Page 209 - The Jet Engine

19: Noise suppression

Contents Page

Introduction 199
Engine noise 199

Methods of suppressing noise202

Construction and materials 205

3. Airframe self-generated noise is a factor in an
aircraft's overall noise signature, but the principal
noise source is the engine.

INTRODUCTION ENGINE NOISE
1. Airport regulations and aircraft noise certification 4. To understand the problem of engine noise
requirements, all of which govern the maximum suppression, it is necessary to have a working
noise level aircraft are permitted to produce, have knowledge of the noise sources and their relative
made jet engine noise suppression one of the most importance. The significant sources originate in the
important fields of research. fan or compressor, the turbine and the exhaust jet or
jets. These noise sources obey different laws and
2. The unit that is commonly used to express noise mechanisms of generation, but all increase, to a
annoyance is the Effective Perceived Noise deciBel varying degree, with greater relative airflow velocity.
(EPNdB). It takes into account the pitch as well as Exhaust jet noise varies by a larger factor than the
the sound pressure (deciBel) and makes allowance compressor or turbine noise, therefore a reduction in
for the duration of an aircraft flyover. Fig. 19-1 exhaust jet velocity has a stronger influence than an
compares the noise levels of various jet engine equivalent reduction in compressor and turbine blade
types. speeds.

199