Hear a faint hiss in your recordings? The sound could be coming from within the mic! The self-noise of a microphone is exactly what it sounds like (see what I did there?): the noise created by the mic itself. However, there's more to it than this obvious answer.
What is microphone self-noise? Self-noise is the faint sound produced, mainly by the active circuitry inside microphones. Active mics contain electronics that add considerable “self-noise” to the mic signal, calculated as the self-noise specification. Brownian movement and thermal noise also add self-noise to a lesser extent.
Self-noise is also known as equivalent noise level and may be labelled as such on certain microphone spec sheets. That's all fine and well, but what does self-noise actually mean to microphone users? Let's answer this question throughout this article!
Table Of Contents
- What Is Microphone Self-Noise? (Equivalent Noise Level)
- What Are A-Weighted Decibels?
- What Factors Create Microphone Self-Noise?
- Why Is Low Self-Noise Important?
- Is There Ever An Advantage To High Self-Noise?
- How Do Manufacturers Measure Self-Noise Values?
- The Signal-To-Noise Ratio
- A Note On Noise In Passive Microphones
- Reducing Microphone Noise
- Related Questions
What Is Microphone Self-Noise? (Equivalent Noise Level)
Microphone self-noise is the noise generated by the microphone itself. All microphones have self-noise to some extent. The self-noise of some microphones is negligible, while other mics are avoided in quiet situations due to their overly loud self-noise ratings.
Some manufacturers refer to self-noise as equivalent noise level (EIN). This is because the natural output level of the mic is equal to its output level when recording a sound source level the matches its equivalent noise level value.
Self-noise is an important specification for active microphones (mics that require power to function) and is typically specified in A-weighted decibels (dBA).
As mentioned, active microphones have active circuit boards that actually produce noise when powered. The circuitry effectively adds noise to the mic's signal and, in the process, reduces the signal-to-noise ratio of the mic.
Passive microphones (mics that do not require power to function) do not have any active components in their circuitry and therefore are not affected by the noise of active circuitry. It's for this reason that passive microphones do not have self-noise specifications.
For a deeper read into passive and active microphones, check out my article Do Microphones Need Power To Function Properly?
Noise degrades audio signals, and so it's critical to maintain a usable signal-to-noise ratio. Self-noise is a factor we should keep in mind!
What Are A-Weighted Decibels?
Self-noise is specified in “A-weighted” decibels. What are they?
A-Weighted decibels (dBA) express the perceived relative loudness of air vibrations to the human ear. In other words, dBA represents the way we hear sound waves.
The human ear does not have a flat frequency response, so our hearing is more sensitive to some frequencies than others. The dBA scale aims to flatten things out for us.
As with all decibel values, the dBA measures the intensity of a sound by comparing it with a given level on a logarithmic scale.
dBA is always relative to the threshold of human hearing. So 0 dBA would be complete silence with no detectable noise whatsoever.
To illustrate further, a 100 Hz tone at 100 dB SPL is perceived by humans as having equal loudness to a 1,000 Hz tone at 80 dB SPL.
100 dB SPL at 100 Hz and 80 dB SPL at 1,000 Hz have the same perceived loudness and, therefore, the same dBA value (even though their dB SPL values differ by 20 dB!).
Once again, dBA is a weighted scale based on perceived loudness rather than the actual vibration of air or audio voltages!
As an aside, dBA are typically smaller than “regular” dB SPL values (there are many frequencies we can't perceive fully). So dBA, in effect, makes the self-noise seem less than it actually is!
For reference, 25 dBA would be like a whispering person across a silent room and is, in my opinion, the cutoff point for a self-noise value in a microphone. At this point, the microphone isn't worth having around in quiet studio settings.
For more information on the complex nature of decibels, check out my in-depth article What Are Decibels? The Ultimate dB Guide For Audio & Sound.
What Factors Create Microphone Self-Noise?
The main source of microphone self-noise is from active circuitry. However, there are three factors that create microphone self-noise:
Noise From Active Circuitry
The main source of self-noise is the active circuitry of condenser mics and other active microphones.
The noise from the active circuitry is twofold:
- Active circuitry introduces noise to the audio signal passing through it.
- To a lesser extent, active circuitry emits sound. The sound is then picked up by the microphone diaphragm and turned into an audio signal.
For this reason, condenser microphones and active ribbon microphones typically have specified self-noise values on their datasheets.
The noise comes from internal amplifiers and impedance converters (vacuum tubes, transistors, and other internal circuit components).
To learn more about these active internal amps and impedance converters, check out the following articles What Are FETs & What Is Their Role In Microphone Design? and What Are The Differences Between Tube & FET Microphones?
