What Does A Microphone’s Maximum Sound Pressure Level Actually Mean?

How loud of a sound can a microphone handle? When looking at a microphone's spec sheet, “maximum SPL” may be listed with a specific value. That being said, a microphone's maximum sound pressure level can be a misleading piece of information.

So what does a maximum sound pressure level actually mean? The maximum sound pressure level of a microphone is not the pressure level that will destroy the microphone. Rather, max SPL is the sound pressure threshold at which a microphone's output signal begins to distort. 

This article will explain max SPL ratings in detail to deepen our understanding and lessen our fear of blowing up our mics!

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Table Of Contents

• What Does Maximum Sound Pressure Level Actually Mean?
• What Is Distortion?
• What Is Total Harmonic Distortion?
• What Causes Microphone Distortion?
• The Definition Of Maximum Sound Pressure Level.
• How Do Manufacturers Test For Max SPL?
• A Bullet List Of Typical dB SPL Values.
• How Are Max SPL Values Of Microphones Calculated?
• When Is It Important To Pay Attention To The Max SPL Rating?
• How To Avoid Exceeding Max SPL Values.
• Related Questions.

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What Does Maximum Sound Pressure Level Actually Mean?

So what happens when we push things too far? What happens when we exceed the maximum sound pressure level?

The microphone typically doesn't get destroyed!

Of course, I have to say “typically” here since some diaphragms can be damaged by excessive sound pressure levels (or by the gusts of air that are sometimes produced along with excessive sound pressure levels).

The point is that Max SPL is not the breaking point of a microphone.

This is a falsity that I thought was true for years… but the truth is:

The maximum sound pressure level of a microphone is the sound pressure level threshold at which its signal becomes distorted. 

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What Is Distortion?

Distortion is any deformation of an output signal relative to its input signal.

I'm sure we've all heard distortion before. It can be obvious (like digital clipping or electric guitar distortion) or subtle (like the warm saturation of a nice tube microphone).

Digital Audio Distortion

Digital distortion or “clipping” often sounds harsh, over-compressed, and severely damages the original digital audio signal. Audio engineers typically avoid digital clipping like it's the plague unless used for special effects.

Digital clipping drastically alters an audio waveform. There is a ceiling that cannot be broken with digital audio: 0 dBFS or 0 decibels Full Scale. As this ceiling is met, digital clipping happens as the signal is effectively flattened.

Distortion can take the signal on the left and turn it into the signal on the right:

Analog Audio Distortion

Analog distortion is a bit more forgiving and can cause a slight compression and saturation of the signal. This is part of the reason why analog audio can sound so “warm.”

The signal is still changed with analog distortion, but there is much less sharpness in the alteration of the audio signal with analog distortion.

Saturation is an intrinsic part of distortion. Saturation of a signal means harmonics are either created in the signal and/or the existing harmonics are increased in level.

Microphone Distortion

Microphones are inherently analog devices. So if we are to record a source louder than a microphone's max SPL, we will get analog distortion in the mic's circuitry and output.

Analog distortion isn't always an unwanted occurrence. The compression and saturation that come with slight analog distortion may even improve the subjective quality of the microphone signal!

Personally, I try my best not to subject microphones to sound sources above their maximum SPL. Other issues could arise with overloading their circuitry, and I prefer to have my microphones perform the way they're meant to.

Analog distortion and saturation are complicated to measure holistically. Therefore, microphone distortion is most often measured as total harmonic distortion.

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What Is Total Harmonic Distortion?

Total harmonic distortion (THD) is the ratio of the sum of the powers of all harmonics to the power of their fundamental frequency. In other words, a specific measurement of saturation in a sine wave signal. 

The particularity of THD distortion is that it applies to signals that cover full frequency ranges. However, calculating THD becomes overly complicated unless measured from a single-frequency sine wave.

Sine waves are used because they only emit only one frequency: the “fundamental frequency.” 

If the sine wave signal is distorted, it will contain harmonics (integer multiples above the fundamental frequency). As the original sine wave signal input becomes distorted, more harmonics are added to the signal output. 

The measurement of max SPL rating is based on the calculation of a certain percentage of THD at a specific sine wave frequency.

We'll discuss THD and harmonic distortion (saturation) in further detail in the section How Do Manufacturers Test For Max SPL?

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What Causes Microphone Distortion?

Microphones are transducers that convert sound (mechanical wave energy) into audio (electrical energy). Mics do not always represent the sound waves with perfect accuracy, though they often come close.

