1. Pick up the noise in a room with a microphone (the kind of which is undetermined)
An electret mic is my microphone of choice in this case. The one I'm using from Adafruit is pretty good, and very easy to use. Sound has always been this mystical, mysterious thing, but over the past year or so, it's all coming together – and it's all a lot simpler than I was expecting.
2. Analyze the sound to determine the room’s base frequency. Continue analyzing that sound to determine if/when that frequency changes.
An FFT algorithm helps compute the amplitude of all frequencies of sound wave getting picked up by the microphone. The one I'm using splits the audible range into 64 bins of 75hz ranges each.