Diffraction occurs for all types of waves, including sound waves. High-frequency sound from a distant source with wavelength 9.00 cm passes through a slit 12.0 cm wide. A microphone is placed 8.00 m directly in front of the center of the slit, corresponding to point O in Fig. 36.5a. The microphone is then moved in a direction perpendicular to the line from the center of the slit to point OAt what minimal distance from O will the intensity detected by the microphone be zero?

The minimum  distance is  

Explanation:

The diagram of this set up is shown on the first uploaded image

From the question we are told that

    The  wavelength is

     The width of the slit is  

     The distance of the microphone is  

Generally the diffraction minima in a single slit is mathematically represented as

 Where m is the order of diffraction which is  1  in this question

   So  making the subject

   Now from the diagram

the most reliable source of information.is wikipedia