The increased performance of a blocked line of sight baffle configuration is clearly evident from the performance data presented in Fig. Various curves and staggered patterns have been designed and are commercially available. The performance of absorptive silencers can be sharply improved if the line of sight through the silencer is blocked or eliminated. The resultant plane wave motion presents essentially a grazing incidence to the absorbing treatment and hence little sound is absorbed. The nonlinearity is due principally to the rapid absorption of high-order transverse modes in the first few feet of the silencer, leaving only a plane-wave-type of sound propagation. Note that the performance is not a linear function of length i.e., doubling the length does not double the attenuation. 26.3 is the performance of baffles 6 inch thick, 12 inch on centre (50 per cent open area) for silencer lengths of 4, 8 and 12 ft. With respect to the length of parallel baffles, the acoustical performance increases as length increases. Note also that better performance at lower frequencies is obtained as the thickness of the absorbing material is increased. Note that the attenuation increases sharply at high frequencies as the spacing is narrowed. The effect of baffle thickness and spacing can be seen in Fig. However, the grouping herein will be on the basis of the major noise reduction mechanism. It should be emphasised that even the basic forms cannot always be strictly divided as purely absorptive or purely reactive. However, before discussing the more complex combinations, each type will be broken down into its simplest form and the basic characteristics of each examined. High-performance silencer designs generally combine both absorptive and reactive elements in their construction. We shall see that many of the inherent penalties of passive devices are avoided with the active noise control approach.
It should be emphasised that the basic approach of active noise control is not new, but it is only recently that the control theory and microelectronic components have reached the state of the art to produce practical results. With careful attention to matching phase and sound pressure amplitude, a cancellation process ensues (interference), with resultant lower noise levels. Here, noise reduction is achieved by generating an ‘anti-noise’ field which is superimposed on the source field. Showing extreme promise as a noise control measure is a concept called active noise control. Difficulty generally is found not with finding a silencer with adequate acoustical performance but with dealing with problems such as size, weight, aerodynamic pressure losses, etc. With a basic understanding of the acoustical properties of each type, the noise control engineer can usually select a silencer or combination of silencers which will effectively provide noise reduction regardless of the character of the noise. In any case, they are usually installed in pipes or ducts to reduce sound transmission from one section of a gas flow system to another. In addition, other functions such as water separation, filtering, spark arresting, heat recovery or exchange, etc., may also be present. Some silencers combine the elements of two or more types for extended performance. The noise reduction of dispersive silencers usually comes from diffusing a high-velocity gas flow into smaller lower-velocity streams. Reactive silencers contain no absorbing material but depend on the reflection or expansion of the sound waves with corresponding self-destruction as the basic noise reduction mechanism.
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