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It might get loud! What's maxSPL and THD?

Jul 8, 2021 2 min read

Thomas K.T.
I like words.

In this blog, we’re tackling the topic of maximum sound pressure levels. We’ll measure different things, and you’ll also learn which mics are best for loud sources like drums or guitar amps.

Microphone manufacturers state the maximum sound pressure level a microphone can handle before the signal starts to distort.

The dB scale

Just a quick reminder, the dB or decibel scale is a logarithmic scale.  It allows you to compare two values easily, often related to sound pressure or voltage.

As the dB scale is a logarithmic or non-linear scale, the values increase exponentially.

For example, if you boost the gain on your preamp by 6 dB, you’re increasing the signal strength by factor 2 and with + 24 dB, you’ll increase it by factor 16. 

We are really used to think in linear scales, so it takes some time to get used to this. But some things, we will never get used to, especially the fact that you need to restart ProTools after switching your audio device. 

THD - Total Harmonic Distortion

 

In sound reproduction, we often aim for a “pure”, undistorted signal.

For our microphones, we state the max SPL at 0.5% THD, measured at 1kHz. What does THD stand for? Total Harmonic Distortion – Damn, that’s a great name for a band.

 

At very high sound pressure levels, some components of the microphone start behaving non-linearly. 

This manifests as harmonic distortion. A 0.5% THD ratio means that the harmonics produced by the distortion represent 0.5% of the total output of the mic.

You can find THD specifications not only for microphones but also in preamps and loudspeakers for example.

With tube mics or preamps, the story is a bit different. Tubes want to be driven. Why? When you think of tube saturation, you actually want harmonics to be audible so that you get that characteristic sound.

Condenser mics can take loud levels.

Btw, you won’t break your condenser mics with high sound pressure levels. Worst case, the membrane sticks to the backplate, and you’ll have to turn off phantom power and wait until it becomes loose again. Today, professional condenser microphones can handle very high sound pressure levels. 

The LCT 640 TS for example handles a maxSPL of 152 dB. With the PAD engaged. So, it’s not that.

It‘s because condenser mics produce high outputs. And this can lead to clipping further down in your recording chain, like your preamp for example.

Before you blame your sensitive mic, check your input levels. It’s very likely that your preamp or your converter may be clipping.

If your levels are too high, there are ways to fix this:

  • If your mic has a PAD, use it to lower the mics‘ sensitivity.
  • You can also lower the gain on your preamp.
  • Or you can use the preamp‘s PAD, if the signal is still too hot.

That doesn’t mean you shouldn’t use dynamic mics, but sometimes you want more detail in your recording. For example, when you mic the bottom of a snare drum, but also for wind instruments condenser mics make sense when you want the details to shine. 

 

What about ribbon microphones?

Ribbon mics on the other hand are much more fragile, and sadly they can break. Ribbon mics are dynamic mics btw. 


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