An Introduction to Sound and Noise

Transcription

1 An Introduction to Sound and Noise Chapter 1 Unless you re unfortunate enough to be deaf, sound is such a common part of everyday life that we rarely appreciate all of its functions. It provides us with pleasure - such as that obtained by listening to music or the gentle sounds of a rain forest. It allows us to communicate with other human beings and animals. It can be used to alert or warn us - for example a ringing telephone means that someone wished to talk to us, or a wailing siren signifies danger. Sound being one of our principle senses also permits us to make quality evaluations and diagnoses. We can hear when our car engine misfires telling us it needs tuning, a squeaking valve on an instrument means it needs lubrication; a heart murmur tells us we are sick; and so on. Not all sounds are associated with pleasure. Many of the sounds associated with our modern society annoy us. This is of course a subjective judgement. For example some people enjoy heavy metal music, whilst others think that its only function is to kill the neighbours grass if it is played loud enough. Any sound that is unpleasant or unwanted is called noise. The level of annoyance of noise depends not only on the quality of the sound, but also our attitude towards it. The sound of a jet aircraft taking off may be pleasant to the ears of the pilot and passengers, but will be ear-splitting agony for the people living near the end of the runway. Sound doesn't need to be loud to annoy. A rattling window on a quiet night, or a dripping tap can be just as annoying as a jet aircraft. Remember the Chinese water torture used repetitive quiet sounds from a dripping tap. Loud sounds can damage and destroy. A sonic boom can shatter windows and shake plaster off walls. But the most unfortunate case is when sound damages the delicate mechanism designed to receive it the human ear. When this happens our hearing is impaired. The loss may be temporary, or permanent depending on how often and for how long we are exposed. What is Sound, What is noise? When an object moves backward and forward in a repetitive motion it is said to vibrate or oscillate. If this occurs in a medium such as air the particles near the object are affected and start vibrating at the same rate, producing variations in the normal air pressure. This disturbance spreads, and eventually may reach a human ear, which translates these vibrations into a sensation that we call sound.

2 The simplest definition of sound is therefore any pressure variation or oscillation in a conductive medium that may be detected by the human ear. The most common conductive media are air or water, although sound may be conducted directly through solid structures such as metal pipes etc. This definition does not include any oscillation that cannot be detected by the human ear. An example of this would be fluctuations in air pressure caused by weather changes. These can be accurately measured by a barometer, but are too slow to be picked up by a normal human ear, with the exception of when we move rapidly from one height to another, such as in an aircraft. In this instance we do not hear sound but rather experience a slow pressure build up in the ear. Noise is characterised by a lack of order or regularity in its sound wave. This generally (but not always) results in tones that are less pleasing to the human ear. Musical instruments by contrast produce regular patterns in the waveforms that they generate. A reasonable definition of noise is therefore a complex sound that lacks regularity or order in its waveform. It may also be defined more simply as any unwanted or undesired sound, or even as any sound not native to the environment although this last definition would make all musical instruments noise a view with which many would not concur. Figure 1.1 A typical noise waveform. Note the random variations in shape of waves. Important terms associated with sound and noise In order to be able to explain concepts associated with sound and noise it is important to define some of the most essential terminology used in the area. These are given below, but more extensive definitions are given in the sound and noise dictionary at the end of this resource package. Frequency and Pitch The human ear requires pressure variations of at least 20 times per second in order for us to be able to perceive them as sound. The number of pressure variations (or complete 7

