{"id":633,"date":"2013-08-10T18:57:11","date_gmt":"2013-08-11T01:57:11","guid":{"rendered":"https:\/\/inkofpark.wordpress.com\/?p=633"},"modified":"2024-09-01T20:10:24","modified_gmt":"2024-09-02T02:10:24","slug":"that-noisy-fancalibrated-measurement","status":"publish","type":"post","link":"https:\/\/www.inkofpark.com\/?p=633","title":{"rendered":"That Noisy Fan\u2013Calibrated Measurement"},"content":{"rendered":"

I am working with the freetronics microphone module<\/a>, which is described as having a sensitivity of \u201c-40 dB typical\u201d\u2014let us assume it is dB (SPL) referenced to 20 micropascals root mean square (rms) at 1 KHz. When I think of an electronic element\u2019s sensitivity, though, I\u2019m thinking volts per micropascal and this is not provided.<\/p>\n

The microphone\u2019s schematic suggests the SPL measurement is the rms average over 3 milliseconds, and that the signal is proportional to rms pressure level. This suggests a log scale for display (a change from previous work).<\/p>\n

The upshot is that the fan generates about 62 dBA; but to get that conclusion, I had to perform a spectral correction to match measurements from a more calibrated sensor.<\/p>\n

\"sound_level_dba\"<\/a><\/p>\n

Remember that I made the measurements with the RIMU data logger I built. That data logger has a digital low-pass filter on the microphone SPL. The low-pass filter has much too slow a response, but the basic result is OK. The RIMU, shown in the next picture, is the instrument.<\/p>\n

\"20130803-24\"<\/a><\/p>\n

I borrowed an SPL meter from my father (thanks!). It is, unfortunately, a C-weighted measurement, measuring in dBC. The C-weighting is occasionally a very useful measurement of sound level. Most measurements are done A-weighted, which is similar to human hearing. My challenge is to convert a measurement made with an unweighted microphone in arbitrary units to A-weighted measurement in dBA. The answer is to take a measurement with the instrument in dBC, and record the sound a with a sound recorder, figure out what converts C-weighting to A-weighting for this signal.<\/p>\n

\"20130803-25\"<\/a><\/p>\n

In the spectra below you can see that the spectrum recorded without weighting, by the Zoom recorder. I then applied an A weighting and a C weighting. What\u2019s important is the conversion between the rms value for A and the rms value for C, which is a 6 dB correction in one case, and a 13 dB correction in the other.<\/p>\n

\"fanandwater-psd\"<\/a><\/p>\n

 <\/p>\n

\"fanonly-psd\"<\/a><\/p>\n

So, to rehash the steps<\/p>\n