Two fire alarms are placed in a building, one 3.1 m north of the other. A person sitting at a desk which is 51.2 m east and 9.9 m north of the point mid-way between the alarms does not hear them when they sound. What is the lowest frequency that the alarms can be sounding at assuming that the alarms are both perfectly in phase?
[speed of sound in air=330m/s]
For real fire alarms dead zones like this are not a problem, why?