By Charley Beeman
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.
The omnidirectional microphone is one of the basic microphone patterns. Traditional MEMS microphones are omnidirectional. The omnidirectional microphone collects sound equally from all directions.
Dipole:Omni Ratio: 0:1
The subcardioid microphone is weighted further toward the omnidirectional signal than the dipole signal resulting in a less directional polar pattern without a null point and 10dB attenuation to the back. This polar pattern might be used when a wider pickup angle is required than a cardioid, but some background attenuation is still beneficial, such as recording a large group
Dipole:Omni Ratio: 1:2
The cardioid polar pattern is weighted equally between an omnidirectional and dipole microphone resulting in a microphone with the strongest rear rejection and a wide pickup angle. A cardioid is best suited when you need to reject the maximum noise directly from the rear, such as when two subjects are sitting across the table from each other and speaking into separate microphones.
Dipole:Omni Ratio: 1:1
The supercardioid weighs the dipole signal more heavily than the omnidirectional signal. This creates a small rear lobe but has a higher DI and UI than either a cardioid or a dipole. Supercardioids are useful in applications where there is not direct interfering noise behind the microphone, but the subject still needs to be able shift around, such as when filming.
Nulls: 127°, 233°
Dipole:Omni Ratio: √3:1
The hypercardioid polar pattern has the highest DI and will reduce the most ambient noise. This is useful when most of the background noise is not direct and maximum noise rejection is required. A hypercardioid polar pattern would be beneficial for someone talking on the phone using wireless headphones in a crowded environment.
Nulls: 110°, 250°
Dipole:Omni Ratio: 3:1
The dipole microphone is the other basic microphone pattern. A dipole collects sounds equally in the front and back and most strongly rejects sound from the sides. Directional MEMS microphones from Soundskrit are dipole microphones.
Nulls: 90°, 270°
Dipole:Omni Ratio: 1:0