Graphs: P. Minary / Editor: R. Lloyd-Faulconbridge
The external ear is the visible part of the ear, which picks up the airborne sound waves.
External ear: scheme
The external ear plays the role of an acoustic antenna: the pinna (together with the head) diffracts and focuses sound waves, the concha and the ear canal act as a resonator.
Acoustically, the eardrum is the final part of the external ear, which thus functions as a tube open only at one end
Transfer function of the external ear
Both the sound pressure levels and the phase of the acoustic waves change whilst being propagated from the free field environment, via the external ear, to the eardrum. These changes vary with the frequency of the sound and for each direction of the acoustic waves in the horizontal and vertical planes. They correspond to the transfer function (TF) of the external ear.
Ex. Amplitude and phase changes ( transfer function) of a pure tone, output (red) versus input (blue). The signal is amplified by 2 = + 6 dB ; its phase is shifted by - p/2 (phase lag of 90 degrees, or a quarter of a cycle).
Acoustic amplification of the external ear
Influence of the pinna (p) and of the ear canal (c) on the amplitude of the signal reaching the ear drum (incidence: 45 degrees in the horizontal plane). At 3000 Hz, the final amplification (t) is 20 dB (10 times the free field level).
External ear and sound localization
Result of the transfer function: for a given acoustic source in the free field, there is a difference between the two ears in both the sound pressure level (when f > 500 Hz, see later), and in the phase (or the time of arrival, see below).
The maximum time difference between the ears is 760 microseconds (see later).However, we are able to pinpoint a sound in front of the head within 1 - 2 degrees.
This corresponds to a time difference of only 13 microseconds. The cells within the auditory system which analyze acoustic information are sensitive to these microchanges!