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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.

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.

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.

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.

A single-wheel trailer has been designed and built to study the origins of tire noise and its basic characteristics. The single test tire, nominally the 10.00/20 size usually mounted on large trucks and semitrailers, is located 12.2 m (40 ft) behind the rear axle of the towing vehicle to isolate it from other noise sources. Reflective surfaces that could interfere with noise measurements are minimized by the high, single-beam construction of the trailer. The towing vehicle is modified to reduce its noise and wake in the vicinity of the test tire, which can be loaded to 22.2 kN (5000 lb) by dead weights and rolled at expressway speeds. Because of its unusual configuration, the dynamic behavior of the trailer was studied prior to design to help determine several design parameters and show that the trailer would follow well. Extensive stress analyses of the trailer beam and other structural elements were also required.

Tests of laser ignition are conducted in a combustion bomb. A range of fuels is investigated comprising isooctane, cyclohexane, n-heptane, n-hexane, clear indolene, and No. 1 diesel fuel. The ignition characteristics of laser-induced sparks are compared with sparks generated with a spark plug for different air/fuel ratios. The power density required to produce laser induced sparks is investigated. Although laser ignition appears to be impractical as an ignition device because of its low efficiency and high cost, it presents some interesting possibilities compared to the standard spark plug in that the laser spark is electrodeless and can be positioned anywhere inside the combustion chamber. Its primary use appears to be as a research tool.

The results of an experimental study of air cushion noise and overpressure are presented, and methods to reduce these effects are investigated. Free-field studies of inflator noise are made in an open anechoic room, and various silencing devices are tested. Studies are also made of the combined inflator noise and overpressure in a closed passenger compartment. Tests are made for different window openings in the compartment. Results are presented for a new experimental air cushion that draws in air from the passenger compartment as it inflates. Some of the implications of the data with regard to risk criteria for damage to human hearing are discussed.