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9.3 - THE NOISE FLOOR
In the Rycote presentation of data, the specification of the noise floor is called 'Self Noise' i.e. the noise generated by the microphone system. In practice this is applicable to the noise generated by any internal amplification system incorporated into the microphone housing, usually the microphone preamplifier. It is unfortunate that the use of only this definition of the noise floor excludes the measurement of noise induced in the electrodynamic microphone due to surrounding magnetic fields - typically mains induced 50/60Hz 'hum'. This characteristic is, of course, of particular interest in the construction of a quality moving coil or ribbon electro-dynamic microphone.
Nowadays Self Noise in an electrostatic microphone is mostly generated by the microphone capsule, however it is unfortunately rare to see the self noise measured with respect to the capsule and the preamplifier together. The specified 'Self Noise', usually generated by the preamplifier in an electrostatic microphone, is expressed as 'an equivalent acoustic noise value'. In other words, the level of acoustical stimulus that will produce an electrical signal equivalent in the microphone to the electrical noise generated by the microphone. This is obviously a good representation of the noise floor of the microphone, when making a comparison with the acoustic noise level in the studio, or with the lower end of the dynamic range of a musical instrument or other type of sound source. However this does not help us to position this lower limit of the dynamic range in relation to the following amplification stages in the recording chain. So again we must transform this 'Self Noise' value into the other useful measurement units in the acoustical and electrical domain.
The subjective impact of noise also depends very much on the eventual level at which the noise is heard. Low level noise will not create the same subjective impression as noise at a higher level. In particular our reduced perception of low level noise in the bass frequency range (as shown by the psycho-acoustic measurements of hearing sensitivity published by Fletcher & Munson, Dadson & King, ..) would seem to indicate that the CCIR-468 measurement of the 'Self Noise' characteristic would be more appropriate to basic operational conditions, as it is a good approximation to the reception curve of the human ear at very low sound pressure levels. The DIN/IEC "A- weighting" measurement (db-A) measurement is closer to hearing reception at levels of about 40dB SPL or more correctly 40 phons. However, both the CCIR and the DIN/ IEC specification of 'Self Noise' have been supplied by certain manufacturers.
Again we must transform the specification of this specific characteristic into our usual set of measurement values in the acoustical and electrical domain. Let us consider that our example microphone has a 'Self Noise' value of '14dB SPL'. This is 80dB lower than the standard acoustic stimulus value for the measurement of output sensitivity, or 10000 times lower than 1Pa i.e. at 10-4Pa. So we would expect the output voltage of self noise from the microphone also to be some 80dB below the output sensitivity value, or 10000 times less. The Output Sensitivity of our hypothetical example microphone is 7.75mV/Pa or -40dBm/Pa, therefore the 'Self Noise' voltage would be:
0.775µV (7.75mV divided by 10000)
or -120dBu (-40dBu minus 80dB).
This is shown clearly in the following diagram:
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Max-SPL could therefore also be specified as 134dB SPL (94dB SPL + 40dB).
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If we now bring the maximum SPL calculation together with the noise floor calculation we can now see the total dynamic performance of the microphone:
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