1 FUNDAMENTALS OF ACOUSTICS - World Health …

1 1 FUNDAMENTALS OF ACOUSTICSP rofessor Colin H HansenDepartment of Mechanical EngineeringUniversity of AdelaideSouth Australia aspects of ACOUSTICS are presented, as they relate to the understanding andapplication of a methodology for the recognition, evaluation and prevention or control of noiseas an occupational hazard. Further information can be found in the specialised literature listedat the end of the PHYSICS OF SOUNDTo provide the necessary background for the understanding of the topics covered in thisdocument, basic definitions and other aspects related to the physics of sound and noise arepresented. Most definitions have been internationally standardised and are listed in standardspublications such as IEC 60050-801(1994). Noise can be defined as "disagreeable or undesired sound" or other disturbance.

2 From theacoustics point of view, sound and noise constitute the same phenomenon of atmosphericpressure fluctuations about the mean atmospheric pressure; the differentiation is greatlysubjective. What is sound to one person can very well be noise to somebody else. Therecognition of noise as a serious Health hazard is a development of modern times. With modernindustry the multitude of sources has accelerated noise-induced hearing loss; amplified musicalso takes its toll. While amplified music may be considered as sound (not noise) and to givepleasure to many, the excessive noise of much of modern industry probably gives pleasure tovery few, or none at all. Sound (or noise) is the result of pressure variations, or oscillations, in an elastic medium( , air, water, solids), generated by a vibrating surface, or turbulent fluid flow.

3 Soundpropagates in the form of longitudinal (as opposed to transverse) waves, involving a successionof compressions and rarefactions in the elastic medium, as illustrated by Figure (a). Whena sound wave propagates in air (which is the medium considered in this document), theoscillations in pressure are above and below the ambient atmospheric Amplitude, Frequency, Wavelength And VelocitySound waves which consist of a pure tone only are characterised by: the amplitude of pressure changes, which can be described by the maximum pressureamplitude, pM, or the root-mean-square (RMS) amplitude, prms, and is expressed in Pascal(Pa). Root-mean-square means that the instantaneous sound pressures (which can be positiveFundamentals of acoustics24 Figure Representation of a sound wave.(a)compressions and rarefactions caused in air by the sound wave.)

4 (b)graphic representation of pressure variations above and belowatmospheric negative) are squared, averaged and the square root of the average is taken. The quantity,prms = pM; the wavelength ( ), which is the distance travelled by the pressure wave during one cycle; the frequency (f), which is the number of pressure variation cycles in the medium per unittime, or simply, the number of cycles per second, and is expressed in Hertz (Hz). Noise isusually composed of many frequencies combined together. The relation between wavelength and frequency can be seen in Figure the period (T), which is the time taken for one cycle of a wave to pass a fixed point. It isrelated to frequency by:T = 1/fFigure Wavelength in air versus frequency under normal conditions (after Harris1991).The speed of sound propagation, c, the frequency, f, and the wavelength, , are related by thefollowing equation:c = f the speed of propagation, c, of sound in air is 343 m/s, at 20(C and 1 atmosphere other temperatures (not too different from 20(C), it may be calculated using:c = 332 + of acoustics25c RTk/M(m s 1)(1)Figure Sound generation illustrated.)

5 (a) The piston moves right, compressing air asin (b). (c) The piston stops and reverses direction, moving left and decompressing air infront of the piston, as in (d). (e) The piston moves cyclically back and forth, producingalternating compressions and rarefactions, as in (f). In all cases disturbances move to theright with the speed of sound. where Tc is the temperature in (C . Alternatively the following expression may be used forany temperature and any gas. Alternatively, making use of the equation of state for gases, thespeed of sound may be written as: where Tk is the temperature in (K, R is the universal gas constant which has the value per mole(K, and M is the molecular weight, which for air is kg/mole. For air, theratio of specific heats, , is of the properties just discussed (except the speed of sound) apply only to a pure tone (singlefrequency) sound which is described by the oscillations in pressure shown in Figure , sounds usually encountered are not pure tones.)))

