11/01/15

TESTING MICS AGAINST WIND NOISE

This article presents how handheld microphones are tested against wind noise. It highlights the difference in performance between pressure and pressure gradient mics in windy situations.
By Eddy B. Brixen, audio specialist

The purpose of a microphone is to catch the desired sound and nothing else — not wind, not handling noise.

The displacement of air molecules caused by a sound wave is rather small. In the case of normal speech, at a distance of 1 meter, the sound pressure level (SPL) is in the range of 60 dB. This sound pressure will only cause a displacement of the microphone membrane in the range of 0.0001-0.001 mm. In contrast, wind often creates considerably more movement. This means that wind-generated noise is much louder than speech, which makes it difficult for the microphone to pick up the desired sound.

So the art of designing microphones, especially for outdoor use, is about getting the desired sound as clear and natural as possible. At the same time, unwanted sound generated by wind should be reduced.
 

Pressure mics perform better in wind

Omnidirectional (pressure) microphones are, in general, less sensitive to wind compared to directional (pressure gradient) microphones. The difference is based on design — the pressure mic’s membrane is less compliant than that of the pressure gradient’s. The higher stiffness of the pressure mic’s membrane provides approximately 10 dB lower sensitivity to wind noise compared to the pressure gradient mic.
 

Generating wind in the lab

A microphone must eventually prove its worth in the field, but it is of course necessary to build test facilities in the lab as well.

There are different ways to generate wind in a test facility. The most commonly used methodology, which is also suggested by the IEC standards organization, is to use a ventilation device with an axial fan. The fan is programmed to generate various wind speeds. However, generating wind in this way is loud. The fan, the motor driving it and the controlling mechanism all make noise. To reduce this noise, the fan unit is placed in a remote room and the wind is channeled through an acoustically-damped duct.
 

Measuring wind data

Microphone manufacturers often discuss whether the wind generated should be chaotic or just a laminar (streamline) flow. In the development process a laminar flow is preferred, as it easier to investigate how a microphone will react to wind from different directions — one direction at a time. In this way, it is possible to produce ‘polar patterns’ of the wind’s influence on the mic from different directions. If there is a need for a single wind sensitivity number, summing the noise figures of all measured directions is one solution. Alternatively, the data of the worst direction can be presented.
 

d:facto™ and the wind

While developing the d:facto™ Vocal Microphone Series, DPA Microphones has looked into the mechanisms that generate unwanted wind noise and has performed intensive testing. The test results show that the two versions of d:facto™ provide advanced performance in windy situations:

d:facto™ Vocal Microphones are less sensitive to pop-noise compared to competing mics
d:facto™ Interview Microphones are less sensitive to wind noise than competing mics

When designing the d:facto™, attention has been placed on stopping the wind as far from the capsule as possible by introducing various grid designs. These protective grids are designed to let sound pass on to the microphone capsule.
 

References

Brixen, Eddy B. High wind and rain. AES Convention 119 (2005). Paper No. 6624.
Brixen, Eddy B.; Hensen, Ruben: Wind generated noise in microphones – an overview – Part 1. AES Convention 120 (2006). Paper No. 6635.
Brixen, Eddy B.: Wind generated noise in microphones – an overview – Part 2. AES Convention 121 (2006). Paper No. 6879.

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