Microphone sensitivity


Purpose of a microphone

The purpose of a microphone is to transfer acoustical energy into electrical energy. Most microphones do this by having a relatively large and flexible surface, or diaphragm, that moves in response to local fluctuations in air pressure caused by passing acoustic waves. The diameter of this diaphragm is typically on the order of 1cm, though there are a wide range of models either side of this, including miniaturised models based upon micro-electro-mechanical systems (MEMS) micro-fabrictaion technology.

There are many subtleties and variations in how this is achieved in practice, and we won’t cover them all on this website. The key idea is that microphones are transducers (aka ‘converters’): they transduce energy from one form (acoustical) to another (electrical).

The rest of this page is framed in terms of the most common contemporary use of microphones, i.e., to transduce real-world acoustic signals to the digital (discrete) signal domain, via the analog electrical domain.

Types of microphone

There are 3 main types of microphone:

  • Condensor microhones
    A metal diaphragm acts as a moveable component of a capacitor circuit. The diaphragm is moved through being forced by local acoustic pressure waves. Since the capacitor needs to be electrically charged, such microphones require power, and various accompanying electronics.
  • Dynamic microhones
  • Ribbon microphones

Basic function of a microphone

Regardless of the type of microphone, we can consider that the key function is the same:

  • The production of a varying electrical voltage across the microphone’s output terminals that is analogous to the varying acoustical pressure signal on the diaphragm (i.e., a transduction from acoustical to electrical energy).
  • Such transduced signals are small, of the order of \upmuV (microvolts) to mV (millivolts), and need boosting (aka. amplification) before conversion to the digital signal domain.

Specifying the efficiency (sensitivity) of a microphone

With so many types, technologies, and models of microphone, we need some kind of common language if we are to be able to compare one to another. A key aspect of a microphone’s performance is its efficiency, also known as sensitivity. This describes the voltage swing seen at the output terminals for a given acoustical input, or some equivalent metric.

An efficient, or sensitive, microphone will produce a lot of voltage (relatively speaking) for a given acoustical pressure at the diaphragm. An inefficient, or insensitive, microphone will produce a small voltage (relatively speaking) for the same acoustical pressure. In this context, sensitivity isn’t a statment of quality, it’s a statement of transduction efficiency – in some cases you want a very sensitive microphone (e.g., quiet environments), and in others you want a very insensitive microphone (e.g., very loud environments).

A helpful way to express microphone sensitivity, which emerges intuitively from the above description, is in mV/Pa (millivolts per Pascal).

  • A microphone with a sensitivity of 50mV/Pa will produce 50mV (RMS) of output when placed on-axis in an acoustical field of 1Pa (RMS) consisting of a 1kHz sine tone.

Environmental condsiderations: Free field vs pressure field vs diffuse field

Strictly speaking, when specifying a microphone’s sensitivity, we need to specify the environment in which we are placing the microphone. The two most common environments are the free-field, in which a microphone’s diaphragm is placed at right angles to the main sound source, and the pressure-field, in which a microphone is enclosed in small cavity such as an artificial ear. Most microphone specifications published by manufacturers involve measurements conducted in free-field conditions, such as in an anechoic chamber with the microphone facing a loudspeaker. As such, in this page all terms and descriptions assume free-field condititions.

There is a minor irony here in the context of field recording, in that many field recording situations involve a microphone being placed into something more like a diffuse-field, with multiple sound sources from multiple directions at the same time.

Further information on this is available via the excellent Microphone Handbook: Volume 1 (Theory) by Brüel & Kjær corporation.