Lesson 3 Measurement of sound

Transcription

1 Lesson 3 Measurement of sound 1.1 CONTENTS 1.1 Contents Measuring noise The sound level scale Instruments used to measure sound Recording sound data The sound chamber The anechoic chamber References MEASURING NOISE A variety of methods and scales are used for measuring and putting numbers on noise it depends upon the situation. The measurement of the noise usually includes information about 1. Sound level (energy) of the noise, given in units of decibels db 2. The frequencies (low to high) present in the noise, given in units of Hertz (cycles per second) 3. The duration of the noise, measured over of hours or days A set of measurement is usually processed and combined to give a convenient single number or index. All assessment of noise needs to be compared to human opinion about the annoyance or acceptability of different levels of a particular noise Loudness Level The loudness level of a sound is defined as the sound pressure level in db of a standard frequency, 1000 Hz, pure tone, which is heard with loudness equal to that of the sound, and measured in phon. The curves shown in Figure 1 connect the equal loudness levels of each sound pressure level for each frequency of pure tone and are called equal loudness contours. The figures are based on the average values of tests carried out with numerous young persons aged from 18 to 20 years. We can observe the following in Figure 1 (Maekawa, Rindel and Lord, 1997).. 1

2 Figure 1: Equal loudness contours. 1. Generally speaking, below 500 Hz the auditory sensitivity is reduced with decreasing frequency. At 100 Hz, for example, on the curve for 20 phon the sound pressure level is nearly 30 db higher than at 1000 Hz, which means the sound energy is 1000 times that of the standard; at 40 Hz, times; and at 20 Hz, about times that of the 1000 Hz standard. 2. This fall in sensitivity becomes less pronounced at higher levels of the range. 3. Maximum sensitivity exists between about 3000 or 4000 Hz at the lower level, it reaches 3 6 db from the standard. 1.3 THE SOUND LEVEL SCALE The sound level scale is the logarithm of the ratio of measured sound intensity to the intensity at the threshold of audibility. This scale is also known as the decibel (db) scale. Reminder: 2

3 The logarithm of a number is the exponent by which another fixed value, the base, must be raised to produce that number. For example, the logarithm of 1000 to base 10 is 3, because 1000 is 10 to the power 3: 1000 = = More generally, if x = b y, then y is the logarithm of x to base b, and is written y = logb(x), so log10(1000) = 3. If the number of decibels is given by N, then Where: I = measured intensity of sound I o = reference intensity = l pw/m. For instance, suppose we have two loudspeakers, the first playing a sound with power P1, and another playing a louder version of the same sound with power P2, but everything else (how far away, frequency) kept the same (UNSW, 2013). The difference in decibels between the two is defined to be If the second produces twice as much power than the first, the difference in db is db = 10 log 2 = 3 db. If the second loudspeaker had 10 times the power of the first, the difference in db would be = 10 log 10 = 10 db. This is shown in figure 2: 3

4 Figure 2: Illustration of the decibel scale. The ratio of the power of two sources is plotted on the X axis. Source: UNSW, How big is a decibel? One decibel is close to the Just Noticeable Difference (JND) for sound level. Sound levels that differ by less than 1 db are hard to distinguish. The following example shows the effect of noise decreasing by 3 db steps. Example of broadband noise decreasing by 3 db steps Example of broadband noise decreasing by 3 db steps.mp3 The following example shows the effect of noise decreasing by 1 db steps. Example of broadband noise decreasing by 1 db steps Example of broadband noise decreasing by 1 db steps.mp3 4

