Home' Accord : Accord March 2017 Contents Feature 15
More than a location
Localisation provides a listening edge by helping you separate the
sounds you want to hear from those you don’t. When you meet a friend
for coffee, it’s your localisation abilities that let you hold a conversation
in a noisy café. We call this an auditory figure-ground effect, and it’s the
auditory equivalent of “I Spy”—where we pick out one thing amongst a
background of others. But it’s also known as the cocktail party effect,
and includes the ability to not only find the voice of a speaker, but
focus our auditory attention on one speaker in the midst of others and
background noise (as you’d find at a cocktail party).
Localisation, as demonstrated by the cocktail party effect, makes it
easier to participate in social situations. It’s especially useful in noisy
places. Many of us take this ability for granted; we don’t even
recognise when we separate conversation from noise. But hearing
healthcare patients report conversations in background noise as
difficult, whether or not they use hearing instruments.2
Localisation skills also help map out auditory scenes. They allow you to
know where your cat is, or where you left the cordless telephone.
Externalisation of acoustic images gives the impression that a sound
appears outside the head, providing a realistic awareness of our
environment.3 Environmental awareness helps in many different aspects
of daily life: safety, navigation, and social participation.
Having environmental awareness allows you to avoid being hit by a car
as you cross the street. However, there are additional benefits you might
not notice. People need to be able to tell where things are to walk safely.
If they suddenly see something that they didn’t hear, they may react in
an exaggerated way, which can affect their balance.4 Indeed, a “poor
awareness of the auditory and spatial environment” has been suggested
as a possible contributor to falls.5 In fact, auditory localisation is
especially important to those with visual impairments “who use auditory
localisation to reconstruct inside their heads the world around them.”6
With environmental awareness, we can see again how important localisation
is to social situations. Turning toward your communication partner is not
only socially appropriate, it’s a known enhancer of communicative
effectiveness. Without localisation, it’s difficult to know who you should
face, or that someone is speaking to you at all. The environmental
awareness we derive from localisation can make us hear and experience
sound in a much more natural way, improving our life overall.
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Left ear / right ear
Fig. 1 – Various angles of incidence
of sound produce varying levels of
loudness at all pitches. Below, we
have simplified HRTFs with 3 pitches
highlighted in particular: high=dark
blue, medium=cyan, low=gray.
Each ear’s HRTF is shown separately
but, in reality, they occur at the
same time (see combined). When
they are combined they produce the
interaural information that allows
the auditor y system to determine the
sound’s origin. E .g ., at 45 degrees
there is a 15 dB difference in sound
between the two ears at the high
(dark blue) pitch.
How does localisation work?
The ability to hear sounds binaurally allows us to locate the source of a
sound. Localisation is impaired if there is difficulty in any of the following
areas of the auditory pathway: the outer ears (including pinnae and ear
canals); the middle and inner ears; the auditory nerve and brainstem;
the central auditory pathways and cortex.
The outer ears (along with the head) shape the sound that enters the rest
of the auditor y system. Since it’s created by our unique pinna and ear
canal geometry, the shaping of sound is specific to each individual. Just
as they cast a shadow of light, the head (and pinnae) cast a shadow of
sound. This is shown in a Head-Related Transfer Function (HRTF), which
shows the Sound Pressure Level (SPL) transform from a sound source in
free field to the ear canal. (See Fig. 1 .)
The cocktail party effect is made possible through binaural and spatial
processing, and requires the integrity of the entire auditory pathway.
The auditor y system uses information from both ears to perceptually
separate a signal of interest (a conversation partner) from noise
(background speech and noises).
In addition, signal-to-noise ratio (SNR)—the ratio of the primar y speech
signal to the background noise—can be improved by the normal auditor y
system using the difference in SNRs between ears to improve the overall
SNR. These binaural processes provide an improvement in speech
intelligibility compared to the monaural condition.
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