Maximum sound level for measurement microphones

summary

Levels up to 120dB can be measured with most systems, easily. 

Basic rule

The smaller the membrane area, the larger the maximum level. Therefore, 1/4" or 1/8" microphones are usually more suitable than 1/2" microphones for high sound levels.

However, it is important to ensure that the preamp or ADC does not clip and limit the microphone level.

background

The sound pressure moves the membrane of the microphone and generates a voltage. Excessive sound pressure can simply mechanically destroy the sensitive membrane. The microphones must never be exposed to this sound level. Such high sound levels are not only created by jet engines or explosions, but when a microphone is quickly inserted into a sound level calibrator, enormous pressures can occur.

However, the useful level range is much smaller, since the membrane  movement becomes non-linear at high levels. Distortions occur that modify the measurement result. This limit sound level, at which the distortion factor is still below 3% (at 1kHz), is usually specified in the data sheet.
With a typical 1/2" class 1 microphone (with 50mV/Pa) (the "workhorse" in measurement technology), this value is 146dB RMS or 149dB peak (e.g. Microtech Gefell MK250).

With a 1/4" microphone with 5mV/Pa, the maximum sound level is 168dB RMS (THD 3%) (Microtech Gefell MK301E)

information in the data sheets

It is very important to distinguish between the maximum level that leads to destruction and the limit sound level in the data sheets. Furthermore, a clear distinction must be made between peak and RMS. In any case, you should consider some headroom and not operate the capsules at the limit areas

The microphone pre-amp

A typical 1/2" microphone capsule with 50m/Pa delivers a voltage of 17.7V RMS at 145dB sound pressure (RMS), i.e. +/-25V. This high voltage must also be processed by the preamplifier, otherwise it overdrives.
50V are actually for transistor circuits no problem, but the preamps are usually designed for high sensitivity and low inherent noise.

Therefore the maximum sound level of the capsule is further reduced by the electrical amplification chain. The maximum output voltage or input voltage of the preamplifier is of main importance here.

Typical pre-amplifiers for acoustic measurement technology

Systems operating with constant current (ICP/IEPE) deliver a maximum output voltage of 3-6.5V RMS (Microtech Gefell MV210 6.5V).

Systems running on 48V phantom power deliver about 3V RMS. These preamplifiers come from studio technology and are also used in the NTI AL1, NTI XL2, NTI XL3, among others.

In professional acoustic measurement technology, preamplifiers with a LEMO connector are used. These are operated with a supply voltage of 120V and deliver output voltages of up to 33V RMS (Microtech Gefell MV203)

Overview of the maximum voltage (RMS) of the different preamplifiers

Output voltages of a typical microphone capsule

A typical measuring microphone has a sensitivity of 50mV/Pa. At a sound level of 94dB RMS, the output voltage is therefore 50mV RMS

The table below shows the output voltage for different sound levels

A preamplifier with an output voltage of 3V will therefore reach the limit sooner than the capsule itself. In the case of a digital system, the maximum input range of the AD converter must also be taken into account.

The ADC - Digitization

In general, the signal from the preamplifier is digitized by an ADC. However, these modules only have a limited voltage range. Either the ADC is designed for low noise, but then it will clip at higher levels. Or the ADC is designed for high levels, then the inherent noise increases.

Today's ADCs have a dynamic range of 120dB with very high circuit complexity. High quality devices typically use 100dB. Therefore, most sound level meters use different measurement ranges to cover a wide range of levels. 

MEMS microphones

As a micromechanical system, MEMS microphones have a completely different structure than classic condenser microphones. You can find them in all smartphones and similar devices today. Typical MEMS microphones had a relatively low maximum level of 110dB and high self-noise. However, MEMS microphones with a level of 138dB are now also available.

How can the maximum sound level range be extended?

1. Use of a less sensitive capsule.

2. (Electrical) attenuators (5,10,20dB) are available for 1/2" systems with 60UNS thread, which reduce the output voltage of the capsule. 

3. Acoustic dampers. The sound level at the microphone membrane is reduced by damping measures such as housing or absorbent materials. The problem here is that the directional characteristic changes. In addition, a sound level calibrator cannot simply be plugged in.

basic rule

In general, high sound levels can be measured more sensibly with smaller microphone capsules (1/4"), as these have a higher limit sound level and, due to their lower sensitivity (typically 1-5mV), do not overdrive the subsequent amplifiers so quickly.