Internet users reputedly love lists of the ten coolest or ten worst kind. A regular favourite is ten most amazing optical illusions, featuring diverse arrays of motion aftereffects, waterfall illusions, impossible staircases, and Kanizsa triangles.
Less familiar are auditory illusions. The University of California psychologist Diana Deutsch is responsible for discovering many of the more recent ones, and she's given them intriguing names like the titles of Robert Ludlum thrillers, including the Tritone Paradox and the Cambiata Illusion.
One auditory illusion which is now so familiar that it's taken for granted is stereo sound reproduction. Slight differences in the sound played by two speakers spaced apart conjure up what's appropriately called the phantom image, giving the listener the impression they're hearing a continuous soundfield curving in from the sides.
The stereo experience was once considered exciting enough for Count Basie to perform the piece Stereophonic, and for Cole Porter to write the song Stereophonic Sound for Fred Astaire and Cyd Charisse in the 1957 film Silk Stockings:
If Ava Gardner played Godiva riding on a mare
the people wouldn't pay a cent to see her in the bare
unless she had glorious Technicolor, Cinecolor
or Warnercolor or Pathecolor or Eastmancolor
or Kodacolor or any color
and stereophonic sound
and stereophonic as an extra tonic
Stereo uses differences between its channels to create the illusion of spatial sound. These can include timing, frequency and loudness, although not necessarily together. One technique which combines all three to produce vivid and realistic stereo images is binaural stereo, and some UK Soundmap contributors have been using it.
The phantom image of a good binaural recording is like the glass in Philip Larkin's poem Water in which 'any-angled light would congregate endlessly'. It envelops the listener to create a strong sense of being present at the recording itself. Individual sounds seem to come from precise locations all around.
To record in binaural stereo, a tiny microphone is placed either inside or at the entrance to each canal. In this way they're well positioned to capture the differences which off-centre sounds will present to each ear.
Sound waves from a dog barking to your left will be slightly less intense when they reach your right ear compared to your left ear. They'll also arrive at your right ear up to half a millisecond later. How the brain is able to detect such a small time lag is impressive, especially since even the fastest nerve impulses are propagated at less than a third of the speed of sound.
Lastly, the mass and shape of the shoulders and of the head itself, and the convolutions of the ear lobes, will all produce subtle changes in the frequencies reaching each ear. Our hearing of the world is never disembodied.
Drawbacks inherent to the technique have prevented binaural recording from being widely used. The apparent realism of the stereo image is degraded when played back over loudspeakers. Binaural recordings are best listened to using headphones. With that last caveat in mind, here are some binaural sounds from the UK Soundmap.
One early contribution featured Sheffield's tram system:
The county of Suffolk has yielded some good recordings, including these two by contributor ermine. First, the sounds of a game auction at Campsea Ash:
Next, the twittering of zebra finches at Stonham Barns:
Binaural microphones are relatively cheap, with some pairs priced at well below Â£100. Although one giant electronics corporation registered several binaural patents in the 1970s, the needs of hobbyists are met by a cottage industry mainly based in the United States.
However convincing it might sound, it is a mistake to assume that a binaural recording must be an accurate reproduction of the original soundscape. It is an illusion, but with enough of the right cues for the brain to construct a familiar-seeming perception of another time and place.
Editor, UK Soundmap