Answer - SAQ 1. The intensity, I, is given by: Back

Transcription

1 Answer - SAQ 1 The intensity, I, is given by: Noise Control. Edited by Shahram Taherzadeh The Open University. Published 2014 by John Wiley & Sons Ltd. 142

2 Answer - SAQ 2 It shows that the human ear is less sensitive at lower frequencies than at higher frequencies. This effect is more significant at low (or quiet) sound levels than at high levels. 143

3 Answer - SAQ 3 Total absorption = absorption of floor + absorption of ceiling + absorption of walls Since the floor is hard, its absorption coefficient is assumed to be 0. Hence: Total absorption = (5 6 0) + ( ) + 2( ) 0.4 = = 38.2 m 2 144

4 Answer - SAQ 4 From Equation 15, the room constant R c is given by: Substituting this value into Equation 16, the sound pressure level at 2 m or greater from the machine is: log(40) = 77 db(a) (to 2 significant figures) 145

5 Answer - SAQ 5 Correct: c. The room constant depends on the amount of sound absorption within a room. f. The room constant is expressed in units of area. Wrong: a. The sound pressure level due to a given source at a given distance in a room is less than that from the same source at the same distance outdoors. False: the presence of the room boundaries adds a reverberant contribution to the direct sound at any given reception point. b. The sound pressure level near to any source obeys an inversesquare law with distance. False: near to a source, the finite size of the source will have some effect and the source location will also have an influence. d. The larger the room constant, the greater the sound level at distances of 10 m or more from the source. False: the room constant is a measure of the total absorption in a room. As this increases, the reverberation will decrease and consequently so will the contribution from reverberation to the total sound at a given distance. e. The room constant depends on the position of a sound source within a room. False: the room constant depends only on the surface absorption in a room. Statements (c) and (f) are true. 146

6 Statement (a) is false: the presence of the room boundaries adds a reverberant contribution to the direct sound at any given reception point. Statement (b) is false: near to a source, the finite size of the source will have some effect and the source location will also have an influence. Statement (d) is false: the room constant is a measure of the total absorption in a room. As this increases, the reverberation will decrease and consequently so will the contribution from reverberation to the total sound at a given distance. Statement (e) is false: the room constant depends only on the surface absorption in a room. 147

7 Answer - SAQ 6 Correct: 88 db(a) Wrong: 100 db(a) 73 db(a) 64 db(a) None of the above A-weighted sound power = 10 2 W, so using Equation 9: From Figure 16, for a very live room, surface area S = 6800 m 2 : = Using Equation 15 and total Alternatively, using the graph in Figure 16, for a very live room of total surface area 6800 m 2 : R c 70 m 2 So, at 20 m, using Equation 16: 148

8 Using a more precise value for the room constant would result in more or less the same answer - SAQ 6 - SAQ 6. Since these equations are approximations only, there is no point in using very precise numbers. However, it is good practice to check this. 149

9 Answer - Exercise 1 Using Equation 19, we have: So the level is 89 db to the nearest whole db. Note that T is not the sum of the exposure times but the time period for the average (the 8-hour working day in this case), and this period and the individual exposure times must be in the same time units. 150

10 Answer - Exercise 2 L Aeq,24h is first calculated for each type of train separately and then these individual values are combined. The total duration for each type of event is obtained by multiplying the single event duration by the number of events. Note that the 24-hour period has to be converted to seconds since the individual event durations are given in seconds. The total duration corresponding to noise from the fast trains is 12 seconds per train 120 trains, so using Equation 19: Similarly, for the slow trains: and, for the freight trains: The total L Aeq,24h is obtained by combining these three levels: L Aeq,24h = 10 log( ) = 69.4 db(a) So the combined equivalent continuous level is 69 db(a) to the nearest db. It is also possible to combine these stages in one go: 151

11 152

12 Answer - SAQ 7 Since the ground is flat, the mean height, h m, is given by the mean of the source and receiver heights: h m = ( )/2 = 1.75 Equation 25 therefore gives: Thus the attenuation due to ground effect is 4 db to the nearest whole db. 153

