Scientists discover a new way humans can estimate distances from sound
13 Mar 2018
A study has revealed a new mechanism that allows humans to use hearing to estimate their relative distance from sound sources. The research, led by scientists at LMU Munich in Germany in collaboration with the MRC Institute of Hearing Research in Glasgow, reveals that humans can perform this task more efficiently when they are allowed to move.
The findings are published in the Proceedings of the National Academy of Sciences.
This ability to determine with our hearing which of two sounds is closer uses a phenomenon called ‘auditory motion parallax’. It is similar to how humans can perceive the relative distance of objects using our eyes. A person can observe this ‘visual parallax’ by looking at a distant object, such as a building on the other side of the street, while holding their hand in front of it. If they move their head from side to side, their hand, which is close to their eyes, will seem to move a lot, while the building, which is far away, will appear to move very little.
Co-author Dr Owen Brimijoin, then at the MRC/CSO Institute of Hearing Research 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.”
Professor Lutz Wiegrebe and his team set out to determine whether our hearing system takes advantage of this motion parallax to determine how far away a sound source is. Participants wore blindfolds and motion tracking equipment and they were seated facing a row of loudspeakers that emitted two different sounds. The speakers simulated different distances for the two sounds by using the parallax phenomenon, with the "closer" sound moving more than the "further" sound. The participants’ task was to determine which of the sound sources appeared to be closer.
Participants who moved their upper bodies sideways to the left and right – so that the closer sound moved more than the further sound – were better able to estimate which of two sound sources was closer, demonstrating that humans can use auditory motion parallax to estimate relative distances from sound sources. Participants were able to do so even when the simulated distance between the two sound sources was only 16 cm.
The researchers also found the participants were less able to distinguish the distances of the sounds when they were passively moved left-and-right on a motion platform, or the loudspeakers were moved – they performed best when they moved their heads actively themselves.
Professor Wiegrebe from Ludwig-Maximilians-Universität München in Germany said: “We humans find it difficult to assess, either visually or acoustically, how far away an object is from us. It is not that difficult for us to estimate our distance from a speeding ambulance when we hear its siren. But when the sound is unknown, we cannot tell whether we are hearing a faint sound close by or a louder sound further away.
“This interaction between self-motion and the auditory system is remarkable. 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 the hubbub. Movement alters the spatial sound properties in our ears and this enables us to determine which sound sources are closer to us.”
Dr Bruna Galobardes, programme manager for neurological disorders and sensory science at the MRC said: “Nearly eight million adults (about one in six) in Great Britain suffer from hearing loss, which can compromise interpersonal communication, impact on quality of life and daily living, and limit independence. This research could contribute to improving hearing devices so that they better replicate the hearing experience, which could increase uptake of these devices and improve health and quality of life.”