The question is asked so often that Shure’s engineering department did some tests and wrote a white paper on the subject. They used Shure’s SM58 dynamic microphone, the best selling microphone in the world for many years, to get answers.

As a point of reference, Shure said that 140 dB sound pressure level (SPL) is the accepted threshold of pain for the human ear. The maximum sound SPL from a human voice as measured by Shure is 135 dB at one inch from the mouth.

On the other hand, a kick drum played very loudly may exceed 140 dB SPL, but has never been measured by Shure above 150 dB. The loudest orchestral instrument — a trumpet — can theoretically produce a maximum SPL of 155 dB at one inch, but only in its upper register.

Shure notes that the distribution of energy (sound pressure) in speech, music and noise is dependent on the frequency.

For example, the human voice does not produce much energy below 100 Hz and its frequency of maximum SPL would be higher than 100 Hz. Exactly how much higher depends on the individual voice being recorded.

Unlike a condenser microphone which has internal electronics that may overload, a dynamic microphone distorts when its diaphragm hits a physical barrier — like the magnetic pole piece — and can move no further.

The excursion of the diaphragm is frequency dependent and is greatest at the resonant frequency of the diaphragm. Therefore, the maximum SPL of a dynamic microphone such as the SM58 is frequency dependent. This means that low frequencies will produce distortion at a lower SPL than higher frequencies.

For Shure’s SM58, the frequency range to first exhibit distortion is centered around 100 Hz, close to the resonant frequency of the microphone's diaphragm. At 100 Hz, the measured maximum SPL is 150 dB and the electrical output of the microphone is 0 dB V or 1.0 volts. (Note this is a line level signal, not a mic level signal.)

In the 1 kHz range, the SM58 measured maximum SPL is about 160 dB due to the change in microphone sensitivity at the higher frequencies. The electrical output of the microphone at 160 dB SPL is +10 dBV or 3.2 volts.

In the 10 kHz range, 180 dB SPL is the maximum SPL of the SM58. However, this is a calculated measurement as Shure Engineering had no means to create such enormous and dangerous SPL levels.

For comparison, NASA reports that a space shuttle launch measures 180 dB SPL and higher at 10 meters.

In the 20 kHz range, the maximum SPL is calculated to be around 190, due to the response falloff of the SM58. But now the point of absurdity has been reached because at 194 dB SPL the sound pressure varies from twice normal atmospheric pressure (at the wave peak) to a total vacuum (at the wave trough).

Plus the sound source must be moving at the speed of sound just to generate a wave of this intensity.

In summary, Shure said a well-designed dynamic microphone of professional quality will never reach its distortion point in "normal" conditions. If one does encounter distortion when using a pro-level dynamic mic for an extremely loud source, it is most likely that the electrical output of the microphone is clipping the input of the microphone preamplifier.

To solve this problem, an in-line attenuator ("pad") must be placed before the preamplifier input, or the microphone must be moved farther from the sound source. In general, the sound pressure level will decrease 6 dB for each doubling of the distance.

As one who records professional audio, this is “need to know” information.