Meteor Infrasound

The Elginfield Infrasound Array (ELFO)

The Wind Shelter and Wind Noise


Wind noise generically affects all infrasound measurements. The noise is generated by turbulence at all scales close to the microphone. Shown below is the average over all elements for the first half of the year of the noise at ELFO relative to the International Monitoring System Arrays as described by Bowman et al (2005).



Most of the noise is generated by wind. Shown below is the relative noise averaged over one hour blocks when different wind speeds prevailed across the ELFO array. A clear dependence with wind speed is evident and underscores the need to isolate the microphone from wind as much as possible. Only at the lowest wind speeds are microbaroms (peaking near 0.2 Hz) easily detected by the array, attesting to the generally noisy site characteristics.

Common approaches to wind reduction are use of hollow pipe arrays as is commonly used at IMS arrays. We have used porous garden hoses as a cheap (and effective) alternative. However, wind noise is still significant, even with such hoses. In an attempt to counteract the noise a wind shelter was built in August 2007 and is located at the Northwest Element. Its main purpose is to protect the element from ambient wind noise that can interfere with the infrasonic signal that is being picked up by the element, generating crude and usually inaccurate results. However, with the implementation of the wind shelter at the element, the wind noise can be drastically reduced, yielding better results.

There are many wind shelter design suggestions and if the signal-to-noise ratio (SNR) is to be maximized, it is essential that the wind shelter is optimized for better performance to reduce the noise received by the element. For the best comparison between different noise shelters, they would all need to be tested in the same controlled environment.




How it Works

The wind shelter that is shown above has a sloping design, allowing the wind to move up and out of the way where the element will not detect its interference. This ensures that the infrasound signal is not mixed with any interference the wind may create.



Figure 1