This paper deals with nonporous windscreens used for reducing noise in infrasonic measurements. A model of sound transmission using a modal approach is derived. The system is a square plate coupled with a cavity. The model agrees with finite element simulations and measurements performed on two windscreens: a cubic windscreen using a material recommended by Shams, Zuckerwar, and Sealey [J. Acoust. Soc. Am. 118, 1335-1340 (2005)] and an optimized flat windscreen made out of aluminum. Only the latter was found to couple acoustical waves below 10 Hz without any attenuation. Moreover, wind noise reduction measurements show that nonporous windscreens perform similarly as a pipe array by averaging the pressure fluctuations. These results question the assumptions of Shams et al. and Zuckerwar [J. Acoust. Soc. Am. 127, 3327-3334 (2010)] about compact nonporous windscreens design and effectiveness.

The detection capability of the infrasound component of the International Monitoring System (IMS) is tightly linked to the performance of its wind noise reduction systems. The wind noise reduction solution implemented at all IMS infrasound measurement systems consists of a spatial distribution of air inlets connected to the infrasound sensor through a network of pipes. This system, usually referred to as “pipe array,” has proven its efficiency in operational conditions. The objective of this paper is to present the results of the comparison and validation of three distinct acoustic response models for pipe arrays. The characteristics of the models and the results obtained for a defined set of pipe array configurations are described. A field experiment using a newly developed infrasound generator, dedicated to the validation of these models, is then presented. The comparison between the modeled and empirical acoustic responses shows that two of the three models can be confidently used to estimate pipe array acoustic responses. This study paves the way to the deconvolution of IMS infrasound data from pipe array responses and to the optimization of pipe array design to IMS applications.

Wind noise reduction (WNR) is important in the measurement of infrasound. Spatial averaging theory led to the development of rosette pipe arrays. The efficacy of rosettes decreases with increasing wind speed and only provides a maximum of ~20 dB WNR due to a maximum size limitation. An Optical Fiber Infrasound Sensor (OFIS) reduces wind noise by instantaneously averaging infrasound along the sensor's length. In this study two experiments quantify the WNR achieved by rosettes and OFISs of various sizes and configurations. Specifically, it is shown that the WNR for a circular OFIS 18 m in diameter is the same as a collocated 32-inlet pipe array of the same diameter. However, linear OFISs ranging in length from 30 to 270 m provide a WNR of up to ~30 dB in winds up to 5 m/s. The measured WNR is a logarithmic function of the OFIS length and depends on the orientation of the OFIS with respect to wind direction. OFISs oriented parallel to the wind direction achieve ~4 dB greater WNR than those oriented perpendicular to the wind. Analytical models for the rosette and OFIS are developed that predict the general observed relationships between wind noise reduction, frequency, and wind speed.

Infrasound signals are detectable from many different sources, such as earthquakes and man-made explosions. Wind-generated turbulent noise can mask incoming infrasound signals; however, pipe-array wind-noise-reduction systems (WNRSs) have been designed to reduce the level of noise in the observed pressure time series. Given that the arrival times of the signals need to be well-known to calculate the source back azimuth and trace velocity, the response of the WNRS must be known in magnitude and phase. Previous work has been performed to optimize these systems and effectively model them. The goal of this research is to determine the effects of different defects which may occur during normal operation in typical field-experiment conditions. The models were extended to include the effects of defective systems, such as blockages or leaks. It was found that these models could effectively recreate the responses observed in an experimental setting, and several different defects were tested and are summarized in this paper.

<p>In order to improve the confidence in the results of measurements carried out in the field, on-site metrology is a key step. With the medium-term objective of being able to deploy a portable metrology system on different infrasound stations, CEA-DAM has tested an innovative system for calibrating its infrasound sensors. The first tests were conducted in November 2019 and September 2020 as part of the installation and certification of the IMS IS25 infrasound station in Guadeloupe. A total of 20 microbarometers were qualified on site.<br>We present the equipments deployed, the methods used and the results of the measurements carried out. It appears that the preliminary results show a very good correspondence between the measurements performed in the field, under particular environmental conditions, and the measurements performed in the metrology laboratory. The method will be confronted to the metrology community within the framework of the European Infra-AUV project in 2022.</p>