Acoustics Flashcards
Reverberation Time? Units?
T; sec
Transmission loss
TL; dB
What is the articulation index?
a measure of speech intelligibility calculated from the number of words read from a selected list that are understood by an audience. A low articulation index (less than 0.15) is desireable for speech privacy whereas a high articulation index (above 0.6) is desried for good communication
What is attenuation?
The reduction of sound
What is amplificiation?
The increased intensity of sound by mechanical or electrical means
What is frequency?
The number of pressure fluctuations or cycles occurring in 1 sec, expressed in Hz
What is a Hertz?
The unit of frequency; 1 cycle per second equals 1 Hz
What is Impact insulation class (IIC)?
A single number rating of a floor ceiling’s impact sound transmission performance at various frequencies
What it Noise criteria (NC)?
A set of single number ratings of acceptable background noise corresponding to a set of curves specifying sound pressure levels across octave bands. Noise criteria curves can be used to specify continuous background noise, achivesound isolation and evaluate existing noise situations
What is Noise Insulation class (NIC)`?
A single number rating of noise reduction
What is Noise Reduction (NR)?
The arithmetic difference, in decibels, between the intensity levels in two rooms separated by a barrier of a given transmission loss. Noise reduction is dependent on the transmission loss of the barrier, and the absorption of the surfaces of the receiving room.
What is the Noise reduction coeeficient(NRC)?
The average sound absorption coefficient to the nearest 0.05, measured at the four one-third octave band center frequencies of 250, 500, 1000, and 2000 Hz
What is Reverberation?
The persistence of a sound in a room after the source has stopped producing sound
What is a Octave band?
A range of frequencies in which the upper frequency is twice that of the lower
What is a phon?
A unit of loudness level of a sound equaltot he sound pressure level of a 1000 Hz tone judged to be as loud
What is Sound transmission class (STC)?
A single number average over several frequency bands of a barriers ability to reduce sound. The higher the STC rating, the better the barrier’s ability to control transmission
What is Transmisison Loss (TL)?
The difference, in decibels, between the sound power incident on a barrier in a source room and the sound power radiated into a receiving room on the opposite side of the barrier. The transmission loss varies with the frequency being tested
What are the 3 basic qualities of sound?
Velocity, frequency, and power
What is the velocity of sound in air?
1130 ft/sec
At what absorption coefficient is a material considered absorbing?
Above 0.2
What is the equation for acoustical absorption (A)?
A=Sa
What should the average absorption coefficient be for a room?
.20
Are higher or lower absorption coefficient suitable for large rooms?
Lower
What effect does each doubling of the amount of absorption have on a room?
3dB difference
If additional absorptive material is being added to a room, the total absorptions should be increased by how much to be noticeable?
3 times
Each doubling of the absorption in a room reduces reverberation by how much?
One-half
Where are absorptive materials more effective when placed in large rooms?
Ceiling
Where are absorptive materials more effective when placed in small rooms?
Walls
Generally absorption increase with an increase in what of a poruous absorber?
Thickness
The amount of absorption of a porous type of sound absorber such as fiberglass or mineral wool is dependent on what 4 factors?
1) thickness; 2) density; 3)porosity; 4) orientation of fibers in material should be composed of open, interconnected voids
What equation is ued to find reverberation?
T=.05(V/A)
What are the 3 primary ways sound can be controlled in a space?
Reducing sound source, modifying absorption in space, introducing nonintrusive masking sound
What are the two measures used to evaluate open office acoustics?
Articulation class (AC); articulation index (AI)
What is articulation class (AC)?
Gives rating of system component performance and does not account for masking sound
What is articulation index (AI)?
Measures the performance of all the lements of a particular configuration working together (ceiling absorption, space dividers, furniture, light fixtures, partitions, background masking sound, HVAC)
What 3 aspects of speech privacy does AI evaluate?
Compare relative privacy between different pairs of workstations, evaluate how changes in open office components affect speech privacy, measure speech privacy objectively for correlation with subjective responses
What is the value range of AI?
0-1; 0=completely private 1=no privacy
What are the 5 important factors in designing for speech privacy?
1) ceiling must be highly absorptive;
2) space dividers must reduce transmission of sound from one space to adjacent space;
3) other surfaces (furn, lights, windows) must be designed or arranged to minimize sound reflections;
4) activites should be distanced to take advantage of the normal attenuation of sound with distance;
5) the must be a properly designed background masking system. Background noise must maintain good signal-to-noise ratio
What is impact noise?
Sound resulting from direct contact of an object with a sound barrier
How if impact noise quantified?
Impact insulation Class number
What is mechanical noise
Sound that occurs when a vibrating device is in continuous direct contact with the structure
How can mechanical noise be transmitted?
