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9.2 - MAXIMUM SPL

Maximum SPL is an indication of the maximum high level acoustic sound pressure that can by transformed by the microphone into an electrical signal before the onset of distortion. This distortion can be due either to the physical limitations of the capsule, or, to the initial amplification stage within the microphone. Unfortunately the max- SPL as expressed by each manufacturer is not always made within the same set of measurement conditions. As explained by Chris Woolf (paragraph ‘max SPL’ of “What the fields mean” on the Rycote Microphone Data), it is important to verify the “percentage of distortion” at which this maximum SPL is measured, different manufacturers will specify different percentage of Total Harmonic Distortion levels for the measurement of max-SPL, and, in many cases, do not indicate any %THD value at all!

For high quality studio microphones, a value of 0.5% THD would seem to be an acceptable common denominator. As suggested by Chris Woolf, an approximation to 0.5% THD can be obtained by reducing the max-SPL specified for 1% THD, by 6dB. For microphones without any indication of %THD in the measurement of max-SPL, the user would be excused for assuming that the ‘max SPL’ specified is just before drastic clipping. Let us hope that manufacturers can be persuaded to be more explicit in their specifications. For instance, measurement at two values of THD (0.5% and 2%) might clarify the situation for most users. Obviously the origins of this distortion will differ considerably depending on the type of microphone under consideration. In fact with electro-dynamic microphones, whether moving-coil or ribbon, the maximum SPL is usually not specified. Except for electro- dynamic microphones used in very special circumstances, this characteristic is therefore only of interest in the analysis of the performance of electrostatic microphones.

Our transformation of this ‘max SPL’ value is again a process of establishing the correspondence between all the different measurement units, be it in the acoustical or the electrical domain. This time, as the characteristic is expressed in “Pascals”, we need to establish the equivalent value in ‘SPL’ in the acoustical domain, and in ‘mV’ and ‘decibels’ in the electrical domain, in order to have an overall view. Let us take as an example a max-SPL value of 100 Pa. This is some 100 times or 40dB greater than the standard acoustic stimulus value (1 Pa or 94dB SPL) for the measurement of output sensitivity.

claculmation of a maximum SPL corresponding to 134dB

Max-SPL could therefore also be specified as 134dB SPL (94dB SPL + 40dB).

shows relation between maximum SPL in the acoustical domaine and the reference output sensitivity

This value can also be expressed in the electrical domain. The output voltage generated by this high level acoustic stimulus will also be 100 times greater or 40dB above the output sensitivity value. If this microphone has an output sensitivity of 7.75mV/Pa or – 40dBu/Pa (the example used previously) the ‘max SPL’ would generate an output voltage of 775mV or 0dBu. This is clearly shown in the following diagram :

shows relation between maximum SPL in the acoustical domaine and the electrical domaine w.r.t. the reference output sensitivity

It is now a simple matter to check the specification of the next amplification stage to see if it will accept such a voltage level without generating distortion.