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 & Tripod

So, you want to experiment with using a parabolic reflector for the first time — where do you start? Who sells them and how do they work? A reflector used for field recording is essentially a large, lightweight plastic dish which looks like an oversized contact lens. Typically around 22 inches (56cm) in diameter — more on that later — they can be held or mounted on a tripod, and the latter method of course ensures that the creaking and popping of tired wrist and elbow joints stays out of your recordings. Twenty‑two‑inch models available from two of the most popular manufacturers, Telinga and Wildtronics, generally retail for £350 to £1000 but sometimes more, depending on the model/kit you go for. Some models are sold with the manufacturer’s own microphone, with mono and stereo options, while others allow you to place your own pencil condenser mic, such as a Sennheiser MK8020, Schoeps CCM 4 or Rycote CA‑08, inside the dish. On most kits, the microphone mounting apparatus and handle/cable are detachable, and can be stored separately in a backpack, and some dishes are, like my own, made from a flexible polycarbonate blend. This allows them to be rolled up and placed in a bag for easy transport during field recording trips (this is purely a functional benefit: as long as the reflector is well manufactured, in my own experience rigidity/flexibility doesn’t affect sound quality).

As long as incoming sound waves run parallel to the axis of symmetry, they will be reflected directly onto the focal point of the dish. One significant drawback of a parabolic reflector is the amount of coloration that occurs with off‑axis sources; for this reason, it doesn’t work particularly well in densely populated spaces where lots of sources blend and move around (eg. overgrown woodland). A reflector is usually at its best in calm, open or semi‑open spaces, pointed towards a single source.

As with any field recording methods, there are various technical considerations. First, note that wavelengths approaching the diameter of the dish can’t be reflected very well, and this means that unwieldy sizes are required if you want to record anything with significant low‑frequency content accurately. For example, with a standard‑size 22‑inch reflector, a source frequency of around 600Hz is a reasonable, mathematical ‘lower limit’ to bear in mind. Trying to record anything approaching, or below, this threshold generally won’t produce satisfying results (most birdsong and sporting activity contains frequencies well above this).

Second, it’s also worth noting that whilst in theory the acoustical amplification a dish provides increases with frequency (at roughly 6dB per octave), in practice it actually tails off — this will become apparent from about 5kHz onwards, when using a 22‑inch reflector with a pencil‑type omnidirectional microphone. This attenuation is due to the size of the ‘globular focus’, which shrinks with higher frequencies: the acoustical energy at the focal point of higher frequencies can’t move a microphone membrane as effectively. Phase cancellation (if the mic isn’t precisely centred within this smaller focus) and atmospheric attenuation of higher frequencies are also contributing factors.

Microphone choice has an influence on the result too, of course. The two most sensible polar patterns to go with are cardioid and omnidirectional. Both types’ capsules are placed at the focus, with cardioids always pointing ‘into’ the dish. An omni tends to sound more natural, since direct (non‑reflected) sounds are also captured. A cardioid, on the other hand, isolates the reflected subject very well, rejecting direct sources, but it has less overall sensitivity due to its decreased pickup at the sides. Having said that, do bear in mind that omnis become increasingly directional at higher frequencies so differences in overall sensitivity aren’t as significant as you might imagine.

As long as a sound source has a reasonable amount of high‑frequency content, a reflector will amplify it acoustically, meaning less electronic amplification is required.

The key point to take away from all of this is that as long as a sound source has a reasonable amount of high‑frequency content, a reflector will amplify it acoustically, meaning less electronic amplification is required. This obviously means less inherent signal noise, and this can give the recordist a realistic prospect of capturing subjects at a distance of 100m or more.

Getting Crafty

Field recording requires just as much mastery of technique as of technology. There’s very little point in knowing how a parabolic reflector works if you don’t know how to utilise it in real‑world situations, and I can’t emphasise this enough when it comes to wildlife sound recording, which is arguably the most challenging variant of field recording out there. Fieldcraft can really only be learnt properly through direct experience, and one of the best resources for this is the Wildlife Sound Recording Society, of which I am a member: www.wildlife-sound.org. But having said that, here are some helpful points to bear in mind as you venture out with your big dish for the first time:

1. Wind: Most models of reflector will require some kind of wind cover, which can be stretched over the front. This helps with wind shielding and camouflage (dark greens and browns are good choices). Just be sure not to take it off in a midge‑dominated environment, since the little sods will get trapped inside the dish when you put it back on, which makes for interesting listening! It’s worth noting that the rear, curved structure of a reflector can make a good windshield; if a moderate wind is blowing, try standing with your back to it. Also, stay away from woodland when it’s windy, since moving vegetation sounds terrible when recorded parabolically.

2. Rain: For obvious reasons, reflectors are virtually useless in rain, which impacts the plastic structure and sounds utterly apocalyptic through headphones.

