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A microphone is an instrument that picks up sound by converting it into electrical energy. This means that when a person speaks or sings, the microphone will transform their voice into something that can be sent and amplified through speakers. Microphones come in many shapes and sizes, making them suitable for various different types of applications.
In addition to their size and shape, the sensitivity of a microphone will also affect how it picks up sound. Different microphones may produce different output levels when exposed to the same sound source. This means that some microphones are more sensitive than others.
Microphone sensitivity is the amount of output signal for a given input. Different mics have different gains in response to input signals. For example, you have a new microphone and you find that it records the signal at a low level, therefore it’s considered a low-sensitivity microphone.
On the other hand, another mic may record at a loud level in the same signal, thus it has a high sensitivity rating. Simply put, the microphone’s sensitivity rating tells you what level of sound or gain the mic records at.
Before we dive into a more detailed explanation of a microphone’s sensitivity, it’s important to define certain terms to understand everything better.
The different forms of energy at a microphone’s input and output are measured using the following units of measurement:
Microphone sensitivity ratings are standardized and measured at a certain SPL with a certain tone. Typically, a microphone input is measured with a 1 kHz tone at a 94 dB SPL or 1 pascal (Pa) pressure. The microphone output, which is the voltage across its output connection, is typically measured in mV or dBV, compared to the given sound pressure level.
The microphone input impedance is often 10 times greater than the output impedance of a microphone. Thus, it may be regarded as an “open circuit”. Some companies rate microphone sensitivity using an open circuit output voltage, which is the output the microphone will deliver with a stated sound pressure level input. The open circuit output voltage measurement is very useful when comparing microphone sensitivity as it provides a consistent standard.
As the reference level used is well above the microphone output level, the resulting sensitivity specification will be a negative number. This means that when we compare microphone sensitivity in regards to dB value, the closer the sensitivity rating is to zero, the greater the signal provided to the input terminals of the microphone.
For instance, using the reference of 94 dB SPL, one microphone with a sensitivity rating of -40 dB is more sensitive than -55 dB, and -55 dB is more sensitive than -60 dB, and so on. This measurement allows users to easily estimate how much gain is required for an average medium-level sound source.
It’s worth noting that some microphone manufacturers prefer to specify microphone sensitivity using mV/Pa. As stated earlier, Pa or Pascal is the sound pressure a microphone diaphragm is subject to. A microphone is exposed to a certain level of sound and the amount of voltage a microphone produces in relation to that sound is its gain. If two microphones are exposed to the same exact sound pressure level, the mic with a higher sensitivity will produce a greater output voltage.
In this case, the sensitivity rating isn’t measured using negative numbers. When mV/Pa is used to measure the sensitivity rating, the higher the voltage is, the higher the mic signal level.
Microphone sensitivity is determined by the effectiveness of the microphone as a transducer. There are three critical processes in the conversion of energy within a microphone that affect the sensitivity of the microphone. These are:
The two most common microphone transducer types are the dynamic microphone and the condenser microphone. These two types can be further classified into three categories: moving-coil dynamic microphones, dynamic ribbon microphones, and condenser microphones.
Moving-coil dynamic mics are made of a coil of copper wire attached to a movable diaphragm in a magnetic field. The audio signal is produced when the copper wire moves back and forth through that field. A step-up transformer is often used to increase the output level of moving-coil dynamic microphones. However, overdoing the step-up can make the frequency response worse. For these mics, the typical sensitivity specs are 1 to 6 mv/Pa or -60 to -44 dBV/Pa.
Dynamic ribbon microphones are less sensitive than moving-coil dynamic mics. For these microphones, their capsule is made of a thin corrugated aluminum ribbon diaphragm suspended between two magnets.
There are two classes of ribbon mics: passive ribbon microphones and active ribbon mics. Passive microphones utilize passive step-up transformers to increase their output signal, while active microphones use active electronics to increase their output level. Passive mics typically have sensitivity specs of 0.5 to 6 mV/Pa or -60 to -44 dBV/Pa while active mics have specs of 8 to 32 mV/Pa or -42 to -30 dBV/Pa.
Condenser microphones are made using two parallel conductive plates that form a capacitor. One is stationary while one is a moveable diaphragm. When the diaphragm moves in reaction to varying dB SPL, the gap between the plates changes. Condenser mics utilize active electronics to increase the output voltage from the capacitor diaphragm. Condensers typically have a sensitivity of 8 to 32 mV/Pa or -42 to -30 dBV/Pa.
Regarding sensitivity ratings, there is no single answer to which one is better as it almost always depends on the application you will be using the mic. A mic with higher sensitivity isn’t always better than mics with low sensitivity. High-sensitivity microphones may need less preamp gain but they may also have less headroom than a low-sensitivity mic.
The right microphone sensitivity rating for you depends on the recording application. Sometimes a higher-sensitivity microphone will be better, other times a lower-sensitivity microphone will be more suited.
If you will be recording isolated, loud sounds, low-sensitivity mics are generally better to use. If you will be recording ambient, quiet sounds, high-sensitivity mics are ideal.
A mic low in sensitivity typically has a higher maximum SPL than a high-sensitivity mic. Low-sensitivity mics are good for recording applications such as vocals in a loud environment, guitar and bass amps, and high SPL sounds like gunshots, vehicles, and more. Professional mics that have a low sensitivity spec include the Shure SM7B, Shure SM58, and Rode Procaster.
On the other hand, if you’re recording acoustic, nature sounds, voice-over, and such, you should make use of high-sensitivity mics. One microphone that’s well-known to be on the higher spec is the Rode NT1-A. At 94 dB SPL, it will have an output of -32 dBV/PA.
A microphone’s sensitivity depends on the job that you need it for. For instance, you might think that a microphone producing a warm sound is good but if you don’t consider its sensitivity rating and the application you will be using it in, your warm tone may get buried in noise instead.
Knowing and understanding microphone sensitivity helps not only yourself but also the audience and the audio engineers as well. When you know the sensitivity rating of your microphone, you will know how much audio signal to expect out of it. Sometimes, a microphone with a lower sensitivity will be good, and other times, a high-sensitivity microphone will be better.
Shopping for a new microphone but you’re not sure if the one you’re getting is good? One of the first places that you can start with is knowing how to read microphone specifications such as dynamic range, SPL, noise, sensitivity, and such. Doing so can get you closer to achieving the sound that you want. Out of numerous specifications, the sensitivity rating is one that needs a lot of consideration when selecting a microphone to use for any application.
The right kind of microphone should have a balanced signal-to-noise ratio. This means that the microphone signal should be high enough to avoid requiring too much gain, but low enough to prevent distortion. This is where the sensitivity specification comes in.
Sensitivity ratings measure a microphone’s ability for converting acoustic energy into an output voltage. To measure mic sensitivity, most use a standard reference of a hypothetical microphone that produces 94 dB SPL, and a resulting sensitivity specification is usually a negative number. There are also regions in the world where mV/Pa is used to measure sensitivity.
When buying a professional microphone for any kind of usage, always consider its sensitivity as this factor could make or break your audio.