Infrasound generator, designed and built for this experiment
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.
Equipment from the National Physical Laboratory is used to test the frequency response of the pipe
