In order to generate enough infrasound to fill the Purcell Room, we had to build an infrasonic generator – ordinary speakers couldn’t play low frequencies at the volumes we wanted to achieve.
Our generator was an extra-long stroke subwoofer in a sewer pipe that was around 6 metres long. It was driven by a sine wave generator, amplified by a power amp that had a flat frequency response down to 10Hz.
For acousticians: Tests at the National Physical Laboratory showed this was performing as a quarter-wavelength pipe. The pipe had a resonant frequency of 17.4Hz.
We realised such a large pipe would be difficult to move from venue to venue so we employed Tim Hunkin (engineer, cartoonist and sculptor) and Graham Norgate to make a portable version of the pipe, according to our designs. Tim and Graham are both excellent craftsman – and Tim has the added advantage of a owning the biggest shed we’ve ever seen! (pictured above).
Richard Lord (NPL) and I carted this sturdy pipe around the country for several years. It’s only recently been put out to grass (2008). For those of you interested in its welfare: I’ve recycled the speaker, power amp and other electronics. I sent the pipe itself to the local recycling store where it’s being chopped up and resold. I recently spotted something that looked very much like the remains of the pipe in the Freebutt, Brighton. It had been recycled to make bar stools.