13 Answer - SAQ 8 Correct: d. For a given power input, processes involving squeezing and bending are less noisy than processes involving impact and shock. Wrong: a. Noise control by design of plant depends only on improving the mechanical efficiency. False: the choice of process is also important. b. In a situation where there are five noise sources, each producing a different noise level at the measuring position, but where one source produces a noise level that is 10 db(a) more than any of the others, the overall noise level may be 3 db(a) higher than the highest of the individual levels. False: the resultant overall noise level will be insignificantly greater than the highest. c. Noise control of rotating machinery requires only that the speed of rotation be reduced. False: altering speed affects frequency spectrum but not necessarily overall level, and at some speeds may result in an increase in sound level. Statement (d) is true. Statement (a) is false: the choice of process is also important. Statement (b) is false: the resultant overall noise level will be insignificantly greater than the highest. Statement (c) is false: altering speed affects frequency spectrum but not necessarily overall level, and at some speeds may result in an increase in sound level. 154

14 Answer - SAQ 9 Correct: b. It depends on the size of the enclosure, the amount of absorption within it, and the transmission loss of the walls and roof. d. It is improved by vibration isolation of the enclosed machine. Wrong: a. It is equal to the difference between the sound levels inside and outside the enclosure. False: noise reduction is defined as the difference in levels before and after the enclosure is built. c. It is improved by the presence of openings. False: openings tend to reduce sound transmission loss. e. It is unlikely to exceed 5 db(a). False: enclosures can produce reductions in the order of 30 db(a). Statements (b) and (d) are true. Statement (a) is false: noise reduction is defined as the difference in levels before and after the enclosure is built. Statement (c) is false: openings tend to reduce sound transmission loss. Statement (e) is false: enclosures can produce reductions in the order of 30 db(a). 155

15 Answer - SAQ 10 First calculate the total surface area of the enclosure: S = 4 (5 3) + (5 5) = 85 m 2 (Note that this total does not include the floor area, since this is occupied by the enclosed machine.) Also: A = 4 (5 3) 0.9 = 54 m 2 So, by Equation 37: This would be a reasonable target for reduction at 1000 Hz. 156

16 Answer - SAQ 11 Correct: b. Propagation of sound from one room in a building to another via a corridor is an example of airborne flanking transmission. d. Vibrations in a dwelling due to passing vehicles on a road nearby may be due to indirect, airborne and structure-borne sound paths. e. Closing a window in a room could have an effect on the noise level in that room due to a source in an adjacent room. Wrong: a. Structure-borne sound results only from contact between a structure and a vibrating object. False: airborne sound can induce structural vibration. c. A noise problem on the third floor of a five-storey building due to an air-conditioning plant mounted on the roof is a consequence of a direct airborne sound path. False: structure-borne sound is more likely to be involved. Statements (b), (d) and (e) are true. Statement (a) is false: airborne sound can induce structural vibration. Statement (c) is false: structure-borne sound is more likely to be involved. 157

17 Answer - SAQ 12 Direct airborne through the open toilet door into the hall, landing, etc. Flanking airborne under or through gaps in the door, out through the window and back in again through an adjacent one. Flanking structure-borne directly from the toilet to the floor or wall and to the room(s) below or to the side, and from water pipes or drain pipes in direct contact with the toilet and the structure. 158

18 Answer - SAQ 13 Wall area = 4 15 m 5 m = 300 m 2 Floor area = ceiling area = 15 m 15 m = 225 m 2 Since the absorption of the different areas before treatment is small, we can use Equation 38: so R c1 = A 1 = m 2 After treatment, however, the mean absorption coefficient is not small enough for us to use Equation 38, so we use Equation 15 (Section 2.6) for calculating the room constant: so by Equation 15 Thus: 159

19 or 13 db to the nearest db. 160

20 Answer - SAQ 14 Correct: b. An air gap behind a porous layer has the same effect on its absorption as increasing its thickness. d. Panel absorbers can be tuned to particular frequencies. Wrong: a. The low-frequency absorption of a porous material is greater than that at high frequencies. False: porous materials absorb better at high frequencies. c. Making road surfaces porous has no effect on traffic noise. False: porous road surfaces reduce traffic noise by reducing generation of road-tyre noise, as well as absorbing generated noise. e. Adding a porous absorbing layer behind a panel absorber increases its effectiveness at the tuned frequency. False: an absorbing layer makes it effective at most frequencies. An air gap reduces noise at the tuned frequency. Statement (b) is true (see Figure 40). Statement (d) is also true. Statement (a) is false: porous materials absorb better at high frequencies. Statement (c) is false: porous road surfaces reduce traffic noise by reducing generation of road-tyre noise, as well as absorbing generated noise. Statement (e) is false: an absorbing layer makes it effective at most frequencies. An air gap reduces noise at the tuned frequency. 161

