说New words

•        Regulation法规

•        Certification认证

•        Suppression抑制

•        Unit 单位

•        Annoyance烦恼

•        EffectivePerceived Noise deciBel 有效感知噪声分贝(EPNdB)

•        Pitch 音高

•        Soundpressure 声压

•        Duration持续时间

•        Flyover飞越

•        Airframe机身

•        Signature特征

•        Source来源

•        Originate起源于

•        Obey 服从

•        Vary 变化

•        Equivalent等价物、相等的、相同的

•        Shocknoise 激波噪声

•        Violent暴力的、强烈的

•        Shearingaction 剪切作用

•        Frequency频率

•        Downstream下游

•        Additionally另外

•        Discrete离散的

•        Tone 音调

•        Selective选择性

•        Amplification放大

•        Interaction相互作用

•        Wake 尾流

•        Distinct明显的、截然不同的、确实的

•        Broadband宽频

•        Harmonics谐波

•        Intensity强度

•        Dependent依赖

•        Row 排

•        Intense激烈的

•        Plainnozzle普通喷管

•        Suppressornozzle 消音喷管

•        Landing-approach着陆进近

•        Diminish减少

•        Predominant主要的

•        Nevertheless尽管如此

•        Propagating传播

•        Acoustically声学上

•        Absorbent吸收的

•        Lining内衬

•        Advantageously有利的

•        Audible听得见

•        Corrugate  波纹的

•        Lobe-type波瓣型

•        Promote提升

•        Extend延伸

•        Penalty处罚

•        Incurred招致

•        Compromise妥协

•        Sacrifice牺牲

•        Aim 目的

•        Eject 弹出

•        Inherently固有的

•        Desirable向往的

•        Confine限制

•        Expel 驱逐

•        Perforateface sheet 带孔面板

•        Composite复合

•        Honeycomb蜂窝状

•        Backingsheet 底板、背板

•        Porous多孔的

•        Adjust调整

•        Calibrate校准、标定

•        Category类别

•        Region地区

•        Fibrous-metallicmaterial纤维-金属材料

•        Parent metal母材

读Sentence

•        1.Airport regulations and aircraft noise certification requirements, all of whichgovern the maximum noise level aircraft are permitted to produce, have made jetengine noise suppression one of the most important fields of research.

•        2. Theunit that is commonly used to express noise annoyance is the EffectivePerceived Noise deciBel (EPNdB). It takes into account the pitch as well as thesound pressure (deciBel) and makes allowance for the duration of an aircraftflyover. Fig. 19-1 compares the noise levels of various jet engine types.

•        5. Jetexhaust noise is caused by the violent and hence extremely turbulent mixing ofthe exhaust gases with the atmosphere and is influenced by the shearing actioncaused by the relative speed between the exhaust jet and the atmosphere. Thesmall eddies created near the exhaust duct cause high frequency noise butdownstream of the exhaust jet the larger eddies create low frequency noise.Additionally, when the exhaust jet velocity exceeds the local speed of sound, aregular shock pattern is formed within the exhaust jet core. This produces adiscrete (single frequency) tone and selective amplification of the mixingnoise, as shown in fig. 19-2. A reduction in noise level occurs if the mixingrate is accelerated or if the velocity of the exhaust jet relative to theatmosphere is reduced. This can be achieved by changing the pattern of the exhaustjet as shown in fig. 19-3.

•        9.Listed amongst the several other sources of noise within the engine is thecombustion chamber. It is a significant but not a predominant source, due inpart to the fact that it is 'buried' in the core of the engine. Nevertheless itcontributes to the broadband noise, as a result of the violent activities whichoccur within the combustion chamber.

•        10.Noise suppression of internal sources is approached in two ways: by basicdesign to minimize noise originating within or propagating from the engine, andby the use of acoustically absorbent linings. Noise can be minimized byreducing airflow disruption which causes turbulence. This is achieved by usingminimal rotational and airflow velocities and reducing the wake intensity byappropriate spacing between the blades and vanes. The ratio between the numberof rotating blades and stationary vanes can also be advantageously employed tocontain noise within the engine.

•        12. Inthe corrugated nozzle, freestream atmospheric air flows down the outsidecorrugations and into the exhaust jet to promote rapid mixing. In the lobe-typenozzle, the exhaust gases are divided to flow through the lobes and a smallcentral nozzle. This forms a number of separate exhaust jets that rapidly mix withthe air entrained by the suppressor lobes. This principle can be extended bythe use of a series of tubes to give the same overall area as the basiccircular nozzle.

•        14.The high by-pass engine has two exhaust streams to eject to atmosphere.However, the principle of jet exhaust noise reduction is the same as for thepure or low by-pass engine, i.e. minimize the exhaust jet velocity withinoverall performance objectives. High by-pass engines inherently have a lowerexhaust jet velocity than any other type of gas turbine, thus leading to aquieter engine, but further noise reduction is often desirable. The mostsuccessful method used on by-pass engines is to mix the hot and cold exhauststreams within the confines of the engine (fig. 19-5) and expel the lowervelocity exhaust gas flow through a single nozzle (Part 6).

•        17.The corrugated or lobe-type noise suppressor forms the exhaust propellingnozzle and is usually a separate assembly bolted to the jet pipe. Provision isusually 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 lossesby guiding the exhaust gas smoothly through the lobes to atmosphere. Thesuppressor is a fabricated welded structure and is manufactured fromheat-resistant alloys.

读全文并回答以下问题：

•       Whatis the principal noise source for an aircraft?

•       Whatare the significant noise sources for an engine?

•       Describethe two distinct types of noise in compressor and turbine.

•       Whatare the main noise sources for pure jet engine and the by-pass engine?

•       Describe the noise absorbing lining.

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