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A prediction model for noise produced by an air conditioning system in an aircraft pressurized compartment is described. It is based on analysis of the sound flow energy in air conditioning system channels, including both the generation attenuation effects of the acoustic oscillations within the system. Differences in calculations from those usually used in stationary air conditioning systems for industrial, public, and residential buildings are discussed. The mechanism of noise generation by compressors, cooling turbine, separated flows in regulator, and jet flows from the pipe nozzle are investigated. The calculation results are obtained and carried out for different operational airplane conditions. Optimum tasks for operation conditions of an air conditioning system for noise impact criterion are formulated and solved.

With 6.5 million people living on some 1000 square kilometers of land, about 70% of which are hilly terrains, one could imagine how difficult it would be in planning compatible land use. High‐rise residential blocks surrounding sports ground are not uncommon in Hong Kong. They should co‐exist harmoniously if the latter is intended solely for recreational sports. Hong Kong, being a cosmopolitan city, hosts many kinds of outdoor entertainment activities of cultures and many are held frequently in these sports grounds owing to the absence of purpose‐built outdoor venues for these activities. With increasing environmental expectations, complaints against noise from these activities are on the rise. Special considerations will have to be struck between efficient use of land and adequate noise protection of the nearby noise‐sensitive receivers. This paper describes the various outdoor entertainment activities being held in Hong Kong, their noise generation characteristics, and the approaches taken to control the noise.

An interengaging fastener member having a plurality of headed elements, which can reduce the noise generated due to the relative movement between the headed elements engaged with each other. The fastener member 10 is made of polymeric material as a one-piece molded article, and includes a generally flat base 14 and a plurality of headed elements 18 projecting from the base 14. Each headed element 18 has a stem 22 arranged on and projecting from the base 14 in a predetermined spaced relationship, and a head 24 formed at the end of the stem 22 and having a contour swollen from the periphery of the stem 22. The head 24 is defined by a generally flat surface 26 adjacent t the stem 22 and extending radially outward from the periphery of the stem 22, a curved surface 28 bulging out with an apex and arranged at an opposite side of the generally flat surface 26 from the stem 22, and a transition surface 30 formed from a second curved surface for connecting the generally flat surface 26 with the curved surface 28.

Continuous plumes of bubbles were entrained by a jet of water impinging on a horizontal surface in both fresh and salt water environments. Several hours of acoustic time series were digitally recorded in both cases. Many of the prominant features of the sound generated in salt water were explained by modeling the plume as a cylindrical resonant cavity at the surface of a shallow‐water waveguide. However, for salt water, the extreme low‐frequency limit, the dominant mechanism for creating sound, was determined to be hydrodynamic turbulence. In addition to these two mechanisms of noise generation, in the fresh water case there were also large emissions resulting from the growth and break up of bubbly structures around the edges of the main plume—a mechanism that was absent in the salt water environment. Individual bubble oscillations could also occasionally be heard in fresh water at frequencies below 10 kHz. Details of these findings and the theoretical models will be presented, highlighting the important ...

The rotation of N-bladed propellers of a multi-engine aircraft is controlled by phase angle synchronizer 30 such that a predetermined phase angle offset φ3 is maintained between the blades of a first propeller 10 and the blades of a second propeller 20, at which offset cabin noise is reduced to a minimum and cabin vibration to a near minimum. The controller 40 determines the optimum phase angle offset φ3 to be that phase angle of the set of N periodic phase angles: φ1+K(360/N), where K ranges in integer steps from 0 to N-1, each characteristic of minimum cabin noise generation, beginning with the minimum first phase angle, φ1, lying between 0 and 360/N degrees, which is closest to the second phase angle, φ2, characteristic of minimum cabin vibration.

The observation that oceanic ambient noise is wind dependent over a broad frequency range [G. Wenz, J. Acoust. Soc. Am. 34, 1936 (1962)] suggests a major role for gas bubbles as sources of this sound. Bubble noise generation mechanisms in a variety of modalities and under both passive and active surveillance are examined. The preliminary studies have examined topics such as collective oscillations of bubble clouds, bubble entrainment by capillary waves, and precipitation noise. A brief overview of this research is presented and the most recent results are described in some detail. [Work supported by ONR and ONT.]

