Introduction To Parabolic Reflectors

Photo: Mark Ferguson, except where otherwise stated

The parabolic reflector is the ultimate directional microphone setup for outdoor recording. Here’s how to get the best from it.

Let’s say you’re a sound designer. A new client has just flung a rapid‑turnaround promo film your way, which happens to contain three close‑up shots of singing UK/European bird species: blackbird, song thrush and blue tit. You ask a few friends for recorded materials and scour online sample libraries, but these sounds sit uncomfortably in the mix and don’t sync well. So you make some shotgun mic recordings in the local park, but they sound terrible when you up the volume and EQ them as needed. You haven’t the time (or trust in the public!) to leave your microphones hidden in the bushes while you wait for the birds to come close enough, so to deliver the sound quality needed you need a way to isolate each species as quickly as possible, at a distance, and with a high signal‑to‑noise ratio. Well, one possible way to do this is with a parabolic reflector...

Historical Reflections

Current research suggests that the principles behind parabolic curves were proven by the Greek mathematician and geometer Diocles (circa 240‑180 BCE). In his text On Burning Mirrors, he described the properties of a parabola, noting that it always reflects incoming light running parallel to its axis of symmetry to a focal point and, today, the Olympic torch is traditionally ignited using this principle — sunlight is focused on to the head of the torch — before it begins its journey in the hands of enthusiastic runners.

We’re working with sound rather than light, of course, but sound waves can be focused in much the same way onto the capsule of a microphone. It’s not entirely clear when people started experimenting with reflectors to gather sound, but interest seems to have taken off in the early 20th Century, most notably when the British developed a series of concrete ‘sound mirrors’ to track enemy aircraft before they reached land. Some of these formidable structures exist to this day, by the way: some of the best examples can be found at Denge, near Dungeness in Kent. Portable listening horns, quasi‑parabolic reflectors and similar devices, some of which sat rather comically over the user’s ears, were also developed for aircraft tracking purposes by Germany, the USA and other nations throughout the First World War and into the 1930s, until the invention of RADAR rendered them obsolete.

The concrete ‘acoustic mirrors’ at Denge, Kent, constructed between 1928 and 1935. Microphones were moved around the mirrors to pinpoint incoming enemy aircraft..Photo: Wikimedia Commons

Interest in wildlife sound documentation seems to have kickstarted the development of smaller reflectors for field recording purposes during the 1930s, especially for recording avian sounds. As far as historical records suggest, in May 1932 Professor Peter Paul Kellogg of Cornell University (in collaboration with student Peter Keane) became the first person to successfully record a bird using a parabolic reflector: the song of a yellow‑breasted chat, Icteria virens. As the decades advanced and reflectors became somewhat lighter and more portable, they were utilised (and sometimes even built from scratch) by formative wildlife sound practitioners around the world. Today, parabolic recordings are frequently used by natural history post‑production studios, and are also employed by broadcasters to capture competitive sporting action from the sidelines, notably in American football.

By Hand &...