As discussed, self-noise is typically given in A-weighted decibels. Some manufacturers give additional readings in different units.
Moving-coil dynamic and passive ribbon microphones rarely, if ever, have self-noise specifications.
Johnson-Nyquist noise or “thermal noise” is caused by the agitation of electrons inside an electrical conductor. Electrical conductors are used dynamic and ribbon microphone transducers as well as in microphone transformers. Dynamic microphones (both moving coil and ribbon types) have inherent thermal noise.
Thermal noise has a similar sound to white noise, having roughly equal intensities across all audible frequencies, though the two are completely independent of one another.
The lower a dynamic microphone's output impedance, the lower its thermal noise. Thermal noise is also a function of temperature, but the influence of temperature on the overall noise floor is not typically a major factor in most recording situations.
That being said, though the impedance of many professional dynamics mics is nominally 150 Ω on their datasheet, very few truly are. Impedance takes [DC] resistance and [AC] reactance into account and is, thereby, a function of frequency. The DC resistance of the capsule's voice coil produces most of the impedance, but not all.
As mentioned, resistance generates thermal agitation or Johnson-Nyquist noise predictably according to the temperature. In frequency regions of 500 Ω or more (which can happen at certain points in many dynamic mics), the thermal noise of the mic cartridge may generate as much or more than the first-stage transistors in a good mic preamp.
The thermal noise produced by their electromagnetic conductors is very quiet and largely considered negligible. However, it's worth noting in this article.
If you're experiencing noise in the audio signal from a dynamic microphone, chances are the noise issue is happening within the mic cable or the mic preamp!
The third source of self-noise comes from Brownian movement.
Brownian movement is the random movement of particles suspended in a fluid. In the case of the vast majority of microphones, this refers to the molecules in the air.
Brownian movement affects condenser and ribbon microphones more than moving-coil dynamics. This is because condenser and ribbon diaphragms are considerably lighter than moving-coil diaphragms and have lower inertia.
However, similar to the Johnson-Nyquist noise, Brownian movement is practically negligible, especially when calculating noise for specifications sheets.
Different types of microphones have self-noise for different reasons, but the noise from active circuitry is really the only remarkable source.
To learn more about the microphone types discussed in this section, check out the following My New Microphone articles:
• What Is A Condenser Microphone? (Detailed Answer + Examples)
• The Complete Guide To Ribbon Microphones (With Mic Examples)
• The Complete Guide To Moving-Coil Dynamic Microphones
Why Is Low Self-Noise Important?
First and foremost, low noise in a microphone is critical in technical/clinical measurement situations. If we're concerned with measuring the faintest sound vibrations in a medium, any inherent noise in the measurement microphone will have adverse effects on the results, making them inaccurate at best.
Noise is also generally unwanted in mixes and recordings. Too much noise can be distracting to the listener and negatively affect your mix.
Noisier microphones will need to be placed closer to their sound sources to produce an audio signal with a decent signal-to-noise ratio.
Low self-noise mics, on the other hand, can be placed further away. This is important in a variety of situations:
- A vocal performance further from the mic is often better: the performer can physically express themselves a bit more (“elbow room”). Additionally, because they are further from the mic, the chance of plosives “pops” overloading the mic capsule is reduced.
- Parabolic or shotgun mic situations in long-distance recording, like in sporting events: if we're trying to capture sounds that are far away, we need a low self-noise to achieve a usable signal-to-noise ratio.
- When recording ambiences and quiet sounds: a low self-noise is paramount in order to get a clean, quality recording.
Related My New Microphone articles:
• Why Are Parabolic Microphone Dishes Used In Football?
• Best Parabolic Microphones
• What Is Ambient Miking & What Is An Ambient Microphone?
• Best Microphones For Recording Ambience
With that being said, the vast majority of environments will have some noise. Yes, even soundproof rooms have a low level of ambient noise. If the environment's dBA value exceeds the microphone's self-noise rating, there's really no benefit.
Is There Ever An Advantage To High Self-Noise?
As much as self-noise is demonized, there are situations where a microphone with more self-noise could be preferred over a low self-noise microphone.
To further understand, let's think in terms of the alternate name of self-noise: equivalent noise level.
The term equivalent noise level translates to the following: the microphone outputs a self-noise signal equal to the signal it would produce from a given sound source at its equivalent noise level value.
For example, if a mic has an ENL of 15 dBA, its output signal voltage in absolute silence is equal to its output signal voltage when subjected to a sound at 15 dBA.
In other words, the “noise floor” of the mic is 15 dBA, and the microphone won't register sounds under its noise floor of 15 dBA!