When subjected to loud enough sound pressure, a microphone's output audio signal deforms relative to what it would otherwise be at lower SPLs. This may happen for a few reasons:

• Overloading the microphone's electronic circuitry (most common).
• Overloading of the microphone's capsule (less common).
• Damage to the microphone.

Overloading The Microphone's Electronic Circuitry

The analog circuitry inside active microphones has inherently limited headroom.

Specifically, the impedance converter (transistor or tube) and amplifier electronics are only capable of handling so much signal before distortion. This threshold is determined by the internal power rails and the impedance of the microphone.

Microphone impedance converters are typically FETs or vacuum tubes. Learn more about each in the following My New Microphone articles:
• What Are FETs & What Is Their Role In Microphone Design?
• What Are The Differences Between Tube & FET Microphones?
• Do Microphones Amplify Sound And/Or Audio?

To overcome this, many active microphones have attenuation pads built into their designs. These pads effectively decrease the signal voltage from the capsule before the signal gets to the electronics. This helps to reduce the likelihood of distortion by effectively reducing the maximum sound pressure level of the microphone.

Overloading Of The Microphone's Capsule

Although possible, sound pressure levels will rarely ever overload microphone capsules. This is especially true when recording any practical sound sources.

Dynamic microphones do not contain active circuitry, and therefore their max SPL is determined by the overloading of their diaphragm. This is why max SPL ratings are rarely explicit with dynamics. They're just too high!

Dynamic microphones distort when their moving coil hits against the capsule magnets or casing. This is most likely to happen at the microphone's resonant frequency, but still, most dynamics will never overload in any practical applications.

Condenser microphone diaphragms are very robust and unlikely to be overloaded. They are stretched tightly and move very little when subjected to low and high sound pressure levels.

Ribbon microphone diaphragms are fragile, but sound pressure itself typically won't overload them. Rather, it's the gusts of moving air that have the potential to overload and tear these sensitive ribbons.

To learn more about microphone capsules, check out my article What Is A Microphone Capsule? (Plus Top 3 Most Popular Capsules).

Damage To The Microphone

A damaged microphone may very well still work but with a distorted output.

Damaged diaphragms, electronics, or output connections can all lead to subpar performance and microphone distortion. More often than not, though, serious damage will cause much worse than just signal distortion in a microphone.

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The Definition Of Maximum Sound Pressure Level

In the above paragraphs, we've learned the following points about max SPL:

• The max SPL value of a microphone is the threshold at which the output signal starts to distort.
• Overloading a microphone diaphragm to the point of distortion requires an impractically high sound pressure level.
• Overloading the active circuitry of a condenser or active ribbon mic is certainly possible.

To sum it all up, max SPL is typically determined by the mic's electronics and not the mic's capsule/diaphragm.

For this reason, max SPL ratings are rarely, if ever, given to dynamic microphones. Even passive ribbon microphones won't generally have a listed max SPL.

However, condenser and active ribbon mics will almost certainly come with this specification, and it's important to make a note of it to help avoid distortion in your recordings.

Let's take a look at 4 microphone examples and their maximum sound pressure level ratings:

Microphone	Mic Type	Max SPL Rating
Shure SM57	Dynamic (Passive)	N/A (though approximated by Shure to be around 160 dB SPL @ 1 kHz)
Royer R-121	Ribbon (Passive)	>135 dB @ 20 Hz
Royer R-122	Ribbon (Active)	135 dB @ 30 Hz
Neumann TLM 102	Condenser (Active)	144 dB (THD 0.5% @ 1 kHz)

Note that the SM57 moving-coil dynamic mic has the highest max SPL rating.

The passive ribbon has a very high but measurable maximum SPL. Note that the ribbon mics will begin distorting at lower frequencies before the mid and upper frequencies. Therefore, “>135 dB @ 20 Hz” means the R-121 can capture higher SPLs of sound sources with higher frequency ranges.

The R-122 active ribbon mic has a defined max SPL rating that is relatively low.

The TLM 102 condenser mic has a measured max SPL of 144 dB, which is high enough to capture most practical sound sources without overloading the mic circuitry and distorting the output signal.

Let's now talk about how the max SPL is measured and how we can better use it as audio enthusiasts!

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How Do Manufacturers Test For Max SPL?

So we know that the max SPL rating of a microphone is the SPL at which its signal will start to distort but what constitutes distortion, and how do we reliably create these sound pressure levels to test for max SPL?

Total Harmonic Distortion

As mentioned, THD is a measurement of the level of harmonic distortion. It refers to a specific measurement of saturation in a sine wave signal. 

THD is presented as a percentage. This percentage is the ratio of the sum of the powers of all harmonic frequencies above the fundamental frequency to the power of the fundamental frequency.