5 The following example shows the effect of noise decreasing by 0.3 db steps. Example of broadband noise decreasing by 0.3 db steps Example of broadband noise decreasing by 0.3 db steps.mp What is 0 db? 0 db does not mean no sound, it means a sound level where the sound pressure is equal to that of the reference level. This is a small pressure, but not zero. It is also possible to have negative sound levels: - 20 db would mean a sound with pressure 10 times smaller than the reference sound pressure. Different combinations of frequencies and levels of sound produce the same sensation of loudness. This is due to the variation of the sensitivity of the ear with frequency. Thus loudness cannot be directly measured by instruments. Loudness is determined by referring to the loudness or phon scale which shows sounds of various levels and frequencies which are perceived as of the same loudness The phon scale The phon is a unit of loudness level for pure tones. A unit used to describe the loudness level of a given sound or noise. The system is based on equal loudness contours, where 0 phons at 1,000 hz is set at 0 decibels, the threshold of hearing at that frequency (see graph). The hearing threshold of 0 phons then lies along the lowest equal loudness contour. If the intensity level at 1,000 Hz is raised to 20 db, the second curve is followed. It will be noted, therefore, that the relationship between the decibel and phon scale at 1,000 Hz is exact, but because of the way the ear discriminates against or in favour of sounds of varying frequencies, the phon curve varies considerably. For instance, a very low 30 Hz rumble at 110 decibels is perceived as being only 90 phons (see graph); for its effect, however, see infrasonic. Two sounds of equal intensity do not have the same loudness, because of the frequency sensibility of the human ear. A 80 db sound at 100 Hz is not as loud as a 80 db sound at 3 khz. A new unit, the phon, is used to describe the loudness of a harmonic sound. X phons means as loud as X db at 1000 Hz The Sone Scale The sone is a unit of perceived loudness proposed by Stanley Smith Stevens in The sone is derived from psychophysical measurements that involved volunteers adjusting sounds until they judge them to be twice as loud. This allows one to relate perceived loudness to phons. Experimentally it was found that a 10 db increase in sound level corresponds approximately to a perceived doubling of loudness. So that approximation is used in the definition of the phon: A loudness of 1 sone is equivalent to the loudness of a signal at 40 phons, the loudness level of a 1 khz tone at 40 db SPL. Each 10 phon increase (or 10 db at 1 khz) produces almost exactly a doubling of the loudness in sones. 0.5 sone = 30 phon 1 sone = 40 phon 2 sone = 50 phon 4 sone = 60 phon 5

6 1.4 INSTRUMENTS USED TO MEASURE SOUND The most common instruments used for measuring noise are: 1. The sound level meter (SLM) 2. The integrating sound level meter (ISLM) 3. The noise dosimeter 4. Extensometers The sound level meter (SLM) Sound level meters are handheld instruments used to measure sound, or noise. Sound level meters come in various shapes and forms, but they have common points. They tend to have a pointy bit at the top to stop the sound reflecting back at the microphone. Some lower cost sound level meters, do not have a pointed top but instead have the microphone on an extension to get it away from the case, again to reduce reflection. This method is used to keep the case design cost down. The Sound level meter consists of a microphone, electronic circuits and a readout display. The microphone detects the small air pressure variations associated with sound and changes them into electrical signals. These signals are then processed by the electronic circuitry of the instrument. The readout displays the sound level in decibels. The SLM takes the sound pressure level at one instant in a particular location. Figure x: Sound level meter schematic. To take measurements, the SLM is held at arm's length at the ear height for those exposed to the noise. With most SLMs, it does not matter exactly how the microphone is pointed at the noise source. The SLM must be calibrated before and after each use. With most Sound Level Meters, the readings can be taken on either slow or fast response. The response rate is the period over which the instrument averages the sound level before displaying it on the readout. A standard SLM takes only instantaneous noise measurements. This is sufficient in workplaces with continuous noise levels. But in workplaces with impulse, intermittent or variable noise levels, the SLM makes it difficult to determine a person's average exposure to noise over a work shift. 6

7 Plate x: Sound Level Meter Plate x: Sound Level Meter 7

8 Plate x: Sound Level Meter Plate x: Sound Level Meter 8

9 1.4.2 The integrating sound level meter (ISLM) The integrating sound level meter is similar to the dosimeter. It determines equivalent sound levels over a measurement period. The major difference is that an integrating sound level meter does not provide personal exposures because it is hand-held like the sound level meter, and not worn. The integrating sound level meter determines equivalent sound levels at a particular location. It yields a single reading of a given noise, even if the actual sound level of the noise changes continually. It uses a pre-programmed exchange rate, with a time constant that is equivalent to the SLOW setting on the sound level meter. Plate x: Integrating Sound level Meter 9

