Aerodynamic measurements: Boundary layer components

 

Understanding laminar flow, turbulent flow and the transition region

As examined in the What is the difference between acoustic and aeroacoustics? article, the studies of aerodynamics and aeroacoustics are bound together. Using microphones, surface- or flush-mounted, in a wind tunnel focuses on measuring the characteristics of air pressure in the boundary layer of an object in flow through a medium (or the medium around the object).

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Public service announcement for small-diameter microphones:

 

Remove the grid for better data

Measurement microphones are highly specialized and fine-tuned devices designed for sensing minute dynamic variations in the ambient pressure. These variations are so small that measurement microphones use a very thin (only a few micrometers thick) metal foil as a diaphragm to be able to sense those variations. The thinness of the diaphragm allows it to deform even when the excitation is extremely small, making the detection of pressure variation possible even at very low sound pressure levels.

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From ¼″ to ⅛″ microphones

 

Why so small?

The Accuracy at high frequencies is vital, and... post, deals with the benefits of using a ¼″ microphone compared to the more commonly used ½″ mics and measurement scenarios where those benefits outweighed the drawbacks, or where the drawbacks weren’t relevant. The obvious question, of course, is why stop at the ¼″ microphone? If there are benefits to smaller microphones, and the areas of interest in noise mitigation and sound quality trending to hi-res and higher frequencies, how small can microphones get and still be viable?

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Accuracy at high frequencies is vital, and...

 

Microphone size matters

In any measurement scenario, the measurement equipment and the measurement environment interact in two ways. The first and simplest way is how the microphone ‘adds’ data to the environment, and the second is how the microphone ‘extracts’ data from the environment. And size has a great impact in both directions.

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