Noise is an inherent issue in the world of audio. Maintaining a high signal-to-noise ratio is critical to getting a clean audio signal fro your recordings, sound reinforcement, and playback. A good signal-to-noise ratio starts at the microphone.
What is a good signal-to-noise ratio for a microphone? Ideally we want no noise in the mic signal and, therefore, a signal-to-noise ratio (SNR) or infinity. A low self-noise rating is essential when capturing quiet sound sources (15 dBA or less) and so a “good” SNR would be 79 dB or more.
In this article, we’ll discuss what a signal-to-noise ratio specification actually means, what constitutes “noise,” and how to improve the SNR in your mic signals.
What Does The Signal-To-Noise Ratio Rating Of A Microphone Actually Mean?
Signal-to-noise ratio (SNR) is technically the ratio of the power of the signal level to the noise power that corrupts that signal.
Therefore, the higher the ratio, the less noise will be apparent in the overall microphone signal.
But the signal-to-noise ratio needs to be measured somehow, so what does SNR really mean as a specification?
First, the signal-to-noise ratio, as a specification, applies only to active microphones. This is because active mics have inherent “self-noise” that is omnipresent in their signals regardless of external noise factors.
SNR is calculated against a reference acoustic level of 1 Pascal (94 dB SPL). This is standardized as a 94 dB SPL 1 kHz tone at the mic’s capsule.
Subtracted from this standardized value is the microphone’s self-noise rating. Generally speaking, self-noise is given in decibels A-weighted (approximating how we hear noise) rather than in decibels SPL (since self-noise really only affects the mic signal). However, because we use the 1 kHz tone (which our ears are sensitive to), we can make the subtraction with these differing units.
For example, if a microphone had a self-noise of 14 dBA, that microphone’s specified SNR would be 80 dB (94 dB SPL minus 14 dBA).
By the way, a SNR of 80 dB would be considered good!
A self-noise of 14 dBA (SNR of 80 dB) will be indiscernible from the natural ambience of most studio rooms and will not be noticeable in the context of a full mix.
To read more about microphone self-noise, check out my article What Is Microphone Self-Noise? (Equivalent Noise Level).
What Is A Good SNR For A Microphone?
I would consider a SNR of 74 dB and higher to be “good.”
When recording louder sound sources (kick drums, trumpets, etc.), we may be able to get away with smaller SNR ratings since the signal will be hotter.
When recording softer sound sources (ambience, conversation, etc.), we would likely want a SNR even larger than 74 dB.
Ideally, we do not want any noise in the signal unless used for special effect. In this case, the SNR would be rated as 94 dB, but this is impractical even in passive microphones.
If an active microphone has a SNR spec of 84 or above, we can count on that microphone to be fairly silent, even when recording softer sound sources.
But true signal-to-noise ratio is not the theoretical 94 dB minus the self-noise. True SNR is that actual ratio of signal level to noise level and this affects all microphones.
Like mentioned above, a SNR of 74 dB and higher would be considered good.
Microphones operate in the real world and the real world has ambient or environmental noise. What we consider noise is subjective, but can be summed up in the following:
- Signal: the part of the electrical signal that represents the intended sound source.
- Noise: the part of the electrical signal that represents every other sound.
If we can get the signal to be 74 dB (or more) louder than the noise, we’re in really good shape!
So now that we’ve discussed what SNR is and what a good SNR is in a microphone, let’s discuss noise and signal in more detail.
The Noise Of The Microphone
The noise of the microphone, as we’ve discussed, is referred to as self-noise.
Self-noise really only applies to active microphones. The powered components in these mics produce noise.
This can be noise that is introduced directly into the mic signal as it passes through the active component (transistors, vacuum tubes, and printed circuit boards). It can also be the sound that is emitted by these components which is then, in turn, captured by the microphone capsule.
Note that self-noise can also be attributed by random air molecules hitting the diaphragm and Brownian movement.
Regardless, this self-noise is a major contributor to the microphone noise in the signal-to-noise ratio.
