd noises. It works like a wind instrument, such as a flute. The wind may need to be a specific direction for such sounds to occur. Therefore, the weird noises may be intermittent, so it may not always be picked up on a single vigil.
Some hear it, some don't!
On vigils, sometimes some people hear a sound and others don't! It might be paranormal or the Cocktail Party Effect.
See here for causes and types of haunting sounds.
© Maurice Townsend 2009,2010
Weird (ghostly) sounds
Sometimes people report sounds they don't recognise and consider they may be paranormal. This may just be because the listener is unfamiliar with an otherwise completely normal sound (the same point applies to things people see). However, it can also be a familiar sound that has been modified to sound strange.
Human's sensitivity to sound increases at times of stress, making previously unheard background noises suddenly prominent.
As well as disturbances in the air, such as the wind (see top right), sound may be modified by objects between the listener and the source. So, a sound from another room in a building may be modified (made fainter and some frequencies absorbed) by a wall to sound quite different. In particular, higher frequency sounds tend to be absorbed more readily (both by the air and solid materials). Sound from other rooms will often be missing higher frequencies and so sound unusually low pitched. If the same source of the sound was in the same room as the listener it might be perfectly recognisable. Heard from two rooms away it might sound weird and ghostly. The same effect occurs out of doors where low frequency sounds carry further.
Where is that sound coming from?
Humans detect the direction of sound sources by two methods. At low frequencies (below about 1500 Hz) the phase difference in the sound waves is used. At higher frequencies (above about 4000 Hz) , the difference in sound intensities caused by the head acting as a sound shadow prevails. With a sensitivity ranging from 20 to 20000 Hz, it comes as no surprise that the reliability of this direction-finding system varies according to the frequency of the sound. In addition, faint sounds are difficult to locate because the sound differences across the head are too small to be detected by ear. To make matters worse, sounds in buildings can be reflected off walls and diffracted around obstacles. This means that the waves can approach the listener from a different direction from the actual source.
Thus it can be difficult to accurately locate the source of sounds, particularly when there are objects between the listener and the source (which may have modified the direction and frequency). Observers should therefore be cautious when declaring where a sound is coming from (see the figure top left)! Sounds are often reported from empty rooms on vigils. But why are observers so certain they know where the sound was coming from?
In EVP (electronic voice phenomena), paranormal voices are recorded even though they were not heard at the time of the recording. In fact, some of these sounds may be perfectly natural but were just forgotten at the time. We tend to get used to background noises and ignore them. However, they may appear more prominent if we're listening to many hours of recording with not much happening on it. Visit our EVP gallery.
The sound we hear is a movement in air - a compression wave in fact. Sound can also traverse other materials. Just as with light, sound diminishes with distance from its source and is absorbed or reflected by any materials in the way. However, unlike light, air carries sound whereas with light it merely gets in the way. This is important but disturbances to the air (such as winds) can distort and modify sound in a way that would leave light largely unaffected.
Sound is heard better downwind of a source than upwind. This is not because the air is 'carrying' the sound, rather it is a wind shear effect. Sound will also carry longer distances when there is a temperature inversion in the atmosphere (with higher temperatures above the ground). With gusty wind, sounds from a long way off may be fragmented and appear to come and go (like bits of EVP).
One strange aspect of sound propagation concerns very loud noises, like those from explosions. They may be heard for a long distance (kilometres) but there can be a circle of silence some way out where nothing is heard, even though the sound is audible further away. This effect is caused by the sound being reflected back from air layers high in the atmosphere. It could explain some anomalous sounds which cover a wide area and appear to have no obvious source.
Perhaps the most common reports of paranormal sounds are ghostly footsteps and 'someone moving around' in an empty room. These are both simple sounds which could be misinterpretations of mundane phenomena partly absorbed by intervening walls. They may be assigned to the wrong room because listeners' direction finding is impaired by frequency shifts (see left).