The cylindrical microphone array: A proposal for use in international standards on sound power level measurement

The article considers the previously presented manipulating mechanism for positioning of a microphone during acoustical measurements in anechoic chamber. Usually the aims of acoustical measurements in anechoic chamber are: estimation of Sound Power Level of the noise source, measurement of directional characteristics of an electroacoustical transducer, measurement of the sound diffusion characteristic of a given structure and a measurement of Sound Pressure Level on a given measurement grid. The specific of that kind of measurements brings up the need of measurement microphone positioning in many points of the measurement space accordingly to relevant standards. In most cases during the tests it is necessary to position the microphone in certain points on the hemisphere. In such cases utilizing of typical microphone stands impedes the measurement and extends the time needed for the tests. The presented manipulation system for a measurement microphone allows positioning the microphone on the hemisphere around the tested object as required by the standards on the Sound Power Level measurement. Its construction is a simple, rigid form aiming at little effect on the acoustic field inside the chamber whereas the control system and the software are targeted at the maximal flexibility that allows not only standard testing but also scientific research in freely selected scenarios. Since its initial introduction the system has been extended by an additional axis that is used for rotating the microphone, which allows its positioning on the line that is coincident with the centre of rotation of the turntable. Such an extension eliminates the problem of use of the corrections of the directional characteristics of the microphone when measuring the sound signal. The microphone can be positioned directly towards the source of the sound. The article briefly reviews the mechanical construction of the positioning mechanism and focuses on the structure of the control section of the drive system constructed in the manipulator. The method for cooperation of actuators and the control system is presented. Also the description is given for the internal structure of the multi-level control circuits built in the applied drives. Finally the structure of the control application is presented.

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Three-Dimensional Sound Directivity around a Helicopter Turboshaft Engine

Journal of the American Helicopter Society ◽

10.4050/jahs.57.042002 ◽

2012 ◽

Vol 57 (4) ◽

pp. 1-10

Author(s):

Helene Gounet ◽

Serge Lewy

Keyword(s):

Microphone Array ◽

Power Level ◽

Three Dimensional ◽

Sound Field ◽

Broadband Noise ◽

Sound Power ◽

The Novel ◽

Sound Power Level ◽

Directivity Patterns ◽

Turboshaft Engine

Turboshaft engines can be the main source of noise due to a helicopter at takeoff. Some new silencing designs of the inlets and of the ejector were tested on a Turbomeca Arrius 2B2 engine in an open-air static facility. Intake and exhaust are not axisymmetric, and conventional directivity patterns of sound field on a horizontal arc of circle are insufficient. A special microphone array on a vertical half-circle translating axially was built. Data processing has been implemented to plot maps of sound pressure levels in third-octave bands and to compute sound power levels. Intake and exhaust radiations are separated thanks to mufflers on the other side. The lined fins in the secondary lateral inlet well reduce the compressor tone, which is largely dominant in intake radiation. Its sound power level is decreased by 7 dB. The novel ejector is also successful to reduce exhaust broadband noise above 1 kHz with a gain of 5 dB on sound power level. Finally, the benefit on the acoustic emission of the helicopter in flight is appraised.

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