Maximum Sound Pressure Level

What is Maximum Sound Pressure Level?

An absolute limit for maximum sound pressure which can be recorded by a microphone does not exist since there is no value at which the transducer would fail completely (at least within practical limits). The maximum sound pressure level is also not the point at which the microphone can be damaged - far before this is the case, the microphone begins to distort the signal.

The maximum sound pressure level is thus a defined sound pressure level in dB, at which the sound generates a distorted signal at the output of the microphone. "Distorted" means that harmonic distortion exceeds a certain threshold. The harmonic distortion, also referred to as THD or Total Harmonic Distortion, is given in % and describes the ratio of the sum of the powers ​​of the harmonic overtones (see Fig. 1) to the power of the fundamental. For the specification of the maximum sound pressure level, the threshold should be defined with a low total harmonic distortion of 0.5%.

Some microphone manufacturers provide maximum sound pressure levels at higher THD values, or omit information about THD altogether, which then leads to supposedly better values. Therefore, when comparing microphones it is crucial to observe not only the value of max SPL itself, but also the total harmonic distortion at which max SPL was taken. There can be large differences (5 - 20 dB are common) among these specifications.

The Measurement

According to the standard IEC 60268-4, the maximum sound pressure is measured by increasing the sound pressure of a purely sinusoidal sound (from a loudspeaker) until the distortion at the output of the microphone reaches the specified value. The measurement is taken at the incidence angle at which the strongest distortion occurs.

The problem with this method is the nonlinearity of the sound source and the air, which strongly influence the measurement. The measurement can therefore be carried out differently: distortion caused by the acoustic part of the capsule usually occurs only at much higher sound levels than in the electronics of the microphone amplifier behind the capsule. Thus, the capsule can be assumed to be ideal and instead, the maximum input level of the amplifier can be used to obtain this value. For this, the amplifier is fed with an electrical signal of increasing amplitude, until harmonic distortion at its output reaches the specified value. The voltage at which this occurs together with the sensitivity of the capsule in mV/Pa can be used to calculate the maximum sound pressure of the whole microphone.

For measuring the maximum input level of the amplifier, the same specified THD value must also be set. The maximum sound pressure level is a logarithmic value given in dB with the reference value of 20 μPa.

The load impedance, i.e. the input impedance of the device on which the microphone is operated when measuring the maximum sound pressure level, should correspond to the smallest manufacturer's recommended load impedance of the microphone.

In Practice

Typical values ​​of modern condenser microphones for the maximum sound pressure level at 0.5% THD lie in the range of 130 dB (e.g., SCHOEPS MK 4 + CMC 6: 131 dB). These are sound pressure levels that correspond approximately to the pain threshold of the human ear, i.e. very loud sound events, which occur only rarely when recording instruments. For example, a very close-miked snare drum produces sound levels in this range; a human voice at very close range only in exceptional cases. The vocal microphone V4 from SCHOEPS, with an even higher maximum sound pressure level of 144 dB at 0.5% THD, can even record a rifle shot from a distance of less than one meter without distortion.

For microphones with an amplifier whose maximum input level is lower, it is recommended to reduce the level before the capsule electronics with an attenuator (e.g. PAD 10 C). Since an attenuator reduces the sensitivity of the microphone and thus increases the noise, it should only be used for very loud sound events. In general, as a customer, you should be more careful when microphone manufacturers advertise a high limit sound pressure level. It should not be forgotten that even the sound itself is already heavily distorted at such high sound pressures.

If a sound event is very loud, in addition to the microphone itself, it can also cause clipping at the input to which the microphone is connected (such as the preamp or mixer), which is often mistakenly attributed to the microphone. In such case, it is recommended to generate a reduction of the level by means of a so-called "pad circuit". Ideally, this circuit is close to or in said input device. This reduces the amount of noise induced in the microphone cable relative to the useful signal.