You and a friend are talking on the lawn outside of a musical performance hall. You hear a sustained pure frequency of a sound wave due to someone inside the hall testing the acoustics. The sound exits a single open doorway of width 1.23 m and makes its way to you. Your friend pulls out his smartphone and uses an app to instantly determine the frequency of the sound. He challenges you to guess the frequency. Not having the same app, you can't do the same thing on your smartphone, but you can apply your physics understanding. You walk over until you are standing straight out and far away from the open door on a line perpendicular to the wall of the building. You take a reading on your smartphone compass. Then you start walking in a circular arc centered on the open doorway. At each point on the arc, you can aim your smartphone at the door, perform a compass reading, and convert that to an angular position along the arc measured from the line straight out from the doorway, which you call 0°. You find that there are minima in the sound intensity at angles of 17.1°, 35.9°, and 61.6° from the line straight out from the doorway. You perform a calculation on the calculator app on your smartphone and proudly tell your friend what the frequency of the sound is (in Hz). What would be the frequency?