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Measurement microphones generally
don't make good testing mikes. They have omni directional characteristics, so most of the
measurement is about the noise environment and room reflections rather than the
test speaker. In addition they are expensive, frequently need
dedicated pre-amplifiers and power supplies, and are just not robust enough for
the production environment. The linearity and long term stability that make a measurement microphone a measurement microphone are not needed with comparative testing. Changes in your microphone's sensitivity and linearity are accommodated by the periodic re-calibrations. Any reasonable quality microphone with a cardioid characteristic and an acceptably extended high frequency response is suitable. We used to recommend the Audio Technica ATM33a. Mikes with a hyper cardioid characteristic do give greater noise isolation but they tend to be less linear at high frequencies. We have found cardioid mikes the best compromise. Forget all about 1 metre on axis. To get the best signal to noise ratio the mike needs to be close to the speaker. To reduce the accuracy with which the test speaker has to be placed on its bench mark and to get a reasonable integration of the drivers in a system the mike needs to be placed away from the test unit. By cut and try you find the best solution for your environment. Try to arrange things so that systems fire horizontally and away from the operator. This arrangement minimises the effect of the operator on the measurement. Test the speaker in its "normal" position, ie with its bottom at the bottom, otherwise you will have scratched cabinets! Forget having the speaker on its back with the mike above. Guaranteed bad news - connecting up is a faff, and the operator's head becomes the biggest variable in the measurement process. |
When
production testing you need to think about the
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