Some values are critical to understand when browsing for the best microphone for a job or looking through a mic's specifications/datasheet. Knowing and comprehending the following specs will help tremendously in choosing the right microphone for you and your application.
The top 5 microphone specifications you need to understand are:
- Frequency response
- Polar response
- Maximum sound pressure level
This article will explain in detail what these qualities are in a microphone and share microphone examples. Whether you're looking for clarification on 1 spec or all 5, this article should further your understanding of microphones.
Table Of Contents
This will be an in-depth read, so a table of contents will help you find what you're looking for.
- What Are Microphone Specifications?
- The Example Microphones
- 1. Frequency Response
- 2. Polar Response
- 5. Sensitivity
- 6. Maximum Sound Pressure Level
- 7. Self-Noise
- Related Questions
What Are Microphone Specifications?
Let's start by briefly discussing what a microphone specification is.
What are microphone specifications? Microphone specs are values assigned to various qualities of a microphone. These qualities are quantitative measurements found during manufacturer testing of a mic. Specs have to do with how the mic is built, how it reacts to sound, and how it produces and outputs electrical mic signals.
The Example Microphones
Learning is always better with examples! To better explain the important specifications in this article, I'll be referencing a moving-coil dynamic mic, a ribbon dynamic mic, and a multi-pattern condenser mic. These microphones are popular examples of their class. They are:
- Shure SM58 (moving-coil dynamic microphone)
- Royer R-121 (ribbon dynamic microphone)
- Neumann U87AI (multi-pattern condenser microphone)
Shure, Royer and Neumann are featured in My New Microphone's Top 11 Best Microphone Brands You Should Know And Use.
1. Frequency Response
What is microphone frequency response? The frequency response of a microphone is the mic's frequency-specific sensitivity. It tells us which sound frequencies a microphone will reproduce and which frequencies the mic is more sensitive to relative to other frequencies.
If we break down the term, “frequency response” means how well a microphone responds to frequencies. Microphones are primarily designed to respond to sound frequencies (20 Hz – 20,000 Hz). Some microphones will also respond well to infrasound (sound frequencies less than 20 Hz) and ultrasound (sound frequencies greater than 20,000 Hz).
Microphone frequency response and polar response are the 2 most important microphone specifications to understand.
Microphone frequency response is provided in two ways on a microphone specifications sheet.
Frequency Response Range
The frequency response range of the microphone gives us a general idea of the lowest frequency a microphone will reproduce accurately to the highest frequency response a microphone will reproduce accurately.
Frequency response ranges are often a simple range from one frequency to another, especially when that range can be expressed as 20 Hz – 20,000 Hz (see Neumann U87AI). This range is commonly expressed since it is the largely agreed-upon range of human hearing, and most microphones are designed to capture sound for human listening.
Other times (and this is more helpful), the range is expressed with a variation in sensitivity within that range. This is when a plus/minus (±) certain decibel (dB) amount is given (see Royer R-121).
Let's take a look at the frequency response ranges of our microphone examples:
- Shure SM58: 50 Hz – 15,000 Hz
- Royer R-121: 30 Hz -15,000 Hz ± 3dB
- Neumann U87AI: 20 Hz – 20,000 Hz
But this only tells a small part of the story…
Frequency Response Graph
A microphone's frequency response graph really gives us a solid understanding of perhaps the most important microphone specification. How well does a microphone reproduce sound frequencies, and which frequencies will the microphone accentuate or de-emphasize relative to other frequencies.
Take special note that a frequency response graph gives us the relative frequency-specific response of a microphone.
A frequency response graph has 2 axes:
- X-axis: the frequencies from 20 -20,000 Hertz (Hz) – (sometimes this range is greater)
- Y-axis: the relative sensitivity given in decibels (dB)
Much of a microphone's “character” comes from its frequency response.
Flat microphones: some microphones are cherished for being “flat” and reproduce all sound frequencies at the same relative sensitivity.
Coloured microphones: other mics are cherished for being “coloured.” The frequency responses of coloured mics aren't as “accurate” as the flat mics, but how they shape the sound adds a certain character that many people like. Some microphones are so coloured that they are often marketed for specific applications only (kick drum microphones, for example).
As an interesting aside, human hearing is not flat but coloured. Our hearing is the most sensitive in the presence range (3 kHz to 7 kHz), while our sensitivity drops off toward the boundaries of our range (20 Hz and 20,000 Hz). To add to the confusion, our “biological frequency response” changes with sound intensity and is even altered as we age.
