The RTA420 microphone is "pin 3 hot", what does that mean?

The Rational Acoustics RTA 420 microphone (listed on our store here) is Pin 3 hot. This means that the polarity of the microphone is reversed compared to other microphone standards.

If you are using a single microphone, this should not cause any differences in your measurement.

However, if you will be comparing a microphone that is not pin 3 hot to the RTA 420, this is something to be aware of and you will need to make adjustments accordingly. 

For more technical details, refer to the following article: https://en.wikipedia.org/wiki/XLR_connector#Three-pin_in_audio_use