As I mentioned briefly in Pt. I of Capturing Sound, polar patterns (or directional response) determine the pick up pattern of microphone. Another way to look at this is if you think of your ear as a microphone (again) and the way you ‘collect’ sound – you tend to hear sound better in front of you as your ears ‘cup’ the sound in front of you and diminish the sound behind. This makes your ears like directional microphones, focussing on the sound in front of you and rejecting the sound behind.
“It should be noted that all mics respond differently at different frequencies. For example, a mic can be very directional at one frequency (usually higher frequencies) but virtually omnidirectional at another.” – Bobby Owsinski
These are two of the three main types of microphone polar patterns:
- Cardioid (directional)
- Figure of 8
This is the simplest of polar patterns – a condenser microphone is omnidirectional in its original state. Ideally, this type of polar pattern will pick up sound equally from all points around it, as seen from the diagram below. Although they are not ideal in a live situation (high probability of feedback), in the studio using them as an ambient microphone to capture the room’s reverb is very useful.
Examples of Omnidirectional Microphones:
MicW N101 – Omnidirectional Condenser
“The N101 is a very good microphone, and although it’s not quite as smooth and controlled at the high end as the industry benchmarks, I felt it gave little away in terms of clarity or resolution, and managed a similar performance in terms of noise and maximum SPL figures.”
“Good value for money.”
Earthworks TC20 – Omnidirectional Condenser
“A mic like this really excels as a drum overhead or percussion mic, but it is far more flexible than you might imagine, and it doesn’t produce a clinical or unexciting sound just because it is accurate. I wouldn’t buy a mic like this specifically for recording quiet acoustic instruments, but if you have one around, don’t exclude it: you might just be pleasantly surprised.”
As the bracketed name suggests, this type of polar pattern has a strong pick up focus in front of it, while rejecting most of the sound behind. This pattern does not completely reject the sound from behind but rather reduces it making the pattern a kind of heart/squished kidney bean shape. In the live environment this becomes very useful as it will reject the noise from the audience while also reducing the chance of feedback.
Examples of Cardioid Microphones:
Neumann TLM102 – Cardioid Condenser Microphone
“For those project-studio owners who want just a single decent mic that can tackle a variety of sources as well as vocals, this should prove a very practical choice.”
Audio-Technica AT5040 – Large-Capsule Capacitor Microphone
“For the AT5040 they wanted to combine the familiar benefits associated with a large-diaphragm design — which include high sensitivity and very low self-noise — with those of a small-diaphragm mic, such as the better high-frequency extension and precision, greater ruggedness, and better temperature stability.”
As I mentioned previously, in its native state, a condenser microphone is omnidirectional, but this can be altered. By punching holes in the back of the backplate any sound occurring behind the microphone will reach the back at the same time as the front, therefore cancelling it out and making the microphone directional. The size and position of the holes determines what frequencies are cancelled and if done so correctly, a Hyper-Cardioid can be created – basically a cardioid that rejects much less behind it and focuses more on what is in front.
III. Figure of 8 (Figure-8 or bidirectional if you’re feeling fancy)
The figure of 8 polar pattern is exactly how it sounds – it picks up audio in a figure of 8 pattern with the microphone in the middle (see diagram below). This means that it will pick up audio occurring in front of the microphone and behind it, but will reject mostly anything coming from either side. In general, the front and back have the same pick up level, but it is common for the front to have a brighter frequency response. Commonly found in ribbon microphones, the sound picked up from the rear of the microphone will generate an inverted signal compared with the front. This becomes much more relevant and useful when we look at Pt. III of this blogging trilogy – Microphone Techniques.
Extra: The Proximity Effect
This strange effect occurs in cardioid and hyper-cardioid as you get closer to the sound source. The difference in their near and far response is that when the microphone is close to the source, it generates a build up of low frequencies. This proximity effect is normally used to the engineer’s advantage to provide warmth to the source, but if it is not considered, it can provide an unbalanced frequency response.
Check out this Sound on Sound article on using polar patterns effectively (it provides the basics behind the patterns, but also puts these theories into practice)
And as always, Bobby Owsinski’s The Recording Engineer’s Handbook
Any questions, just ask :]