Directionality of microphones: how it works

The microphone seems like a simple device until it comes to practice. Which side to direct it to the sound source, at what angle and at what distance? How to choose a microphone based on the situation? All this determines its focus. Of all the characteristics, it is she who raises the most questions. We will talk about it.

Here are the basic things to know to choose the right microphone for your situation:

Microphones are: hand-held – to read reports and sing karaoke, desktop – for blogs and streams, suspended – to record instruments, such as a clothespin – to hang on the interlocutor and record interviews, on-camera – to hang on a video camera.
Professional microphones are connected by XLR cable to a preamp, audio card, mixer or d-box. One of these devices prepares the signal to a digestible form and feeds it to an analog-to-digital converter (ADC) to obtain zeros and ones. But there are microphones that already have a built-in preamp and ADC – just connect them to a computer via USB and you’re done.
Microphones differ in design – they are dynamic, condenser, ribbon and electret. In dynamic you need to speak closely. Capacitors are more sensitive – they can be placed a couple of meters from the sound source, but to amplify a weak signal, it needs + 48V phantom power. Electret – cheap microchip microphones found in all headsets and phones. Ribbon – specific studio microphones.
Microphones vary in directivity, with the main microphones being unidirectional (cardioid), bidirectional and omnidirectional.
Directionality is one of the main determining factors when choosing a microphone. On stage, a cardioid one is needed so that it does not catch too much – only vocals, for an interview bi-directional is suitable, with a recording of a whole group in a room or ambient sounds such as rustling leaves and rain noise, omnidirectional is best suited. It’s easy to get confused, so you need to talk more about focus.

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Omnidirectional

The first carbon and condenser microphones were omnidirectional. They catch everything in a horizontal plane. These microphones are essentially a very sensitive barometer, which consists of a box with a thin diaphragm inside. Even the smallest sound pressure from either side vibrates the diaphragm. Speech to omnidirectional microphones can only be written in a well-muffled room, otherwise the noise of a car outside the window and the knock of a neighbor on a pipe will be recorded along with the voice.

Directional pattern of the omnidirectional Neumann M50

Yet sometimes they are used to record vocals in the studio. Due to their design, they are less susceptible to explosive consonants “p” and “b” than other microphones, as well as to the howling of the wind. They also lack the effect of approaching – most other microphones produce more low frequencies as the sound source approaches the diaphragm. Omnidirectional ones record the same timbre at different distances.

Omnidirectional microphones provide the least color to the sound, so samples and instruments recorded on them sound natural and natural. They are best suited for recording:

Groups of musicians with acoustic instruments – small orchestras, folk music, folk and ethnics.
Large acoustic instruments – from acoustic guitar to bagpipes, piano, grand piano and organ.
Samples – the rustle of the forest, the murmur of a stream, the sound of a car starting, etc.
ASMR is a specific genre on YouTube about goose bumps from rustling sounds, inherently related to samples. Here more about how to choose the microphone to ASMR.
Voices , especially when the microphone is a lavalier. Then it does not need to be precisely positioned, and the proximity to the sound source cuts off unnecessary things.

Bidirectional

Omnidirectional microphones are good for recording music in the studio, but for radio presenters, the pie chart is not very convenient. The microphone catches everything around, but it should – only the speaker’s speech. Therefore, shortly after the release of omnidirectional microphones, RCA invented a bi-directional microphone – it only caught from the front and rear.

Bi-directional microphones are equally sensitive to sounds from the front and rear, but have areas of absolute silence on the sides. Their design is based on the differential pressure principle. The diaphragm is open on both sides, and the device detects the difference in pressure between one side and the other. If we speak to the membrane from the front, the signal will be with positive polarity, if from the back – with negative polarity. A bi-directional microphone pattern is sometimes referred to as a figure-of-eight.

AEA R84 Bi-Directional Microphone Pattern

Such microphones usually do not perceive sounds from the sides – at 90 and 270 degrees.
The sound from these positions falls on the membrane at the same time from the front and from the rear, two identical signals with different polarities are obtained, which ultimately gives zero. This creates a dome of silence, which is convenient to use in practice – in the studio or in the field. Some microphones for video cameras are bi-directional so you can record clear sound for reporting in difficult conditions.

Microphone Audio-technica ATR6550x

A fairly narrow signal reception angle is useful for isolating a voice or instrument surrounded by other sound sources. Bidirectionality is useful for capturing two sources when they are opposite each other. But it’s worth remembering that bi-directional microphones have a stronger effect of proximity than others. They are also very sensitive to explosive consonants.