For more information on this, please check out the Fletcher-Munson Equal-Loudness Curves.
Let's take a look at our 3 microphones and their frequency response graphs. In doing so, we'll see that the frequency response ranges do not serve us all that well in determining a mic's true frequency response. I'll restate their frequency response ranges.
Shure SM58 Frequency Response Graph
Shure SM58 frequency response range: 50 Hz – 15,000 Hz
Though the SM58 will reproduce frequencies from 50 Hz – 15,000 Hz, we see that, at these limits, its sensitivity is greatly reduced (nearly a 13 dB difference between the mic's most sensitive frequencies and least sensitive frequencies). This means that frequencies near the extremities will not be represented all that well in the SM58's mic signal.
There is quite the boost between 4 kHz and 7 kHz with another boost around 10 kHz. This is generally known as the “presence range,” which helps to accentuate human speech. The SM58's increased response to frequencies in this range accentuates the human voice, which is a big reason why this microphone is the most popular live performance vocal mic in history.
Read more into why the Shure SM58 is such an awesome vocal microphone in my article on the Best Microphones For Live Vocal Performances.
The Shure SM58 is, without a doubt, a “coloured” microphone, which is why it makes such a great live vocal microphone. Let's discuss why
- It accentuates the frequencies responsible for speech intelligibility (4 kHz – 7 kHz).
- It rolls off the low end, ridding of much of the low-end rumble and stage noise that could otherwise overload the microphone in live situations and cause feedback.
- It rolls off the high end and rids of mic signal harshness while reducing the likelihood of microphone feedback.
For more information on microphone feedback, check out my article What Is Microphone Feedback And How To Eliminate It For Good.
Royer R-121 Frequency Response Graph
Royer R-121 frequency response range: 30 Hz -15,000 Hz ± 3dB
Compared to the Shure SM58, the Royer R-121 has a fairly flat response. However, this microphone would most often be considered coloured. Why?
First, it has a low-end and a high-end roll-off (30 Hz and 15,000 Hz, respectively), which means it doesn't cover the entire range of human hearing accurately. Second, within its range, some frequencies are better represented than others.
A third reason, which is a big selling point of the Royer R-121 and ribbon microphones in general, is the gentle and gradual decrease in sensitivity in the mic's upper frequency response. This yields a character and sound that is best described as “warm,” “natural,” and “close to the way our ears hear sound naturally.”
Neumann U87AI Frequency Response Graphs
Neumann U87AI frequency response range: 20 Hz – 20,000 Hz
As we see above, the Neumann U87AI has 3 different frequency response graphs. 1 for each of its switchable polar patterns.
As we plainly see in each of the U87's frequency responses, the microphone has a slight roll-off at the high and low-ends of its response, as well as a slight boost in the upper/presence frequencies.
Even with these slight variations in frequency-dependent sensitivity, the Neumann U87AI is often considered to have a flat frequency response. Across the lower and upper mid frequencies, we see that the response is extraordinarily flat (60 Hz – 5 kHz).
Because much of the harmonic content of sound rests in the mid-frequency ranges, and the U87AI is flat in these ranges, we can say the U87AI is “flat.” It is certainly more flat and sounds truer than the aforementioned SM58 and R-121 dynamic mics.
We notice, too, that there are two curves for each of the U87AI's frequency response graphs. The line that rolls off the low-end more drastically represents the frequency response of the Neumann mic when its built-in high-pass filter is engaged.
Engaging a microphone's high-pass filter (HPF) will alter its frequency response. HPFs are often engaged to reduce low-end rumble and noise in a microphone signal and reduce the proximity effect in directional microphones.
For more information on microphone high-pass filters and proximity effect, check out my articles:
• What Is A Microphone High-Pass Filter And Why Use One?
• Audio EQ: What Is A High-Pass Filter & How Do HPFs Work?
• What Is Microphone Proximity Effect And What Causes It?
How Is Microphone Frequency Response Tested?
How do microphone manufacturers accurately test the frequency responses of their microphones? Mic manufacturers typically project pink noise from a calibrated loudspeaker toward their microphone in an anechoic chamber to test the microphone's frequency response. Calibration mics are often used as a reference. Frequency response graphs are then created according to the results.
Frequency Response Generalities
- Frequency response specs are generated for on-axis axis sounds. Some manufacturers (Electro-Voice comes to mind) will include a second frequency response curve for their cardioid mics that juxtaposes the mic's sensitivity at 180° off-axis (the rear of the mic). All mics (yes even omnidirectional mics) will have a changing frequency response as a sound source moves off-axis.
