1969-02-01

Noise Reduction Techniques as They Apply to Engine-Generator Design and Treatment 690755

Small engines may require soundproofing to eliminate one or more of the following effects: hearing loss, speech interference, community annoyance, detectability, and psychological disorientation. Detectability criteria are frequently associated with military applications and may require the use of a soundproof enclosure in addition to other engine treatments.
Acoustical noise sources are conveniently classed as either aerodynamic or mechanical. Aerodynamic sources are predominant on small engines. Treatment of exhaust noise by individual components, e.g., muffler, is inadequate; a system approach, through the use of an electro-acoustic analog computer, has proved to be a much more satisfactory procedure.
To develop a soundproof enclosure for a 0.3 kw military engine-generator set several control techniques were required; (1) the enclosure had to be decoupled from the engine, (2) the panel structures had to have a high transmission loss (achieved through the careful selection of damping material), (3) cooling air inlet ducts and exhaust gas outlet ducts had to be properly treated to minimize noise conduction. The degree of noise reduction attainable was limited by specifications for size, weight, engine cooling at high ambient temperatures, and engine accessibility for starting and fueling.

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