So to measure THD, we need to use a pure, undistorted sine wave rather than an instrument or sound we'd actually want to record.

Yes, this gives a slightly unnatural max SPL rating, but it gives a measured rating, which is important to have. It's important to note that the max SPL at a given frequency will not be the max SPL at other frequencies.

The 1,000 Hz Tone

Most manufacturers use a 1 kHz tone to calculate the max SPL of their microphones.

They do so by emitting the tone immediately in front of the microphone diaphragm, increasing the amplitude of the tone until distortion (THD) happens at the mic's output.

Please note the maximum sound pressure level is frequency-dependent. However, the 1 kHz tone provides a close approximation of the max SPL across the overall frequency spectrum.

Also, note that the aforementioned Royer microphones (R-121 and R-122) have max SPL ratings measured at 20 Hz and 30 Hz, respectively. This is due to the fact that ribbon microphones are more likely to distort at lower frequencies before higher frequencies when subjected to a given SPL. These low-frequency ratings basically tell us that the rest of the frequencies will be perfectly fine at these max SPL ratings.

Back to the point. So the 1 kHz tone is captured by the microphone. The maximum sound pressure level is the level at which the microphone output shows 0.5% total harmonic distortion (or 1% THD, depending on the manufacturer). 

This is how max SPL is calculated, but it's not always easy…

How Do Manufacturers Create The Tone Necessary For Testing Max SPL?

How do we acoustically recreate a tone as loud as the max SPL of a given microphone? These Max SPL ratings are often well above 130 dB SPL.

Recreating a pure 1 kHz tone at these levels on typical loudspeakers will result in significant loudspeaker distortion, which is obviously a big problem.

A distorted signal at the input inevitably means a distorted signal at the output, even if the microphone itself does not distort. Testing in this manner would give us a false reading for max SPL.

So why do loudspeakers tend to distort at SPL values above 130 dB SPL?

It's because they're expensive to make and not designed to be louder than they need to be. Additionally, dB SPL values this high will cause ear damage! 

Loudspeakers are designed to be listened to, not to injure people. To help illustrate this, I'll include a quick reference to dB SPL values:

A Bullet List Of Typical dB SPL Values

• 0 dB SPL – Threshold of hearing. Anything below 0 dB is not perceivable to human ears.
• 20 dB SPL – Bedroom when no one's home.
• 40 dB SPL – Refrigerator hum.
• 60 dB SPL – Normal conversation.
• 65 dB SPL – Business Office.
• 85 dB SPL – Average City Traffic.
• 100 dB SPL – Jackhammer.
• 110 dB SPL – Chainsaw or Rock Concert.
• 120 dB SPL – Ambulance Siren.
• 125 dB SPL – Jet Engine from 100 meters away.
• 140 dB SPL – Threshold of pain. Anything this loud or louder will cause immediate hearing damage and will be very painful.

Suppose we look at any given condenser microphone's spec sheet. We'll typically see a max SPL rating near the threshold of pain.

Think of how we use microphones on a kick drum, for example. We close-mic the kick to capture its sound in isolation. However, if we were to put our ear inside the kick drum in place of the microphone, it wouldn't be the best sound at all. In fact, it would damage our ears!

Loudspeakers typically won't recreate sounds this high. And so, we must find alternatives to calculate the maximum sound pressure levels of microphones.

Decibels are fairly complicated to understand. If you'd like to know more about what decibels are and how they work with sound and audio, check out my article What Are Decibels? The Ultimate dB Guide For Audio & Sound.

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How Are Max SPL Values of Microphones Calculated?

That's a good question. Let's discuss the best answers for passive microphones and active microphones.

Measuring Max SPL Of Passive Microphones

Passive microphones are not overly susceptible to distortion.

Moving-coil dynamic mics, for example, won't even have a maximum SPL rating listed on their spec sheets. This is because you'll likely never be tasked with recording a sound source loud enough to distort a passive dynamic microphone.

A passive mic's electronics are durable and resistant to overloading. Therefore, its max SPL is a physical limitation. To distort a passive microphone, you'd have to have such a great sound pressure level that the diaphragm bangs against the capsule/cartridge, in which case it can move no further.

Where is the loudest place you'll put your passive dynamic microphones? For most studio engineers, the answer would be inside a kick drum. But even with the heaviest foot, the likelihood of the sound exceeding 155 dB SPL is slim-to-none.

So unless you're trying to close-mic a space shuttle, I wouldn't worry about the max SPL of passive microphones. They won't distort in normal contexts!

