Soundwalking with balloons and a banjo
Some of the best sounds in cities can be found in places that at first might seem unlikely sites for an aesthetic wonder. For the Manchester Science Festival I took a group of people to explore underneath dingy canal and railway arches built at the height of the industrial revolution. This active soundwalk was inspired by Davide Tidoni, an artist I found out about while researching Sonic Wonderland / The Sound Book.
The reverberant arch
Start in the Castlefield arena and walk under the arch shown in the photo above and below. You need to make some sound as you walk under to notice how suddenly the sound changes. Tim played the banjo, and as I walked under the arch I could hear the sound reflecting and reverberating. As I entered under the arch, it was like someone had flipped a switch to turn the acoustic effect on. This is what a balloon burst below the arch sounds like:
A quirk of geometry keeps the sound bouncing underneath the arch. In Architectural Acoustics, this persistence of sound is quantified through a parameter called the reveberation time. Underneath this arch the reveberation time is near four seconds, a value more commonly found in a large church.
Go to the neighbouring arch which is right by the canal. Clapping hands or bursting a balloon gives a clear slap-back echo. This creates the slow fluttering repeats of the balloon burst I recorded :
From the shape of the arch, it is likely that the sound follows the paths shown in this animation (originally made to explain what happens at Echo Bridge, MA).
The sound skims around the underneath of the arch as well as bouncing straight back and forth just above the water. The echo repeats every 0.17 s, meaning the sound undergoes a roundtrip of nearly 60m as it bounces back and forth across the canal. If you sang here, you might want to choose an old rock and rock number like Blue Moon, because what the arch is generating is like the electronic slapback echo used to give Elvis his distinctive sound in Sun Record recordings.
The best arch
This arch has the best acoustic. It is part of a car park on the opposite side of the canal to Dukes 92, and so often there are cars parked underneath ruining the acoustic effect .
The arch focuses the sound with the focal point being at the ground. This means the sound keeps bouncing back and forth between the floor and ceiling adding a richocheting sound to the balloon burst. The repeat happens every 0.03 seconds, as the round trip from floor to ceiling and back again is about ten metres . Below is a plot of the recording of a balloon burst, the first big peak is the initial bang reaching the microphone. The regular peaks which gradually die away are the focussed echoes.
I also played a short extract from Barry Cockcroft’s Ku Ku under the arch. In the second phrase it sounds like two people playing. It is quite difficult to not be off-put by the strong echo while playing. I think the nasty harsh tone of the high notes is caused by the colouration added by the arch (and maybe not helped by the poor intonation at one point).
Do you know of any other arches worth visiting?
 It is 3.8 seconds at mid-frequency.
 I measured the round trip to be 10.1 m
 (c) Manchester Science Festival 2013, Chris Foster
 Balloon sounds available to download from freesound.