Arthur C Clarke's short story Silence Please, published in 1957, tells the tale of a young scientist, Rupert Fenton, who invents a machine, the Fenton Silencer, which creates "complete silence over a wide area". Thrilled by the prospect of it relieving the "outraged ears of suffering humanity", he sets it running in a concert hall and confounds the audience by completely silencing the performance of an opera. In reality this feat would be technically impossible, but the theory behind the Fenton Silencer is identical to the "active noise-control" technology used in noise-cancelling headphones. Noise can be effectively suppressed by anti-noise, which sounds almost identical to the original noise, but is actually a mirror-image of its sound wave. In experiments this year, scientists have been pushing the boundaries of what might be possible, with active noise-control – but could it ever be used to successfully silence traffic, muzak or nuisance neighbours?
Can true silence ever be achieved?
The theory of active noise-control was first devised in the mid-1930s, and it’s beautifully simple. If two precisely opposite sound waves combine, the peaks of one cancel out the troughs of another. It’s called destructive interference, and if it’s done perfectly, the result is total silence. However, achieving that silence outside the laboratory is extremely difficult. Take the case of the concert hall in Clarke’s story: with sound waves rebounding in different directions from various surfaces, every person in the auditorium would hear a slightly different sound, and one machine could never cancel out all of them. “In some sense, noise-cancellation is about solving equations,” says Sheng Shen, from the Electrical and Computer Engineering department at the University of Illinois. “Imagine you want to create silence at one particular position. That’s one equation. But at any other position, it’s a different equation.”
When anti-noise is generated, anything less than total precision across both space and time, may have the opposite effect. Rather than achieving silence, the noise could end up doubling in volume. As a result, successful use of active noise-control has, thus far, been restricted to very specific circumstances: either at the source of the noise (for example placing speakers in an air conditioning duct) or in our ears, which is where noise-cancelling headphones have been such a great success. In these devices, a headset-mounted microphone picks up external noise, inverts the waveform and plays it through the headphones, cancelling out some of the external sound. But it’s still a complex process. Anyone who has used these devices on a long haul flight will know how good they are at suppressing the low rumble of the aircraft, but higher frequency sounds are relatively unaffected, and merely get dampened by “passive noise-control” – i.e. your ears being physically covered up.
“The low frequency noise in an aircraft is repetitive,” says Shen, “but the high-frequency sound of a baby crying is not predictable! The major difficulty in the noise-cancellation industry is that you have to do everything really fast. If you generate the anti-noise too late, it won’t cancel the noise out. So it’s a challenge for companies to design really fast hardware.”
How does wireless technology help?
Shen and his colleagues at the University of Illinois have developed an ingenious approach to this problem by using wireless technology. “Wireless signals travel a million times faster than sound,” he explains. “It’s similar to the speed difference between seeing lightning and hearing thunder.” By placing wireless-enabled microphones around a local environment – say in a noisy office corridor – and sending those noise signals to a headset worn by someone sitting in the office itself, the information needed to create anti-noise moves faster than the sound itself. “So now we have a relatively large time gap,” says Shen. “Our earpiece has a head start, and has adequate time to cancel out complex sounds – including high frequencies.”
This system, known as Mute, is so adept at blocking higher frequencies that the headsets can have a “hollow ear”, ie the ear canal doesn’t need to be blocked; anti-noise just needs to be played next to the ear. Mute’s bubble of noise-reduction around the human head has outperformed noise-cancelling headphones in recent tests, but Shen is clear about the system’s limits. “We’re not claiming that it can reduce noise levels across a room,” he says. “There’s just better cancellation at your ear. And we also lose portability, as you can’t carry our system around with you, for example if you’re running on the street.”
What has been tried?
Downplaying the capabilities of active noise-control is perhaps a wise approach. In recent years, a number of inventions have seemed to promise effective noise suppression to the public, but have fallen way short. One notable example, called Muzo, raised more than half a million US dollars (Dh1.8m) on Kickstarter in 2016 with its promise of creating a “personal zone” with “noise-blocking technology”. When the device finally launched, it merely used masking techniques to play sounds that distracted from the unwanted noise. Backers were far from impressed. “It produces a humming noise when the silent mode is activated, but background noises don’t disappear,” said one, “and I will file a request for a refund.” Shen laughs at the mention of Muzo. “We knew way before they manufactured the product that it wasn’t going to work,” he says. “The idea of putting a device on a bedroom window that cancels all the noise from the street is a bright vision, but it’s not going to work.”
Nevertheless, attempts to harness the potential of active noise-control are ongoing. Back in May, researchers at Nanyang Technological University in Singapore produced a window-mounted prototype that they claim is able to reduce outdoor sounds by as much as a half, indicating that the science fiction dream of electrically generated silence is evidently still alive and kicking, regardless of the scientific challenges.
Clarke’s short story ends with the Fenton Silencer exploding and killing its inventor, but the biggest hazard to today’s scientists is perhaps over-optimistic expectations. “We all agree that noise is a big problem in society,” says Shen, “and in some sense our unrealistic hopes emphasise why we need noise-cancellation so much.”