Noise suppression Introduction

Noise suppression

Introduction 
Engine noise 
Methods of suppressing noise
Construction and materials 

INTRODUCTION
1. Airport regulations and aircraft noise certification requirements, all of which govern the maximum  noise level aircraft are permitted to produce, have made jet engine noise suppression one of the most important fields of research.

Fig. 19-1 Comparative noise levels of various engine types.

2. The unit that is commonly used to express noise annoyance is the Effective Perceived Noise deciBel (EPNdB). It takes into account the pitch as well as the sound pressure (deciBel) and makes allowance for the duration of an aircraft flyover. Fig. 19-1 compares the noise levels of various jet engine types.
3. Airframe self-generated noise is a factor in an aircraft's overall noise signature, but the principal noise source is the engine.

Introduction Noise suppression, Noise suppression Engine noise, Noise suppression Methods of suppressing noise, Noise suppression Construction and materials, Comparative noise levels of various engine types, 

Thrust distribution Diffuser duct

Diffuser duct
13. The conditions at the diffuser duct inlet are the same as the conditions at the compressor outlet, i.e. 19,049 lb. Therefore, given that the diffuser--

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.