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1. What wall construction blocks more noise?

    a. A 6 foot thick wall built from chicken feathers

b. A 12” thick concrete block wall

c. A 1/8” mass loaded vinyl

    d. Any of these could be the right answer

1. D. Not enough information is provided about the material and construction of the wall. Wall transmission loss is dependent on mass per unit area, stiffness, and details of fabrication such as presence or lack of direct transmission paths.

2. Through which medium does sound travel the fastest?

a. Steel

b. Wood

c. Water

d. Air

 

2. A. The speed of sound is governed by the density (large density lowers the speed of sound) and stiffness (high stiffness raises the speed of sound) of the medium in which it travels. Relative to the other choices the combination of density and stiffness leads to steel having the highest speed of sound (around three times the speed of sound in water and fifteen times the speed of sound in air).

3. What will happen to the intensity of a sound when you move from 10 ft. to

20 ft. away from the source?

a. The intensity of the sound will double

b. The intensity of the sound will be divided by two

c. The intensity of the sound will be divided by four

d. The intensity of the sound will be the same

 

3. C. The Inverse Square Law teaches us that for every doubling of the distance between sound source and recipient, the sound pressure is divided by two and the sound intensity is divided by four. This is true because a simple noise source radiates outward in spherical waves.

4. What function do most standard ceiling tiles perform?

a. Block sound transmission into the room from above

b. Block sound transmission out of the room through the ceiling

c. Absorb sound in the room

d. All of the above

 

4. C. Standard ceiling tiles do not carry enough weight to deaden noise bleed in either direction through the ceiling. There are weighted finished ceiling tiles that can perform this feat, but standard ceiling tiles do not. They will absorb sound reflections in a room, to varying degrees depending on the quality of the tile.

5. You’re at a party, and you can’t hear the person next to you talking; why not?

a. Too many people talking at the same time

b. The surfaces in the room are too reflective

c. The person speaking isn’t talking loud enough

d. George Clooney just walked into the room

 

5. B. Although any of our options here may cause a hearing problem, the important problem is the sound reflections from the surrounding walls and ceilings in the room. These reflections compete with new sound signals, causing a blurring of speech clarity making it more difficult to carry on a conversation. This is often referred to as the “cocktail party” effect.

6. What travels most easily through a common wall assembly?

a. High frequency

b. Medium frequency sound

c. Low frequency sound

d. They all travel equally through a wall

 

6. C. In general, low frequency noise is the most difficult to block with traditional wall constructions, although at isolated higher frequencies transmission of sound may not be blocked efficiently.

7. A quiet library will have a Sound Pressure Level (SPL) in decibels on the A scale (dBA) of:

a. 10-20 dBA

b. 30-40 dBA

c. 40-50 dBA

d. 85 dB dBA

 

7. C. To rate architectural spaces for their acoustic performance acousticians will use a single number rating system such as the Balanced Noise Critera (NCB) that takes account of the frequency and amplitude characteristics of the acoustic environment in the space. An NCB rating of 30-40 is considered appropriate for a library. A more easily obtained measurement of the noise is in terms of decibels on the “A” scale, or dBA. The dBA description of acoustic Sound Pressure Level accounts for human response to noise. A rule of thumb is that for typical amplitude/frequency characteristics of noise the A weighted Sound Pressure Level (measured with a simple sound level meter) is 8 dBA higher than the NCB level. The answer 40-50 dBA is therefore consistent with this.

8. Which of the following statements is most true relative to applying sound absorbing material in a room to control reverberation?

a. Doubling the absorption of panels will double the sound absorption results

b. By installing 1/3 of a recommended panel treatment, 1/3 of the required improvement in acoustic environment will result

c. Wall treatments will absorb more sound than ceiling treatments

d. There is a break-even threshold in your ability to improve the acoustic environment

 

8. D. Each room targeted for a reduction in sound reverberation has an optimum point that is a compromise between perceived improvement in the acoustic environment and the amount and cost of material that can installed. Theory suggests that doubling the amount of absorption halves the reverberation time in a room. Successive doubling of absorption by adding more material is neither economical nor physically practical and successive halving of the reverberation time is not matched by improvement in perception of the acoustic environment, and may in fact be counter productive. A general rule of thumb is to attempt to install absorbing material as uniformly as possible.

9. Regulations for implementation of hearing protection programs for work place noise assume that permanent hearing loss begins to appear for an individual exposed to

a. 85 dBA

b. 100 dBA

c. 120 dBA

d. 160 dBA

 

9. A. Government standards require hearing protection for factory workers who are exposed to 85 decibels or higher over an 8 hour work day.

