This article provides an overview on 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 microphone 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)
Impedance
Impedance is how much a device resists the flow of an AC current (such as audio signal) and is measured in ohms. Typically when referring to microphones, "low impedance" is considered anything under 600 ohms. "Medium impedance" would be 600 ohms to 10,000 ohms and "high impedance" would be greater than 10,000 ohms. All microphones have a specification regarding their impedance - sometimes the value is written on the mic somewhere, other times you might need to consult the technical manual or manufacturers' website to determine the number. Generally speaking, low impedance microphones are better than high impedance, and quite often you can use impedance as a rough gauge when determining overall quality. The advantage of low impedance microphones is that they can be used with very long cable runs and negligible signal loss. Mics with hardwired cables and a 1/4" jacks are high impedance, while mics that require a balanced audio cable and xlr connector are low impedance.
When connecting your microphone it's important to know the corresponding ohm level of the sound mixer or amplifier. A low impedance microphone should always be connected to an input with a higher impedance value, otherwise signal loss will result. Typically "low impedance" audio mixers have inputs with impedance levels between 1000 and 2000 ohms, and are designed to work with the lower level microphone impedance levels.
Self Noise Level
Self noise is the electrical hiss that a microphone produces. Typically the self noise spec is "A weighted", meaning that the lowest and highest frequencies are flattened in the response curve, to better simulate the signal response of the human ear. (We tend to perceive mid range sound frequencies as louder.) As a general guideline, an A Weighted self noise spec of 18dB SPL or less is excellent (very quiet), 28dB SPL is good, while anything over 35db SPL is not well suited for quality audio recordings.
Because dynamic microphones do not have active electronics (no phantom power requirements) they have very low self noise when compared to condenser microphones. Most spec sheets for dynamic microphones do not include self noise measurements.
Signal to Noise Ratio
The signal to noise ratio (S/N) is the difference in dB between a microphone's sensitivity and self noise. A higher S/N means that the signal is cleaner (less noise) and that the microphone has more "reach". Reach can be defined as the accurate pickup of quiet/distant sounds due to high S/N. Typically reach is not listed as a metric on a tech sheet as any microphone can pick up a distant sound if the source is loud enough. For example, even a very inexpensive mic can pick up a thunderclap from far away.
As a general rule when evaluating S/N ratios, given 94dB SPL, anything over 74dB is excellent, a S/N spec of 64dB is considered good.
Summary
Hopefully these definitions have helped to provide some understanding about typical microphone specifications. The truth is that there is no "ideal" microphone that is perfect for every situation. Manufacturers design their microphones with specific audio applications in mind - such as live performances or studio reproduction, and as a result have tailored the specifications so that the mic sounds the best that it can with a given hardware configuration. Generally speaking, more expensive microphones are engineered with better hardware, which results in better specifications and performance.