- The proximity effect will alter the bass response of directional microphones. Some manufacturers include various graphs for sound sources at different distances from their directional mics.
- Lavalier microphones often come with various capsules/grilles specifically made to alter the frequency response, though the microphone transducer itself does not change.
- Moving-coil dynamic microphones typically have poor upper-frequency sensitivity and therefore low values for the upper-end of their frequency responses.
- Ribbon dynamic microphones typically have gentle roll-offs in their upper frequency responses which gives them their warm and natural “ribbon sound.”
- Large-diaphragm condenser microphones are fairly flat and often have a slight upper-frequency boost.
- Small-diaphragm condenser microphones have the widest, most extended frequency responses.
- The frequency response graphs of reputable companies are typically accurate, though smoothed out for easier visualization. The graphs of cheaper mics from lesser-known companies may or may not be accurate.
For more information on microphone frequency response, check out my article What Is Microphone Frequency Response?
2. Polar Response
What is a microphone polar response/pattern? A microphone's polar response/pattern represents its directional sensitivity. In other words, how well the mic responds to sounds from various directions. Omnidirectional, cardioid and bidirectional are common qualitative polar patterns, and each mic has its own quantitative polar pattern.
If we break down the term, “polar response” means the microphone's response to sounds about a polar graph. Yes, microphones capture sound in 3-dimensions and a polar graph is a 2-dimensional coordinate system. However, the information gathered from a microphone's polar response gives us a solid understanding of that mic's relative directional sensitivity.
Note that, because microphones act in 3-dimensional space, we must envision microphone polar patterns as spherical (or ovoidal) with rings/cones of silence rather than circular (or oval-shaped) with null points.
Microphone polar response and frequency response are the 2 most important microphone specifications to understand.
Microphone frequency response is provided in two ways on a microphone specifications sheet.
Polar Pattern Types
There are 3 general microphone polar pattern patterns:
- Omnidirectional: omnidirectional microphones are, at least in theory, equally sensitive to sounds coming from all directions.
- Bidirectional: bidirectional microphones are equally sensitive to sounds from the front and rear of the mic capsule/element and have a “ring of silence” which rejects sound sources from the sides of the capsule/element.
- Unidirectional: unidirectional microphones are most sensitive in one direction. These are the cardioid-type polar patterns which very greatly in their directionality and null points of maximum rejection.
For an in-depth discussion of microphone polar patterns, please read through my article The Complete Guide To Microphone Polar Patterns.
Let's touch on these three polar pattern types and elaborate particularly on the unidirectional type and its many versions.
Most omnidirectional microphones work on the “pressure” acoustic principle. This basically means that their diaphragms are only open to exterior sound pressure variations on one side.
Note that sometimes microphone manufacturers add acoustic principle as a specification in and of itself for their mics.
Alternatively, in multi-pattern condenser microphones, omnidirectional polar patterns are often achieved by combining 2 back-to-back cardioid diaphragms in-phase with one another. Multi-pattern condenser mics typically have a capsule with two diaphragms separated by a short distance that share a backplate. Combining the signals of the 2 capsules in varying phases and amplitudes yield the multiple polar patterns.
Bidirectional microphones (commonly referred to as “figure-8” mics) work on the “pressure-gradient” acoustic principle. Unlike the pressure principle, the pressure-gradient principle has both sides of the diaphragm open to external sound pressure variation.
This means the diaphragm moves according to the difference in sound pressure between its front and back sides.
A bidirectional mic has both its sides equally open to external sound pressure variations (sound waves).
This naturally means it's equally sensitive to the front and to back.
Sounds directly from the sides, however, hit both sides of the diaphragm simultaneously and cause equal pressure on the front and back. This causes no movement in the microphone diaphragm, and therefore no signal. Therefore, bidirectional mics have a “cone of silence,” which rejects sound from their sides.
In multi-pattern microphones, the 2 aforementioned cardioid capsules are summed together but in opposite polarity in order to achieve a bidirectional “figure-8” polar pattern.
Equal sensitivity from the front and back that slowly diminishes until a null point at the microphone's sides causes a figure-8-looking polar pattern, hence the name “figure-8.”
Unidirectional microphones, like bidirectional microphones, work on the pressure-gradient principle. The difference with unidirectional microphones is that both sides of the diaphragm are not equally exposed to exterior sound pressure.