10. Your eardrum can rupture at:

a. 85 dB

b. 100 dB

c. 120 dB

d. 160 dB

 

10. D. The eardrum probably will rupture at 160 dB. 120 dB is more or less the threshold of feeling at which a tickling sensation will begin to be felt and 140 dB is more or less the level at which pain begins.

11. A rock concert can peak at:

a. 85 dB

b. 100 dB

c. 120 dB

d. 160 dB

 

11. C. Rock concerts can average 120 dB, and can peak up to 140 dB.

12. Foam absorbs sound reflections by performing what function:

a. The sculpted surface deflects sound waves into multiple directions

b. The foam traps the sound inside the foam

c. The foam pores convert the sound waves to heat

d. The foam serves as a storage locker and stores the sound

 

12. C. Sound waves are attenuated inside porous media by at least two mechanisms that convert acoustic energy to heat. In one mechanism the sound waves cause the porous media to compress, expand, and shear and this absorbs energy from the acoustic wave. In a second mechanism the acoustic wave creates motion of the air inside the porous medium that consists of countless tortuous passages. This fluid motion results in loss of acoustic energy.

13. Which of the following statements is most true:

a. Low frequency noise is louder than high frequency noise

b. High frequency noise is louder than low frequency noise

c. Either of these could be true depending on the intensity of the sound source

d. None of the above

 

13. C. Loudness is the perception of the level of noise. It depends on both amplitude of the noise and the frequency of the noise. In general, the human ear perceives low frequency noise to be less loud than higher frequency noise, if the measured sound pressure level in dB is the same.

14. What is the speed of sound in air?

a. 760 miles per hour under all conditions

b. 343 meters per second under all conditions

c. Speed will vary depending on temperature of the air

d. All of the above

 

14. D. The speed of sound will vary based on temperature.

15. Which of the following is most true?

a. The more a surface is able to vibrate, the less noise will pass through

b. The less a surface is able to vibrate, the less noise will pass through

c. The vibration of a surface is unrelated to the noise that is passed through

d. The less a surface is able to vibrate, the more noise will pass through

 

15. B. By impairing a surface’s ability to conduct vibration, you control the amount of sound wave transfer through structural vibration. Vibrations can be controlled by adding weight, increasing the stiffness, and disconnecting direct transmission paths in the structure.

16. Which wall assembly will block the most noise (the surface mass of each construction is estimated in each case)?

a. Standard 2 x 4 studs on 18 inch centers on a 2 x 4 plate with 3/8 inch drywall on each side (approximately 8 lb/square ft)

b. Staggered standard 2 x 4 studs on 18 inch centers on a 2 x 6 inch plate with 3/8 inch drywall on each side (approximately 11 lb/square ft)

c. Six inch concrete block wall (approximately 32 lb/square ft)

d. Double wall construction. Each wall has standard 2 x 4 studs on 18 inch centers on a 2 x 4 plate. 3/8 inch drywall only on outer surfaces. Wall structures are not in contact (approximately 13 lb/square ft)

 

16. D. Double walls and double glazed windows have higher transmission loss than single layer structures with similar mass per unit area. The structural void between the layers disconnects the structural transmission path. This type of structure can exceed the transmission loss of a structure with a much higher mass per unit area but with less attention paid to disconnecting direct paths of sound transmission.

17. A 6 decibel drop in Sound Pressure Level or Sound Intensity Level represents which of the following?

a. Dividing the sound pressure by 10

b. Dividing the sound pressure by 6

c. Dividing the sound pressure by 4

d. Dividing the sound pressure by 2

 

17. D. Decibel levels are logarithmic. A common analogy is the Richter scale that measures earthquakes. Doubling of sound pressure corresponds to an increase of 6 dB in Sound Pressure Level. Conversely, halving the sound pressure reduces the Sound Pressure Level by 6 dB.

18. For a multiple purpose auditorium used for speech and music a target range for mid-frequency (average of 500 and 1000 Hertz) reverberation time should be:

a. 0.2-0.5 seconds

b. 1.4-1.9 seconds

c. 2.5-3.0 seconds

d. 3.0-3.5 seconds

 

18. B. Absolute consensus is difficult to achieve, but with perhaps some uncertainty at the upper and lower extremes of the range it would be generally recommended that for a multi-purpose auditorium (volume from approximately 60,000 cubic feet to 600,000 cubic feet) intended for both speech and music the 15

reverberation time should be between 1.4 and 1.9 seconds. Within this range acceptable reverberation time increases with room volume. Other common scenarios may be well outside this range. For example in a classroom (volume between 10,000 and 40,000 cubic feet) the reverberation time for primarily speech should be in the range 0.6-0.8 seconds. For a concert hall (volume 300,000 cubic feet to 1,000,000 cubic feet) an appropriate range might be 2.0 to 2.4 seconds, again increasing with volume.

