Binaural recordings are reproductions of sound that create a three-dimensional effect which in turn provides the listener with the sensation of being immersed within an environment or scene. Effective binaural audio creates convincing impressions of 360º sound direction. Recordists are able to create this effect through successfully capturing key elements of physical acoustics within their recordings which provide the human auditory system with information that helps with deducing the location of the sound source without visual aid.
Francis Rumsey in his book, Spatial Audio, explained that “Binaural approaches to spatial sound representation are based on the premise that the most accurate reproduction of natural spatial listening cues will be achieved if the ears of the listener can be provided with the same signals that they would have experienced in the source environment or during natural listening.” (2001).
So, what are the all important binaural cues needed for accurate spatial perception? Here I will begin to outline them for you.
Interaural Time Difference (ITD)
The term ITD is used in the field of Acoustics to describe the difference between the time taken for a sound to arrive at the ear which is closest to the source and the time taken for it to reach the ear which is furthest away. These time differences are an auditory cue relied upon by the human auditory system to accurately locate sound sources. As human ears are separated by the head, the time taken for a sound wave to reach both ears will differ in proportion to the angle, distance and direction of the sound source in relation to the listener.
The sound waves emitted from a source located directly in front or behind the listener will arrive at both ears simultaneously. If the source deviates from these positions, time differences between both ears will occur, as illustrated in Figure 1.
As well as the time difference introduced by the angle of incidence, additional time delays are created by the head, which sound must travel around in a curved path to reach the furthest ear. For a particular angle in relation to the head, there is a maximum frequency beyond which ITD is no longer a reliable cue for sound localisation. Above this maximum frequency the head begins to interfere with sound waves more significantly, preventing them from diffracting efficiently and thus attenuating sound intensity level. According to Howard and Angus (2006: 100), for steady state signals, the Interaural Time Difference is analysed as an Interaural Phase Difference (IPD) by the auditory system when determining the direction a sound is coming from. Howard and Angus (2006: 101) calculated that, when the angle of incidence is 90°, the maximum frequency that can be successfully located using IPD is 743Hz. Jan Mohamed and Cabrera (2008: 2) also state that the effectiveness of the IPD cue is limited to below 700Hz , supporting Plack’s theory (2014: 161) that ITDs work best for low-frequency tones.
Interaural Intensity Difference (IID)
This cue is relied upon by the auditory system for locating relatively high-frequency sounds. A listener’s head acts as a physical barrier for these sounds, creating interpretable intensity level differences at each ear.
This effect could be described as an acoustic shadowing effect because sound waves are unable to pass through the head to the ear furthest away from the source, thus leaving it ‘in the shade’ (Figure 3). Figure 2 illustrates how the angle at which a sound source is situated in relation to a listener’s head affects intensity level (amplitude). When on the median plane, the sound emitted from the source is received at equal levels at each ear, whereas amplitude progressively increases at one ear and decreases at the other as the source is moved further away.
According to Howard and Angus (2006: 102), an object does not act as a significant sound diffuser until it’s size is around two-thirds of the wavelength (λ) of a particular sound, therefore IID is less useful as a localisation cue below a minimum frequency. Figure 2 demonstrates how lower frequency sounds (250 Hz) are less inhibited by the head, leading to less pronounced IIDs.
Additional phenomena worth noting are:
• Larger heads create a greater IID at a given frequency.
• Higher frequencies create larger IIDs at a given angle
Look out for the next post which will cover additional cues and examples of when, even with the aid of ITDs and IIDs, locating a sound source can be very dificult.
Howard, H. M., Angus, J. A. (2006). Acoustics and Psychoacoustics. Oxford: Focal Press.
Jan Mohamed, M.I., Cabrera, D, (2008) Human Sensitivity to Interaural Phase Difference for Very Low Frequency Sound. Acoustics 2008: Proceedings of the Australian Acoustical Society Conference. Geelong, Australia.
Plack, C. J. (2014) The Sense of Hearing: Second Edition. New York: Psychology Press.
Rumsey, F. (2001) Spatial Audio. Oxford: Focal Press.
Shannan, B. (2010) Audiology Update. Available from the Scottish Sensory Centre, University of Edinburgh website: http://www.ssc.education.ed.ac.uk/courses/deaf/dnov10i.html.