3 waves that pass a given point) per second is called the frequency of the sound. It is measured in units called Hertz, which are commonly shortened to Hz. A scream produces a high pitched - or high frequency noise. This makes our eardrums vibrate rapidly. A growl produces a low pitched - or low frequency sound, which makes our eardrums vibrate at a much slower rate. Hence we can see that pitch is related to frequency the greater the frequency, the higher the pitch. Note that this relationship is not linear in that a twofold increase in frequency does not double the pitch of the sound. Pitch is most easily defined as the subjective response to frequency. Frequencies that are too high to be detected by the human ear are called ultrasound. These are commonly used in medical diagnosis and imaging. Wavelength As you probably remember from your childhood it is possible to estimate the distance of a thunderstorm by using a general rule - for every three second gap between the thunder and the lightning, the storm is one kilometre away. To be more accurate we can say that the speed with which sound spreads throughout a medium depends on the mass and elastic properties of the medium. In air at around 25 C, sound travels at around 344 m/sec. (or 1238 kilometres per hour). In other media such as water speeds are different. Sound generally moves much faster in liquids and solids than in gases. The speed of sound in water, for example, is slightly less than 1,525 m/sec at ordinary temperatures but increases greatly with an increase in temperature. The speed of sound in copper is about 3,353 m/sec at ordinary temperatures and decreases as the temperature is increased (owing to decreasing elasticity); in steel, which is more elastic, sound moves at a speed of about 4,877 m/sec. Sound is propagated very efficiently in steel. If we know the speed and frequency of sound we can calculate the wavelength of the sound waves that is the distance from the top of one wave or pressure peak to another. This is done by using the equation; ( λ ) Wavelength = Speed of Sound Frequency Using this equation we can see that at 20 Hz one complete wavelength is about 17.2 meters, whilst at Hz it is only 1.72cm. From this it might be obvious that low frequency sounds have long wavelengths and high frequency sounds have short wavelengths. This is demonstrated in figure

4 Figure 1.2 The relationship between sound frequency (Hz) and wavelength (from Brüel and Kjær Measuring Sound 1 ). Sounds that are made up of a single frequency are referred to as pure tones. These are quite rare, but one example is the tuning fork used to tune the keys on a piano. The middle C key is exactly Hz. Another example is the test tones used to calibrate audio equipment. Commonly 1000Hz and 10000Hz are used. Most sounds are made up of a combination of many frequencies. Even the notes of musical instruments are complex waveforms made up from many different frequencies. Most everyday noise consists of a wide mixture of frequencies. This is commonly referred to as broad band noise. When the noise has frequencies evenly distributed throughout the audible range it is known as white noise. This sound is somewhat like the sound of water near a waterfall. Amplitude, Intensity, Loudness and Sound Pressure Some sounds are so loud that we need to cover our ears whilst others are so soft that we can barely hear them, but this will depend on the person, the type of sound and the environment in which it is heard. Loudness is a very subjective measurement in that if the sound is pleasant, then it may not be perceived as being too loud, whilst if it is undesired, even a much lower loudness level may be too loud. Loudness also depends on the response of the ear to sound, and the human ear is not equally responsive (sensitive) to all frequencies. For example the ear does not respond at all to frequencies lower than 20Hz or above 20000Hz, but is very responsive to those in the region Hz. Loudness is best defined as the subjective judgment of intensity of a sound by humans, and depends on the sound intensity and frequency of the stimulus. Sound pressure (or sound intensity) depends on the amplitude of the vibration of the particles in the medium. 9

5 Amplitude is simply how far a particle moves from the rest position when a wave passes, or more simply - the pressure fluctuation. The greater the particle moves from the rest position the larger the sound wave and the greater the amount of energy that is carried by the wave. If you have trouble with this concept think about the last time you were at the beach. The small waves on the shore lap up against your feet, but do not cause any movement. If you go to Hawaii however, and jump into the big surf one of those 6 meter waves have enough energy to break every bone in your body. In the case of sound waves the energy is passed on as increases in air pressure. Figure 1.3 demonstrates the relationship between amplitude (sound intensity or pressure) and wavelength. Figure 1.3 The relationship between sound pressure and wavelength (from Brüel and Kjær Measuring Sound). Over much of the frequency range it takes about a threefold increase in sound intensity (a tenfold increase in acoustical energy, or, 10 db see this section for definitions) to produce a doubling of loudness Compression and Rarefaction The air vibrates in response to pressure variations in the air (or other medium transporting the sound) when these vibrations are rapid enough and intense (loud) enough. These pressure variations are caused by a series of compressions and rarefactions moving out from the sound source to make the sound wave. The best way to explain the meaning of these terms is to look at the relative density of air particles in a tube. If we place a piston in the tube, but do not move it, the air particles are evenly spread throughout the tube. If however the piston is pushed forward, the particles immediately in front of the piston are compressed together. If the piston is stopped this 10