Measuring Max SPL Of Active Microphones

Unlike their passive counterparts, active microphones will always come with a max SPL rating.

Active mics include condensers and active ribbon mics.

However, like the dynamic mic diaphragm, the condenser diaphragm is not likely to distort.

Any distortion from exceeding the maximum SPL value of a professional-grade condenser microphone happens due to electrical overload. Basically, the signal from the capacitor capsule overloads the active circuitry (notably the amplifier stage and impedance converter immediately following the capsule).

We safely assume distortion in an active mic comes from overloading the active circuitry rather than the nonlinear movement of its diaphragm.

So when measuring for max SPL, some manufacturers simulate microphone distortion by directly injecting a 1 kHz tone into the condenser's circuitry, bypassing the diaphragm altogether.

Increasing the voltage of this pure signal until 0.5% THD gives a voltage value. Converting the applied voltage to a theoretical SPL (measured in dB SPL) is then easily calculated if we know the microphone's sensitivity rating.

The sensitivity rating tells us the mic signal level relative to a 94 dB SPL (1 Pascal) 1 kHz tone. We can cross multiply with the max SPL voltage to find the actual max SPL value in dB SPL.

With that calculation, we would see exactly at what dB SPL the circuitry would be sufficiently overloaded (when subjected to a 1 kHz tone). Therefore, this dB SPL value would be equal to the maximum sound pressure level of that microphone!

I'll restate here that this method assumes that the diaphragm assembly is not producing distortion!

Oftentimes a condenser will be designed with an attenuator switch (pad) to increase the SPL the mic will effectively capture before distorting. Once again, this is because the diaphragm typically handles a high SPL easily, and it's the overloading of the circuitry that will cause distortion in the signal.

For more information on active and passive mics, check out my article Do Microphones Need Power To Function Properly?

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When Is It Important To Pay Attention To The Max SPL Rating?

The only time we really need to pay attention to a microphone's Max SPL rating is when we're choosing a microphone for a kick drum, guitar cabinet, or another scenario where we're miking a loud sound source. Generally speaking, we only need to double-check these maximum ratings with condenser microphones or active ribbon microphones.

So, as an example, if we want to mic up a kick drum with a condenser microphone, we should take precautions.

Check the max SPL to ensure it can handle the SPL of the kick drum. Of course, the sound pressure level of this kick drum will vary depending on the drum design, the drummer, and the genre of music being played.

A condenser mic is safer outside the kick drum and away from the hole of the front head (where the sound pressure level is lower).

This is not to say that sticking a condenser mic inside a kick drum will damage it, but you may run the risk of capturing a distorted signal.

When in doubt, I personally opt for dynamic mics so that I don't have to worry about the max SPL rating!

Here is a short list of considerably high SPL instruments to be cautious about:

• Trumpet @ 0.5 meters = up to 130 dB SPL
• Loudest human voice @ 1 inch distance = up to 135 dB SPL
• Inside a kick drum = up to 145 dB SPL

To learn about my recommended microphones for each of the above sound sources, check out the following My New Microphone articles:
• Best Microphones For Trumpet
• Best Studio Microphones For Recording Singing
• Best Studio Microphones For Recording Scream Vocals
• Best Studio Microphones For Recording Rap Vocals
• Best Microphones For Live Vocal Performances
• Best Kick Drum Microphones

So in most cases, you do not need to worry about the maximum sound pressure level value of your microphone. If there does happen to be distortion in the audio signal for a given microphone, chances are it'll be a clipping issue with the preamplifier or analog-to-digital converter!

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How To Avoid Exceeding Max SPL Values

• Choose a high max SPL microphone or a dynamic microphone.
• Engage the pad if the microphone has one.
• Move the microphone further from the sound source.
• Tilt the microphone off-axis (this also helps protect the diaphragm from gusts of air).

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Related Questions

How much sound pressure can a microphone handle before breaking? It depends mostly on the diaphragm. Dynamic mics handle any practical sound pressure they're subjected to. Condenser diaphragms are more sensitive, and ribbons are especially fragile. Sound pressure alone won't typically break a mic, but transient gusts of air from loud sound sources will.

What is the maximum sound pressure level possible on Earth? The Max SPL at standard atmospheric pressure is 194 dB SPL. 194 dB SPL is equal to 101,325 Pascals, which is atmospheric pressure. The troughs of a 194 dB SPL sound wave literally create a vacuum. Sounds louder than 194 dB have distorted waveforms at their troughs. These are known as shockwaves.

For information on all the possible microphone specifications, please continue to my article Full List Of Microphone Specifications (How To Read A Spec Sheet).