Combining Microphone Polar Patterns
By Charley Beeman
2023-06-12
Introduction to Microphone Polar Patterns
Microphones are categorized by their polar pattern. A microphone polar pattern describes how the sensitivity of a microphone varies based on the signal’s direction of arrival (DoA). Designing a system to have a specific polar pattern is useful to control what sounds are picked up so that the signal of interest is most prominent in the recording. The basic polar patterns for microphones are omnidirectional and dipole as seen below:
A microphone with an omnidirectional polar pattern picks up sound equally from all directions. A dipole picks up sound from the front and back equally and rejects sounds from the sides. Dipoles have a null, where sound is rejected the most, at 90º and 270º. The polar pattern also shows where sound begins to drop off. In a dipole, there is a -3 dB gain at ±45º compared to 0º. The Soundskrit SKR0400 microphone has a dipole polar pattern.
Directivity Index and Unidirectional Index of Microphone Polar Patterns
It is useful to describe microphone polar patterns with a single figure that represents how much noise they reject and from where. The two primary measures are the directivity index (DI) and unidirectional index (UI).
The directivity index measures the ratio of the microphone output for a sound positioned directly in front of the microphone (𝜃 = 0⁰) versus sound with the same amount of total acoustic power coming from all directions equally. The DI of an omnidirectional microphone is 0 and the DI of a dipole microphone is 4.8 dB.
The unidirectional index measures the ratio of the microphone output for a sound positioned in front of the microphone (-90⁰ < 𝜃 < 90⁰) versus sound with the same amount of total acoustic power coming from the back of the microphone (90⁰ < 𝜃 < 270⁰) . Both omnidirectional microphones and dipole microphones have a UI of 0 dB.
Combining Basic Microphone Polar Patterns
A cardioid polar pattern rejects noise coming from the backside. A cardioid microphone will have a DI of 4.8 dB and a UI of 8.4 dB.
A cardioid microphone can be created by combining the signals from an omnidirectional microphone and a dipole microphone located near each other. An omnidirectional microphone outputs the same signal regardless of the direction of the sound. The polarity of the signal from the dipole is inverted depending on whether the signal comes from the front or rear of the microphone. The rear lobe of the dipole will have a negative polarity compared to a signal coming from the front.
Due to this inverted polarity, the summation of an omnidirectional and a dipole microphone will create a cardioid polar pattern. This summation creates a cardioid as when sounds come from the rear, the signal from the dipole will be inverted compared to the omnidirectional microphone and the signals will destructively interfere and cancel. Sound coming from the front will have the same polarity on both microphones so the signals will constructively interfere and sum. Signals coming from the sides will be rejected by the dipole and will not impact the signal from the omnidirectional microphone. This summation of microphone polar patterns is illustrated below:
By adjusting the ratio between the omnidirectional and dipole microphones, we can adjust the microphone polar pattern to be more or less directional depending on the situation. For example, adjusting the polar pattern to be weighted more towards the dipole will reintroduce a rear lobe but narrow the front lobe. In the tool below you can see how adjusting the ratio between the dipole and omnidirectional microphones impacts the final polar pattern.
Standard Microphone Polar Patterns
While the range between omnidirectional and dipole is infinitely adjustable, there are several cardioid-like microphone polar patterns commonly referred to. These are enumerated below with their DI, UI, and the ratio of omnidirectional to dipole.
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