Just after sunrise on a misty spring morning last year, my fellow acoustician at the University of Salford, Bruno Fazenda, and Rupert Till of the University of Huddersfield, UK, could be found wandering around Stonehenge popping balloons. This was not some bizarre pagan ritual. It was a serious attempt to capture the “impulse response” of the ancient southern English stone circle, and with it perhaps start to determine how Stonehenge might have sounded to our ancestors.
An impulse response characterises all the paths taken by the sound between its source â€“ in this case a popping balloon â€“ and a microphone positioned a few metres away. It is simply a plot of the sound pressure at the microphone in the seconds after the pop. The first, strongest peak on the plot represents the sound that travelled directly from the source to the microphone. Later, smaller peaks indicate the arrival of reflections off the stones. The recording and plot shows the impulse response Bruno and Rupert measured with a microphone positioned at the centre of Stonehenge and a popping balloon at the edge of the circle.
This impulse response represents an acoustic fingerprint of the stones. Back in the lab, it can be used to create a virtual rendition of any piece of music or speech as it would sound within the stone circle. All that is needed is an “anechoic” recording of the raw music or speech â€“ a recording made in a reflection-free environment such as the open air or, better, a specialist anechoic chamber such as we have at Salford. The anechoic recording and the impulse response can then be combined using a mathematical operation called convolution.