Choosing Vocal Microphones – Understanding Technical Specifications

This article provides an overview of five important measures that are typically listed in microphone specification sheets: frequency response, sensitivity, impedance, self-noise level, and signal to noise ratio. Understanding these specs can help when trying to decide on the best omnidirectional boundary microphones to purchase for a specific application.

Frequency Response

Frequency response measures how a microphone reacts to different sound frequencies. An ideal “flat” response (equal sensitivity) microphone would respond equally to all frequencies within the audible spectrum. This results in a more accurate reproduction of sound and produces the purest audio. The truth is that even microphones which are advertised as having a “flat response” can deviate somewhat at certain frequencies. Typically spec sheets will list frequency response as a range like “20Hz to 20kHz”, meaning that the microphone can reproduce sounds that fall within that range. What this does not explain is how accurately the various individual frequencies will be reproduced. Some microphones are purposely designed to respond differently to certain frequencies. For example, instrument microphones for bass drums are generally engineered to be more responsive to lower frequencies while vocal microphones would be more responsive to the frequency of a human voice.

As a general rule of thumb, condenser microphones have flatter frequency responses than dynamic. This means that a condenser would tend to be the better choice if accuracy of audio reproduction is the main goal.

Sensitivity

Microphone sensitivity measures how much electrical output (measured in “millivolts” mV) is generated for a given sound pressure input. Typically when measuring microphone sensitivity the mic is placed in a reference sound field where a sound pressure level (SPL) of 94 dB (1 Pascal) at 1000 Hz is maintained at the microphone. (Some vendors like Shure use 74 dB 0.1 Pascal). The distinction is that 94 dB SPL is the typical sound intensity of someone speaking twelve inches away while 74dB SPL would be the same speaker one inch away. A typical condenser microphone might have a value listed either like “7mV/Pa” or -43dBV in the technical specification. These two values mean the same thing – they’re just expressed differently.

If two microphones are subject to the same SPL and one generates a higher output voltage, that microphone is said to have a higher sensitivity rating. Although knowing how to read/compare microphone sensitivity (output) is important, the actual sensitivity rating usually is not a major consideration in mic selection. Typically the design of a microphone for a particular application plays a role when manufacturers determine the appropriate output level. For example, dynamic microphones are typically less sensitive than condenser mics as they’re generally used fairly close to the sound source. Listed below are the typical specifications for three different microphone transducer types:

Condenser: 5.6mV/Pa (high sensitivity)
Dynamic: 1.8mV/Pa (medium sensitivity)
Ribbon: 1.1mV/Pa (low sensitivity)