From Suh's total carrier number fluctuation model of 1/f noise generation mechanism, a model that seems to be able to describe the behavior of 1/f noise in various semiconductor devices is presented. The model is formulated in a simple logical procedure without any ambiguous or obscure assumptions and expresses the voltage fluctuation spectral density and the current fluctuation spectral density in terms of the short-circuited channel current fluctuation spectral density. The obtained expressions seem to fit in various experimental results.

The lively discussion that followed Dr Lotz's presentation focused primarily on wheel/rail rolling noise: mechanisms for noise generation, the relative importance of the wheel and the rail as sources of noise, means for control of the noise, etc. The report paraphrases, as faithfully as possible, the comments of all participants. The writer's opinions are confined to a brief section at the end.

When a train is running over sharp curves, the noise is mainly caused by the flexural vibration of wheels. The noise radiating area of a spoke type wheel is smaller than that of a convensional web type wheel. Then, the spoke type wheel is supposed to reduce the noise at curves. In this paper, the characteristics of the noise radiation of the spoke type wheel is experimentally investigated, and it is found that the noise generated by the spoke type wheel is considerably smaller than that by the web type wheel. Its flexural vibration is analyzed theoretically. In this analysis, the wheel is assumed to be an annular Mindlin plate which is elastically supported by springs which simulate the spokes.

An efficiency rating system is presented to aid in the selection of hand held air guns, nozzles, and ejectors. This guide is based on noise levels, force or thrust, and air supply pressure. In addition, air consumption, blocked static air pressure, noise directivity patterns and spectra are presented. Finally, noise control techniques in the use and installation of nozzles and ejectors are reviewed.

A structural–acoustical system has been established to model theoretically the noise generation of a six‐cylinder diesel engine. In this system, six‐cylinder pressures have been treated as six mutually correlated inputs. The noise measured 3 ft away from the engine has been treated as the single output of the system. Six structural–acoustical frequency responses with respect to the six inputs were evaluated experimentally. The multiple coherence function between all the inputs and the single output has also been evaluated experimentally. Using the multiple coherence function, the engine noise which is coherent to the cylinder pressures was separated from the total engine noise. It has been found that, for the V‐6 diesel engine studied, the coherent noise is almost equal to the total noise. From the structural–acoustical frequency responses, the contributions of the individual cylinder pressures to the coherent engine noise were assessed.Subject Classification: 50.50; 60.20.

The Food and Agricultural Organization of the United Nations granted a fellowship to the author to study, at several European ship research institutes, the problem of noise generation and propagation on fishery vessels. He later took part in the design of the fishery research vessel Prof. Siedlecki, which included the implementation of a series of antivibration and antinoise precautions. This paper compares vibration and noise levels on two fishery research vessels—the conventional RV G.O.Sars and RV Prof. Siedlecki. The paper is divided into two parts. In the first part, mechanical vibrations of the elastically supported propeller are dealt with and conditions for minimum vibration response are determined. In the second part, structural vibrations in the ships and noise propagation from the ships to the water are described. The paper concludes with data on structural vibrations and waterborne noise propagation. This paper, and the paper published in the June 1984 issue of JOURNAL OF SHIP RESEARCH [1],2 completes the author's approach to propeller vibrations.

The noise generation mechanism of flat belts on a timing pulley, trapezoidal tooth profile timing belts and circular tooth profile timing belts has been experimentally studied. The results of this study are as follows. (1) The noise generation mechanisms of the three kinds of belts are all the same. The noise is mainly an impact sound due to the collision of the belt and the top land of the pulley tooth at the entry point of the driving pulley. (2) The impact noise is reduced by the interference of either the belt and the pulley tooth flanks or the top land of belt tooth and the bottom land of the pulley tooth groove. (3) The noise level of the trapezoidal tooth profile timing belt is high when the belt pitch, expanded by the tension, is equal to the pulley pitch.

Measurements are reported of the pressure fluctuations in the near field of a submerged jet. The mechanisms by which the pressure fluctuations may be generated are discussed and estimates are presented of two of these, namely the transient oscillations of micro-bubbles of air excited by the pressure drop in the nozzle, and the cavitation of micro-bubbles by the turbulent pressure fluctuations in the jet mixing region.