21 Answer - SAQ 15 So the value is 5 db to the nearest whole db. Note that I have assumed the mean absorption coefficient is small enough for us to use the approximate formula to calculate A 2. In this case the mean value is 0.283, which is a borderline case. 162

22 Answer - SAQ 16 Correct: a. The attenuation offered by a noise barrier is limited by diffraction. d. Noise barriers are most effective if they are close either to the source or to the receiver. Wrong: b. The effectiveness of noise barriers continues to increase as their density is increased. False: diffraction is the limiting factor. c. Barriers are only effective if they are made entirely from soundabsorbing materials. False: barriers are effective even if made from a (dense) reflecting material. However, they can be more effective if made from a soundabsorbing material, as long as it is dense enough. e. Lightweight acoustic barriers can be very effective. False: effective barriers must have a surface density of at least 10 kg m 2. Statement (a) is true. Statement (d) is also true (because the path difference is then greater). Statement (b) is false: diffraction is the limiting factor. Statement (c) is false: barriers are effective even if made from a (dense) reflecting material. However, they can be more effective if made from a sound-absorbing material, as long as it is dense enough. Statement (e) is false: effective barriers must have a surface density of at least 10 kg m

23 Answer - SAQ 17 Indoors there are more likely to be reflecting surfaces to provide sound paths round the barrier other than those corresponding to diffraction and direct transmission. 164

24 Answer - SAQ 18 In Figure 49, find the curve that meets the left-hand vertical axis at 40 db. This is the difference (50 10) between the insulation in db of the two surfaces. Follow the vertical line up from 4 on the horizontal axis (which is the ratio of high to low insulation in this case) until you intersect the curve. The net loss of insulation is the reading on the right-hand vertical axis at this point, i.e db. This means that the composite wall (including window) can be expected to offer at most ( ) db = 17.5 db insulation at 1 khz. 165

25 Answer - SAQ 19 Cavity brick walls don t offer a full sound reduction because of weak links, such as windows, doors and air bricks. 166

26 Answer - SAQ 20 Correct: a. In the frequency range through which the mass law holds, either doubling the superficial weight of a wall or doubling the frequency produces an increase in transmission loss of between 5 and 6 db. Wrong: b. The increase in insulation resulting from a cavity wall construction compared with that of one of the single wall components is explained by the mass law. False: the increase is also as a result of cavity resonance (the air acting as an isolating spring). c. Deviations from the mass law behaviour of a single wall are caused only by flanking transmission. False: wall vibrations (bending waves) and stiffness have an effect. d. An air gap accounting for 5% of the area of a wall that otherwise offers 50 db(a) sound reduction at 2 khz will reduce the sound reduction at 2 khz to 25 db(a). False: according to Figure 49, a 5% (1/20 area) gap of 0 db insulation will reduce the wall insulation by nearly 37.5 db, i.e. to around 12.5 db(a). Statement (a) is true. Statement (b) is false: the increase is also as a result of cavity resonance (the air acting as an isolating spring). Statement (c) is false: wall vibrations (bending waves) and stiffness have an effect. Statement (d) is false: according to Figure 49, a 5% (1/20 area) gap of 0 db insulation will reduce the wall insulation by nearly 37.5 db, i.e. to around 12.5 db(a). 167

27 Answer - SAQ 21 Earmuffs will be more effective at 1000 Hz than at 100 Hz. From Figure 54 (b), good earmuffs will have attenuations of about 18 db at 100 Hz and 40 db at 1000 Hz. The relevant attenuations for earplugs are, from Figure 52(b), 15 db at 100 Hz and 25 db at 1000 Hz. 168

28 Answer - SAQ 22 Earmuffs will be unsuitable in hot conditions or where the noise is predominantly low frequency. 169