Instead, carefully designed acoustic labyrinths are designed to offset the phase of sound waves as they hit the [open] front of the diaphragm and travel through the labyrinth to the back of the diaphragm.
By varying the time, it takes a sound wave to hit the front and then the back (or vice versa) of the diaphragm, we achieve unidirectional polar patterns.
Let's take the cardioid polar pattern, for example (which is, by the way, the most commonly used polar pattern).
Cardioid microphones employ acoustic labyrinths that delay sounds from reaching the rear diaphragm by precisely the amount of time it takes sound to travel from the rear port to the front of the diaphragm.
This means that sounds directly from the rear of a cardioid mic reach the back and the front of the diaphragm at the same exact time, causing no diaphragm movement. These sounds cancel each other out at the mic capsule, which explains the rear null point of the cardioid polar pattern.
Let's describe each of the cardioid-type patterns according to their null points and lobes of sensitivity relative to 0° on-axis:
- Cardioid: 1 null point at 180° with no lobes
- Hypercardioid: 2 null points at 109.5° and 250.5° with rear lobe
- Supercardioid: 2 null points at 126.9° and 233.1° with rear lobe
- Shotgun/lobar: typically 4 null points at 45°, 135°, 225°, and 315° with two side lobes and one rear lobe.
Shotgun/lobar polar patterns take the idea of an acoustic labyrinth a step further by introducing an interference tube. An interference tube is a long slotted tube attached to the front of the mic capsule designed carefully to cancel out sound waves across the frequency spectrum from off-axis directions.
The result is a highly directional polar pattern with a large rear lobe along with smaller side lobes.
Polar Pattern Diagrams
Polar pattern diagrams provide a visual aid and help tremendously to convey the true directional response of a microphone.
Let's take a look at the aforementioned polar patterns in graphic form to better understand what they are:
The above diagrams give us a great idea of what the ideal polar patterns look like graphically. However, in reality, polar patterns rarely portray the ideal.
Microphones naturally become more directional at higher frequencies and, conversely, become less directional (more omnidirectional) at lower frequencies.
Let's take a look at each of our 3 example microphones:
Shure SM58 Polar Pattern Diagram
The Shure SM58 is a cardioid microphone and has the following polar pattern diagram:
The Shure SM58 has a cardioid pattern that is apparent at 500, 1,000, and 2,000 Hz.
However, at 125 Hz, the pattern seems more omnidirectional, while at 4 and 8 kHz, the pattern looks a bit more supercardioid.
This is a great example, showing that microphones get more directional at higher frequencies and less directional at lower frequencies.
Royer R-121 Polar Pattern Diagram
The Royer R-121, like the vast majority of ribbon microphones, is bidirectional. It exhibits the following polar pattern diagram:
The R-121 polar pattern gets slightly tighter at higher frequencies.
Again, this increase in directionality is common among all microphones.
Neumann U87AI Polar Pattern Diagrams
The Neumann is a multi-patter microphone with omnidirectional, cardioid, and bidirectional options. The following polar pattern diagrams represent each of these options, respectively:
How Are Polar Patterns Tested And Measured?
How do microphone manufacturers accurately measure the polar patterns of their microphones? Mic polar patterns are measured by projecting a tone from a loudspeaker and rotating a mic 360° about the centre of its capsule. The varying output signal strength is measured, and a polar plot is created with the data. This process is often repeated with different tones to provide more information.
Polar Pattern Generalities
- Ribbon microphones are, by nature, bidrectional.
- Multipattern microphones are typically condensers.
- Though supercardioid, hypercardioid, and lobar patterns are film and television essentials, they are rarely used in studio environments.
- Microphones naturally become more directional at higher frequencies
- Microphones naturally become less directional (more omnidirectional) at lower frequencies.
- Top address and pencil microphones cannot truly be bidirectional.
- Bidirectional microphones exhibit the most proximity effect.
- Omnidirectional microphones exhibit no proximity effect.
- Omnidirectional microphones are practically immune to plosives.
What is a microphone's sensitivity rating? Mic sensitivity rating measures the microphone's output signal level at a given acoustic input. Basically, it tells us how effective the mic is as a transducer of acoustic energy to electrical energy. Mic sensitivity is typically rated in mV or dBV per 1 Pascal or 94 dB SPL.
Basically, when you're recording audio with microphones. You'll likely notice that some mics require more gain than others to get to a healthy level. The microphones that require more gain are less sensitive, while those mics that require less gain are more sensitive.
