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9.4 - A WORKED EXAMPLE

To pilot you through your first set of calculations, I will take as a worked example the microphone ‘Neumann U87’, after that, you are on your own.

The output sensitivity specified in the Neumann U87 data sheet is 28mV/Pa. So the output sensitivity entry becomes :
output sensitivity of a Neumann U87

output sensitivity of a Neumann U87

Max SPL is specified as 117dB SPL. This is 23dB higher than the reference level for the measurement of output sensitivity of 94dB SPL
117dB SPL – 94dB SPL = 23dB

The start of our max SPL line is now :

output sensitivity of a Neumann U87

There are two ways to find the equivalent value of this SPL in Pa, either to transform 23dB into a ratio :
ratio = 10 (23 / 20) = 14.1
therefore 1Pa multiplied by 14.1 = 14.1 Pa

or to transform 117dB SPL into Pa with respect to the reference 0dB SPL of 2 x 10-5 Pa:
max SPL in Pa = (2 x 10-5) x 10 (117 / 20 ) = 14.1 Pa


The specification chart reads as follows:

shows acoustical domaine for a U87

We can use both the ratio of 14.1 and the decibel difference value of 23dB to fill in the values in the electrical domain.
shows acoustical domaine and electrical domaine for a U87

The noise floor (self noise) is specified as 23dB, this is 71dB less than the reference level for the measurement of output sensitivity of 94dB SPL, equivalent to a ratio of 3548.
94dB SPL – 23dB SPL = 71dB ratio = 10 (71 / 20) = 3548

shows noise floor in the acoustical domaine for a U87

Both the ratio of noise floor value and the decibel difference can be used to complete the self noise entry line:
shows noise floor in the acoustical domaine for a U87


The total dynamic range is now simply a matter of calculating the number of decibels between the Max SPL and the Noise Floor.
Total Dynamic Range = 23dB + 71dB = 94dB

The calculation of a ‘specification chart’ (as illustrated above) for each individual microphone, facilitates the comparison between the different microphone specifications. The total dynamic range available for each microphone is of course an important factor in the selection process. However the race for the microphone with the most spectacular specification of dynamic, should not outweigh a balanced appreciation of all the other characteristics.

MATCHING

The total dynamic range of the sound source must be carefully matched to the microphone’s own dynamic range, and then on to the next amplification stage, usually the input module of the mixing desk. This would seem obvious with respect to the sound source working into a microphone, but the use of the sensitivity adjustment on the input module seems to be still rather intuitive, not to say haphazard. Careful consideration must be given to the sensitivity settings of the input module so as to make optimum use of the microphone’s wide dynamic range. Too low a sensitivity will need extra amplification at a later stage, and therefore increase the level of the noise floor, whereas too high a sensitivity will increase distortion at high signal levels and may introduce clipping. This is particularly important in analogue mixing desks which can have a dynamic range inferior to 100dB. Digital mixing desks with either 20bit or 24bit digital resolution will have a theoretical dynamic range of about 120dB and 144dB respectively - this is not a reason to squander the advantages, by not matching the dynamic range of the sound source (through the microphone) to the input stage.

OTHER CHARACTERISTICS

Many microphones are made for a specific type of utilisation. A hand held voice microphone being a common example. This type of microphone, usually pressure- gradient, generally compensates for the proximity effect by a low frequency roll-off. In addition the microphone must have some efficient anti-plosive protection and be insensitive to noise transmitted through the microphone housing (handling noise). This type of characteristic although not shown in the Rycote data sheets may possibly be indicated in the short description that accompanies the specification of each microphone. And of course another important characteristic is of course - reliability! In the field of outside broadcasting and news gathering, reliability is sometimes more important than any other characteristic. Would it be too much to hope for that some day an indication of a microphone’s sensitivity to shock, temperature and humidity could be published. This type of information would probably not go amiss in the studio context also.

The next and last section is a blank specification chart that should help you with your own calculations.