The diagram accompanying this post shows a cylindrical column bored at right angles to a flat surface. Let’s say the material is a block of brass with a hole drilled in it. The arrow represents a fast moving stream of compressed air. As the air passes over the top of the column a high pitched sound is generated. The question is what determines the pitch of the sound produced, is it behaving like a Halmholtz resonator or is it behaving like a closed ended resonant air column, what equation can be used to accurately determine the frequency produced? Consideration: Take a drinking straw and insert it into a glass of water. Now blow across the top open end of the straw and the water will rise up the straw. The harder you blow the grater the vacuum generated thus the higher the water rises (venture effect). The venture effect in the above set-up generates a vibrating vacuum which is responsible for the sound generated.