There are few ways that microphone manufacturers explain the sensitivities of their microphones. As mentioned above, mic sensitivity is typically given in millivolts (mV) or decibels relative to 1 volt (dBV) per 1 Pascal or 94 dB SPL.
What does that mean? Let's break it down.
Let's start with the acoustic reference sound.
As mentioned, there are two values for the acoustic reference sound for measuring microphone sensitivity:
- 94 dB SPL
- 1 Pascal
Fortunately, 94 dB SPL (decibels sound pressure level) equals 1 Pascal of sound pressure, so these reference levels are the same. They represent the intensity of an acoustic sound wave (sound pressure) at the microphone capsule.
It's also standard in testing for mic sensitivity that a 1 kHz tone be used at the microphone capsule. The 1 kHz tone is 94 dB SPL (1 Pascal) loud.
The output measurement is trickier to understand.
As mentioned, two measurement units describe a microphone's output when the mic is subjected to a 1 Pascal or 94 dB SPL sound waves at its capsule:
- In millivolts (mV)
- In decibels relative to 1 volt (dBV)
Our measurement in millivolts is easier to understand. For a given sound pressure level, the microphone will output X millivolts of electricity (analog mic signal).
The easiest part of measuring in millivolts is that, like the Pascal, it's linear. However, it doesn't really make things easy when it comes time to apply gain to mic signal since mic preamp gain is measured in dB (a logarithmic scale).
dBV (also known as dBv) relates a microphone's output to an imaginary microphone that outputs 1 volt per 94 dB SPL or 1 Pascal. No microphone is that sensitive. Our theoretical microphone would output 0 dBV, and so our real microphones would have negative dBV values.
dBV, like all decibel values, are logarithmic rather than linear. dBV is more difficult than mV to grasp conceptually but works extremely well for estimating the amount of preamp gain a microphone would need.
Generally speaking, microphones output mic levels (-60 dBV to -40 dBV). In order to be used in professional audio equipment, they need to be boosted to professional line level (+4 dBu or 1.78 dBV).
I go into greater depth about mic levels, and line levels in my article Do Microphones Output Mic, Line, Or Instrument Level Signals?
If you're interested in calculating between dBV and mV or dB SPL and Pascals, here are the formulae:
dBV = 20•log10(V)
V = 10(dBV)/20
dB SPL = 20 • log10(P/Pref)
P= Pref • 10(dB SPL)/20
Note that Pref = 2 × 10−5 Pa.
Here are the sensitivity ratings of each of our 3 example microphones:
- Shure SM58: –54.5 dBV/Pa (1.85 mV) @ 1,000 Hz open circuit
- Royer R-121: -47 dBv Re. 1v/pa
- Neumann U87AI: values given at 1 kHz into 1 kΩ
• omnidirectional: 20 mV/Pa
• cardioid: 28 mV/Pa
• bidirectional: 22 mV/Pa
The above values are according to the microphone specification sheets. We see that Shure and Royer (North American manufacturers) subscribe to the dBV measurement while Neumann (European manufacturer) gives us sensitivity ratings in mV.
Let's quickly run through our example microphones in dBV and mV from most sensitive to least sensitive.
- Neumann U87AI in cardioid mode: 28 mV/Pa or -31 dBV/Pa
- Neumann U87AI in bidirectional mode: 22 mV/Pa or -33 dBV/Pa
- Neumann U87AI in omnidirectional mode: 20 mV/Pa or -34 dBV/Pa
- Royer R-121: 4.5 mV/Pa or -47 dBV/Pa
- Shure SM58: 1.85 mV/Pa or -54.5 dBV/Pa
How Are Microphone Sensitivity Values Measured?
How do microphone manufacturers accurately measure the sensitivity of their microphones? Microphone sensitivity is typically measured by projecting a 94 dB SPL (1 Pascal) 1,000 Hz tone at the microphone capsule. The mic output is then measured in millivolts or dBV (at open circuit or into a specified load).
Microphone Sensitivity Generalities
- Active microphones are more sensitive than passive microphones.
- Passive ribbon mics generally have the lowest sensitivity ratings even through their diaphragms are the most “sensitive.”
- The higher a microphone's sensitive rating, the less preamp gain it requires.
- Sensitivity is more critical in low-sensitivity dynamic mics that depend on clean mic preamp gain to boost their signals.