3. Handling Noise: Handling noise can be an issue, especially with cardioid arrangements. Thick gloves help, but in most cases, a tripod is the answer. Look for something lightweight and durable like the Slik Pro series, and make sure it can be attached to your kit before purchasing.

4. Suitability of Location: Reflectors tend to sound very ‘muddy’ in enclosed spaces (eg. thickly vegetated, deciduous woodland). Using them near running water can also be problematic. In my own experience, they sound best in calm, open environments, focused towards a single point like the top of a tree, branch edge or fence post. To illustrate what’s possible in an ideal situation, here’s my own recording of a song thrush vocalising from the very top of an English oak, in a Cotswold meadow: https://tinyurl.com/5n76yz5f.

5. Narrow Focus: Related to the previous point, reflectors excel at individual species capture but are less useful for groups (eg. flocks of birds), since sources within these groups tend to move off‑axis. If you are recording large groups of animals, consider doing so at a fair distance, since the whole scene will narrow to a more manageable point for parabolic capture.

6. Working With Animals: Remember that if you’re recording living things they can react to your very presence! Think about how you are going to approach your target species. With birds, approach respectfully, slowly and quietly at a diagonal (never head‑on), and don’t wear any white or bright clothing. One of the advantages of a reflector is that you can record species at great distances with relatively little disturbance; this is something to be exploited, rather than worked against.

7. Routes & Distractions: Reflectors are difficult to carry through overgrown habitats, so think about your walking route before you go out. In urban/suburban spots, they tend to attract lots of public curiosity; people regularly mistake mine for some sort of experimental radio antenna or drone. Just answer all questions honestly, and folks tend to move on. Also bear in mind your own safety if you head out to public parks early in the morning to capture birdsong: always tell someone where you are going.

8. Take A Minute: Finally, because of their incredibly high directivity and inherent requirement for headphone monitoring, parabolic reflectors tend to skew your awareness of the wider environment. It’s all too easy to spend long periods with headphones on, focused on a particular area and waiting for the relevant species to appear. Whilst this is inherent to the craft of parabolic recording, it’s good to take your headphones off for a while to recalibrate your ears to the wider soundscape, and locate new recording opportunities.

9. Mono Or Stereo: I generally prefer working in mono, but stereo capture is also possible. One method involves the placement of a dividing baffle vertically along the axis of symmetry, bisecting the focus. Small microphones can then be placed either side of the baffle in a kind of quasi‑Jecklin disk arrangement: reflected sound is recorded in mono, while environmental sounds are captured in stereo. Since the mics are mounted close to the baffle, this technique can also take advantage of a pressure‑zone amplitude boost. Other stereo methods involve mounting a Mid‑Sides configuration internally (with omni or cardioid at the focus, and figure‑eight resting just above or below), and mounting two miniature omnis (eg. DPA 4060s) externally on the edges of the dish, to complement an internally mounted mono mic.

The author, recording a Eurasian skylark (Alauda arvensis) in an open field. This species is very challenging to record well, since it starts singing from first light and typically flies upwards in a quasi‑spiral as it does so. This necessitates an early start with reflector in hand, which rarely ends well in terms of handling noise. As the photo illustrates, however, it is possible to wait for skylarks to sing from the ground (or a rock, fence post, etc) and opt for a tripod‑mounted approach. This kind of knowledge highlights just how important it is to complement technical know‑how with fieldcraft when recording wildlife.

Above The Mic Locker

Hopefully, it’s clear that the studio sound design dilemma I set out at the top of this article (or other similar ones) could be solved effectively and efficiently with a parabolic reflector. As mentioned previously, it’s also an incredibly useful piece of kit for recording action at outdoor sporting events, such as football kicks, running, tackling and more. Just make sure to get permission before you turn up at the sidelines! In short, if you can think of any sound source with a reasonable amount of high‑frequency content that you need to capture at a distance for your projects, a reflector can help. It has certainly been an invaluable addition to my own field recording arsenal. I regularly used one throughout my PhD research to record birdsong, fox and deer barks, bush crickets, grasshoppers and bumblebees. These sources were successfully worked into large‑scale stereo and multi‑channel electroacoustic compositions (see: https://tinyurl.com/mtax246y), and lent themselves to all sorts of experimental processing.

Much has changed since the early days of wildlife sound recording, when heavy, unwieldy reflectors had to be hefted through all kinds of habitats by the recordist. Now, you can simply fold one up and assemble it when you get there. And if you do need to carry it fully assembled on a tripod, you no longer require the physique of a special forces operator to manage a hike with one! That said, a good level of physical fitness still helps. Aside from benefitting your own audio work, a high‑quality library of parabolic wildlife recordings (with accompanying weather, GPS and observational data) makes a wonderful contribution to bioacoustics research, and many organisations — notably the British Library — are only too happy to receive donations of wildlife recordings made with reflectors. So consider popping one above your mic locker. You won’t regret it!