Sound point source

In making sound measurements, several factors regarding the nature of the source should be considered. Whether the source is a true point source in space radiating spherically, or whether it is a hemispherical source close to one flat surface or a quarfer sphere between two flat surfaces, and so on, if will make a difference in how the measurement needs to be performed. However, a detailed discussion of measurement issues is beyond the scope of this chapter and the reader may use some of the more comprehensive... 

Larson, T.V., al. "The Influence of a Sulfur Dioxide Point Source on the Rain Chemistry of a Single Storm in the Puget Sound Region", Water, Air and Soil Pollution, 4, 1975, pp. 319-328. 

As seen from Figure 13.1, sound generated by a point source with power W propagates as a spherical wave so the sound intensity is inversely proportional to the square of a distance from the sound source. 

In a free space, the sound source can be considered as a point source (see Figure 13.1). In practical industrial applications, however, sound is either radiated from the source of definite size (e.g., loudspeaker membrane) or, more frequently, reflected from the point source by surfaces of different shapes such as horn, paraboloid, ellipsoid, etc. In both cases such sound radiation can be regarded as coming from a plane source. This results in a specific pattern of sound intensity in the zone near the sound source, the sound intensity is constant (Fresnel zone), whereas outside this zone (the Fraunhofer zone) the sound intensity decreases inversely with the square of the distance from the plane source, i.e., in the same way as for a point source (Figure 13.4). 

In a gas, the attenuation of the intensity of sound S in dB emitted by a point source (S) at a distance x is given by the exponential equation below ... 

The sound generation/detection regions are small, and the system can be modeled as a point source/point receiver system. 

In a free field, sound emitted by a point source travels uniformly in all directions, i.e., the wavefronts are spherical, while the sound level decreases by 6dB per doubling of the distance from the source. 

Echo directivity was experimentally studied for surface SH Wave probes and SH Wave angle probes. Frequencies used in the experiment were 5MHz and 2MHz, the angles of refraction 90°and 70°, the crystal size 10X 10mm and 5X5mm. The echo directivity was evaluated, using side drilled holes of various depths. The experimental results showed consistency with the calculation based on a point sound source assumption on the test surface in different phases. 

The sound pressure P was produced at the point D by the ultrasonic sound source which was generated at the point B in "n" order of the munite area AA by the incident beam. The sound pressure P is given by the equation(l)... 

The calculation based on a point sound source on the test surface in different phases showed consistency with the experiment. The assumption was proved to be reasonable and correct. 

It should also be pointed out that the levels from ventilation noise in, for example, workshop premises are often lower than those emanating from other sources, for example machines of various kinds. It is not uncommon for the ventilation noise from industrial premises to cause more disturbance in adjacent offices than in the industrial premises themselves. The fact that ventilation noise propa gates in this manner is due to its pronounced low-frequency character. The more low-frequency components in the noise, the greater the propagation. Sound radiation from industrial premises, via duct openings in facades and roofs, may in this manner also cause disturbances in nearbv residential accommodations. 

In a fixed point in space, the magnitude of pressure will change according to the nature of the sound source,... 

Tw o noise sources produce 69.2 and 69 dB pressure levels respectively (Table 9.16) at the same space point. The composed sound at said point will have a total pressure level of 72.1 dB, calculated by adding 2.9 dB to 69.2 dB. In a similar way, at 250 hertz, for example, if each of the sources has a pressure level of 60 dB, the total level will be 63 dB (sum of 3 dB and 60 dB). 

Sound power is the total energy emitted from a fan that is a function of the fan s speed and point of operation and is independent of the fan s installation and surrounding environment Sound power level is the acoustical power expressed in decibels (dB) radiating from a source. Sound power can be converted into predictable pressure levels (dBA) after the acoustical environment surrounding the fan is defined. Sound pressure for a specific fan varies with... 

Model predictions of numerous point and area source strategies were completed, an economic analysis of the alternatives was developed, and the results placed before a public advisory committee. The cost-effectiveness and fine particle benefits identified for each strategy have provided the public with a sound basis for their recommendations. Based on an early staff analysis, the most cost effective alternatives included ... 

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