When we think of self-noise as the noise floor, we can imagine a higher self-noise being a better choice in certain situations. A high noise floor means a microphone will effectively reject quieter sounds while capturing loud sounds.
High microphone self-noise may be better suited for these situations:
- Miking kick drums (and other loud percussion instruments)
- Miking horns
- Loud concerts
- Noisy environments
Low microphone self-noise is preferred in the following situations:
- Quiet instruments
- Symphony/Orchestra performances
- Any work in iso-booths, anechoic chamber, and other acoustically “dead” spaces
To learn more about my recommended microphones for the above-listed applications, check out my Recommended Microphones And Accessories.
How Do Manufacturers Measure Self-Noise Values?
So how do manufacturers get a true reading of self-noise in their microphones? There are two main methods:
The Soundproof Container Method
This method of testing self-noise involves placing the microphone in a soundproof container and recording it from within. Of course, having absolutely no sound within the container is a difficult (and expensive) task to accomplish, but the technology is available.
These tests are often conducted in anechoic chambers which have absolutely no ambient noise.
When we remove external sound stimulus, the active microphone only outputs the sound of itself.
Some of the noise could be from the electronics producing sound that is then captured by the diaphragm.
The bulk of the noise, however, is simply from the audio signal passing through the circuitry itself. This leads us to the next method.
The Capsule-Less Record Method
Another, more affordable, way of measuring the self-noise of an active microphone is to record it without its capsule.
This way, we only send the self-noise of the electronics, even if there is sound in the exterior environment.
The capsule-less record method is not the best way to measure self-noise since the microphone isn't complete. It often yields lower self-noise values for the microphone specifications sheet.
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What Is A Good Self-Noise Spec For A Microphone?
So what kind of self-noise rating should we be looking for in a microphone? Let's look at the outer limits here:
- 25 dBA and above is practically unusable in professional studio settings
- 0 dBA is ideal but not a reality in microphones (or recording environments, for that matter)
Let's break down some ranges of self-noise dBA values, and you can rate them as good or bad!
≤ 10 dBA: Extremely Low Noise
With modern, large-diaphragm condenser microphones, it is possible to reach these low levels of self-noise. Is there any added benefit the lower we go here? Well, typically, the ambient noise in studio rooms and other recording environments will exceed 10 dBA, and so the benefit will be lost.
That being said, less than 10 dBA of self-noise is truly remarkable!
The Rode NT1-A is a large-diaphragm electret condenser that has a self-noise of only 5 dBA:
The Rode NT1-A is also featured in My New Microphone's 50 Best Microphones Of All Time (With Alternate Versions & Clones).
11 dBA – 15 dBA
This is the territory of many large-diaphragm condensers, good small-diaphragm condensers, and really well-designed large-diaphragm tube condensers.
Typically the self-noise in this range will be indiscernible from the natural ambience of a room and will not be noticeable in the context of a full mix.
The Neumann KM 184 is a small-diaphragm condenser with a self-noise rating of 13 dBA:
The Neumann KM 184 is also featured in My New Microphone's 50 Best Microphones Of All Time (With Alternate Versions & Clones).
16 dBA – 19 dBA
This is a good rating for older microphones, and nearly all modern active studio mics are designed to be in this range or lower.
This self-noise may be heard in really quiet performances, but like the 11-15 dBA range, it will get lost in a full mix pretty quickly. We can hear it, but it's not usually an issue.
The Behringer C-2 is an excellent budget-friendly small-diaphragm electret microphone and has a self-noise rating of 19 dBA:
20 dBA – 23 dBA
The self-noise here is clearly audible. Mics in the range still work fine for recording loud sources (with perhaps less gain at the preamps). A conscious effort must be put into creating a workable signal-to-noise ratio.
However, I'd caution against using these microphones for voice work or capturing other relatively quiet sources as the noise will be apparent in the recordings.
≥ 24 dBA
These mics could work fine in noisy live situations but are typically seen as “unfit” for studio applications.
The Signal-To-Noise Ratio
The signal-to-noise ratio (SNR) plays a big role in the quality of a recording.
Expressed in decibels (dB), the SNR compares the level of “signal power” to the level of “noise power.” The higher the SNR, the more true signal is coming through relative to the signal noise.
When talking about the quality of an audio signal, the SNR plays a huge role. There are many ways that noise can get into a signal, and self-noise is, of course, one of them!
Therefore, we want the self-noise of a microphone to be minimal to capture the cleanest recording possible. Capturing clean audio is often a game of inches!
To get a good idea of the signal-to-noise ratio at a given sound level, we can subtract the microphone self-noise from the sound pressure level the microphone is picking up.
