Microphone Responses & Measurements
Microphone have various properties which determines how they interact with sound.
There are five key properties to consider, including:
The frequency response is the output level or sensitivity of a microphone over its operating range from lowest to highest frequency.
A frequency response curve is a graphical representation of how a microphone will respond in the audio spectrum and so, how it will affect a signal’s overall sound timbre.
On a frequency response curve graph the x-axis represents the signals measured frequency, and relative response in decibels (dB) on the y-axis.
Microphone manufacturers list the frequency response, such as 20-20,000 Hz, and usually provide a frequency response curve graph.
A microphone that has a flat frequency response produces equal output at all frequencies. This type of microphone is known as a flat response microphone. On the graph the curve will be drawn as a flat line. Flat response microphones are usually desirable for recording natural instruments such as a piano or acoustic guitar.
A microphone that has a varied frequency response produces differing output of frequencies. This type of microphone is known as a shaped response microphone. On the graph the curve will contain peaks and troughs. A shaped response is designed to enhance or detract certain applications. For example, a microphone may have a peak around 5 KHz to increase the presence of a vocal. Shaped response microphones are especially useful for reducing pickup of unwanted sound and noise outside the frequency range of an instrument.
A transient is a short duration, high level peak, such as a hand-clap or snare drum hit. Transient response refers to the ability of a microphone to respond to these transient peaks.
Dynamic and condenser microphones respond differently to transients and therefore have different transient responses.
For any microphone to convert sound into electrical energy, the sound must move the internal diaphragm.
The transient response for a condenser microphone is quick and accurate due to the light mass of the diaphragm. In contrast, dynamic microphones have a heavy diaphragm along with the additional weight of the coil of wire. It therefore takes longer for the diaphragm to react to a sound wave due to the effects of inertia hence causing a less accurate recording.
Generally, a Dynamic microphone’s transient response is not as efficient as the transient response of a condenser. This means condenser microphones are better suited to instruments with sharp attacks or extended high frequency output. However, condenser microphones are not able to withstand extreme sound pressure levels due to their high sensitivity. Dynamic microphones are able to withstand high sound pressure levels due to their robust diaphragm, making them a more suitable choice for instruments such as the kick drum or bass guitar.
The self-noise is the sound level that creates the same output voltage as the inherent noise of the microphone. This represents the lowest point of the microphone's dynamic range, and is particularly important if recording sounds that are quiet; the lower the number the better the microphone.
The maximum SPL (sound pressure level) the microphone can accept is measured for particular values of total harmonic distortion, typically 1%. This is generally inaudible, so one can safely use the microphone at this level without harming the recording.
The dynamic range of a microphone is the difference in SPL between the noise floor and the maximum SPL. If stated on its own, for example "120 dB", it conveys significantly less information than having the self-noise and maximum SPL figures individually.