Audio For Broadcast: Microphones Part 1 - Polar Patterns

Here we look at microphone polar patterns and what to consider when planning how to capture sounds to create gripping broadcast content.

There are a huge variety of microphones on the market, it’s totally overwhelming, and with good reason. Choosing a good microphone isn’t like choosing a good bottle of wine.

You know if you have a good bottle of wine straight away; if you like how it tastes you will like how it tastes in a restaurant, in a bar or in your kitchen. Similarly, you might like the sound of a particular microphone on location in a stadium, but it doesn’t mean you will like it on a Vox Pop, or in a studio, or in front of a drum kit. Or in a kitchen.

Broadcast is never that simple and microphone choice is always dependent on the application.

Everything about the broadcast will inform your decision, and questions need to be asked before you even get to work; how noisy is the environment? Do the mic’s need to be hidden? Are there any physical restrictions in place, will there be audience participation, how much of an issue is foldback, how skilled is the talent, is it live, do you need a backup, what is the weather like, what is the format of the show, what is the talent comfortable with?

The application and what the broadcaster is trying to achieve will always be the first consideration, and answers to questions like these will provide the necessary guidance to mic selection for any environment. For this reason broadcast facilities stock a range of microphones with different polar patterns, electronics and form factors, and before settling on any of them the sound recordist’s first job should be a discussion with the production team to ascertain what the story is about.

The Capture

Let’s look at some fundamentals.

Sound is the vibration of particles in the air. It happens all around us all the time, and a falling tree in a forest will still vibrate air particles whether we are there to hear it or not. Microphones all do exactly the same thing, converting those vibrations into electronic signals which can be fed into a mixer or a speaker for amplification or manipulation.

But they are all very different and have characteristics which can help define a mix and adapt to your broadcast environment.

Polar patterns are a good place to start. A polar pattern determines the directional frequency response of the mic’s so you can work out which are best suited to your needs; in other words, it tells where it will pick up sound from. For live TV and radio, the aim is generally to capture speech and reduce room noise, so the most common mic patterns used broadcast tend to be unidirectional.

Polar Patterns

Unidirectional microphones are known as cardioid’s.

In geometry, a cardioid is a heart-shaped plane, and similarly a cardioid mic has a maximum response on axis and a maximum rejection at 180° off axis; from above it looks like a heart.

Cardioid Polar Pattern.

Cardioid Polar Pattern.

This rejection of off-axis noise makes it a good choice for many broadcast applications, as it is geared to pick up whatever is in front of it and not much from the sides or the back. It is a popular all-rounder for broadcast as it rejects a lot of ambient noise, such as an audience or other guests on a panel but is still somewhat forgiving if the main subject is a little off axis.

A lobar pattern takes off-axis rejection to another level, and mics which use this pattern are often used in broadcast. These are sometimes referred to as super-cardioid mics and you will see them all over broadcast productions. They are used on boom poles held above an audience or in the street on a live interview, and they are used to capture on-field action at sports events. Often referred to as shotgun microphones due to their long and thin appearance like a gun barrel, they are even more adept at rejecting ambient noise and off-axis sound sources than a cardioid by providing an even, narrow pickup angle.

The level of the sound source within this pickup angle is consistent, while sounds coming from outside this angle are reduced. The width of the angle is dependent on the length of the mic; a longer mic barrel produces a tighter pickup angle. This can be useful on outdoor shoots as it means the mic can be located further away, and it works well on audience participation shows when a boom operator can target someone in an audience asking a question, while rejecting the ambient noise from the rest of the room.

Shotgun Polar Pattern.

Shotgun Polar Pattern.

Shotguns In Sports

Outside broadcasts, especially sports, will also often make use of shotgun mics. You can spot them mounted to poles when you watch a UK Premiership match, covered with fluffy windshields to reduce wind noise and pointing into the pitch from the edge of the field of play (usually one behind each goal, one on each corner, four around the 18-yard boxes and two on the centre lines). The placement allows these highly directional mics to pick up the action across the whole pitch and allow the sound mixer to cross fade between them to best pick up the sound on the field, such as the ball being kicked, while keeping crowd noise at a minimum.

The Sound Is All Around

An omni-directional mic has none of the above traits – it picks up noise from all directions, including any ambient noise, so it provides a very natural representation of the environment.

Omni Polar Pattern.

Omni Polar Pattern.

For this reason, omni mics are more often used where the environment can be controlled, or in settings where the environment is part of the story, such as a location shoot in a large space like a church where the acoustics of the room can be used as part of the narrative.

Conversely, omni mics can also be a good option for close miking and they can be useful when working with talent who are untrained in broadcast techniques. While a cardioid mic provides more control over the subject and the environment, close miking a presenter with an omni mic doesn’t make much difference and may result in a cleaner sound, and it’s even more forgiving than a cardioid mic as there is no off-axis position at all.

Bafta award-winning sound supervisor Robert Edwards notes that when you have a presenter engaging directly with people in an audience, “a cardioid mic must be in close and tight, but if the presenter likes to waft it about a bit, an omni mic is better. It’s much better at coping with people.”

In The Field

In American Football, omni mics are also used to capture action from pitchside but unlike Premiership football, American football has an entirely different rhythm. It doesn’t flow the same; it’s fast, it’s brutal and it’s visceral, and plays go in violent and crunching waves.

To capture on-field action sound supervisors will often use parabolic microphones at pitchside, which is a semi-spherical bowl attached to an omnidirectional mic. The bowl acts like a satellite dish to capture highly directional audio and it reflects the sound waves directly into the mic. As it only reflects what it is pointed at it is perfect for capturing the on-field communication and tackles.

And while it seems somewhat counter-intuitive using an omni mic on a highly directional piece of equipment, it works better because not all the reflected signals will hit the mic at the same angle. 

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Polar patterns play a big part in designing the sound for a programme, and they apply to all microphones. The next step is to ascertain what kind of microphone to use.