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May 31, 2010

What is Binaural Recording?

by soundlandscapes

This blog centres around binaural recording. So what is binaural recording?

Binaural recording is a method of recording sound that uses a special microphone arrangement and is intended for replay using headphones. Dummy head recording is a specific method of capturing the audio, generally using a bust that includes pinnae (outer ears). Because each person’s pinnae are unique, and because the filtering they impose on sound directionality is learned by each person from early childhood, the use of pinnae during recording that are not the same as the ultimate listener may lead to perceptual confusion.

The term “binaural” has frequently been confused as a synonym for the word “stereo”, and this is partially due to a large amount of misuse in the mid-1950s by the recording industry, as a marketing buzzword. Conventional stereo recordings do not factor in natural ear spacing or “head-shadow” of the head and ears, since these things happen naturally as a person listens, generating their own ITDs (interaural time differences) and ILDs (interaural level differences). Because loudspeaker-crosstalk of conventional stereo interferes with binaural reproduction, either headphones are required, or crosstalk cancellation of signals intended for loudspeakers. As a general rule, for true binaural results, an audio recording and reproduction system chain, from microphone to listener’s brain, should contain one and only one set of pinnae (preferably the listener’s own) and one head-shadow.

With a simple recording method, two microphones are placed 18 cm (7″) apart facing away from each other. This method will not create a real binaural recording. The distance and placement roughly approximates to the position of an average human’s ear canals, but that is not all that is needed. A typical binaural recording unit has two microphones mounted in a dummy head, inset in ear-shaped moulds to fully capture all of the audio frequency adjustments known as head-related transfer functions that happen naturally as sound wraps around the human head and is “shaped” by the form of the outer and inner ear.

Today, “in-ear” or “near-ear” microphones are now readily available and can be linked to a portable digital recorder, bypassing the need for a dummy head by using the recordist’s own head. Clip-on binaural microphones using the recordist’s own head are offered by Core Sound, Sound Professionals and Soundman amongst others.

Once recorded, the binaural effect can be reproduced using headphones. Binaural reproduction does not work with mono playback; nor does it work while using loudspeaker units, as the acoustics of this arrangement distort the channel separation via natural crosstalk.

The result is a listening experience that spatially transcends normally recorded stereo, since it accurately reproduces the effect of hearing a sound in person, given the 360° nature of how human ears pick up nuance in the sound waves. Binaural recordings can very convincingly reproduce the location of sound behind, ahead, above, or wherever else the sound actually came from during recording.

Any set of headphones that provide good right and left channel isolation are sufficient to hear the immersive effects of the recording, and anyone who has even a cheap set of headphones can enjoy the recordings. As with any playback, higher quality headphones will do a better job of creating the illusion.

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