- Any active microphone with a sensitivity rating greater than 8 mV/Pa will have an excellent noise performance with any quality preamp.
- Digital USB microphones do not have sensitivity values. This is because they do not output analog mic signals (voltage) but rather digital information. If a 0 dBFS mic output limit is measurable at a certain dB SPL acoustic input, then a sensitivity rating could be calculated from 94 dB minus the 0 dBFS's corresponding acoustic input level. The result would be measured in dBFS (decibels full scale), which is a digital signal relationship.
For more information on microphone sensitivity, check out my article What Is Microphone Sensitivity And Why Does It Matter?
4. Maximum Sound Pressure Level
What is a microphone's maximum sound pressure level? A mic's max SPL rating is the sound pressure level (measured at the mic capsule) at which the microphone's output signal starts to distort. Max SPL ratings are typically measured at a 0.5% total harmonic distortion of a single-frequency tone and are generally due to overloading a mic's circuitry.
Let's add an additional definition for total harmonic distortion: Total harmonic distortion (T.H.D) is a measurement of distortion is a signal that represents the ratio of the sum of the powers of all harmonic components to the power of the fundamental frequency. T.H.D of a single-frequency tone is used to help calculate the max SPL of a microphone.
So really, the maximum sound pressure level is not the absolute limit a microphone can handle before destruction. Rather, it's the point where distortion becomes significant enough to be audible.
Maximum sound pressure level is a specification that isn't often found for moving-coil dynamics, sometimes found for ribbon dynamics, and always found for condensers.
This is because it's extremely difficult to actually overload a microphone's capsule/element, while it's relatively easy to overload an internal amplifier/impedance converter.
In order to overload and distort a microphone capsule, the diaphragm would have to hit against the housing of the capsule in some fashion. While not impossible, the chances of this are incredibly low in any practical application.
Because of the thinness and delicate nature of ribbon diaphragms, lower frequencies may cause non-coincidental vibrations in the ribbon. This does show up as distortion in the mic signal but is generally unheard of since humans have difficulty hearing these low frequencies.
Before we get into the max SPL ratings of our 3 example microphones, let's talk about what we could expect to hear at varying dB SPL values:
|0 dB SPL||Threshold of Hearing|
|10 dB SPL||Calm breathing|
|20 dB SPL||Soundproof recording studio|
|30 dB SPL||Quiet whisper @ 1.5 meters|
|40 dB SPL||Silent auditorium|
|50 dB SPL||Silent bedroom|
|60 dB SPL||Normal conversation|
|70 dB SPL||Household vacuum cleaner|
|80 dB SPL||Cabin of jet aircraft travelling|
|90 dB SPL||Lawnmower|
|100 dB SPL||Chainsaw|
|110 dB SPL||Pneumatic Drill|
|120 dB SPL||Car horn @ 1 meter|
|130 dB SPL||Threshold of Pain|
Loud trumpet @ 0.5 meters
|140 dB SPL||Rifle fired @ 1 meter|
|150 dB SPL||Jet engine @ 1 meter|
|160 dB SPL||Shotgun blast @ ear|
|170 dB SPL||Shotgun blast @ ear|
|180 dB SPL||Rocket Launch @ rocket|
|194 dB SPL||Max SPL for a sound wave |
(at Atmosphere Pressure)
So chances are you won't be recording something with excessive SPL (for example, above the threshold of pain at 130 dB SPL).
The loudest human voice was about 135 dB (@ 1 inch for the mouth), but even loud vocals are typically topping out around 115-120 dB SPL. Kick drums can get loud if the drummer has a heavy foot (140 dB SPL inside the kick drum). Trumpets have been recorded as loud as 155 dB SPL in the upper register, but this is, once again, uncommon.
Notice that some of the above examples have distances? This brings up another critical point in understanding max SPL ratings. This point can be summed up with the inverse-square law:
What is the inverse-square law of sound waves? The inverse-square law states that a sound wave's intensity will drop by 6 decibels (dB) for every doubling of distance. This decrease in intensity happens as a sound wave propagates spherically, and the sound energy is distributed over the ever-increasing surface diameter of the wavefront surface.
So basically, if we have a microphone with a distorted signal because we're over its max SPL rating, there's a simple fix: distance the microphone from the sound source!
Here are the maximum sound pressure levels of our 3 examples microphones:
- Shure SM58: none given
- Royer R-121: >135dB @ 20 Hz
- Neumann U87AI: 0.5% T.H.D @ 117 dB (cardioid) or 127 dB with pad
Let's talk about each of these mics and their max SPL rating in more detail.
