The Shure SM91 is a dynamic microphone in the form of a boundary microphone, also known as PZM (pressure zone microphone.) The Shure SM91 and mics like it are designed to be placed on top of or near the signal you want to pick up and will pick up almost everything around it. Placing this microphone on the floor will pick up everything in the room, making the Shure SM91 also great for conferences and for recording speech.
I’ve been using the Shure SM91 for a number of years and it is probably my favorite PZM microphone because of the wide range it has in terms of the sound it picks up. It comes in crystal clear and I usually use these mics are room mics, or to record kick drum and as a supplement microphone for recording everything and anything. I also have great results with the Shure SM91 for recording acoustic pianos by putting the microphone inside of the piano. In addition to this I’ve gotten some pretty awesome and interesting sounds by taping the Shure SM91 up onto a Leslie cabinet for recording B3s are other organs. No matter what you are recording with the Shure SM91, I find that I am usually happy with the results. The price of the Shure SM91 is pretty reasonable, but I would really only recommend it to professionals or if you have a very specific use for this, as there are other microphones that I would recommend before this if you are just starting up your microphone collection. You know you can trust what you are getting from Shure, as they have a long track record of making great microphones at reasonable prices. For what it is, the Shure SM91 is just about as good as you can get and must be tried if you are in the market for a PZM mic.