SNR = SPL − SN
- SNR: signal-to-noise ratio
- SPL: sound pressure level at the microphone diaphragm
- SN: the microphone's self-noise
Note that there could certainly be other factors that induce noise in the mic signal (electromagnetic interference, ambient noise, preamp gain, etc.), so the above equation is hopeful at best.
For signal-to-noise ratio microphone specifications, the mic “pickup” level is set at 94 dB SPL (1 Pascal). The SNR is, therefore, the difference between 94 dB SPL and the microphone's self-noise rating.
Oftentimes the self-noise dBA value is subtracted from the dB SPL value. This is not exactly accurate, but it does yield a seemingly higher SNR rating.
Reputable microphone manufacturers like Neumann often give A-weighted and non-A-weighted values for their microphone self-noise and signal-to-noise ratio values. Let's look at the Neumann KM 184, for example:
The equivalent noise level (self-noise) ratings of the Neumann KM 184 are given as 22 dB (CCIR) or 13 dBA (A-weighted).
Therefore, the signal-to-noise ratio of the KM 184 is given as 72 dB (CCIR re. 94 dB SPL), calculated as 94 dB – 22 dB = 72 dB.
Alternatively, the SNR is given as 81 dBA (A-weighted re. 94 dB SPL), calculated as 94 dB – 13 dBA = 81 dB.
In this way, Neumann is honest with its microphone users. This is one of the many reasons why Neumann is one of My New Microphone's Top 11 Best Microphone Brands You Should Know And Use.
To learn more about microphone SNR, check out my article What Is A Good Signal-To-Noise Ratio For A Microphone?
A Note On Noise In Passive Microphones
As we've discussed, passive microphones (dynamic mics and most ribbon mics) do not have self-noise ratings. This is because they do not have active amplifiers in their design.
Because they have no internal amplifiers, passive microphones naturally output lower signal levels than active microphones. A microphone preamplifier will typically have to apply more gain to a passive mic's signal to bring it up to line level than it would an active mic's signal.
Any time we apply gain to a signal, we risk introducing noise. Noise is inherent in the internal amplifiers of active microphones (which produce self-noise) and external microphone preamplifiers.
Therefore, after the mic preamplifier gain stage, the passive microphone signal may have just as much or even more noise than its active microphone counterpart. It all depends on the amount of preamp gain applied to the signals and the “cleanliness” of the preamp gain.
To learn more about microphone preamplifiers and mic gain, please read the following My New Microphone articles:
• Do Microphones Amplify Sound And/Or Audio?
• What Is A Microphone Preamplifier & Why Does A Mic Need One?
• Best Microphone Preamplifiers
Reducing Microphone Noise
So self-noise is inherent and practically impossible to get rid of. However, microphone noise also comes from other sources as well.
The most obvious method to help reduce microphone noise is to reduce the noise in the ambient environment. Choose quiet recording spaces when possible, use acoustic treatment to deaden the room to reduce reflective noise if necessary.
Electromagnetic interference (EMI) from other electrical devices also causes microphone noise. Though most professional mics use balanced audio to mitigate EMI, the shielding and common-mode rejection don't make them absolutely immune to the effect. Fix grounding issues in the electrical system and avoid running audio lines alongside power lines if possible. If the EMI, or radio frequency interference, is overly noticeable, consider using an EMI filter like the Shure A15RF.
To learn more about microphone noise and how to reduce it, check out my article 15 Ways To Effectively Reduce Microphone Noise.
If noise is unavoidable or you find yourself working with noisy audio signals, there are methods of reducing noise in post-production with audio plugins.
For more information on this process, check out my article The Top 7 Best Noise Reduction Plugins For Your DAW.
Which active microphones have the lowest self-noise? Any spec under 10 dBA renders the self-noise negligible compared to most ambient noise sources. However, some mics are well below this value. Here are a few common low noise condenser mics:
- Lewitt Audio LCT 550 = 3 dBA
- Nevaton MC50-Quad = 4 dBA
- Rode NT1-A = 5 dBA
- AKG C 414 = 6 dBA
- Neumann TLM103 = 7 dBA
What are all the factors that cause noise in a microphone signal? The factors that cause noise in a microphone signal include:
- The ambient noise of the environment
- Self-noise from active electronics
- Random molecules hitting the mic diaphragm
- Thermal noise of dynamic moving coil/humbucker (negligible)
- Interference in microphone
- Interference in the mic cable
- Mic preamp noise
For information on all the possible microphone specifications, please continue to my article Full List Of Microphone Specifications (How To Read A Spec Sheet).
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.