In urban rapid transit systems the squeling noise of railway wheels during the passage through narrow curves is very troublesome to passengers and city dwellers. Frequency analyses of measured curving-noise levels show that the bending vibrations of the wheel disks excited by stick-slip motion between the wheel rim and the rail are the main sources of this sound radiation. To get insight into the dynamics of the noise generation it is necessary to understand the bending vibration behaviour of a wheel disk as well as the sound radiation mechanism and the generation of selfsustained oscillations in the contact area. The present paper deals with a theoretical model which comprises all three parts of the problem. In the first part the natural bending vibrations of the wheel are calculated by means of a finite element model. In the second part the forced and self-sustained oscillations are determined using modal expansion techniques. The third part deals with the calculation of the radiated sound pressure and sound power due to wheel oscillations. Here, an acoustical quality level is introduced to classify the various types of wheel disks. The major assumptions in the present paper are rigid rails and point contact between wheel and rail.

The theory of non-stationary motion around aerofoils has many important applications in the field of aeronautics and turbomachines. The most significant problems are flutter, fluctuating loads experienced by blades of a typical turbomachine which move through a non-uniform field of flow, disturbed by the induced effects of other stationary and moving blade rows. It is important to predict the fluctuating lift force on a blade or aerofoil due to a flow disturbance, because both the blade vibration and noise generation are related to fluctuating lift and moment.

The performance of a prototype combination cotton gin stand and lint cleaner was evaluated. Fiber quality parameters are compared to those from a conventional ginning and cleaning system. A successful combination gin-lint cleaner would eliminate much of the energy use, noise generation and air pollution associated with current ginning and lint cleaning systems

A unified aeroacoustic jet noise prediction method has been developed based on the modeling of principal noise generation and emission mechanisms from first principles. It is demonstrated herein that this jet noise prediction method is a useful diagnostic tool for assessing the relative importance of the various mechanisms for a given nozzle type. The relative contributions of 1) turbulent mixing noise suppression, 2) shock-cell broadband noise suppression, 3) convective amplification suppression, and 4) fluid shielding attenuation have been evaluated for a high element number multichute suppressor to arrive at a plausible explanation for how multielement suppressors suppress jet noise. This explanation, an alternative view to historical conceptions of jet noise suppression, suggests an approach to designing low noise suppressor nozzles.

Measurements of the at-ear helmet-generated aerodynamic noise and helmet insertion loss were carried out for the two major types of motorcycle helmets. From these data and existing information on noise generation by flow around a bare head it was found that for quiet motorcycles at typical operating speeds a significant part of the riders at-ear noise is generated by the air flow. An assessment of the possibility of hearing damage was then carried out. It was found that only with extremely high usage would there be a significant risk of hearing damage for either the bare headed or helmeted rider. Helmets did, however, give significant protection. Detection of warning signals was then considered. It was found that under none of the conditions investigated here did the helmet put its wearer at a disadvantage compared with the bare headed rider, and at typical constant speeds the helmet gave a rider an advantage in the detection of warning signals.

The noise generation by the turbulent mixing of a heated, subsonic jet and its cold surroundings is discussed, with the help of the method of matched asymptotic expansions, separately for an inner region where hydrodynamical characteristics dominate and for an outer region where sound fields prevail. The actual calculations then can be confined to the inner region since the first approximation of the outer solution depends only on the multipole-character of the inner solution. For a first inner approximation compressibility effects are negligible, and the flow field can be described by the incompressible approximation of the flow equations. With account taken of matching conditions, a U 0 4 -power law can be realized if monopole sources with a strength ∼ U 0 2 exist. Such monopole-like sources are possible only if there is a mass flow between the heated and the cold gas flow due to the diffusion caused by temperture gradients. This effect can be included by abandoning the usually applied assumption d S /d t =0 ( S being the entropy). A qualitative assessment of the order of magnitude of these effects is given.

No studies have been published of noise variation in Indian cities and very few investigations of the phenomenon have been made anywhere. Using a set of six recording sites, arbitrarily cho...

