Learning to Listen

What is sound? Well, if I wave my hand about in mid air I'm pushing some of the air around, creating pressure waves that radiate outwards like the ripples on a pond at over 700 miles per hour.

If I were able to waggle my hand fast enough (say a thousand times a second) we would hear a musical tone. As it is, you'll have to take my word for it, but next time you play your stereo you should be able to see (or gently feel with your finger) the cones of the speakers vibrating as you hear your music. They are doing the same as my hand was, only a lot faster. Hearing is the mechanism whereby these pressure waves are converted into the nerve impulses that result in the subjective experience we call "sound". It's the operation of a cluster of mechanical and electrochemical processes that sit between the outside world and the parts of the brain where that world is perceived and understood.

The pressure waves are transmitted as vibrations from the ear drum via a complex mechanical linkage to the 'organ of Corti' - an array of specialised cells within a little snail-shaped structure called the cochlea. These cells convert the vibrations into electrical impulses which are transmitted via the cochlear nerve to the primary auditory cortex of the brain. If the pressure waves fluctuate at between about 20 and 20 thousand times per second, they can ultimately give rise to the sensation of sound.

Human hearing is extremely sensitive. The minimum detectable pressure variation (or 'sound pressure') in air is about plus or minus seven hundred-thousand-millionths (that's eleven decimal places of zeros) of atmospheric pressure, equating to an air movement in the order of a few billionths of a metre - in the same order as the diameter of the gas molecules in air. Indeed, if it were any more sensitive, we would notice a constant background hiss due to Brownian motion - the random movement of the molecules in the fluid of the inner ear.

So our hearing is at best as sensitive as the laws of physics allow. But most of us don't make use of anything like this level of sensitivity. Nevertheless, in 1962 pioneering otologist Samuel Rosen found that such levels of auditory sensitivity were indeed a reality among the Mabaan tribe of south-eastern Sudan,1 albeit in what Rosen described as 'a dramatically quiet, almost silent atmosphere' (to him that is - it may have seemed less quiet to the Mabaan themselves). The Mabaan, who then lived a basic pastoral existence and did not even use drums, also seemed to retain this sensitivity into old age, unlike western urban populations that typically show gradual hearing loss from the age of around twenty.

But fortunately for us, human hearing is also pretty robust - capable of withstanding short-term sound pressures up to about three million times greater than the minimum detectable before the pain threshold is reached. The auditory response is also non-linear - if the average sound pressure is small, small changes in pressure result in large changes in perceived loudness, but if the average sound pressure is large, large changes in pressure yield small changes in loudness. This may partly explain why we tolerate escalating levels of noise.

This was recorded on a weekday afternoon on the Marylebone Road near Baker Street, London. Passers-by were apparently oblivious of the noise - some even engaging in conversation. Our hearing is almost constantly bombarded with such loud sounds, yet we still seem able to concentrate on our work, communicate and conduct our personal lives. So something rather interesting must be happening.

It turns out there's much more to the perception of sound than mere hearing. We must make a distinction between 'hearing', and 'listening'. Hearing is unavoidable, involuntary, and has objectively consistent ranges of performance. Listening is voluntary in that we can and do influence it intentionally, but it also adapts to our environment both in the short- and long term without our conscious intervention. The brain is ultimately more important than the ear.

The perception of sound is largely a matter of experience, and it relies on an assumption of meaning. Philosopher Matthew Nudds of Edinburgh University, has proposed 'it is a commonly held view that auditory perception functions to tell us about sounds and their properties. ... I argue that this common view is mistaken and that auditory perception functions to tell us about the objects that are the sources of sounds.' 2

That makes sense - from the evolutionary perspective, survival depends on recognising the significance of objects and happenings in our surroundings. Sounds are merely some of the sources of information about those objects and happenings. So we get to noise - essentially, sounds that have no meaning for us: that we aren't interested in or which get in the way of sounds we are interested in. Being interested or not is fundamental to our sensory experience of the world. We learn to suppress our responses to stimuli that seem irrelevant to us. And as our urban environment bombards us with masses of irrelevant information, most of what impinges on our senses day to day counts as noise. How do we cope then? The most important mechanisms we make use of are selective attention and categorisation.

