Acoustics and Noise Control

R J Peters

• Publisher: Routledge
• Year: 2013
• ISBN:
  9781317902980
• Edition: 3
• Topic: Tecnología e ingeniería
• Subtopic: Construcción e ingeniería arquitectónica

Acoustics and Noise Control provides a detailed and comprehensive introduction to the principles and practice of acoustics and noise control. Since the last edition was published in 1996 there have been many changes and additions to standards, laws and regulations, codes of practice relating to noise, and in noise measurement techniques and noise control technology so this new edition has been fully revised and updated throughout.

The book assumes no previous knowledge of the subject and requires only a basic knowledge of mathematics and physics. There are worked examples in the text to aid understanding and a range of experiments help students use complicated apparatus.

Thoroughly revised to cover the latest changes in standards, codes of practice and legislation, this new edition covers much of the Institute of Acoustics Diploma syllabus and has an increased emphasis on the legal issues relating to noise control.

Chapter 1 The nature and behaviour of sound

1.1 A qualitative picture of wave motion

Acoustics is the science of sound, and sound is a wave motion. In a wave a change or disturbance in some physical property of a medium is transmitted through that medium. For example when a sound occurs in air the sound wave causes the particles in the air to move to and fro (i.e. to vibrate), and because the particles are elastically connected (air being an elastic medium) this vibration is transmitted through the air. The vibrating layers of air contain energy and so another feature of all waves is that they contain energy. The essential features of a medium which is able to transmit sound waves are that it must possess elasticity and inertia (mass); sound waves can travel through solids, liquids and gases but not through a vacuum. In any real medium there will always be some frictional processes at work so that some of the energy of the vibrating particles of the medium will be lost to the sound wave and turned into heat, a process known as sound absorption.

The two simplest types of sound waves are spherical waves and plane waves (see Figure 1.1), and it helps to understand them if we consider the analogy of waves on the surface of water. If we drop a small object such as a stone into water we see circular ripples travelling outwards. The invisible spherical sound waves in air are their three-dimensional counterparts. If the stretch of water is linear, e.g. a canal, and the stone is replaced by a long plank of wood we would see plane ripples move along the water surface.

The wavefront represents the leading edge of the wave, i.e. it tells us how far the wave has travelled and the rays, always perpendicular to the wavefronts, indicate the direction in which the wave is travelling.

These two forms of wave are idealized models of wave propagation and are useful because waves for sound sources can often approximate to one of these models. Sound from a loudspeaker tends to radiate equally in all directions at low frequencies (i.e. like spherical waves) but be much more directional (i.e. more like plane waves) at high frequencies.

Plane waves travelling in one direction only are the simplest form of waves and can be used to explain frequency and wavelength.

Figure 1.1 Sketch illustrating rays and wavefronts for spherical and plane waves

In a sound wave in air, as a result of the to and fro motion, sometimes the air particles are bunched together, causing a very slight increase in pressure in the atmospheric pressure (a compression) and sometimes causing them to be spaced further apart, causing a very slight reduction in pressure (a rarefaction). This is shown in Figure 1.2 where compressions and rarefactions from the vibrations of a tuning fork are shown travelling in one dimension (down a tube or pipe for example). These very small fluctuations in pressure in the tube constitute the sound pressure caused by the passage of the sound wave down the tube.

The disturbance caused by the sound waves could be described in terms of the vibrations of the air particles, either as a displacement, as a velocity or as an acceleration, and these alternatives will be discussed in more detail in Chapter 7 on vibration. However, since these movements cannot be seen, and since our human ears and our microphones respond to the changes in pressure caused by sound waves it is more usual to measure and describe sound waves in terms of sound pressure, in pascals (Pa.).

Figure 1.2 Propagation of a one-dimensional sound wave

The simplest form of plane wave occurs when the vibration of the air particles causes a sinusoidal variation in sound pressure with time and can be used to explain frequency and wavelength. The sound pressure in such a plane wave varies with both distance and time, as shown in Figures 1.3 and 1.4. This sinusoidal variation of sound pressure with time represents a sound with a single frequency, called a pure tone.

1.2 Frequency and wavelength and sound speed

After a certain amount of time, called the period, T, of the motion, the cycle repeats itself (Figure 1.3). The frequency, f, of the vibration and of the wave is the number of cycles of the motion which occur in one second:

ƒ = 1/T

Thus frequency, ƒ, is measured in cycles per second or hertz (abbreviation Hz).

The wavelength, λ, is the minimum distance between points on the wave where the air particles are vibrating in step or in phase as shown in Figures 1.1 and 1.3.

The relationship between sound speed, frequency and wavelength

In order for air particles, which are one wavelength apart, to be in phase, it must be the case that the wave travels one wavelength in the time that it takes for any one of the particles to complete one cycle of motion. Since the number of such cycles completed in one second corresponds to the frequency of the wave, and since the wave velocity is the distance travelled by the wave in one second, it follows that frequency, ƒ, wavelength, λ, and wave velocity, c, are related by the well known equation:

c = f λ

Figure 1.3 Graph showing variation of sound pressure with time (at one position in space) for a pure tone

Figure 1.4 Graph showing variation of sound pressure with position (at one moment of time) for a pure tone

For sound waves in air the speed of sound ranges, approximately, between 330 and 340 metres per second, depending upon air temperature. Thus for a frequency of 100 Hz, at the lower end of the audio range the wavelength will be about 3.3 metres, whereas at the much higher frequency of 1000 Hz it is about 0.33 metres, i.e. the lower the frequency the greater the wavelength, and vice versa.

Note that the frequency of the wave is de...

Table of contents

• Cover Page
• Half Title Page
• Title Page
• Copyright Page
• Brief contents
• Contents
• Preface
• Authors’ acknowledgements
• Publisher’s acknowledgements
• Terminology and notation
• 1 The nature and behaviour of sound
• 2 Sound propagation
• 3 Human response to noise
• 4 Environmental noise
• Room acoustics
• 6 Sound insulation
• 7 Vibration
• 8 Measurement and instrumentation
• 9 Noise control
• 10 The law relating to noise
• Appendix 1 Glossary of acoustical terms
• Appendix 2 List of formulae
• Appendix 3 Suggested list of experiments, tests and observations
• Appendix 4 Some electrical principles
• Answers
• Bibliography
• Index