A close connection between microseisms and ocean‐wave activity has been recognized for many years. Various mechanisms have been proposed to explain the interaction, most favored being nonlinear interactions between ocean surface waves. Interest in these processes has increased in recent years as underwater acousticians have extended their investigations to infrasonic frequencies. This contribution builds on a study recently reported that confirmed the role of nonlinear wave–wave interactions at infrasonic frequencies but left some questions unresolved.

The propagation of acoustic waves and energy absorption in a medium of air containing suspended water droplets have been investigated experimentally. It has been found that an increase in atomisation pressure or mass flow ratio reduces the size of droplets, thus yielding higher noise reduction. It should be possible to develop an appropriate system, for reducing the noise of hammers or other impact machinery, where the creation of a temporary air-water skirt around the noise source can be synchronised with the blow rate, i.e. the timing of the noise generation.

A review is presented of wheel/rail noise research studies, published since 1976. The indications are that a forced vibration model for the mechanism of wheel/rail noise generation is consistent with the results obtained by various researchers. Further work is needed on the parameters governing the magnitudes of the forces in the wheel/rail contact zone, however, before a complete understanding of noise generation can be achieved, and hence control at source.

Electrochemistry provides a powerful set of tools for analysis. Its power derives from the fact that electrochemistry is inherently chemical in nature. This instrumentation article describes developments in current-carrying electrochemical detectors. It is possible to argue that these detectors, out of all other developments in electroanalytical chemistry, have had the greatest impact on real analytical chemistry since the development of ion-selective electrodes. Within the context of electrochemical detectors, the authors will discuss three topics that are important to all analytical chemists: signal and noise generation and signal-to-noise ratio (S/N), the improvement of qualitative information content, and control of selectivity of the detector. In each area there are many opportunities to increase our knowledge; they describe some relevant research. They demonstrate that there is much to learn and that the electrochemical approach can be made more versatile and powerful that it is today.

The aerodynamic noise generated by a subsonic jet impinging on a flat plate is studied from measurements of near-field and surface-pressure fluctuations. The far-field noise measured at 90° to the jet axis is found to be generated by two different physical mechanisms. One mechanism is the impinging of the large coherent structures on the plate, and the other is associated with the initial instability of the shear layer. These two sources of noise radiate to the far field via different acoustical paths.

Most of the theories available on noise in IMPATT diodes yield good qualitative understanding of noise characteristics in avalanche diodes under large signal conditions. However there is no correlation in the theories at quantitative level, due to differences in the approximations made. A theory is formulated after reexamination of the theories available on the noise properties of IMPATT-Oscillator at large signal levels. This theory is based on the work of Kuvas i.e. on the linearization technique for calculating the spectrum of homogeneous noise with linear damping, resulting in the separation of large signal and noise problems. The analysis takes into account the signal dependence of the noise generation process and the performance characteristics of noise are analysed up to the first four harmonics of the operating frequency. Analytical expressions for AM and FM noise, which are directly measurable quantities for quantitative comparisons are also derived. The theory is applied for quantitative results to n-type GaAs diodes and the effect of unequal ionization rates for electrons and holes in GaAs is presented. The results so obtained are compared with the experimental results in the Part 2 of the paper, which shows the validity of the present analytical treatment. The analysis should be useful in understanding of the diode operating conditions and designing diodes for optimum power noise performance. Die meisten der vorhandenen Theorien uber Rauschen in IMPATT-Dioden ergeben ein gutes qualitatives Verstandnis der Rauschcharakteristiken in Lawinendioden unter Grossignalbedingungen. Es existieren jedoch keine Korrelationen in den Theorien auf quantitativem Niveau, was auf die Unterschiede der durchgefuhrten Naherungen zuruckzufuhren ist. Es wird eine Theorie nach erneuter Diskussion der vorhandenen Theorien uber die Rauscheigenschaften von IMPATT-Oszillatoren bei hohem Signalniveau formuliert. Diese Theorie basiert auf der Arbeit von Kuvas u. a. zur Linearisierungsmethode fur die Berechnung des homogenen Rauschspektrums bei linearer Dampfung, die eine Separation des Grossignals und des Rauschproblems ermoglicht. Die Analyse berucksichtigt die Signalabhangigkeit des Rauschgenerationsprozesses und die Rauschkennlinien werden bis zu den ersten vier Harmonischen der Arbeitafrequenz analysiert. Analytische Ausdrucke fur AM- und FM-Rauschen werden als direkt mesbare Grosen fur quantitative Vergleiche abgeleitet. Die Theorie wird zur Erzielung quantitativer Ergebnisse an n-leitenden GaAs-Dioden benutzt und der Einfluss ungleicher Ionisierungsraten fur Elektronen und Locher in GaAs wird dargelegt. Die so erhaltenen Ergebnisse werden mit den experimentellen Ergebnissen von Teil 2 der Arbeit verglichen und zeigen die Zuverlassigkeit der vorgelegten analytischen Methode. Die Analyse kann fur das Verstandnis der Betriebsbedingungen der Diode nutzlich sein sowie fur den Entwurf von Dioden mit optimalen Rauachleistungsparametern.

