Microphones

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The objective of this course is to understand the various features indicated in the technical documentation of microphones.

Sensitivity

It will determine the efficiency of transduction since it is the level of output voltage from the microphone based on the received acoustic pressure on the diaphragm. Therefore, the louder the sound, the higher the resulting voltage will be (the microphone will generate more electricity).

This value often varies from one microphone to another because the microphone's design has a significant influence on sensitivity. Thus, manufacturers provide us with the ratio of output voltage for a given acoustic pressure; this is the microphone's sensitivity.

To simplify the comparison of different microphone sensitivities in technical specifications, this information is standardized. It is the resulting voltage when subjected to an acoustic pressure of 1 Pa (94 dB SPL) at a frequency of 1 kHz (calculated at 1 meter in the axis of the capsule).

Within various technical documentations, this value will be expressed in mV/Pa or directly in dBu or dBV (output voltage level of the microphone, see course on levels).

Directivity (or polar pattern)

It is the variation in microphone sensitivity depending on the angle of the sound source relative to the diaphragm. It is measured by moving a (point-source) sound source in a free field along a circle with the microphone at the center. Generally, when the source is aligned with the microphone's axis, its sensitivity is maximum. Directionality is represented on a polar pattern diagram (in two or three dimensions), where the center is the diaphragm, and the curve represents the microphone's output level as a function of the angle of incidence.

Technologically, directionality is influenced by the diaphragm's mode of vibration (pressure, pressure gradient, or a combination) and by the microphone's body. The acoustic or electrical processing of the capsule directly affects directionality.

Omnidirectional

Polar pattern omnidirectional

As the name suggests, a microphone with omnidirectional directionality receives sound waves in the same way, regardless of their angle of incidence relative to the diaphragm's axis. It is sensitive to pressure variations uniformly at 360 degrees (all around it). Technologically speaking, this is referred to as a pressure microphone. On a two-dimensional diagram, the sensitivity curve is represented by a circle. In three dimensions, it forms a sphere.

In practice, we will see that its directionality depends on the frequencies of the received sound wave and the shape of the microphone. An ideal omnidirectional microphone would be infinitely small or have a body that has no impact on the received sound waves.

Bidirectional

This type of microphone uses a capsule that operates on pressure gradient. The resulting voltage here is the result of the pressure differential between the two sides of the diaphragm (see study of mechanical acoustic transduction). In a bidirectional microphone, the directionality curve is symmetrical relative to the diaphragm and is characterized by the presence of two lobes on each side. The pickup angle is 90 degrees in the forward and backward directions. This directionality, therefore, favors sound sources placed in its axis on each side of the diaphragm (0° and 180°). However, a source located laterally to the plane of the diaphragm will not generate any signal because the pressure difference between the two sides of the diaphragm will be zero.

Unidirectional

This microphone type is highly directional and is primarily used to discriminate sounds coming from the rear of the microphone. It comes in several variants.

Cardioid

Polar pattern cardioid

Its name comes from its heart-shaped sensitivity lobe (cardio), favoring sounds emitted in the microphone's front axis (one of the two sides of the capsule). The cardioid directionality is a pressure gradient sensor with an added acoustic labyrinth to control one of the lobes of directionality (bidirectional) to the point of canceling it completely. There is also an electrical method to recreate this directionality (double capsule electrostatic technology). For a cardioid microphone, the pickup angle is 130°.

The design of this directionality is more delicate and less effective (at the same price) than an omnidirectional sensor. This is partly due to the acoustic or electrical processing of the capsule. Indeed, these sensors often exhibit frequency response flaws (significant coloration is common, especially in low-end models). A high-quality cardioid capsule is often quite expensive.

Hypercardioid and Subcardioid Variants

There are other unidirectional polar patterns similar to the cardioid microphone. These variants are created, for example, by modifying the design of the acoustic labyrinth (see the study of mechanical acoustic transduction). These variants include subcardioid, supercardioid, and hypercardioid microphones, which are derivatives of the cardioid.

Compared to their cardioid counterpart, hypercardioid and supercardioid microphones are more directional. However, these variants often exhibit more or less pronounced sensitivity lobes on the rear side of the diaphragm (a consequence of the pressure gradient). This intrinsic characteristic of these microphones can pose a serious constraint in sound capture.

The difference between a supercardioid and a hypercardioid microphone lies in the shape of their directional pattern and their pickup angle. Typically, the pickup angle for a supercardioid is 115°, while for a hypercardioid, it is 105°.

Frequency Response

A comparison of the far field on-axis frequency response of the Oktava 319 and the Shure SM58

The frequency response curve of a microphone represents the theoretical variation of sensitivity (or more often sensitivity level) with respect to the frequency of sound. The tested microphone is subjected to sound waves ranging from a frequency of 20Hz to 20kHz emitted by a source located in its axis. In the technical documents provided by manufacturers, it is represented as a graph with frequency scale on the x-axis and variation in decibels on the y-axis. The reference point is typically set at 0 dB relative to the value measured at 1000 Hz. The ideal theoretical curve would, of course, be a horizontal line (the microphone responds with the same sensitivity to all frequencies).

However, it is important not to rely solely on this characteristic because it can be easily influenced by numerous factors. The microphone's design itself (size and mass of the diaphragm) can lead to resonance effects or diffraction effects. Distance also plays a significant role. Similarly, it is often calculated in an ideal situation (free field, treated studio, standardized sources, etc.). In a professional setting, the frequency response curve may not be as faithful.

Proximity Effect

Microphones operating on pressure gradient are subject to a phenomenon called the proximity effect. When the microphone is close to the source, the curvature of the spherical wave leads to an increase in sensitivity, especially pronounced for longer wavelengths (low frequencies) and when the distance to the source is short. To address this issue, manufacturers sometimes incorporate a "low-cut" filter into the microphone body to compensate for the increase in low frequencies.

Some microphones are equipped with switches that allow you to influence the frequency response curve to better adapt to the source and its placement.

Microphone Bandwidth

From this curve, the microphone's bandwidth is determined as the frequency range in which the level variation is less than a limit (for example, set at ± 3 dB or ± 5 dB). The closer the response curve approaches a horizontal line, the wider the bandwidth.