How a London Sewer Inspired a Search for Unusual Sounds
What might be considered a sound “defect” can be fascinating to listen to.
The following is an excerpt from The Sound Book: The Science of the Sonic Wonders of the World, by Trevor Cox.
“Is it safe?” A noxious odor was invading my nostrils as I stared down the open manhole. The metal ladder disappeared into the darkness. I had assumed a radio interview on the acoustics of sewers would involve an official and authorized visit. Instead, it started with a walk into a London park on a summer’s evening. Bruno, the interviewer, produced a large key from his knapsack, opened up a convenient manhole cover, and invited me to climb down. Was it legal to wander around the sewers without permission? What if the tunnel suddenly flooded? What about a canary to warn of poisonous gases? Meanwhile, strolling commuters ignored us as we gazed into the gloom.
I repressed my anxieties and climbed gingerly down the ladder to the sewer about 6 meters (20 feet) below. This was a storm drain built in Victorian times, a long cylindrical tunnel lined with bricks. The floor was treacherous and slippery, and the odor made my skin crawl. I clapped my hands as best I could with rubber gloves on and started to count in my head very slowly—“one, two, three, . . .”—timing how long it took the sound to die away. After 9 seconds a distant rumbling echo returned to me. Sound travels a kilometer (about a half mile) every 3 seconds, so my clap had traveled a roundtrip of 3 kilometers (1.9 miles). Later on, far away down the tunnel, we discovered the staircase off which the sound had bounced; it was draped in disgusting debris.
I found it difficult to avoid head-butting the stalactites hanging from the low ceiling. Sadly, these were not brittle rock, but crusty, fatty deposits clinging to the bricks. These foul stalactites broke off, worked their way down the back of my shirt and scraped my skin. Since I’m tall, my head was very close to the ceiling—the worst place for the revolting stalactites, but the optimal position for observing an unexpected acoustic effect. As the radio interview started, I noticed my voice hugging the walls of the cylindrical tunnel and spiraling into the distance. Speech spun around the inside of the curved sewer like a motorcyclist performing in a Wall of Death.
While every other sense was being overwhelmed with revulsion, my ears were savoring a wonderful sonic gem. This impressive spiraling toyed with me as I tried to work out what was causing the effect. It was so different from anything I had experienced before that I started to doubt what I was hearing. Was it just an illusion, with the sight of the cylindrical sewer fooling my brain into thinking the sound was curving? No; when I closed my eyes, the reverberance still embraced my voice and twisted it around the tunnel.
What was causing the sound to stay at the edges of the sewer and not cross into the middle? I have worked in architectural acoustics for twenty-five years, yet the sewer contained a sound effect I had not heard before. I also noticed that Bruno’s voice was embellished with a metallic twang as it echoed in the sewer. How was that possible in a place devoid of metal? We were surrounded by bricks.
During those hours listening to the sewer, I had an acoustic epiphany. My particular expertise is interior acoustics—that is, the way sound works in a room. Most of my work has focused on discovering ways to mask or minimize unwanted sounds and acoustic effects. Not long after completing my doctorate, I pioneered new ways of shaping room surfaces that now improve the sound in theaters and recording studios around the world. Above the stage of the Kresge Auditorium at the Massachusetts Institute of Technology, you can see the gently undulating reflectors I designed to help musicians hear each other. For a rehearsal hall at the Benslow Music Trust in Hitchin, England, I designed corrugations to adorn a concave wall in order to stop sound reflections from all being focused onto a single point in the room and thereby altering the timbre of the musical instruments.
In recent years I have been researching how poor acoustics and high noise levels in classrooms affect learning. It seems obvious that pupils need to be able to hear the teacher and have a certain amount of quiet to learn, yet there are architects who have designed schools that are acoustic disasters. My bête noire is open-plan schools, where doors and walls are dispensed with, resulting in the noise from one class disturbing others because there is nothing to impede the sound. The Business Academy Bexley in London opened in 2002 and was short-listed for the prestigious Royal Institute of British Architects’ Stirling Prize. The open-plan design caused so many noise problems, however, that the school and local education authority had to spend £600,000 ($0.9 million) installing glass partitions.
Part of my research into schools involved playing noise at pupils as they tried to complete simple tasks involving reading comprehension or mental arithmetic. In one test, playing the babble of a noisy classroom at a cohort of fourteen- to sixteen-year-olds lowered their cognitive abilities to those of a control group of eleven- to thirteen-year-olds who were working in quieter conditions.
I am currently working with colleagues to improve the quality of user-generated content online. I started the project after getting frustrated listening to distorted and noisy soundtracks on Internet videos. We are developing software that will automatically detect when an audio recording is poor—for instance, checking whether there is wind noise whistling past a microphone. The idea is to alert users to poor sound conditions before they start recording, or to use audio processing to weed out some of the interference, just as a digital camera looks for flaws and automatically adjusts exposure time and focus. But before we can write the software, we are grappling with people’s perceptions of audio quality. When you record your child playing in a school concert, does the quality of the recording matter very much? My personal feeling is that audio distortions can be much more important than visual ones. A blurry video with a clear recording of a loved one singing captures that special moment much better than a clear video in which the lyrics are unintelligible and the voice distorted.
But as I splashed about in the sewer, I realized that distortions can sometimes be wonderful. Despite having studied sound intensely for decades, I had been missing something. I had been so busy trying to remove unwanted noise that I had forgotten to listen to the sounds themselves. In the right place a “defect” such as a sound focus, or the metallic, spiraling echo in the sewer, could be fascinating to listen to. Perhaps ugly, strange, and distorted sounds could teach us something about how acoustics works in everyday situations or even how our brain processes sound. By the time I emerged out of the sewer through a manhole in a leafy suburban street, I decided I wanted to find more such unusual acoustic effects. And not just the ugly ones. I wanted to experience the most surprising, unexpected, and sublime sounds—the sonic wonders of the world.
Excerpted from The Sound Book: The Science of the Sonic Wonders of the World, by Trevor Cox. Copyright © 2014 by Trevor Cox. With permission of the publisher, W. W. Norton & Company, Inc.
Trevor Cox is author of The Sound Book: The Science of the Sonic Wonders of The World (W.W. Norton, 2014) and an acoustic engineering professor at the University of Salford in Salford, United Kingdom.