6 In general, sounds are complexmixtures of pressure variations that vary with respect to phase, frequency, and amplitude. Forsuch complex sounds, there is no simple mathematical relation between the differentcharacteristics. However, any signal may be considered as a combination of a certain number(possibly infinite) of sinusoidal waves, each of which may be described as outlined above. Thesesinusoidal components constitute the frequency spectrum of the illustrate longitudinal wave generation, as well as to provide a model for the discussionof sound spectra, the example of a vibrating piston at the end of a very long tube filled with airwill be used, as illustrated in Figure the piston in Figure move forward. Since the air has inertia, only the air immediatelynext to the face of the piston moves at first; the pressure in the element of air next to the pistonincreases.

7 The element of air under compression next to the piston will expand forward, FUNDAMENTALS of acoustics26ppptttfff1f1f2f3 Frequency bands(a)(c)(e)(b)(d)(f)p2p2p2 Figure Spectral analysis illustrated. (a) Disturbance p varies sinusoidally with time tat a single frequency f1, as in (b). (c) Disturbance p varies cyclically with time t as acombination of three sinusoidal disturbances of fixed relative amplitudes and phases; theassociated spectrum has three single-frequency components f1, f2 and f3, as in (d).(e) Disturbance p varies erratically with time t, with a frequency band spectrum as in (f).displacing the next layer of air and compressing the next elemental volume. A pressure pulse isformed which travels down the tube with the speed of sound, c. Let the piston stop andsubsequently move backward; a rarefaction is formed next to the surface of the piston whichfollows the previously formed compression down the tube.

8 If the piston again moves forward,the process is repeated with the net result being a "wave" of positive and negative pressuretransmitted along the the piston moves with simple harmonic motion, a sine wave is produced; that is, at anyinstant the pressure distribution along the tube will have the form of a sine wave, or at any fixedpoint in the tube the pressure disturbance, displayed as a function of time, will have a sine waveappearance. Such a disturbance is characterised by a single frequency. The motion andcorresponding spectrum are illustrated in Figure and the piston moves irregularly but cyclically, for example, so that it produces the waveformshown in Figure , the resulting sound field will consist of a combination of sinusoids ofseveral frequencies. The spectral (or frequency) distribution of the energy in this particular soundwave is represented by the frequency spectrum of Figure As the motion is cyclic, thespectrum consists of a set of discrete some sound sources have single-frequency components, most sound sourcesproduce a very disordered and random waveform of pressure versus time, as illustrated in FigureFundamentals of Such a wave has no periodic component, but by Fourier analysis it may be shown that theresulting waveform may be represented as a collection of waves of all frequencies.

9 For a randomtype of wave the sound pressure squared in a band of frequencies is plotted as shown; forexample, in the frequency spectrum of Figure is customary to refer to spectral density level when the measurement band is one Hz wide,to one third octave or octave band level when the measurement band is one third octave or oneoctave wide and to spectrum level for measurement bands of other special kinds of spectra are commonly referred to as white random noise and pinkrandom noise. White random noise contains equal energy per hertz and thus has a constantspectral density level. Pink random noise contains equal energy per measurement band and thushas an octave or one-third octave band level which is constant with Sound Field Definitions (see ISO 12001) Free fieldThe free field is a region in space where sound may propagate free from any form of Near fieldThe near field of a source is the region close to a source where the sound pressure and acousticparticle velocity are not in phase.

10 In this region the sound field does not decrease by 6 dB eachtime the distance from the source is increased (as it does in the far field). The near field is limitedto a distance from the source equal to about a wavelength of sound or equal to three times thelargest dimension of the sound source (whichever is the larger). Far fieldThe far field of a source begins where the near field ends and extends to infinity. Note that thetransition from near to far field is gradual in the transition region. In the far field, the direct fieldradiated by most machinery sources will decay at the rate of 6 dB each time the distance from thesource is doubled. For line sources such as traffic noise, the decay rate varies between 3 and Direct fieldThe direct field of a sound source is defined as that part of the sound field which has not sufferedany reflection from any room surfaces or Reverberant fieldThe reverberant field of a source is defined as that part of the sound field radiated by a sourcewhich has experienced at least one reflection from a boundary of the room or enclosurecontaining the Frequency AnalysisFrequency analysis may be thought of as a process by which a time varying signal in the timedomain is transformed to its frequency components in the frequency domain.