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Despite the use of windscreens, the measurement of ambient sound levels or noise emissions in quiet environments can be adversely affected by wind blowing over the microphone. This is especially true when environmental impact assessments are being carried out for proposed wind turbine power projects where the objective is to determine the level of background masking noise available as a function of wind speed, since any potential noise impact from the project will only occur under moderately windy conditions. Under calm conditions the project will produce no noise at all. A number of windscreen products are commercially available for short and long-term sound level monitoring in adverse weather conditions. Generally, these windscreens vary by physical size and the method of preventing water from reaching the microphone. High frequency attenuation effects are usually available from the product suppliers but, in general, low frequency turbulence effects are not available. Consequently, a controlled laboratory test program was carried out in a state-of-the-art wind tunnel at the Fraunhofer Institut fur Bauphysik in Stuttgart, Germany to quantify the level of low frequency interference (down to 6.3 Hz) associated with a number of different foam windscreens and an aerodynamic microphone nose cone. A total of nine configurations were tested with "quiet" airflow only, artificial noise only and noise plus airflow to evaluate both low frequency wind induced noise and high frequency attenuation effects. The test program demonstrated that the largest size foam-based windscreens provided the most protection from flow induced noise due to wind. Flow induced noise by air flow alone was estimated from the study results and compared to community noise measurements at a typical wind turbine site. It was determined that flow induced wind noise does not have a significant or detrimental effect on the measurement of A-weighted sound levels under wind conditions of concern as long as the suggested measurement techniques described herein are followed

Low frequency noise measurements in reverberation and free-field rooms require special attention as the sound field in this frequency range may be dominated by only a few modes (eigenfrequencies). This paper discusses how reverberation rooms may be adequately damped to smooth the spatial sound field and to increase measurement reproducibility and repeatability without violating standards. Absorber modules which are only 250 mm thick are described and their operating principles are discussed. The modules are used as linings for test cells and wind tunnels to provide free-field conditions down to 50 Hz for 1/3-octave band measurements. Their unprecedented acoustic effectiveness is demonstrated by fundamental investigations in well-conditioned test rooms and final verification in a number of rigorously silenced and anechoic facilities completed in the research and development centers of 5 different automobile manufacturers in Europe and the United States. Since the novel acoustic materials and modules also provide several advantages concerning spaciousness, cleanability and durability their practicability is demonstrated in a few typical examples of approximately 50 completed measuring environments

In Teil 1 dieses Innovatoriums wurden zunächst passive faserige/poröse Materialien beschrieben, denen wohl mehr als 90 % der Bauteile für den Schallschutz ihre Wirksamkeit bei hohen Frequenzen, bevorzugt oberhalb 500 Hz, verdanken. Teil 2 beschäftigte sich mit reaktiv wirksamen Masse‐Feder‐Systemen, die vor allem bei tiefen Frequenzen, etwa zwischen 125 und 250 Hz, als Schallabsorber und Schalldämpfer verbreitet Einsatz finden. Neben auch höher abstimmbaren Folien‐Absorbern wurden in [2] sehr vielfältig in der Raumakustik einsetzbare neuartige Verbund‐Platten‐Resonatoren mit einer sehr breitbandigen Wirksamkeit von 50 bis 500 Hz umfassend beschrieben. Teil 3 stellt attraktive Fortentwicklungen der klassischen Hohlkammer‐Resonatoren zu innovativen Schlitz‐ und Membran‐Absorbern sowie Rohr‐Schalldämpfern vor. Die beiden letztgenannten sind wieder typische Vertreter der vom Fraunhofer IBP in den vergangenen 10 Jahren verstärkt propagierten Alternativen Faserfreien Absorber ALFA‐Familie, von denen sich besonders die Module 2 bis 7 mit kompetenter Unterstützung starker Lizenz‐ und Kooperations‐Partner im Markt durchzusetzen beginnen.

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