Foundations of Engineering Acoustics

Frank J. Fahy

• Éditeur: Academic Press
• Année: 2000
• ISBN:
  9780080506838
• Sujet: Technology & Engineering
• Sous-sujet: Mechanical Engineering

Foundations of Engineering Acoustics takes the reader on a journey from a qualitative introduction to the physical nature of sound, explained in terms of common experience, to mathematical models and analytical results which underlie the techniques applied by the engineering industry to improve the acoustic performance of their products. The book is distinguished by extensive descriptions and explanations of audio-frequency acoustic phenomena and their relevance to engineering, supported by a wealth of diagrams, and by a guide for teachers of tried and tested class demonstrations and laboratory-based experiments.

Foundations of Engineering Acoustics is a textbook suitable for both senior undergraduate and postgraduate courses in mechanical, aerospace, marine, and possibly electrical and civil engineering schools at universities. It will be a valuable reference for academic teachers and researchers and will also assist Industrial Acoustic Group staff and Consultants.

• Comprehensive and up-to-date: broad coverage, many illustrations, questions, elaborated answers, references and a bibliography
• Introductory chapter on the importance of sound in technology and the role of the engineering acoustician
• Deals with the fundamental concepts, principles, theories and forms of mathematical representation, rather than methodology
• Frequent reference to practical applications and contemporary technology
• Emphasizes qualitative, physical introductions to each principal as an entrée to mathematical analysis for the less theoretically oriented readers and courses
• Provides a 'cook book' of demonstrations and laboratory-based experiments for teachers
• Useful for discussing acoustical problems with non-expert clients/managers because the descriptive sections are couched in largely non-technical language and any jargon is explained
• Draws on the vast pedagogic experience of the writer

1

Sound Engineering

1.1 The importance of sound

Sound is a ubiquitous component of our environment from which there is no escape. Even in the darkness of a deep underground cavern, the potholer hears the sound of the operation of his or her body. In the dark depths of the ocean, creatures communicate by sound, which is the only form of wave that propagates over long distances in water. Only in the reaches of cosmic space, and in high vacuums created on Earth, are atoms so isolated that the chance of interaction, and hence the existence of sound, is negligible.

Sound is one of the principal media of communication between human beings, between higher animals, and between humans and domesticated animals. Sound informs us about our environment; as a result of evolution we find some sounds pleasant and some redolent of danger. The universal importance of music to human beings, and its emotional impact, remain mysterious phenomena that have yet to be satisfactorily explained. Unlike our eyes, our ears are sensitive to sound arriving from all directions; as such they constitute the sensors of our principal warning system, which is alert even when we are asleep.

So, sound is vitally important to us as human beings. But, apart from audio engineers who capture and reproduce sound for a living, why should engineers practising in other fields have any professional interest in sound? The short answer has two parts. On the positive side, sound can be exploited for many purposes of concern to the engineer, as indicated later in this chapter. On the negative side, excessive sound has adverse psychological and physiological effects on human beings that engineers are employed to mitigate, preferably by helping to design inherently quiet machines, equipment and systems: but failing this, by developing and applying noise control measures.

The adverse effects of excessive sound in causing hearing damage, raising stress levels, disturbing rest and sleep, reducing the efficiency of task performance, and interfering with verbal and musical communication, are widely experienced, recognized and recorded. In recent years, noise has become a major factor in influencing the marketability and competitiveness of industrial products such as cars and washing machines, as evidenced by advertising material. Many products are required to satisfy legal and regulatory requirements that limit the emission of noise into work places, homes and the general environment. Failure to meet these requirements has very serious commercial consequences. Aircraft are not certificated for commercial operation unless they meet very stringent environmental noise limits. Road vehicles are not allowed on the road unless they satisfy legally enforced limits on roadside noise. Train noise is currently being subjected to the imposition of noise restrictions.

A less widely known adverse effect of excessive sound is its capacity to inflict serious fatigue damage on mechanical systems, such as the structures of aircraft, space rockets and gas pipelines, and to cause malfunction of sensitive components, such as the electronic circuits of Earth satellites. Sound is vitally important to the military, particularly with the advent of automated target recognition and ranging systems.

Sound is a tell-tale. It gives warning that mechanical and physiological systems are not in good health. Sound generated by the pulmonary and cardiovascular systems provides evidence of abnormal state or operation, as foreseen by Robert Hooke over 300 years ago. The production of equipment for monitoring the state of machinery via acoustic and vibrational signals is a multimillion dollar business. The cost of monitoring is small compared with the cost of one day’s outage of a 600 MW turbogenerator, which runs into more than one million dollars.

Taken together, these different aspects of the impact of sound on human beings and engineering products provide convincing reasons why acoustics is a fascinating subject of study and practice for engineers.