Shure SM58 Max SPL
The Shure SM58, our moving-coil dynamic microphone, does not have a max SPL rating. This is typical of moving-coil dynamics since their capsules are robust, and they have no active circuitry to overload.
So there's no maximum sound pressure level rating on the SM58 datasheet. However, that doesn't necessarily mean that the mic will never distort. Shure actually goes into detail about the SM58 and its max SPLs at various single-frequency tones (read here). Here's a quick recap:
- The SM58 will first start to distort at 150 dB SPL around 100 Hz, which is the resonant frequency of the microphone's diaphragm. At 150 dB, the SM58 outputs a 0 dBV (1,000 mV) signal, which is practically line level.
- Around 1 kHz, the SM58 has a max SPL of 160 dB SPL. The SM58's output at 160 dB SPL is 10 dBV (3,200 mV). It's unlikely the mic will ever be subjected to this level of sound.
- Around 10 kHz, the SM58 has a calculated theoretical max SPL of 180 dB SPL. There's no way for Shure to recreate this dangerous sound pressure level in a controlled environment.
- At 20 kHz, the SM58 has a calculated theoretical max SPL of 190 dB SPL, which is near the upper limit of what's possible for a sound wave (194 dB SPL is the atmosphere's max SPL at atmospheric pressure and is known as a shock wave).
So we see that while the Shure SM58 does have a max SPL rating, it's not really necessary that Shure includes it in its datasheet. There's rarely ever a situation that would call for recording a 150 dB SPL sound source directly at the source, so an SM58 will be able to handle any practical sound source without distorting.
Royer R-121 Max SPL
The Royer R-121 max SPL value is given as >135dB @ 20 Hz, though typically max SPL is assumed to be in relation to a 1,000 Hz frequency.
It's assumed that as the frequency increases, so does the max SPL rating. There is also no percentage of total harmonic distortion. So I believe what Royer is saying is that the R-121 will be able to handle sounds greater than 135 dB SPL at the ribbon.
Repeated high SPL situations will, over time, stretch out any microphone diaphragm. Ribbon diaphragms are the most susceptible to this stretching. Royer mics are built to last and are very resistant to this stretching, but it will still happen (again, over time).
Neumann U87AI Max SPL
The Neumann U87AI is the condenser microphone on this list and, as suspected, has the lowest max SPL rating.
The U87AI's max SPL rating is 0.5% T.H.D @ 117 dB (cardioid) or 127 dB with pad.
The rating tells us most of the practical information we need, except that it omits the other two polar response options. However, because the cardioid pattern is the most sensitive, it has the lowest max SPL and is, therefore, the safest mode to include in the max SPL specification.
So we see that the mic signal of the U87AI, in cardioid mode, will experience 0.5% total harmonic distortion when the mic experiences a 117 dB SPL 1 kHz tone at its diaphragm. The 1 kHz tone is, of course, assumed. The spec tells us that, basically, any sound wave that is 117 dB SPL at the mic diaphragm will cause the signal to distort.
An excellent feature of the Neumann U87AI is its -10 dB pad, which increases its max SPL rating by 10 dB SPL. With the pad engaged, the sound waves would need to be 127 dB SPL at the mic capsule.
How Is A Microphone's Maximum Sound Pressure Level Measured?
How is a microphone's maximum sound pressure level measured? A mic's max SPL is often measured by projecting a 1 kHz tone from a loudspeaker directly into a mic's capsule (as close as possible). The tone's volume is increased until the mic shows signs of distortion (usually 0.5% T.H.D). The dB SPL of the tone is then calculated, and a max SPL rating is given.
Alternatively, if the max SPL of a microphone is too high, other means of testing may be necessary. In this case, an electrical AC voltage may be applied directly after the microphone capsule to simulate a transduced mic signal.
If the microphone's sensitivity is known at a certain frequency (which is calculated using a reasonable 94 dB SPL tone), we can calculate a theoretical maximum SPL of a microphone.
Maximum Sound Pressure Level Generalities
- Maximum sound pressure level is typically never an issue for passive dynamic microphones (of moving-coil and ribbon varieties).
- Exceeding the maximum sound pressure level [within reason] will not damage the microphone. It will only distort the mic signal.
For more information on max SPL ratings, check out my article What Does Maximum Sound Pressure Level Actually Mean?