In practice, these microphones are used:

For interviews and dialogues. An obvious solution for recording two interlocutors.
For backing vocalists , two sing into one microphone.
For percussion in an orchestra – for example, can be positioned between toms and congas.
For a singing pianist , to capture both vocals and instrument.
Cardioid

In fact, the first bi-directional microphone was closed from the rear with a cover, and therefore was, as it were, unidirectional. Soon there was a different approach to creating a unidirectional microphone: two capsules fit in one device – a bi-directional ribbon and an omnidirectional dynamic. Their diagrams are set up so that when combined, they reinforce each other at the front and mutually subtract at the back. This is how we got a real cardioid microphone.

 

Today, cardioid orientation is most common. A microphone with this pattern is most sensitive to sounds from the front and least from the opposite side. On the sides, in the range from 90 to 270 degrees, the sensitivity drops to -6 dB. The result is a diagram that resembles a heart – hence the “cardio” in the name. Such microphones are sensitive to plosive consonants and exhibit an approaching effect – if we speak close to the membrane, there will be more low frequencies.

Shure SM57 cardioid pick-up pattern

Cardioid directivity microphones are the most popular and widely used:

For recording in unprepared rooms and outdoors . A cardioid chart will cut off reflections from walls, the clatter of a neighbor’s child, the noise of cars, or the banging of fingers on the keyboard. They are good for reporting, streaming, podcasting, vlogging. They can be used for recording in difficult and noisy environments.
On the stage. During a live performance, the stage monitor is aimed at the vocalist, through which the performer listens to himself, so any microphone, except for the cardioid one, will start up – catching feedback from the monitor and deafeningly whistle.
In a studio. They allow you to capture instruments individually, cutting off the sound of other sources, so they are indispensable when recording drums and guitars. A combination of cardioid microphones allows you to create your own unique directivity pattern.

Supercardioid and hypercardioid
Left – supercardioid orientation, right – hypercardioid

The supercardioid diagram is highly focused. On the sides, its sensitivity is reduced – up to 10 dB at 90 and 270 degrees, plus it has blind areas from the rear.

It is often confused with the hypercardioid diagram , which is quite similar. But unlike the supercardioid one, the hypercardioid one is even more narrowly focused – the sensitivity on the sides is reduced by 12 dB, while it has a wider rear section of signal reception.

Both diagrams have taken root well in movie mics. Surely everyone saw the footage from the set, where a large hairy microphone on a long pole hangs above the actor. The furry shell is a wind protection, it is stretched over a hollow frame, inside which there is a “shotgun” microphone – in the form of a tube. Their design greatly enhances the direct directivity effect.

Shotgun microphone and directional diagram

This allows you to record the voice of an actor from a distance without unnecessary noise – so that the microphone does not get into the frame (although sometimes it does). The same ones are mounted on some video cameras.

Microphone RODE VideoMic GO

There are also ultra-directional microphones for stage and podcasts. However, they are more difficult to work with than regular cardioid ones. Shotgun microphones are so sensitive to direction that their signal must be constantly monitored in headphones in order to keep them on the course of the sound source.

Multi-directional

Multi-directional microphones let you switch between multiple patterns. They are usually made from two cardioid diaphragms connected together. Different directivity is achieved by combining two signals with different polarities. Therefore, the default choice will be as follows:

Cardioid – One diaphragm is engaged.
Bi – directional – both diaphragms with different polarity.
Omnidirectional – both diaphragms with the same polarity
There are other presets, but all of them are easily achieved by playing with the phase and amplitude of the signal from two diaphragms. It should be remembered, however, that cardioid diaphragms are subject to explosive consonants and zoom effects. Also, the omnidirectional mode will only be an imitation of omnidirectionality, since it is based on a capsule with a different principle of operation. However, multi-directional microphones are very convenient and versatile.

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Stereo, binaural, ambisonic

A stereo microphone consists of at least two diaphragms in one device – usually bi-directional or cardioid. Classic combinations:

XY: Two cardioid capsules at 90-135 degrees to each other.
Center-Sides: The cardioid membrane is directed forward and perpendicular to it is bidirectional.
Blumenstein pair: two bi-directional diaphragms at 90 degrees to each other. They create a very natural-sounding stereo image in which sound sources are located extremely accurately and accurately.
From left to right: XY, center-sides, Blumenstein pair.

Professional stereo microphones are used in audio recording for movies and games. They are also popular in video blogging, especially ASMR videos. Some desktop microphones for podcasting and streaming have stereo mode, for example, the Blue Yeti X uses a combination of XY.

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The combination of multiple diaphragms is used in speakerphones – conference microphones. They pick up sounds in all directions, allowing a dozen other people to use just one microphone.

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Binaural microphones are a special kind of stereo microphones. This is a whole structure – microphones are mounted on an omnidirectional diaphragm in the head or auricle mock-up. The sound is refracted and reflected from the layout in the same way as it behaves with a live listener.