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    Mic Test

    A post on the forum showed how to make an automated microphone test in Tractor last week.

    Recall Tractor is a standalone program used to drive QA401 (and QA450, etc) applications into working as one to automate testing.

    Although the above should work well in a production environment, if you just need to compare two microphones (and a known-good microphone versus a test unit) you can use the QA401 application.

    Below, we'll use the QA401 application standalone to graph the comparison between two microphones using the "MIC Compare To Reference" plugin.

    Configuration of the Hardware

    We'll start with the same hardware setup used in the forum post mentioned earlier. But here's the connection diagram again:

    In accordance with the forum post, a good acoustic environment is needed for testing.

    Essentially, you need a full-range speaker with your two microphones (one a known-good reference and another DUT mic) axially located in the far-field of the speaker.

    When your speaker is 6 inches in diameter, the far-field begins at roughly 5 times that diameter. That means you should position your mics at least 30 inches from the speaker.

    The mics should be centered horizontally and vertically along the speaker's axis.

    To begin this test, we will use two Earthworks M23R microphones. This is a fairly pricey (*US$650) reference mic that's tuned to be ruler flat by mic standards (+/- 0.5 dB).

    In the photo below, you can see the microphones are oriented 1 meter in front of the full-range speaker.

    Note that both mics are trying to measure the same "piece" of air. From the top, you can see this better. The mics are very close together.

    By using the MIC Compare To Reference plug-in, we are able to make a measurement with the hardware setup.

    Let's first reset the QA401 Analyzer software. As a result, we can start from scratch in terms of settings:

    Then bump up the FFT size to 32K by using the up/down buttons in the ACQ SETTINGS box:

    Using the Test Plugins menu option, select the "MIC CompareToReference" plugin:

    Set your plug-in settings similar to what is shown below:

    The test should run and you should see a graph created by the plug-in with the result. With some axis adjustments, that result is shown below for the dual M23R:

    Let's analyze what we're seeing above. The first thing to notice is that one of the M23R mics appears about 0.4 dB "hotter" than the other. The factory cal data on the mics was 36.1 mV/Pa and 35.9 mV/Pa. This is a 0.05 dB discrepancy between the mics. Swapping the QA470 mic preamps (that is, swapping the mic XLR connectors going into the QA470) inverted the curve above.

    That is, instead of a few tenths below zero it shifted to a few tenths above zero. If the mic sensitivity were the same and the QA470 gains were different, then you'd expect the curve to stay the same. But since the curve is inverted, it suggests that the measured difference is coming from the mics. So what is going on?

    There are two calibrations provided with the mic. The first is from a Norsonic 1251. This is a sound level calibrator that generates a 114 dB SPL (+/- 0.2 dB) signal at 1 kHz. The mic is brought into tight contact with the calibrator, and sensitivity can be read. As indicated above, the near-field sensitivity difference between the mics is vanishingly small: just 0.05 dB.

    The second calibration is an "On-axis free-field frequency response." This is a graph that shows the free-field response presumably relative to the response measured by the calibrator.

    A zoom-in of the free-field calibration data for the two mics is shown below: In the plot above, notice the upper mic response at 1 kHz is nearly 3 "dots" above the 0 dB line. Let's say 2.9 dots.

    The lower mic response is nearly 4 "dots" above the 0 dB line. Each dot is roughly 0.6 dB, and so a 1 dot discrepancy would equate to about 0.6 dB. Thus, we can see from the supplied free-field calibration data that indeed there is a ~0.5 dB difference in the mics at 1 kHz and thus the measurement we're seeing is likely correct.