- Rigidly attached equipment can vibrate the building structure or pipes
- Airborne noise of equipment can be transmitted through walls and floors to occupied spaces
- noise transmitted through ductowrk
- movement of air or water through ducts and pipes can cause noise
How can mechanical noise be controlled?
- mechanical equipment should be mounted on springs or resilient pads
- connections between equipment and ducts and pipes should be made with flexible connections
- ducts should be lined or provided with mufflers
- noise producing equipment should be located away from quiet spaces
- walls and ceilings and floors of mechanical rooms should be designed to attenuate airborne noise
- mechanical and plumbing equipment should be designed to minimize high velocity flor and sudden changes in fluid velocity
What is diffusion as it relates to sound?
The random distribution of sound from a surface, occurs when the surface dimension equals the wavelength of the sound striking it
What is diffraction as it relates to sound?
It is the bending of sound waves around an object or through an opening
Name 9 ways to reduce acoustical problems
1) plan similar use spaces next to each other;
2) use buffer spaces such as closets and hallways;
3) locate noise producing areas away from quiet areas;
4) stagger doorways in hallways to avoid straight pathways for noise to go through;
5) locate operable windows as far from each other as possible;
6) locate furniture and other noise-producing objects away from the wall separating spaces;
7) minimize area of the common walls between two rooms where reduction in sound transmission is desired;
8) avoid room shapes the reflect or focus sound (barrel vaulted hallways and circular room;
9) avoid parallel walls with hard surfaces in small rooms
What is the best way to improve TL in a partition?
Add mass and resiliency
NR between two spaces is dependent on what?
transmission loss of the wall, the area of the wall, and the absorption of the surfaces in the receiving room (stiffness/damping qualities)
Two adjacent rooms separated by an acoustical partition are shown. There is a noise producing source in room B.
THe noise reduction in toom A is dependent on all of the following except the
A. area of the partition
B. thickness of the partition
C. transmission loss of the partition
D. absorption of surfaces in room A
B. thickness of the partition
Thickness of a partition or other acoustical separation is irrelevant to the total noise reduction within a space. Factors whcih would influence the effectiveness of the partition are transmission loss, the area of the barrier, and the total sound absorption within the “quiet” space.
The partition assembly shown would be best for controlling which of the following kind of acoustic situations?
A. impact noise
B. excessive reverberation in room A
C. excessive reverberation in room B
D. transmission from room B ro room A
E. transmission from room B to room A
F. mechanincal vibration
C. excessive reverberation in room B
D. transmission from room B ro room A
The sound absorbing panel in room B would help control excessive reverberation in this space. The hard surface on the room A side of the partition would not control excess reverberation in room A. THe double layer of gypsum board would help improve the transmission loff of the wall. Because noise reduction between two spaces is dependent on the transmission loss of the wall, the area of the wall, and the absorption of the surfaces in the receiving room, this assembly would do a better job of reducing sound transmission from room A to room B (because the absoptive panel is in room B) than from room B to rom A. THe partition would do little to control impact noise or mechanical vibration.
A copy machine in an office workroom has a sound intensity level of 65 dB. A computer printer with a sound intensity of 69 dB is added tot he space. What will be the resulting sound intensity level?
71 dB
69 dB -65 dB = 4 dB
69 dB + 1.5 dB = 70.5 dB (71 dB)
Decibel levels are added logarithmically, not arithmitaccly. To express their comined effect. first determine the difference in sound intensity levels between the two sound sources. Then use the table shown to add the apporpriate to the larger dB level.
Equation: coefficient of transmission
Equation: combined transmisison loss
frequency
Equation: inverse square law
Equation: Noise Reduction
Equation: Noise Reduction within a Space
Equation: sabin formula
Equation: Power (W/cm^2))
Equation: sound intesity level (dB)
A copy machine in an office workroom has a sound intensity level of 65 dB. A computer printer with a sound intensity level of 69 dB is added to the space. What will be the resulting sound intensity level?
71 dB
Preferred Noise Criteria (PNC):
is a modification of NC curves that have sound pressure levels lower than the NC curves on the low/high frequency needs of the chart. For example:
Equation for Sound Intesntiy (IL) for multiple sound producing sources
Find the combined intensity level of two office machines, one generating 70 dB and the other generating 76 dB.
77 dB
What would the sound level be in a room of eight typewriters?
82 dB
A conference room and an office are separated by a common wall 13 ft long and 9 ft high with an STC rating of 54. THe total absorption of the office has been calculated to be 220 sabins. What is the total noise reduction from the conference room to the office?
57 dB
What is the combined transmission loss of a wall 9 ft high and 15 ft long with a 3 ft by 7 ft door in it? Assume the TL of the wall is 54 dB and that the door, with full perimeter seals, is 29 dB.
27 dB
t=10^-(TL/10)