19. Which is the greatest contributor to noise bleeding through a common

floor/ceiling assembly?

a. A lack of fiberglass batting stuffed between the floor joists

b. The floor joists themselves

c. The flooring surface up above

d. The ceiling surface down below

 

19. B. Although hardwood floors are worse than carpet, and standard ceiling tiles bleed more noise than finished surface ceilings, the true culprit in sound bleed here is the joists themselves that transmit structure borne energy back and forth between the two floors. The noise control goal is to either raise the floor or lower the ceiling, line with a sound blocking substrate, and impair the surfaces’ ability to conduct vibration. Stuffing fiberglass batting between the joists has less effect unless the primary problems are addressed.

20. Unacceptable background noise is caused by what?

a. Noise due to building services such as air conditioning

b. Traffic noise transmitted into the room through walls and windows

c. The reverberant noise level in the room due to normal activity in the room and poor sound absorption of walls and furnishings of the room.

d. Any of the above

 

 

20. C. Background noise is the noise level in the room exceeded most of the time (say 90% of the time). That is, it is the level of noise when nothing else is going on. Hence if there is a steady drone of air conditioning noise at 60 dBA, or a reverberant noise level of 60 dBA due to other conversations and an acoustically poorly absorbing construction, and a conversation is attempted at average levels of 60 dBA or a bit more, there is a problem with the background noise.

 

21. A noisy compressor powers a machine in a factory. What is the best

noise control method that should be applied?

a. Treat the noise source: enclose the compressor

b. Vibration Isolation: place the compressor on machine mounts

c. Treat the noise path: absorb sound reflections off nearby walls and ceilings

d. Treat the recipient: wear hearing protection headsets

 

21. A. Although all four of these steps are often applied, the most benefit will be gained by attacking the sound source first. If you are able to shut the sound down at its point of origination, you can obtain a much larger decibel level drop. The other steps might also come into play, but the primary treatment should be “source” first and “path” second.

22. The Sound Pressure Level in a room is dominated by a single tone. With suitable treatment it is reduced in Sound Pressure Level by 10 dB. As a first approximation the human response would be that the loudness is

     a. One tenth of the original loudness

     b. One half the original loudness

     c. One fourth the original loudness

     d. Two thirds the original loudness

22. B. As a first approximation perceived loudness of a pure tone increases or decreases by a factor of two for a 10 dB increase or decrease in the Sound Pressure Level. Perception of loudness is considerably more complicated to predict if noise is composed of a wide range of frequencies.

23. In a factory a machine is known to produce a Sound Pressure Level of 85 dB at some location on the factory floor not right next to the machine. A second machine is purchased and installed near the original machine. What will be the Sound Pressure Level at the same point on the factory floor after the installation of the new machine running at the same time as the old one?

      a. 170 dB

      b. 95 dB

      c. 88 dB

      d. 91 dB

23. C. Sound Pressure Level is increased by 3 dB for doubling of sound intensity. Adding the second machine doubles the sound intensity.

24. A noisy machine is operating in a large room with acoustically absorbing panels on the walls. At a large distance from the machine the reverberant Sound Pressure Level is

     a. Dependent on the volume of the room and the amount of absorption

     b. Independent of the volume of the room but dependent on the amount of

absorption

     c. Independent of the volume of the room and independent of the amount of  absorption

     d. Depends on the placement of the machine subject to the restriction that

the listening point is still a long way from the machine

24. B. The reverberant sound level (by definition at a point well away from the machine where the direct field dominates) does not depend on the room volume but decreases with an increase in absorption. At least in theory the reverberant field does not depend on where the noise source is located as long as the listener is not near the machine.

25. The simplest formula used to predict reverberation time in a room shows that  reverberation time

      a. Increases with room volume all else held constant

      b. Decreases with room volume all else held constant

      c. Is independent of room volume

      d. Is independent of the speed of sound in the room, all else held constant

25. A. The Sabine formula, and all other approximations, show that the reverberation time of a room increases as the volume of the room increases as long as the amount of absorption stays the same. The reverberation time decreases slightly if the speed of sound increases (temperature increases).

 

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