What is microphone self-noise (aka equivalent noise level)? Microphone self-noise is the amount of noise a mic produces by itself without any external sound affecting it. Self-noise is created by active circuitry and only affects active mics. A lower self-noise figure means a better signal-to-noise ratio and more clarity when capturing quiet sound sources.
Self-noise is a fairly self-explanatory specification, but it is one of the most important in many instances. It is particularly crucial in film, broadcast, and studio recordings, where a good signal-to-noise ratio is required (nobody wants a noisy recording)!
Note again that self-noise is caused by active circuitry and is therefore only a product of active microphones (condensers and active ribbon mics). Properly powering these active devices within microphones (amplifiers, impedance converters, etc.) is not 100% efficient, so some energy is lost as heat and some as sound.
Passive microphones (most dynamics), on the other hand, do not require power, and their interior mic design does not produce significant noise. As a side note, there is such a thing as Johnson-Nyquist noise, which accounts for the agitation of electrons inside an electrical conductor (moving coil or ribbon). This noise, however, is negligible.
Microphone self-noise is measured in two ways:
- dB (ITU-R 468 noise weighting): This is the standard weighted curve for measuring noise as maintained by the International Telecommunications Union.
- dBA “A-weighted” (IEC 60651 noise weighting): This decibel value is based on the 40 phon equal–loudness contours of the Fletcher-Munson curves and better represents noise as we hear it. Self-noise is most commonly measured in dBA. Self-noise measured in decibels A-weighted always has a smaller (seemingly better) value than the previously mentioned db (ITU-R 468).
The difference between the above 2 decibel values and the real dB SPL values of a sound wave depends on the frequency in question. This makes things quite confusing.
The easiest way of thinking about self-noise is that the mic will not pick up a sound wave quieter than its self-noise rating. This is true regardless of the weighted decibel self-noise measurement.
Sounds waves with intensity above the self-noise rating will be captured by the microphone. The stronger the sound wave, the greater the signal-to-noise ratio will be. Think of self-noise is in terms of signal-to-noise ratio and simply using the weighted decibel ratings as normal dB ratings.
Here are the self-noise values of our 3 example microphones:
- Shure SM58: not applicable
- Royer R-121: not applicable
- Neumann U87AI:
• omnidirectional: 26 dB (15 dBA)
• cardioid: 23 dB (12 dBA)
• bidirectional: 25 dB (14 dBA)
How Is Self-Noise Measured?
How is microphone self-noise measured? The self-noise of a microphone is ideally measured by placing the mic in an absolutely soundproof container and recording its output level. Another common method of measuring self-noise is to measure the output level of a microphone without its capsule.
The first method of the above two is arguably better because it tests the microphone in its entirety. However, this method requires a completely soundproof anechoic chamber, which is a heavy investment.
The second method is much simpler and yields accurate results without the need for an expensive facility.
Microphone Self-Noise Generalities
- Passive microphones do not have noise-making circuitry, and so self-noise is not an issue for them.
- Active microphones produce self-noise due to their internal circuitry.
- A self-noise of less than 10 dBA is rarely noticeable in a mic signal, even in soundproof environments.
- FET condensers are slightly quieter than tube condensers.
- Large diaphragm condenser are slightly quieter than small diaphragm condensers.
For more information on microphone self-noise, check out my article What Is Microphone Self-Noise? (Equivalent Noise Level).
So there you have it, the 5 most crucial microphone specifications to comprehend. I hope you've learned something new and have read through the extra resources if you had any lingering questions.
The top 5 microphone specifications you need to know are, again:
- Frequency response
- Polar response
- Maximum Sound Pressure Level
For information on all the microphone specifications, check out my article Full List Of Microphone Specifications (How To Read A Spec Sheet).
What is the best microphone sensitivity rating? The best sensitivity rating of a microphone is application-specific. Lower sensitivity mics are often preferred for loud sound sources and in less-than-ideal environments. Higher sensitivity mics are preferred when more subtlety is needed, and the environment is more conducive to a quality recording.
What is a unidirectional response mic? A unidirectional microphone response is the same thing as a cardioid-type microphone response. The microphone is sensitive to sound in one direction (on-axis) and rejects sound from other directions. The first commercial unidirectional mic was the Neumann CMV3A (“the bottle”), produced in 1932.
Choosing the right microphone(s) for your applications and budget can be a challenging task. For this reason, I've created My New Microphone's Comprehensive Microphone Buyer's Guide. Check it out for help in determining your next microphone purchase.