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Japan Aerospace Exploration Agency

Noise measurement technologies for wind tunnels

Noise reduction technologies represent one of the biggest demands on today’s aircraft development. Aerodynamic noise normally comes from several locations on an airframe. Although measuring airframe noise with a single microphone makes it possible to evaluate the overall noise level, this approach does not allow researchers to pinpoint exact locations of noise source or the amount of noise emitted from each of them. Therefore, JAXA developed a technology that, using about a hundred microphones simultaneously, identifies noise source locations and sound pressure levels based on how long it takes a given sound wave to propagate from its noise source to each microphone in the array.

Noise source identification technology in a 6.5m x 5.5m low-speed wind tunnel

Sound pressure level distribution from a high-lift half model

Wind tunnel test engineers normally use scale models, which means that, in comparison to actual aircraft, the frequencies of the aerodynamic noise shift toward the high-frequency range in inverse proportion to the scale ratio. To perform evaluations on regulated frequencies with actual aircraft, therefore, one would need to have evaluation technologies capable of obtaining noise data in high-frequency bands. Reducing aerodynamic noise also requires accurate measurement of the sound pressure level of noise which each noise source produces.

JAXA has already developed technologies that can identify the noise source locations in frequency bands up to 80 kHz (the maximum frequency for 1/8 scale models to predict noise emitted by actual aircraft). By using image-processing methods to create high-resolution images of sound pressure distributions and performing simultaneous acoustic and optical measurements, JAXA will develop technologies for identifying the aerodynamic phenomena that noise can cause.