1.2 Acoustics and the engineer

Engineers conceive, model, analyse, design, construct, test, refine and manufacture devices and systems for the purpose of achieving practical ends: and, of course, to make money. This book deals with the concepts, principles, phenomena and theories that underlie the acoustical aspects of engineering. Not so long ago, the acoustics expert was only called in to the chief engineer’s office when something acoustical had gone wrong; he or she was expected to act as a sonic firefighter. Today, major engineering companies involve acoustically knowledgeable staff in all the stages of their programmes of new product development, from concept to commissioning.

The process of predicting the acoustical performance of a product or system at the ‘paper’ design stage is extremely challenging. The task is being progressively eased by the increasing availability of computer-based modelling and analysis software, particularly in the forms of finite element, boundary element and statistical energy analysis programs. However, the ‘blind’ application of these powerful routines brings with it the dangers of unjustified confidence in the resulting predictions. As in all theoretical analysis, it is vital that appropriate and valid models are constructed. The modeller must understand the physics of the problem tackled, particularly in respect of the relative influences on system behaviour of its geometric, material, constructional and operational parameters. Efficient design and development require engineers to identify those elements of a system that are likely to be critical in determining the sensitivity of system performance to design modifications.

A major problem facing the acoustical designer is that details that are apparently of minor importance in respect of other aspects of performance and quality often have a major influence on acoustical performance. This is often not recognized by their ‘non-acoustical’ colleagues who may introduce small modifications in ignorance of their acoustical impact. Unfortunately, it is frequently impossible to predict this impact precisely in quantitative terms because the available models are not capable of such precision. One example in point concerns the design of seals for foot pedals in cars. The acoustical engineer is fully aware of the adverse effect on interior noise of even very small gaps around a seal, but the influence of gap geometry and materialproperties of the seal on sound transmission is very difficult to predict. Another concerns damping, which has a major effect on the influence of structure-borne sound on noise level (see Chapter 10). But it is still not possible to model precisely the magnitude and distribution of damping caused by joint friction and the installation of trim components.

1.3 Sound the servant

One might gain the impression from perusal of the titles and contents of many of the currently available books on acoustics that practitioners are almost exclusively concerned with noise and vibration control. This unfortunately suggests that acousticians spend most of their time preventing undesirable things from happening – or remedying the situation when they do. In fact, engineering for quietness is intellectually and technically challenging, and most beneficial to society. However, there is more to engineering acoustics than noise control, as I hope to convince you in the following paragraphs. Sound and vibration can be put to many positive uses apart from the obvious ones of sound recording and reproduction.

Communication via sound waves is not confined to the air. Marine animals use it for long distance communication. Divers’ helmets largely exclude water-borne sound, so they can use a system in which a microphone in the helmet drives a small loudspeaker that radiates sound into the water. A sensor in the receiver’s helmet creates vibration in a bar held between the teeth, from where it is transmitted by bone conduction directly into the cochlea. Video pictures and data can be transmitted to base from autonomous underwater vehicles used to locate objects and to inspect and maintain offshore oil and gas rigs via acoustic waves. The vehicles can also be controlled using this form of communication.

One of the most important practical benefits of waves is that they can be exploited to investigate regions of space remote from the operator. Passive reception of sound provides information about events occurring in the environment of the receiver. Underwater sound has a particular importance in this respect, because the range of visibility is always short, and negligible in the depths of the ocean. Sound is used to detect and monitor marine animals for census and ecological research purposes. It also signals suboceanic geological activity. Its use in sonar (sound navigation and ranging) systems in the marine military sphere is well known. In a recent development, the reflection from objects of naturally occurring underwater sound provides a means of detection that does not reveal the presence of the listener: this is called ‘acoustic daylight’. Ultrasound cameras for underwater use are under development. Sound is increasingly used to locate and classify military vehicles on the field of battle. The vision system of most robots is based upon ultrasonic sensors. The chambers of nuclear reactors can be monitored for the onset of boiling by means of structure-borne sound transmitted from the fluid along solid waveguides.

Passive sound reception and analysis has been used for centuries as a means of monitoring the activity and state of the internal organs of animals, as exemplified by the sound of turbulence generated by the narrowing of arteries. It is now used to indicate the activity and state of health of the fluid transport systems of trees and tomato plants. Optimal watering regimes are based upon this phenomenon. Machine condition-monitoring systems that utilize sound and vibration signals as one of a setof indicators of machine ‘health’ are of vital importance to industry and system operators because they automatically signal malfunction and provide information about its cause, as well as allowing operators to avoid unnecessary maintenance and outage. Through a phenomenon known as ‘acoustic emission’, the structure-borne sound generated by strain indicates the occurrence of flaws in pressure vessels and other vital structural components. Ultrasonic tension measurement is applied to monitor bolt clamping force more accurately than the conventional torque measurement technique. Leaks in water pipes are detected and located by means of measuring the resulting sound at points on either side. Hardwood being dried in kilns is monitored acoustically to avoid over-rapid drying with consequent splitting. Acoustic detectors are used to monitor the presence of creatures that attack stores of grain in silos. The noise of shingle may be used to monitor transport rates in coastal erosion studies. The electrical response of the brains of persons under anaesthesia to sound impulses provides a good indication of the depth of unconsciousness and minimizes the possibility of conscious awareness of an operation.