The basic tenet of the research reported here has been to develop a fundamental understanding of the mechanism of noise generation in water-lubricated compliant rubber bearings. An experimental model study employed a transparent glass slider rubber against a Buna-N rubber section in order to permit direct measurement of the different aspects of the vibration phenomena, and to be able to observe the type of rubber motion that occurs at the sliding interface. This study has clearly demonstrated that the basic phenomenon is a stick-slip motion of the rubber surface, at times coupled with mechanical resonances of the bearing parts. Shaft torsional resonance does not appear to be involved. The design, geometrical and material conditions that encourage the phenomenon have been established. The direction for improvement is clear, and the paper contains both conclusions and recommendations for corrective means which would lead toward silent operation.

A review of the current state of knowledge of wheel/rail rolling noise is presented. Mechanisms for noise generation, sources of noise, predictive models and means for control are presented. Areas where additional research is required are suggested. While considerable progress in expanding our understanding of wheel/rail rolling noise has occurred in the past ten years, there are still many areas of controversy and many areas where firmly based scientific information is lacking.

A fundamental study of noise generation in complex mechanical systems is undertaken. The objective of the study is the development of design guidelines for the prediction of mechanical system noise levels. Recently developed dynamical procedures are used to obtain the motions of linked mechanical systems with elastic elements and connection clearances. An analytical procedure for modeling the acoustical noise generated by complex mechanical systems is applied to a typical linked system, a four-bar mechanism. The far field acoustic pressure distribution is calculated and shown to be a function of many of the parameters under the control of the system designer. To aid the designer in producing quieter systems, acoustic design functions are developed. In particular, the system sound power and modal power efficiencies are found to be promising design tools.

The resistance and 1 ƒ noise of ruthernium-oxide based thick film resistors were studied over the temperature range of 77–425°K. The experimental results show that the resistance and the 1 ƒ noise have minimum values around T = 350 K and T = 280 K respectively. The 1 ƒ noise follows the relation S v (ƒ) = (K 1 V 2 ƒ ) . A noise generation model based on the fluctuation of tunneling current in metal-insulator-metal units was used to explain the observed frequency and temperature dependence of the 1 ƒ noise.

The purposes, problems, and progress of radiated noise, self‐noise, and ambient noise research are reviewed. Purposes are related primarily to national defense, but applications to fishery and to the utilization of other natural resources are also noted. Basic problems, most of which were recognized 20 years or more ago, involve ascertainment of properties of the noise, identification of noise sources and mechanisms of noise generation, and the discovery and definition of noise dependencies on environmental factors. Many radiated and self‐noise sources and mechanisms have been identified. Major problems are those of noise measurement, noise reduction, and prevention. In the field of ambient noise, most measurements have been of sound‐pressure level. Some of the noise sources and environmental factors have been identified, and a capability for qualitative and gross prediction has been achieved. Recommended are further investigations of the variation of ambient noise with receiver depth, directionality of t...

