Noise suppression CONSTRUCTION AND MATERIALS

CONSTRUCTION AND MATERIALS
17. The corrugated or lobe-type noise suppressor forms the exhaust propelling nozzle and is usually a separate assembly bolted to the jet pipe. Provision is usually made to adjust the nozzle area so that it can be accurately calibrated. Guide vanes are fitted to the lobe-type suppressor to prevent excessive losses by guiding the exhaust gas smoothly through the lobes to atmosphere. The suppressor is a fabricated welded structure and is manufactured from heat- resistant alloys.
18. Various noise absorbing lining materials are used on jet engines. They fall mainly within two categories, lightweight composite materials that are used in the lower temperature regions and fibrous- metallic materials that are used in the higher temperature regions. The noise absorbing material consists of a perforate metal or composite facing skin, supported by a honeycomb structure on a solid backing skin which is bonded to the parent metal of the duct or casing. For details of manufacture of these materials refer to Part 22.

Noise suppression CONSTRUCTION AND MATERIALS,Introduction Noise suppression, Noise suppression Engine noise, Noise suppression Methods of suppressing noise, Noise suppression Construction and materials, 

Noise suppression Noise absorbing materials and location METHODS OF SUPPRESSING NOISE 1

Fig. 19-6 Noise absorbing materials and location.

13. Deep corrugations, lobes, or multi-tubes, give the largest noise reductions, but the performance penalties incurred limit the depth of the corrugations or lobes and the number of tubes. For instance, to achieve the required nozzle area, the overall diameter of the suppressor may have to be increased by so much that excessive drag and weight results. A compromise which gives a noticeable reduction in noise level with the least sacrifice of engine thrust, fuel consumption or addition of weight is therefore the designer's aim.

Noise suppression METHODS OF SUPPRESSING NOISE ypes of noise suppressor

METHODS OF SUPPRESSING NOISE

Fig. 19-5 Types of noise suppressor.

10. Noise suppression of internal sources is approached in two ways; by basic design to minimize noise originating within or propagating from the engine, and by the use of acoustically absorbent linings. Noise can be minimized by reducing airflow disruption which causes turbulence. This is achieved by using minimal rotational and airflow velocities and reducing the wake intensity by appropriate spacing between the blades and vanes. The ratio between the number of rotating blades and stationary vanes can also be advantageously employed to contain noise within the engine.


11. As previously described, the major source of noise on the pure jet engine and low by-pass engine is the exhaust jet, and this can be reduced by inducing a rapid or shorter mixing region. This reduces the low frequency noise but may increase the high frequency level. Fortunately, high frequencies are quickly absorbed in the atmosphere and some of the noise which does propagate to the listener is beyond the audible range, thus giving the perception of a quieter engine. This is achieved by increasing the contact area of the atmosphere with the exhaust gas stream by using a propelling nozzle incorporating a corrugated or lobe-type noise suppressor (fig. 19-5).

Noise suppression ENGINE NOISE

Fig. 19-4 Comparative noise sources of low and high by-pass engines.

6. Compressor and turbine noise results from the interaction of pressure fields and turbulent wakes from rotating blades and stationary vanes, and can be defined as two distinct types of noise; discrete tone (single frequency) and broadband (a wide range of frequencies). Discrete tones are produced by the regular passage of blade wakes over the stages downstream causing a series of tones and harmonics from each stage. The wake intensity is largely dependent upon the distance between the rows of blades and vanes. If the distance is short then there is an intense pressure field interaction which results in a strong tone being generated. With the high bypass engine, the low pressure

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, 

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.