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Measuring the Sound Power of a Reference Source using Sound Intensity

Sonometrics Inc.-Robert Hickling
Published 2009-05-19 by SAE International in United States
Sound intensity is the time average of sound-power flow per unit area in watts/m2. It is generally measured using sound pressure at two closely-spaced microphones. It is commonly believed that it is not possible to measure sound intensity in a reverberation room because multiple reflections in the room create a diffuse pressure field which makes such measurements inaccurate. However there has to be a net flow of sound power in the room from speakers (or other sources) which then passes out through the walls, ceiling and floor. Hence net sound power flow (sound intensity) should be measurable. In a previous paper [1]* it was shown that it is possible to measure the sound power of a reference source accurately in three different reverberation rooms using sound-intensity measurement. Accurate measurements were also made in other work spaces. In this paper the sound-intensity method is compared with the method used by the manufacturer to calibrate a reference source. The calibration procedure uses the far field approximation for intensity in a semi-anechoic room. Results for the sound-intensity method…
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A Plea for Linear Units as an Alternative to Decibels and Octaves

Sonometrics Inc.-Robert Hickling
Published 2005-05-16 by SAE International in United States
Decibels were originally developed in the 1920s by the telephone industry (AT&T and Bell Labs). Initially the unit was the bel, derived from the name of Bell Labs and defined as the logarithm to the base 10 of the transmission loss of electrical power in telephone lines. It was also used for voice signals in telephones where the preferred unit became a tenth of a bel or decibel. The adoption of decibel for sound appears to be due principally to the dominant position of the Bell's acoustical research staff in the 1920s and 1930s. Octaves have their origin in music and were used to facilitate the use of analogue filters in partitioning the frequency scale. Partitioning into octaves divides the frequency scale into bands increasing in width by a factor of 2 with increasing frequency. To obtain greater resolution, the partitioning is often performed in 1/3 and sometimes in 1/12 octaves. However resolution remains poor particularly at higher frequencies. The ear can distinguish frequency differences that are much smaller than 1/3 or 1/12 octaves. Decibels…
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Overpressure and Noise Due to Airbag Deployment

Sonometrics Inc.-Robert Hickling
Bruel & Kjaer Sound and Vibration Measurement A/S.-Gary Newton,
Published 2003-05-05 by SAE International in United States
An overview is presented of the overpressure and noise due to airbag deployment in a passenger car. Overpressure is the low frequency compression of the air in a closed compartment, and noise is the higher frequency sound of bag inflation. The overview is timely, because there is now an accumulation of medical evidence to indicate that the overpressure and noise resulting from airbag deployment can be a threat to hearing, while, at the same time, the growing use of multiple airbag systems increases the threat. This problem can be counteracted by using aspirating airbags that draw in air from the passenger compartment as they deploy. There are two types of aspirating bag: one using suction that requires a compartmentalized bag and the other using entrainment that does not require a compartmentalized bag. The relative merits of suction and entrainment are discussed in the paper. To date compartmentalized bags have not been used. However the bag that appears to be most effective is one with a compartmentalized structure, called the breathing bag originally developed at GM,…
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Determining Sound Power for Automotive Applications

Sonometrics Inc.-Robert Hickling
University of Mississippi-Peng Lee, Alexei Goumilevski, Wei Wei
Published 1997-05-20 by SAE International in United States
Some years ago development, in the auto industry, of routine measurement of sound intensity in vector form, brought about major changes in determining sound power, particularly in improved accuracy and use in an indoor work area. However sound power is not yet being fully utilized. Accuracies within ± 10 % or ± 0.4 dB can be expected and narrow-band spectral data and intensity distributions on the integration surface can be used to identify and quantify component noise sources. In this paper new results are presented. In particular it is shown that sound power based on vector sound intensity can be determined accurately in a reverberation room.
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