Sound waves are used actively to detect the presence and nature of obstacles of all sorts, especially by bats, and under water, as in mine detection. Water flow in the Thames river, which flows through London, is monitored by an acoustic Doppler system. Ultrasound is increasingly exploited in ‘blind vision’ systems. Persons who have become blind as adults say they can ‘see’ better when it’s raining. Why do you think that is? Sound is used in sodar (sound and radar) systems to monitor meteorological phenomena in the atmosphere. The application of ultrasound in medical diagnostics is well known. The Doppler frequency shift of sound reflected from moving surfaces reveals heart motion and blood flow. Intense ultrasound is focused to break up kidney stones in a procedure called ‘lithotripsy’. The sound transmission characteristics of the heel bone provide an early warning of the onset of osteoporosis.

Ultrasound has many industrial applications, including cleaning, cutting, drilling and peening, and, most importantly, in evaluating the quality of welds in thick pressure vessels and gas distribution pipes. It has a host of metrological applications, not only in industry, but, for example, to measure the shape of the cornea of the eye in clinical and surgical work. An acoustic meter of domestic gas flow is currently replacing millions of mechanical systems in Europe. Profiling of the ocean bed is performed by sonar systems. Insonification of chemical mixtures speeds up reactions. Very intense low audio-frequency sound causes particles in the exhaust stacks of power stations to agglomerate so that they may be more easily removed by scrubbers.

Some of the more unusual applications include the following. The ripeness of fruits of various kinds may be evaluated from the speed of sound that passes through them. Pulses of ultrasound, emitted by piezoelectric transducers driven by light transmitted down an optical fibre, are used to actuate pneumatic switches in a few milliseconds. Fishing nets that radiate sound are employed to protect whales that lead fishermen in Canada to fish shoals from becoming enmeshed in the fishing nets. Acoustic shark barriers are also in use near swimming beaches. Acoustic refrigerators are now commercially available and thermoacoustic engines are under development. In Denmark, photo-acoustic sensors are deployed by the civil defence service to detect very small traces of nerve gas. Intense low-frequency sound generated at Heard Island in the Indian Ocean is transmitted around the world and received at a number of stations many thousands of miles away to monitor the temperature of the sea as part of global warming research.

These are but a fraction of the multitude of practical applications of sound. Most of them require a thorough understanding of the physical behaviour of sound for the designs to be efficient and effective. Engineering acousticians will have plenty of challenges other than noise control in the future.

2

The Nature of Sound and Some Sound Wave Phenomena

2.1 Introduction

As a prelude to the analytical expositions presented in the succeeding chapters, this chapter presents a brief descriptive introduction to the nature of sound, qualitatively describes a range of phenomena exhibited by wave fields, and draws the attention of the reader to some examples of acoustic wave phenomena that are experienced in everyday life. Although we usually associate the subject of acoustics with sound in fluids (gases and liquids), sound may also be considered to travel in solid structures in the form of audio-frequency vibrational waves. The characteristics and forms of behaviour of structure-borne waves are more complex and difficult to analyse than those of fluid-borne sound. Structure-borne sound is briefly introduced in this chapter, but a detailed exposition is postponed until Chapter 10. This chapter focuses principally on sound in fluids, particularly in air.

2.2 What is sound?

The phenomenon of sound in a fluid essentially involves time-dependent changes of density, with which are associated time-dependent changes of pressure, temperature and positions of the fluid particles. (The concept of ‘particle’ will be explained more precisely in the next chapter, but for t...

Table des matières

• Cover image
• Title page
• Table of Contents
• Copyright
• Preface
• Acknowledgements
• Chapter 1: Sound Engineering
• Chapter 2: The Nature of Sound and Some Sound Wave Phenomena
• Chapter 3: Sound in Fluids
• Chapter 4: Impedance
• Chapter 5: Sound Energy and Intensity
• Chapter 6: Sources of Sound
• Chapter 7: Sound Absorption and Sound Absorbers
• Chapter 8: Sound in Waveguides
• Chapter 9: Sound in Enclosures
• Chapter 10: Structure-Borne Sound
• Chapter 11: Transmission of Sound through Partitions
• Chapter 12: Reflection, Scattering, Diffraction and Refraction
• Appendix 1: Complex Exponential Representation of Harmonic Functions
• Appendix 2: Frequency Analysis
• Appendix 3: Spatial Fourier Analysis of Space-Dependent Variables
• Appendix 4: Coherence and Cross-Correlation
• Appendix 5: The Simple Oscillator
• Appendix 6: Measures of Sound, Frequency Weighting and Noise Rating Indicators
• Appendix 7: Demonstrations and Experiments
• Answers
• Bibliography
• References
• Index