Noise suppression ENGINE NOISE

ENGINE NOISE

Fig. 19-2 Exhaust mixing and shock structure.

4. To understand the problem of engine noise suppression, it is necessary to have a working knowledge of the noise sources and their relative importance. The significant sources originate in the fan or compressor, the turbine and the exhaust jet or jets. These noise sources obey different laws and mechanisms of generation, but all increase, to a varying degree, with greater relative airflow velocity. Exhaust jet noise varies by a larger factor than the compressor or turbine noise, therefore a reduction in exhaust jet velocity has a stronger influence than an equivalent reduction in compressor and turbine blade speeds.

Fig. 19-3 Change of exhaust jet pattern to reduce noise level.

5. Jet exhaust noise is caused by the violent and hence extremely turbulent mixing of the exhaust gases with the atmosphere and is influenced by the shearing action caused by the relative speed between the exhaust jet and the atmosphere. The small eddies created near the exhaust duct cause high frequency noise but downstream of the exhaust jet the larger eddies create low frequency noise. Additionally, when the exhaust jet velocity exceeds the local speed of sound, a regular shock pattern is formed within the exhaust jet core. This produces a discrete (single frequency) tone and selective ampli- fication of the mixing noise, as shown in fig. 19-2. A reduction in noise level occurs if the mixing rate is accelerated or if the velocity of the exhaust jet relative to the atmosphere is reduced. This can be achieved by changing the pattern of the exhaust jet as shown in fig. 19-3.

Exhaust mixing and shock structure. , Change of exhaust jet pattern to reduce noise level. ,Introduction Noise suppression, Noise suppression Engine noise, Noise suppression Methods of suppressing noise, Noise suppression Construction and materials,