Chapter 14: Acoustics

Question 1

What are the 3 basic qualities of sound?

Answer 1

Velocity, frequency, and power.

Question 2

What impacts the velocity of sound?

Answer 2

Velocity depends on the medium in which the sound is traveling and the temperature of the medium.

Question 3

What is frequency?

Answer 3

The number of cycles completed per second and is measured in hertz (Hz). 1 Hz equals 1 cycle/sec. The sounds that we call high notes or high-pitched sounds have higher frequencies; bass notes have lower frequencies.

Question 4

What is power?

Answer 4

The quality of acoustic energy as measure in watts. It is this power that people perceive as loudness.

Question 5

What is the unit of measurement for the power/intensity of sound?

Answer 5

decibel (dB); 0 dB is the threshold of human hearing and 130 dB is the threshold of pain

Question 6

What is the decibel range for an average office?

Answer 6

60-80 dB

Question 7

What is the decibel range for an average home?

Answer 7

40-60 dB

Question 8

What is the frequency range that a healthy young person can hear sounds?

Answer 8

20-20,000 Hz

Question 9

What is the frequency range that people are most sensitive to?

Answer 9

3000-4000 Hz

Question 10

What is the frequencey range for speech?

Answer 10

125-8000 Hz

Question 11

Is the human ear less sensitive to low or high frequencies?

Answer 11

Low

Question 12

How is sound transmission primarily retarded?

Answer 12

The mass of the partition. The stiffness/rigidity is also important

Question 13

What is transmission loss?

Answer 13

Takes into account only the loss through the partition

Question 14

What is noise reduction?

Answer 14

Dependent not only on the transmission loss but also on the area of the partition separating the two spaces and the absorption of the surfaces in the quiet room (the one not producing the noise).

Question 15

What is the sound transmission class (STC)?

Answer 15

A single-number rating often used to rate the transmission loss of construction. The higher the STC rating, the better the barrier is in stopping sound.

Question 16

What is the STC in which loud speech is not intelligible but can be heard?

Answer 16

35

Question 17

What is the flaw in STC ratings?

Answer 17

They represent the ideal loss through a barrier under laboratory conditions and components built in the field are seldom constructed as well as those in the lab. Also breaks such as cracks, electrical outlets, doors, and the like will less the overall noise reduction.

Question 18

What is a noise criteria curve?

Answer 18

Noise Criteria (NC) curves are primarily used to rate indoor noise levels. It is important that the background noise levels are within acceptable limits to ensure optimal speech communication for structures ranging from residences and offices to hospitals and schools.

Question 19

What is the goal of sound absorption?

Answer 19

Used to control unwanted sound reflections (noise), improve speech privacy, and decrease or increase reverberation.

Question 20

What is the noise reduction coefficient (NRC)?

Answer 20

The average of a material’s absorption coefficient at the four frequencies of 250, 500, 1000, and 2000 Hz rounded to the nearest multiple of .05.

Question 21

What is a coefficient of absorption?

Answer 21

The ratio of sound intensity absorbed by the material to the total intensity reaching the material.

Question 22

What is the sound absorption average (SAA)?

Answer 22

The average of the absorption coefficients for the 12 one-third-octave bands from 200 Hz to 2500 Hz when tested in accordance with ASTM C423.

Question 23

What is the NRC of carpet on concrete?

Answer 23

.50

Question 24

What the NRC of 5/8” suspended acoustic tile?

Answer 24

.60

Question 25

What should the range of the average absorption coefficient of a room be?

Answer 25

.20 - .50

Question 26

What is the order of magnitude that the total absorption of a room should be increased by in order to change the reduction by 5 dB, which is noticable.

Answer 26

Increased by at least 3x

Question 27

Where should absorptive materials be placed to reduce noise in a large room?

Answer 27

Ceiling

Question 28

Where should absorptive materials be placed to reduce noise in a small room?

Answer 28

Walls

Question 29

What is reverberation?

Answer 29

The prolongation of sound as it repeatedly bounces off hard surfaces. Affects the intelligibility of speech and the quality of music. It is technically the time it takes the sound level to decrease 60 dB after the source has stopped producing the sound.

Question 30

What are the 3 primary ways sound can be controlled within a space?

Answer 30

1) By reducing the level of loudness of the sound source 2) By modifying the absorption in the space 3) By introducing nonintrusive background sound to mask the unwanted sound.

Question 31

What is the most effective way to control sound in a space?

Answer 31

By introducing nonintrusive background sound to mask the unwanted sound.

Question 32

What is white sound / random noise / acoustic perfume?

Answer 32

The phenomenon of introducing carefully controlled sound into a space rather than relying only on random background noise. ie. speakers are placed in the ceiling of a space and connected to a sound generator, which produces a continuous, unnoticeable sound at particular levels across the frequency spectrum.

Question 33

What are the different ways to create a sound-resistant partition?

Answer 33

- Add mass to the wall by using more than one layer of gypsum board - Place insulation within the stud cavity - Resilient channels can be used as furring strips on one side of the partition. Only one leg of the channel touches the stud so the wallboard "floats and dampens sound striking it rather than transmitting it to the stud. - Gaps in barriers must be sealed - Electrical outlets should not be placed back to back - HVAC and plumbing equipment should not rigidly be attached to the barrier.

Question 34

What is speech privacy?

Answer 34

A condition in which talking may be heard as a general background sound but not easily understood.

Question 35

How do you achieve speech privacy in an open environment?

Answer 35

- Highly absorptive ceiling - Space dividers that have a combination of absorptive surfaces to minimize sound reflections placed over a solid liner called a septum - Other surfaces (furn, windows, lights, etc) should be arranged to reduce sound reflections. - Background masking system - Activities should be distanced

Question 36

What is impact noise?

Answer 36

Sound resulting from direct contact of an object with a sound barrier ie. shuffled furniture

Question 37

What is the impact insulation class (IIC)?

Answer 37

A single-number rating of a floor/ceiling's impact sound performance. The higher the IIC rating, the better the floor performs in reducing impact sounds in the test frequency range.

Question 38

How do you increase the IIC of a floor?

Answer 38

- Adding carpet - Providing resiliently suspended ceiling below - Floating a finished floor on resilient pads over the structural floor - Providing sound-absorbing material in the air space between the floor and the finished ceiling below.

Question 39

What are some space planning concepts to reduce acoustic problems?

Answer 39

- Plan similar use areas next to each other - Use buffer spaces such as closets and hallways to separate noise-producing spaces - Stagger doorways in halls - Try to locate furniture and other potential noise-producing objects away from the all that is separating spaces - Minimize the area of the common wall between two rooms - Avoid room shapes that reflect or focus sound (ie. barrel vaulted hallways and circular rooms

Question 40

What is the ceiling attenuation class (CAC)?

Answer 40

A single-number measure of the transmission loss through ceiling tiles between two closed rooms where there is no wall or other barrier above the suspended ceiling.

Question 41

What is the articulation class (AC)?

Answer 41

A single-number summation of how effective a ceiling is in absorbing sound reaching it from over low partitions.

Question 42

What is the articulation index (AI)?

Answer 42

Measures the performance of all the elements of a particular configuration working together, including ceiling asorption, space dividers, furniture, light fixtures, partitions, background masking systems, and HVAC system sound. Predicts the intelligibility of speech for a group of talkers and listeners. 0.00 is complete privacy and 1.00 is absolutely no privacy. Unacceptable level of privacy when AI is above 0.30.