We can usually listen quite comfortably to an individual speaker among the numerous competing conversations at a party, provided they're speaking a language we know and they're saying something we can relate to. We ignore the very similar sounds of all the other conversations. But if someone across the room happens to mention our name even quietly, we're suddenly 'all ears'. Then I guess we've all experienced difficulty sleeping the first night in a foreign city. Strange sounds keep us from dozing off even if they're not very loud, whereas the passing traffic outside our home is slept through without the slightest difficulty. That's an example of longer term and more durable selective attention. We gradually cease to pay attention to predictable sounds that experience tells us are irrelevant to us.

The decision to accept or reject sounds - to label them as important or not - resides in the cognitive brain, not in the ear or its raw signal processing systems, and we make such decisions constantly, but usually unconsciously. That's selective attention at work, and it's a necessary part of simplifying a complex environment in order to cope with moment-to-moment living. Selective attention adapts to and recovers from short-term noise swiftly and with ease. But when the noise is a permanent continuous feature of the environment, as much of it has been for the last half century or so, the brain eventually adapts more inflexibly to exclude the noise, and can continue to suppress sounds with similar characteristics even in its absence. Regularly encountered quiet sounds to which we can attach no meaning may also cease to be noticed, even if they are not continuously present.

According to cognitive psychologists David Lewkowicz and Asif Ghazanfar 3 '... broad multisensory perceptual tuning is present at birth, and ... narrows by the end of the first year of life, leaving infants with the ability to integrate only socio-ecologically-relevant multisensory signals.'

So we progressively learn to stop paying attention to what seems irrelevant to us. But as much of what impinges on our senses in our modern urban environment seems irrelevant, we ultimately tune out a greater proportion of the information coming from our surroundings than at any time in the past. And in rejecting the mass of noise we also discard other information that accompanies it, and which might be of value.

Veteran American sound recordist and anti-noise activist Gordon Hempton has lamented there are only a dozen or so places left on the planet that are completely free of human-generated noise for more than fifteen minutes at a time. But I don't believe all is lost.

Although we can't turn the clock back and eliminate the noise, we can learn to adapt our perceptual capacities to accommodate its presence. I'd like to propose that our perceptual tuning is not necessarily frozen at some point in childhood, nor does it necessarily only narrow as Lewkowicz and Ghazanfar suggest. It can be adapted, expanded and tuned at any time by voluntary choice and appropriate action. I don't feel this is too bold an assertion. Indeed it is part of the message of many established meditative disciplines. I also suggest, maybe somewhat more boldly, that, particularly in our over-simplified and noise-dominated environment, it's essential to wellbeing to accomplish this perceptual adaptation. Rosen & Olin said of the Mabaan that 'acute hearing is necessary for survival, so they have learned to listen since early childhood.'  It still is - and as much for the survival of the spirit as of the physical self. By learning to exercise our perceptual capacities to the full we can re-experience subtle aspects of our environment that we have ceased to notice, even in the presence of noise. At the very least, that could make life more interesting, and it could also have other potentially more concrete benefits. So we too need to learn to listen.

1 Rosen, S & Olin, P. 1965. Hearing Loss and Coronary Heart Disease. Bull. N. Y. Acad. Med. Vol. 41, No. 10, October 1965, 1052-1068

2 Nudds, M. 2007. Auditory Perception and Sounds. http://homepages.ed.ac.uk/mnudds/papers/ap.pdf

3 Lewkowicz, DJ & Ghazanfar, AA. 2009. The emergence of multisensory systems through perceptual narrowing. Trends in Cognitive Sciences Vol.13 No.11, 470-478

condensed and adapted from part two of my lecture "The Subtle Sounds of Nature" - The Institute for Cultural Research, June 2010.

Go to Part Three