Reveries of a solitary bat walker

a heterodyne detector

Over the last few weeks I’ve been going on solitary walks with my heterodyne bat detector. These have been in London, Norfolk, Edinburgh, but especially in Deptford/New Cross in preparation for a walk with the New Cross Commoners. Often I would spend up to two hours with headphones plugged in to the device; it is the same basic technology as a radio receiver and the sound produced is thus very similar. Below are reflections on the sometimes fevered experience of solo bat walking, written for an OSA event on distraction.

Psssssss. The constant static of the heterodyne detector is exhausting. Admist the fuzz and whirr coming from my headphones I think I hear distant bat calls. So faint… was that a slap, a pipistrelle sounding out its meal? The blown raspberry of a feeding buzz? No, just a mental artefact, a result of my tired attempts at concentration, a whispery parody of those who found the voices of the dead in the noise of cassette tapes. Sometimes I click my fingers in front of its microphone to remind me of what sound actually sounds like.

This doesn’t help with other mistakes. I follow a constant, rhythmic sound that stays steady as I follow it - till I realise it is the scraping of my keys in my own pocket. Keys are just one of the objects that begin to stand out - not just mine, bags now jangle from across the street. Cars emit a new purr too, bicycles sound like noisy office fans. When walked they are like a tombola. Electrical substations make a strange humm at 40khz, and building sites give out a range of high-pitched tones. What are they? Security systems, special equipment? These sourceless sounds prompt anxiety, whereas the rattles of bicycle wheels and engines are almost warm and reassuring. I turn through frequencies, find little above 70khz; lower down the city’s whispers become audible.

The combination of white noise and headphones muffles my hearing. This, and my diverted attention as I rove the night-time city in search of life, can lead me into some odd situation. I become lost easily - I track my movements with GPS, but the device doesn’t have mobile internet so I can’t correlate it to a map position - it just draws an abstract shape. In a south London meadow, I think I see a bat fly beneath a pathlight. I can’t follow where the bat is going, but I can investigate the direction it flew from. I walk along the edge of the meadow, along a verge of trees. After a minute of walking I look round and see a fox walking directly towards me. I’m spooked - this is not how foxes normally behave. I don’t know what it might do, so I back away. It follows me as I walk, and under the influence of the anxiety inducing static I feel convinced it is firmly escorting me off its territory. Once back on the path, I happen upon a ultrasounic throbbing coming from the concrete rail bridge that borders the meadow. I’m puzzled, unnerved. A second fox appears at the park entrance and looks at me. I stare back a moment, then make a quick pace out through a tunnel. My mind turns; explanations, rationalisations tumble between heart beats.