For more information on microphones and transistors, check out my article Do All Microphones Have Transformers And Transistors? (+ Mic Examples).
For more information on microphone capsules, check out my article What Is A Microphone Capsule? (Plus Top 3 Most Popular Capsules).
The Noise Of The Environment
AN even greater contributor to the noise of a microphone signal is the environmental or ambient noise.
Environment noise comes in many forms:
- Extraneous noise: extraneous noise is really any sound source other than the intended one. In isolation booths, this is largely eliminated. In off-the-floor recordings, this could the sound from other instruments. In non-soundproofed environments, this is likely more noise than is acceptable (fans, traffic, conversations, AC hum, etc.).
- Room reflections: room reflections are created by the intended sound source but often cause noise and phasing issues in the mic signal. You hear this a lot in small rooms with no sound proofing.
- Electromagnetic interference: electromagnetic interference, which includes radio frequency interference, is cause by power mains (broadband interference) and other electromagnetic sources like TV and radio stations, mobile phones, etc. (narrowband interference).
- Mechanical noise: Mechanical noise comes from physical vibrations that reach the microphone. This could be handling noise (in the case of handheld mics) or simply vibrations from the stage or floor that vibrate the mic through the mic stand.
The Strength Of The Sound Source
A major factor in a microphone’s signal-to-noise ratio is, of course, the strength or “loudness” of the intended sound source (and how close the microphone is positioned to that sound source.
When recording loud sound sources (kick drums, trumpets, etc.), we can get away with a noisier microphone. This is because the loud sound source will produce a very strong signal in the microphone.
Since the microphone will require less gain to achieve the same signal level, the noise level will also be boosted less by the gain. Therefore, a better SNR is doable.
When recording quiet sound sources (conversation, for example), a quiet microphone with as little environmental noise as possible is nearly a must.
The conversation mic signal (often in the range of 60-70 dB SPL) will require lots of gain (to become line level for use in audio equipment). This gain will also boost the inherent noise of the microphone and the environmental noise. So a good SNR is a must if we want a clean capture of the convo!
Note, too, that positioning the microphone closer to the intended sound source will improve the SNR of the microphone. This fact brings us to our next section:
Improving The Signal-To-Noise Ratio Of Your Audio Signals
Although we cannot easily change the inherent SNR rating of our microphones, we may be able to lower the amount of noise in our audio signals.
In order to improve the SNR of the microphone signal, we must look at the ways in which noise is introduced into the signal. Once again, they are:
- Inherent microphone self-noise.
- Extraneous noise.
- Room reflections.
- Electromagnetic interference.
- Mechanical noise.
Here is a list of tips and tricks to improve the signal-to-noise ratio of your microphone audio signals:
- Choose A Condenser Or Active Mic With Low Self-Noise.
- Record In Quiet Soundproof Environments.
- Place Mics Close To The Sound Source.
- Do Not Run Mic Cables Alongside Power Cables.
- Use An RF Filter.
- Choose A Dynamic Mic With A Humbucking Coil.
- Use A Shock Mount.
- High-Pass Filter The Mic Signal.
- Use Clean Preamps.
For more tips and tricks to reduce the noise in your microphone signal, check out my article 14 Ways To Effectively Reduce Microphone Noise.
What is microphone self-noise? Microphone self-noise, as the name suggests, is the noise that is made by (and then captured by) the microphone itself. Active microphones have components that produce noise within the microphone and self-noise is the mic specification that tells us how loud this noise will be in the microphone signal.
For more information on self-noise, signal-to-noise ratio, and all the other microphone specifications, please check out my in-depth article Full List Of Microphone Specifications (How To Read A Spec Sheet).
What does a microphone measure? Microphones effectively measure the sound pressure variation around their diaphragms. As the sound pressure changes, the mic diaphragm moves and an electrical audio signal is produced. This electrical signal can then be analyzed carefully to determine the characteristics of the sound pressure.
For more information on microphones and their measurement capabilities, check out my article What Do Microphones Measure And How Do They Measure?