6 compression is transferred out of the tube as a compression wave which the ear can detect as sound if it is of the right frequency and intensity. If the piston is pulled out of the tube it produces a partial vacuum resulting in a lowpressure wave in the air particles near the piston surface. This is referred to as rarefaction. Movement of the piston backward and forward produces successive compression and rarefaction waves which move out from the source through the air layers and we hear as sound. This is exactly how a loudspeaker system works. Figure 1.4 demonstrates this concept. Note that the individual air molecules do not move very far, rather they pass their energy on to the adjacent molecule. To explain this, consider when a rock is thrown into a pool of water, causing a series of waves to move out from the point of impact. A cork floating near the point of impact will bob up and down, with respect to the direction of wave motion, but will show little if any outward or longitudinal motion. A sound wave on the other hand, is a longitudinal wave. As the energy of wave motion is propagated outward from the centre of disturbance, the individual air molecules that carry the sound move back and forth, parallel to the direction of wave motion. Thus, a sound wave is a series of alternate compressions and rarefactions of the air. Each individual molecule passes the energy on to neighbouring molecules, but after the sound wave has passed, each molecule remains in about the same location. Figure 1.4 Compression and rarefaction wave formation 11

7 Sound Power or Acoustical Energy No doubt when you purchased your last stereo system you were confronted by salespersons who extolled the virtues of their goods by saying that they produce zillions of Watts of music power output or more. We interpret this as how loud the system will be when we turn up the volume, but from our ears point of view its how much energy is reaching them as sound pressure waves. As mentioned in the previous section, when a sound wave is propagated energy is transferred through the medium in which it travels, and sound power is a measure of how much energy is passed on by the source as sound waves. The term acoustic energy is also used to describe sound power. Sound Quality If the note A above middle C is played on a violin, a piano, and a tuning fork, all at the same volume, the tones are identical in frequency and amplitude, but very different in quality. Of these three sources, the simplest tone is produced by the tuning fork, the sound in this case consisting almost entirely of vibrations having frequencies of 440 hertz. Because of the acoustical properties of the ear and the resonance properties of the ear's vibrating membrane, however, it is doubtful whether a pure tone reaches the inner hearing mechanism in an unmodified form. The principal component of the note produced by the piano or violin also has a frequency of 440 hertz, but these notes also contain components with frequencies that are exact multiples of 440, called overtones, such as 880, 1320, and The exact intensities of these other components, which are called harmonics, determine the quality of the note 2. How we hear The most obvious answer to this question is with our ears. But the whole process is a little more complicated than this simplistic view. The human ear is constructed of three main sections; the outer ear, the middle ear, and the inner ear. The outer ear consists of a fleshy outer section called the Auricle or pinna and the auditory canal, the hole in the side of our head through which the sound waves enter. The outer ear is responsible for collecting the sound waves. The middle ear, on the inner side of the eardrum, is responsible for conduction of sound waves to the internal ear. This process is often referred to as modulation. It is a narrow passage, extending vertically for about 15 mm and then about 10-15mm horizontally. The Eustachian tube allows the middle ear to directly communication with the back of the nose and throat. This allows air into and out of the middle ear. The middle ear also contains three small, movable bones called the ossicles: the malleus, or hammer; the incus, or anvil; and the stapes, or stirrup. These connect the eardrum acoustically to the fluid-filled internal ear. The inner ear, (sometimes referred to as the labyrinth) is the part of the temporal bone containing the organs of hearing and balance. It is directly connected to the filaments of 12

8 the auditory nerve. It is separated from the middle ear by the fenestra ovalis, or oval window. The inner ear consists of membrane bound canals housed in the temporal bone and is divided into the cochlea, the vestibule, and three semicircular canals. All these canals communicate with one another and are filled with a gelatinous fluid called endolymph. Figure 1.5 shows the structure of the ear. Figure 1.5 The structure of the human ear (from Microsoft Encarta ) Hearing Sound waves are carried through the external auditory canal to the eardrum (also known as the tympanic membrane), causing it to vibrate. These vibrations are transferred by the hammer, anvil and stirrup in the middle ear through the oval window to the fluid in the inner ear. This disturbs the fluid in the cochlea and stimulates the movement of thousands of very sensitive fine hair-like projections called hair cells. Collectively these projections are called the organ of Corti. The hair cells transmit vibrations directly to the auditory nerve, which changes them into impulses, which carry information to the brain. The response of the hair cells to vibrations of the cochlea provides information about sound in a way that is interpretable by the brain's auditory centres. The range of hearing, like that of vision, varies in different persons. The maximum range of human hearing includes sound frequencies from about 16 28,000Hz, but the average healthy person has a normal hearing range of about 20 16,000Hz. This may be extended to 20 20,000Hz in young people. The least noticeable change in tone that can be picked up by the ear varies with pitch and loudness. A change of vibration 13

