Behavior: Basic Psychology of the User - Foundations for Designing User-Centered Systems - page 107

loudness (known as magnitude estimation) is often the best way to quantify
sounds, at least for interface design.
4.6.3 Localizing Sound
People are generally poor at using spatial cues to successfully localize sounds
(Catchpole et al. 2004 ; Kubovy and van Valkenburg 2001 ). The idea that locali-
zation is based on inter-aural time differences at low frequencies and inter-aural
intensity differences at high frequencies is called the 'duplex' theory and dates
back to Lord Rayleigh ( 1907 ), a pioneer in perception. This does not hold for
complex sounds.
We can identify the location of a sound from the time taken for waves to reach
the ears, coupled with information from head and shoulder movements. Sound
reaching the far ear will be delayed in time and will be less intense relative to that
reaching the nearer ear. Thus, there are two possible cues as to the location of the
sound source. Owing to the physical nature of the sounds, these cues are not
equally effective at all frequencies.
Low frequency sounds have a wave length that is long compared with the size
of the head, and this ''bends'' the sound around the head very well. This process is
known as diffraction, and the result is that little or no shadow is cast by the head.
On the other hand, at high frequencies where the wavelength is short compared to
the dimension of the head, little diffraction occurs. A ''shadow'' almost like that
produced by a beam of light occurs.
Inter-aural (between-ear) differences in intensity are negligible at low fre-
quencies, but may be as large as 20 dB at high frequencies. This is easily illus-
trated by placing a small transistor radio close to one ear. If that ear is then blocked
with a finger, only the sound bending around the head and entering the other ear
will be heard. The sound will be much less ''tinny'' because high frequencies will
have been attenuated more than low; the head effectively acts like a low pass filter
(allowing only low frequency sounds). Inter-aural intensity differences are thus
more important at high frequencies than at low ones.
If a tone is delayed at one ear relative to the other, there will be phase differences
between the two ears (the peaks of the waves will arrive at different times). If nerve
impulses occur at a particular phase of the stimulation waveform, the relative timing
of the nerve impulses at the two ears will be related to the location of the sound
source. This is used to locate ''wide'' sounds. However, for sounds whose wave-
lengths are comparable with, or less than, the distance between the two ears there
will be ambiguity. The maximum path difference between the two ears is about
23 cm, which corresponds to a time delay of about 690 ls. Ambiguities occur when
the half wavelength of the sound is about 23 cm, i.e., when the frequency of the
sound is about 750 Hz. A sinusoid of this frequency lying to one side of the head
produces waveforms at the two ears that are in opposite phase (phase difference