10 Plate x: Integrating Sound level Meter The noise dosimeter Plate x: Integrating Sound level Meter A noise dosimeter is a small, light device that clips to a person's belt with a small microphone that fastens to the person's collar, close to an ear. The dosimeter stores the noise level information and carries out an averaging process. It is useful in industry where noise usually varies in duration and intensity, and where the person changes locations. 10

11 Plate x: noise dosimeter 11

12 Plate x: noise dosimeter Plate x: noise dosimeter Plate x: noise dosimeter 12

13 Plate x: noise dosimeter It is used to ascertain the noise exposure of workers during their normal working day. It is a small, light and compact instrument to be worn by the worker. It measures the total A-weighted sound energy received and expresses it as a proportion of the maximum A-weighted energy that can be received per day. This instrument is particularly useful whenever the exposure varies appreciably during the working day. The maximum A-weighted energy that is permitted to be received per day is defined in standards or regulations: it is absolutely necessary that the dosimeter be calibrated on the basis of the adopted standard (e.g. 85 db(a) or 90 db(a) for an 8-hour exposure), including the accepted trading rule, which is 3 db(a) in accordance with the ISO standard (and for most European countries) and 5 db(a) for the OSHA Standard (USA). The 3 db(a) trading rule is consistent with the equal energy principle: 96 db(a) during 2 hr providing the same energy as 93 db(a) during 4 hours or 90 db(a) during 8 hours. The 5 db halving rate assumes that 90 db(a) during 8 hours is equivalent to 95 db(a) for 4 hours or 100 db(a) for 2 hours. Dosimeters are actually sound level meters having a DC output signal converted into a series of impulses which are counted to provide the dose. The technical characteristics of dosimeters must then be the same as for type II sound level meters. The noise dosimeter is clipped to the workers' clothes with the microphone close to the ear, and can be worn without hampering work. The dose provided by the instrument is of course dependent on the duration during which the instrument is used. Therefore, it should first be corrected for an 8 hour period and then converted to the daily noise exposure (L EX, 8) level according to the relevant formula (ISO or OSHA). It is important to know that some old dosimeters do not take into account levels below 89 db (A) or 80 db (A), as they assume that lower levels do not lead to hearing impairment. The L EX,8 is then physically not correct. These dosimeters are obsolete and should be discarded. On certain instruments, a warning marker is activated if the peak level ever exceeds 140 db it is worth noting that the characteristics of the dosimeters have never been standardized. Furthermore, they are extremely limited as they provide one single value at the end. It is strongly recommended to abandon this type of instrument and use the personal sound level meters described in the next section. 13

14 Table 1: Guidelines for Instrument Selection Type of Measurement Appropriate Instruments (in order of preference) Result Comments Personal noise exposure Dosimeter Dose or equivalent sound level Most accurate for personal noise exposures ISLM Equivalent sound level If the worker is mobile, it may be difficult to determine a personal exposure, unless work can be easily divided into defined activities. SLM db(a) This is only useful when work can be easily divided into defined activities and noise levels are relatively stable all the time. Noise levels generated by a particular source SLM db(a) Measurement should be taken 1 to 3 metres from source (not directly at the source). ISLM Equivalent sound level db(a) Particularly useful if noise is highly variable; it can measure equivalent sound level over a short period of time (1 minute). Noise survey SLM db(a) To produce noise map of an area; take measurements on a grid pattern. ISLM Equivalent sound level db(a) For highly variable noise. Impulse noise Impulse SLM Peak pressure db(a) To measure the peak of each impulse Extensometers These are used to measure sound on the sea floor. Extensometers measure distance by carefully recording the travel time of acoustic pulses between pairs of instruments (and the water temperature). 1.5 RECORDING SOUND DATA Recording sound data are a critical part of architectural and environmental acoustics. Even with the simplest sources, care must be taken to follow proper procedure. A meter appropriate to the task must be selected. For environmental survey work a meter, tripod, calibrator, windscreen (to reduce wind generated noise), logbook, distance measuring device (tape or rolling ruler), and watch are the standard kit. For all measurements, a record should be kept, noting the following information where it is relevant: 1. Location 2. Source description 3. Pertinent source details (e.g., manufacturer, model, operating point conditions) 4. Date and time 5. Engineer 6. Source dimensions and the radiating surfaces 7. Distance and direction to the source or a description of the measurement location 14

