Humans can estimate distances to sound sources
better when they move and exploit auditory motion
parallax, a new study carried out under the
direction of Professor Lutz Wiegrebe at the
Department of Biology at LMU in Germany in
collaboration with the MRC Institute of Hearing
Research in Glasgow reveals.
“We humans find it difficult to assess, either
visually or acoustically, how far away an object is
from us,” Wiegrebe says.
“Our visual system makes use, among other things, of
the phenomenon of parallax.
When we move, the apparent position of an object
that is closer to us moves more within our visual
field than an object located further away.
This relative motion provides information about the
relative distance of the two objects.
Localization of sounds is particularly challenging
when the nature of the sound source is not clearly
It is not that difficult for us to estimate our
distance from a speeding ambulance when we hear its
But when the sound is unknown, we cannot tell
whether we are hearing a faint sound close by or a
louder sound further away."
Co-author Dr Owen Brimijoin, then at the MRC/CSO
Institute of Hearing Researchopens in new window in
Glasgow (part of the University of Nottingham),
said: “Visually, the change in the amount of
apparent motion between close and more distant
objects is known to be a very strong cue for
distance, but it’s never been demonstrated in
hearing before. This study shows for the first time
that human listeners use these sorts of cues.
“It highlights the fact that hearing devices may
need to be good at preserving these subtle acoustic
movements if we are to give hearing impaired
listeners the best chance at interacting with and
understanding the space and sounds around them.”
Wiegrebe and his team set out to determine how our
hearing system copes with this situation.
The experiments were carried out in a
non-reverberant chamber to ensure that the
participants could not assess relative distances
from the locations of sounds on the basis of echoes
The experimental subjects wore blindfolding goggles
and their head motions were monitored.
They were seated facing two sound sources that could
be positioned at different distances from the
One of the sources, chosen at random, emitted
high-pitched and the other low-pitched sounds.
The subjects’ task was to determine which of the
sound sources was closer to them. “Participants who
moved their upper bodies sideways – so that the
sound sources were further to the right and then
further to the left – were better able to estimate
the distance between the sound sources.
This result demonstrates that humans can use
auditory motion parallax to estimate relative
distances from sound sources,” Wiegrebe points out.
In fact, subjects were able to do so even when the
distance difference between the two sound sources
was only 16 cm.
The researchers then carried out two further
In one of these, the subjects were passively moved
left-and-right on a motion platform, and in the
other the loudspeakers were moved.
The results showed that the participants performed
best when they were allowed to move actively.
“This interaction between self-motion and the
auditory system is remarkable,” says Wiegrebe.
Clearly, it facilitates processing of the expected
change in the relative positions of the sound
sources in the brain.
This mechanism is also an advantage in situations in
which different sounds impinge on the ears from
different directions. “For example, moving around at
a party helps us to discriminate between sounds in
Movement alters the spatial sound properties in our
ears and thus enables us to determine which sound
sources are closer to us.”
For more information
Psychophysical evidence for auditory motion parallax
Sizing up spaces by ear
LMU - Ludwig-Maximilians-Universität München
MRC - Medical Research Council