Duplex theory  

The Duplex theory proposed by  Lord Rayleigh  (1907) provides an explanation for the ability of humans to
localise sounds by time differences between the sounds reaching each ear
(ITDs) and differences in sound level entering the ears (interaural level
differences, ILDs). But there still lies a question whether ITD or ILD is
prominent.

The duplex theory states that ITDs are used to localise low frequency sounds,
in particular, while ILDs are used in the localisation of high frequency sound
inputs. However, the frequency ranges for which the auditory system can use
ITDs and ILDs significantly overlap, and most natural sounds will have both
high and low frequency components, so that the auditory system will in most
cases have to combine information from both ITDs and ILDs to judge the
location of a sound source.  [1]  A consequence of this duplex system is that
it is also possible to generate so-called "cue trading" or "time–intensity
trading" stimuli on headphones, where ITDs pointing to the left are offset by
ILDs pointing to the right, so the sound is perceived as coming from the
midline. A limitation of the duplex theory is that the theory does not
completely explain directional hearing, as no explanation is given for the
ability to distinguish between a sound source directly in front and behind.
Also the theory only relates to localising sounds in the horizontal plane
around the head. The theory also does not take into account the use of the 
pinna  "Pinna \(anatomy\)") in localisation.(Gelfand,
2004)

Experiments conducted by Woodworth (1938) tested the duplex theory by using a
solid sphere to model the shape of the head and measuring the ITDs as a
function of azimuth for different frequencies. The model used had a distance
between the two ears of approximately 22–23 cm. Initial measurements found
that there was a maximum time delay of approximately 660 μs when the sound
source was placed at directly 90° azimuth to one ear. This time delay
correlates to the wavelength of a sound input with a  frequency
 of 1500 Hz. The results concluded that when a
sound played had a frequency less than 1500 Hz the  wavelength
 is greater than this maximum time delay
between the ears. Therefore, there is a  phase difference
 between the sound waves entering
the ears providing acoustic localisation cues. With a sound input with a
frequency closer to 1500 Hz the wavelength of the sound wave is similar to the
natural time delay. Therefore, due to the size of the head and the distance
between the ears there is a reduced phase difference so localisations errors
start to be made. When a high frequency sound input is used with a frequency
greater than 1500 Hz, the wavelength is shorter than the distance between the
2 ears, a  head shadow  is produced and ILD
provide cues for the localisation of this sound.

Feddersen et al. (1957) also conducted experiments taking measurements on how
ITDs alter with changing the azimuth of the loudspeaker around the head at
different frequencies. But unlike the Woodworth experiments human subjects
were used rather than a model of the head. The experiment results agreed with
the conclusion made by Woodworth about ITDs. The experiments also concluded
that is there is no difference in ITDs when sounds are provided from directly
in front or behind at 0° and 180° azimuth. The explanation for this is that
the sound is equidistant from both ears. Interaural time differences alter as
the loudspeaker is moved around the head. The maximum ITD of 660 μs occurs
when a sound source is positioned at 90° azimuth to one ear.