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How do sound waves bounce? An ERC grantee at Aalto University develops new ways to study acoustics

10 Sep 2018
Sound recording room at the Department of Signal Processing and Acoustics, Aalto University

Acoustics research powers cinema sound and virtual reality gaming. Figuring out how sound bounces of the walls of an auditorium is also essential to designing concert halls

By Gary Finnegan

Stereo sound has transformed how we enjoy movies. Thanks to a system developed in 1977 for the Star Wars movie, cinemas can make it feel like sounds are coming at us from all directions. Video games – and immersive virtual reality technologies – are pushing the boundaries of spatial acoustics.

“With video games and VR, you need to localise sounds – like the sound of somebody closing a door behind you or footsteps approaching from your left,” says Ville Pulkki, an ERC-funded researcher at Finland’s Aalto University. “The problem is that stereo is not good enough: you need to reproduce sound accurately with information about the direction and the size of the room you are in.”’

Pulkki, explaining the work behind Aalto University's research group on spatial sound

Pulkki is going to great lengths to see how sound waves behave in various settings – small rooms, large rooms, under water  and even when traveling at speed strapped to the roof of a car.

An underwater microphone designed at Aalto University is moved inside a pool to find out how sound waves behave under various settings. The audio signals are then fed into an algorithm that can locate the source of the sound

This will help him develop an audio format – like a more sophisticated MP3 standard – that could be used in all music, film and games.

There have already been a number of spin-off technologies from Pulkki’s work on how sound waves behave. For example, acousticians can use this knowledge when designing concert halls. “By clicking their fingers, acousticians can get a rough idea of the acoustics of a room. It tells them something about how sound reflects and reverberates in the space,” he says. “We developed a system that users a flash of laser light to create a small explosion that tells us a lot about the space.”

Working with a laser acoustics group in Helsinki, Pulkki’s team came up with a tiny laser beam about the size of a pen. It sets off a powerful but brief light ‘explosion’ lasting a few nanoseconds. By measuring the shockwave produced by the explosion, they can learn about how waves bounce off the walls. “The shockwave radiates evenly in all directions,” he explains. “This is ideal and allows us to measure the response using a microphone.”

The technology could be used in scale models of concert halls to design a space that gives the audience a perfect acoustic experience – whether they are in the front row or in the cheap seats at the back of the auditorium.

Laser pulses could help acousticians model soundscapes in concert halls, offices and city streets


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