2 Infrasonic filter
Many of the infrasound stations in the global network are built as arrays to filter out unwanted low-frequency noise caused by atmospheric turbulence. An array of pipes connects sound inlets (red circles) to a microbarometer at the centre of each element, and the entire device covers an area of about 40 × 40 m. The background shading represents variations in air pressure caused by an infrasonic signal travelling from the right. When the signal from each element is summed, noise (circular arrows) is largely incoherent between inlets and is therefore averaged out. The signal of interest, however, varies gradually across the filter and is largely retained in the sum.