Wireless technology to measure ancient acoustics

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Does the view equal the sound? That is the question asked by six students in search for ancient greeks’ acousitcs secrets.

Odeon of Herodus Atticus

According to a 2007 study by Nico F. Declercq and Cindy Dekeyser of the Georgia Institute of Technology, the astonishing acoustic properties may be the result of the advanced design of greek theatres. The rows of limestone seats filter out low-frequency sounds, such as the murmur of the crowd, and amplify high-frequency sounds from the stage.

Last March, six Built Environment students for TU/e packed their suitcases and hoped on the plane to Athens. They wanted to know whether there was a scientific explanation for the supposedly excellent reputation of the acoustics in the outdoor theaters, using new technologies. To answer this question, they conducted measuring experiments in the theaters of Argos, Epidaurus, and the Odeon of Herodus Atticus.

For about six days and nights, they explored every corner of three of the largest ancient theatres: the outer ring of Argos is sixty meters away from the stage. Epidaurus could welcome 14,000 spectators and Argos no less than 20,000. When trying to measure data from all those seats, the use of cables is rather inadequate. “Wireless measurements are tough when researching acoustics,” says Built Environment student Bas Peeters. “As far as we know, there’s never been a large-scale outdoor wireless measurement like this. The method developed by Constant Hak, which he tested in the Muziekgebouw during the 2014 Dutch Design Week, enables us to conduct reliable wireless measurements in these theaters.”

Dummy from Eindhoven Epidaurus theatre Greece

Famously, tour guides have their groups scattered in the stands and show them how they can easily hear the sound of a match struck at center-stage: this dummy from Eindhoven University of Technology will tell us !

Sitting on a gold mine

All sounds, including that of dogs, crickets, tourists, traffic, someone spraining their ankle and ‘the worst street performer ever’ were recorded by the twenty microphones the students had at their disposal.

They measured at two hundred positions at each theater, distributed over ten measure lines. For every position they used two sound sources, and by rotating these in five steps, the gathered at least 2,000 source-receiver combinations.

Fewer combinations were unacceptable, as the acoustic field in theaters can differ from seat to seat. And since acoustics are different after a cool night than after a hot day, all measurements were conducted mornings and nights. And now they’re sitting on a gold mine. That is, when all sound recordings have been listened to and the 40-second transmissions have been found, filtered out, downloaded to special software, and converted into impulse responses. The latter are signals shown in a graph that show the response to a transmitted ‘blow’. Processing tdata will take up another few weeks of full-time work for two to three students. The team wants to filter six thousand impulse responses from their recordings.

That number represents a large amount of data, and it’s complete, too: the students have taken into account temperatures, humidity, and wind. Their invaluable results can validate acoustic calculations used for the design of inside and outside spaces.

This outcome – a unique set of useful data – is the reward that kept the students going. As far as research goes, they are not done yet. Apart from the meticulous mapping of the acoustics of an amphitheater, the results will also be used for ongoing research at Building Acoustics on sound propagation in the built environment, and room and stage acoustics. The measuring results will be stored carefully, so future studies can benefit from them as well.

Adapted from article by Norbine Schalij in TU/e’s Cursor magazine

More pictures and information on Eindhoven University of Technology’s website Ancient Acoustics.

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