9 frequency (pitch) corresponding to about 0.03 per cent of the original frequency can be detected by the most sensitive human ears in the range between 500 and 8,000Hz. The ear is less sensitive to frequency changes for sounds of low frequency or low intensity. The sensitivity of the ear to sound intensity (loudness) also varies with frequency. Sensitivity to change in loudness is greatest between 1,000 to 3,000Hz, where a change of one decibel can be detected and becomes less when sound-intensity levels are lowered. The variation in the sensitivity of the ear to loud sounds causes several important phenomena. Extremely loud sounds produce in the ear entirely different tones that are not present in the original tone. These subjective tones are probably caused by imperfections in the natural function of the middle ear. The harshness in tonality caused by greatly increasing sound intensities (as found when a radio volume control is adjusted to produce excessively loud sounds) results from subjective tones produced in the ear. The loudness of a pure tone also affects its pitch. High tones may increase as much as a whole musicalscale note; low tones tend to become lower as sound intensity increases. This effect is noticeable only for pure tones. Because most musical tones are complex, hearing is usually not affected to an appreciable degree by this phenomenon. In sound masking, the production in the ear of harmonics of lower-pitched sounds may deafen the ear to the perception of higher-pitched sounds. Masking is what makes it necessary to raise your voice in order to be heard in a noisy place 2. Prolonged exposure to loud sounds can damage the hair cells, which may result in hearing impairment. Initially damage may only occur to a few hair cells which is hardly noticeable as the brain can compensate for the loss, but as more are damaged it cannot make up for the loss of information and noticeable changes in hearing occur. Most commonly speech and background cannot be distinguished, whilst some words may mix together. By the time this begins to occur it is likely that irreparable damage to the ear has occurred. Hearing loss due to noise exposure is generally greatest at those frequencies where the human ear is most sensitive around 4000Hz. How we measure sound When we wish to measure sound we generally require information about how loud (or intense) a sound is, and which frequencies are being generated. Sound Intensity (Sound Pressure Levels) As we have already stated the human ear is a very sensitive organ. It is capable of detecting a pressure variation of as little as 2 parts in in the air particles around us. This is about 20 millionths of a Pascal (20µPa), or about 5,000,000,000 times less than normal atmospheric pressure. From the chemists perspective, this causes the eardrum to move by less than the diameter of one hydrogen molecule! At the other end of the scale the ear can tolerate sound pressure variations as much as 1,000,000 times greater. This means that if we were to use the normal units for measuring pressure, Pascals, we would end up with very large and cumbersome numbers. To avoid this a special scale has been 14

10 adopted for sound measurement (although it was originally borrowed from electrical communication engineers). It is called the decibel or db scale. The Decibel scale is a measure of the intensity of sound received at a particular point. This must be distinguished from a sound source, which measures its intensity of output in power units or Watts. The amount of sound intensity received depends not only on the power of the source, but also the area over which the energy is received (hence an absolute unit would be watts /m 2 ). The decibel is not an absolute unit of measurement however, but a ratio between a measured quantity and an agreed reference value. The agreed reference level is 20µPa, which is defined as 0dB commonly referred to as the threshold of hearing. In order to cope with the huge range of sound pressure variations detectable by the ear the decibel scale is logarithmic. This means that when we multiply the sound pressure in Pa by 10, we add 20dB to the sound level. Hence 200µPa corresponds to 20dB, 2000µPa to 40dB etc. Thus the db scale compresses a range of one million into 120dB. Figure 1.6 shows the sound pressure levels (SPL) in db of some familiar sounds. Figure 1.6 Sound pressure levels of some familiar sounds (from Fullick, Barker and Krajniak, Occupational Health & Safety in the Laboratory) 15

