Elsevier

Applied Acoustics

Volume 78, April 2014, Pages 11-18
Applied Acoustics

Technical Note
Uncertainties of the frequency response of wet microphone windscreens

https://doi.org/10.1016/j.apacoust.2013.10.005Get rights and content

Abstract

During outdoor measurements, specific windscreens are often used to prevent rain water to penetrate into the microphone and to damage its components, but some water can nevertheless remain in the foam of the windscreen. This paper presents an extensive experimental study of the influence of water in the foam of a windscreen on its frequency response. It is concluded that the presence of water inside a windscreen can significantly change the sensitivity of the system only for rain amount higher than 1 mm and for frequency above 1 kHz. Some disparities can be found between the different types of windscreens. Bias correction values, together with uncertainty estimations are given for different types of windscreens and an estimation of the duration during which correction and uncertainty must be applied is also proposed for several types of common windscreens.

Introduction

Environmental noise measurements often take place over long periods during which acoustic sensors and their outdoor protection are exposed to rainfall. Such situations are very common for example in noise monitoring stations around transport infrastructures. In standards, the influence of rain is only considered by the rain-induced noise that can strongly degrade the signal to noise ratio of the measurement, and common practice is to invalidate the data recorded during a period of rain. Unfortunately, very little information can be found in the literature on the influence of the water that can remain inside the foam of the windscreen, after a rain period, on the frequency response of the windscreen. Currently, this influence is ignored but it could lead to a larger uncertainty in the measurement results.

Very few authors have addressed this issue: Nelson and Godfrey [1] has concluded that there is no significant difference between the frequency response of a wet windscreen and of a dry windscreen. This conclusion was nevertheless based on an experimental study that was concerning only one type of windscreen, that no longer exists now, and only road traffic noise was considered. Moreover, no information on uncertainties is provided in this study.

The purpose of the present paper is to investigate: (a) if the presence of water inside the foam of a windscreen has a significant effect on the frequency response of a windscreen, (b) the significant parameters that could influence this effect (rainfall rate, amounts of precipitation, type of the windscreen …), (c) the uncertainties on an acoustic measurement result to be taken into account for this effect, and finally (d) how long this effect remains after a rainfall period.

An extensive experimental study has been made in the facilities of the Laboratoire National de métrologie et d’Essai (Trappes, F) in order to measure the change in system sensitivity for measurement conditions consistent with dry and wet windscreen. This campaign includes many types of currently available commercial windscreens. The results given here do not depend on a specific environmental noise source and can be applied for any kind of environmental noise measurement for frequencies from 80 Hz to 10 kHz.

The first section deals with the relative frequency response of a wet/dry windscreen and with parameters that can influence this response. The second section presents how to correct measurements in order to take into account the influence of water inside the foam, and also the associated uncertainties due to disparities in windscreens. Many experimental numerical values are provided for practical applications. The last section provides some information about the time span during which correction and uncertainties must be considered after a rain period.

Section snippets

Uncertainties

Depending on frequencies, the water that remains inside the foam can significantly change the frequency response of the windscreen (see Section 2.3). This change may induce a bias in the acoustic measurement result that may not be negligible when considering an uncertainty budget. This bias is also associated with an uncertainty coming from the variability on the frequency response for each type of windscreen, or from the (small) effect of the height of precipitation and of the rainfall rate.

How long is it necessary to take into account uncertainties?

After a rainfall period, the uncertainty must be considered as long as the water inside the foam significantly influences the acoustic response of the windscreen. This duration strongly depends on the drying time of the foam.

An experimental study of the drying behavior of every windscreen has been done with the help of an environmental chamber and of the standardized system of artificial rain generation. Simple empirical relations of the normalized mass of water remaining inside the foam along

Conclusion

An experimental campaign has been carried out in order to investigate the acoustic response of several types of wet windscreen microphones. The effect of the water impregnation of the foam is significant only for frequencies above 1 kHz and for amounts of precipitation above 1 mm. This effect leads to a bias between the wet and the dry situation, and also to a larger uncertainty of the acoustic response of a windscreen. This effect depends on the type of windscreen: the bias can be up to 2 dB for

Acknowledgements

This research was partly financially supported by the French Ministry of Sustainable Development (DGPR, SETRA, IFSTTAR). The authors are grateful to the members of the working group on environmental acoustics uncertainties of the French Association for Standardization (AFNOR) for their valuable comments and especially to M. Lecollinet for helpful discussion about uncertainties. The authors also thank Mr Durocher (LNE) for help on the experimental implementation of ISO/IEC 61672-2.

References (6)

  • P.M. Nelson et al.

    A note on the acoustic sensitivity of microphone windscreens

    Appl Acoust

    (1978)
  • French Standard Pr S 31-115. Guide pour l’évaluation des incertitudes de mesurage en acoustique de l’environnement....
  • International Standard ISO/IEC 61672-2:2003. Electroacoustics – Sound level meters – Part 2: Pattern evaluation tests...
There are more references available in the full text version of this article.

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