15 8. Meter settings 9. Background noise levels 10. Any unusual conditions 11. Time history 12. Measured data Types of Microphones The most common types of microphones in use are: 1. Dynamic 2. Condenser 3. Electret 4. Ceramic, and 5. Ribbon. All microphones consist of a diaphragm, which moves back and forth in response to changes in pressure or velocity brought about by a sound wave, and electronic components that convert the movement into an electric signal. Microphones are characterized by a sensitivity, which is the open circuit output voltage produced by a given pressure, expressed in decibels re 1 V/Pa. A diaphragm moves in response to the changes in sound pressure and is mechanically connected to a coil of wire that is positioned in a magnetic field. The induced current, produced by the motion of the coil, is the microphone s output signal. Both the diaphragm and the coil must be very light to produce adequate high-frequency response. Most dynamic microphones produce a very low output voltage; however, since the electrical output impedance is low, the microphone can be located relatively far away from the pre-amplifier. Dynamic microphones are rugged and are primarily used in sound reinforcement applications, where low fidelity is good enough. The sound level at a given receiver is the reading in decibels of a sound level meter. The meter reading corresponds to a value of the sound pressure integrated over the audible frequency range with a specified frequency weighting and integration time. 1.6 THE SOUND CHAMBER 1.7 THE ANECHOIC CHAMBER An anechoic chamber is a non-echoing or echo-free room designed to completely absorb reflections of either sound or electromagnetic waves. It is a room in which the walls, ceiling and floor are lined with a sound absorbent material to minimise reflections. They are also insulated from exterior sources of noise. The combination of both aspects means they simulate a quiet open-space of infinite dimension, which is useful when exterior influences would otherwise give false results. Anechoic chambers, a term coined by American acoustics experthttp://en.wikipedia.org/wiki/leo_beranek Leo Beranek, were originally used in the context of acoustics (sound waves) to minimize the reflections of a room. More recently, rooms designed to reduce reflection of radio frequencies and external noise have been used to test antennas, radars, or electromagnetic interference. Anechoic chambers range from small compartments the size of household microwave ovens to ones as large as aircraft hangars. The size of the chamber depends on the size of the objects to be tested 15

16 and the frequency range of the signals used, although scale models can sometimes be used by testing at shorter wavelengths. Anechoic chambers are commonly used in acoustics to conduct experiments in nominally "free field" conditions. All sound energy will be traveling away from the source with almost none reflected back. Common anechoic chamber experiments include measuring the transfer function of a loudspeaker or the directivity of noise radiation from industrial machinery. In general, the interior of an anechoic chamber is very quiet, with typical noise levels in the dba range. According to Guinness World Records, 2005, Orfield Laboratory's NIST certified Eckel Industries-designed anechoic chamber is "The quietest place on earth" measured at 9.4 dba. The human ear can typically detect sounds above 0 dba, so a human in such a chamber would perceive the surroundings as devoid of sound. The University of Salford has a number of Anechoic chambers, of which one is unofficially the quietest in the world with a measurement of 12.4 dba Vital statistics for University of Salford Anechoic Chamber 1. Background noise level -12.4dBA 2. Working area 5.4 x 4.1 x 3.3m 3. Cut-off frequency 100Hz Plate x: Anechoic chamber. 16

17 Plate x: Earth's quietest place according to Guinness World Record (2004) with -9.4 dba : The 'anechoic chamber' at Orfield Laboratories, which is per cent sound absorbent and capable of giving you hallucinations 1.8 REFERENCES Maekawa, Z., Rindel, J. H. and Lord, P. (1997). Environmental and Architectural Acoustics. Spon Press, New York. University of New South Wales (2013). db: What is a decibel? School of Physics, Sydney, Australia. Retrieved from 17