11 The decibel scale gives a much better approximation of the human perception of the relative loudness of a sound than the Pascal scale. This is because the ear also reacts logarithmically to sound pressure. Loudness levels Sound pressure levels are highly objective and are measured by the decibel scale. Loudness however, is a subjective quantity and must therefore be measured by a scale that is subjective (requiring human judgement). Loudness is measured in a unit called the phon. One phon is equal to the perceived loudness of a 1 db sound at 1,000Hz. Hence we can see that loudness is frequency dependent, as the human ear is more sensitive to some frequencies than others. For all other frequencies the loudness in phons is decided by comparison with sound at 1,000Hz. It is found that the threshold of hearing, which is assigned to be 0dB at 1,000Hz, is much higher in the lower frequency ranges. For example a 100Hz sound must have a sound pressure level of 40dB before it is audible. Regardless of the intensity for the threshold of hearing for a particular frequency, that loudness value is assigned to 0 phon. This allows a graph to be plotted showing the equal loudness contour for different sound frequencies. This graph also shows us which regions of the sound spectrum to which the human ear is most sensitive. This is generally 2,000 5,000Hz. Figure 1.7 shows an equal loudness contour graph. Sounds below the bottom (MAF) line cannot be heard, whilst those above can. MAF stands for minimum audible field. Figure 1.7 An equal loudness contour graph for 0 phon (from Peterson and Gross, Noise Measurement). Other animals have different responses to frequencies of sound, which is why dogs can hear whistles that are beyond the detection of the human ear. 16

12 At high sound pressure levels the ear is approximately equally sensitive to most frequencies, but at low intensities the ear is much more sensitive to the middle frequencies than to the lowest and highest. For example a 50Hz tone must be 15dB higher than a 1,000Hz tone at a sound level of 70dB in order to give the same perceived loudness. We can compare this to a 1,000Hz tone at a sound level of 10dB. Here a 50Hz tone must have a 35dB higher sound pressure level in order to give the same perceived loudness. This is why sound-reproducing equipment that is functioning perfectly will seem to fail to reproduce the lowest and highest notes if the volume is decreased. Hence manufacturers fit bass and treble boost for use when our stereo is turned down. Figure 1.8 demonstrates this. Figure 1.8 Equal loudness contours for different sound pressure levels (from Brüel and Kjær Measuring Sound 1 ). Figure 1.9 shows the intensities and frequencies of speech and music related to the total human hearing range. If the loudness is increased above about 120dB the perception of the ear is pain rather than sound, and this is less frequency dependant. This is referred to 17

13 as the threshold of pain. Exposure to sound at this level normally results in hearing damage. Figure 1.9 Graph of intensities and frequencies of speech and music related to the total human hearing range (from Brüel and Kjær Measuring Sound 1 ). How do we interpret relative changes in loudness? Basically the answer to this may be given by considering a group of sources all producing the same noise level. In order to reduce the apparent loudness of noise to about one half its original level we would have to reduce the total noise by a factor of 10! In other words, removing 9 of the noise sources would only halve the loudness level. The subjective effects of changes in sound pressure levels are given in Table

14 Table 1.1 Subjective effects of changes in sound pressure levels Change In Sound Pressure Level (db) Decrease Change in Power Increase Change in Apparent Loudness 3 1/2 2 Just perceptible 5 1/3 3 Easily noticed 10 1/10 10 Half or twice as loud 20 1/ Much quieter or much louder Sound Impulses Sounds of very short duration obtain a different response from the human ear than those of longer duration. Sounds of less than one second are referred to as impulsive. Examples of impulsive sounds include gunshots and hammer blows. Any sound with a duration of msec. will be perceived as quieter than the same sound of longer duration. References 1 Brüel and Kjær, Measuring Sound, Nærum, Structure of the Ear," Microsoft Encarta 97 Encyclopedia Microsoft Corporation. All rights reserved. 3 Johnson, C., Keogh, J., and Wood, T., Sound, Heinemann, Richmond, Fullick, G., Barker, D.J., and Krajniak, E., Occupational Health and Safety in the Laboratory, Harcourt- Brace, Sydney, Peterson, A.P. and Gross, E.E., Noise Measurement, 7th Ed., Genrad, Concord,