The sound produced by the piston slap of a diesel engine is investigated by standard methods of sound and vibration analysis augmented by correlation of incidence of individual transient components. This latter technique makes use of information obtained from the tuned outputs of the noise signal, focusing attention on the incidence of single-frequency transients. The contribution of piston slap to the overall noise emitted by an engine is studied analytically by examining equations for the transverse motion of the piston and experimentally by varying engine parameters such as piston-cylinder clearance, gudgeon-pin offset, and piston lubrication. By considering the various distinct characteristics, an insight is gained into the mechanism of noise generation and radiation.

For some time the subject of noise generation in magnetrons has been under study in our laboratory. The unmodulated CW magnetron was chosen as the first subject for investigation, because some of the complications of pulsed or modulated tubes are thus avoided. Since any heterodyning method introduces the noise of another microwave oscillator, a method of direct detection has been employed. This method involves the use of a crystal-video receiver for both AM and FM detection.

As the first step to find the noise reduction methods for toothed belt drive, the authors investigated the characteristics of the noise and its generating mechanism experimentally. The results obtained are as follows: (1) In general running condition the impact sound which is generated periodically with tooth meshing frequency is the most powerful component in the noise. And it was ascertained that the impact sound is caused by the collision of tooth tips of the driving wheel against bottom lands of the toothed belt. (2) When the belt tension is large enough the transverse vibration (string vibration) of belt resonates badly with the meshing frequency and this resonant sound sway the noise level. (3) In a case where both impact and resonant sound are not predominant, the toothed belt drive runs silent enough.

An experimental and analytical study is made of the noise emission from the drawroll assembly of a textile draw-twisting machine. As an alternative to resorting to acoustic absorption techniques or the use of expensive high damping materials, investigations are made into the basic method of noise generation in the assembly. This leads to the incorporation of small design changes and the significantly lower noise emission thereby achieved is compared with the original emission. Aimed at the machine designer, the presentation is made in a straightforward, non-mathematical fashion.

Different aspects of multiple‐pure‐tone (MPT) “broadband” noise generation in high‐speed fans are discussed. An analytical model for multiple‐pure‐tone generation is presented. Experimental evidence indicates that the MPT generation does not seem to be a function of the presence of shock waves. It is also shown that fan “broadband” noise is not broadband in character and could be attributed to the modulation of the blade‐passing‐frequency tones, their sub‐ and higher harmonics.

Rotating blade noise is a topic of wide concern because it is the source of a variety of noise problems. These concerns range from safety, on the one hand, to community acceptability on the other. The purpose of this paper is to provide a general technical background for the problems of noise due to rotating blades. The topics to be covered are the following: the vehicles and components for which these problems are pertinent, the nature of the noise produced, the sources of the noise, concepts of noise generation, identification of the significant parameters in noise generation and reduction, and the methods of noise prediction. Both free rotors and ducted rotors are considered. In addition to the references cited in the text, this paper presents a bibliography of some of the more recent work (since 1968) in the areas of rotor and propeller noise, compressor and fan noise, and duct acoustics.

A study is being undertaken in order to gain a better understanding of how compressor noise is produced. To facilitate mathematical analysis, the noise-generating mechanisms of axial-flow compressor blades have been investigated using flat blades. Analytical expressions are given for calculating the compressor noise due to blade thickness, loading, and vortex shedding. Measured values of compressor noise show good agreement with analytical predictions. It has been found that the lift, or loading noise, dominates the blade thickness noise. Rotor-stator spacing is an important parameter in rotor noise generation.

This paper briefly describes preliminary work in the investigation of the flow in a rectangular cavity formed in one wall of a square channel. The results of visual studies, carried out on a water tunnel and showing the flow pattern for various depths of a cavity of fixed chord, are described. Also wind tunnel data on pressure loss and noise generation are discussed. The flow patterns found in an annular cavity are also described.

An investigation has been carried out into the generation of noise by large natural-draught cooling towers, and the mechanism of noise radiation from them, based on measurements made at four large power stations. An expression relating the A-weighted acoustic power of a tower to its physical characteristics has been derived. This, together with the discovery that the towers behave as area sources at low frequencies and as arc sources at high frequencies, has enabled a complete prediction method to be proposed. This method was used to estimate the noise from three further cooling towers and gave excellent agreement with the levels which were subsequently measured.