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)
What is the single number often used to evaluate the acoustic qualities of partitions?
Sound transmission class (STC)
For sounds of equal energy, are people more sensitive to middle and high frequencies or to low frequencies?
Middle and high frequencies
Most young people can hear sound in what frequency range?
15-25000 Hz
Speech is composed of what sound frequency range?
125-8000 HZ
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.
What are the factos which influence the effectiveness of a noise reducing partition?
Transmission loss (TL)
the area of the barrier
total sound absorption within the “quiet” space
What is the difference between NR and TL.
TL is the difference between the sound power incident on a barrier in a source room and the sound poewr radiated into a receiving room on the opposite side of the barrier. It is typically a laboratory measurement.
NR is the arithmetic difference (in decibels) between the intensity levels in two rooms separated by a barrier having given a transmission loss level.
What is the forumla for calculating NR between two rooms?
NR=TL+10logA/s
NR=noise reduction
TL=transmission loss
A=Absoption of receiving room
s=area of barrier in ft sq
What is inverse square law?
I1/I2 = r1^2/r2^2
What is the formla for calculating NR within a given space?
NR = 10 log A2/A1
What is the formula for TL?
TL = 10log(total area/sigma*t*S)
What is the formula for coefficient of transmission?
t = 10 ^-(TL/10)
What is the formula for sound intensity of multiple objects in a room?
IL =(IL of one obj) + 10log (number of obj)
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.
In order for their benefits to be clearly noticeable, changes to a partition assembly must reduce the sound transmission by hoyw may decibels?
5 dB
An existing paritiion separating two rooms is felt to be insufficient for reducing sound transmission. The partition consists of 4 in metal studs spaced 24 in O.C. with a single layer of 5/8 in gypsum board o each side. There are no penetrations in the partition. TO improve the transmission loss of the partition in the most economical way, which of the following modifications should the architect recommend?
A. Add resilient channels to one side of the wall and attach a single layer of gypsum board to the channels. Glue an additional layer of gypsum board to the other side
B. Add sound absorbing panel to the noisy side of the partition, and add an additional layer of gypsum board to the opposite side
C. Remove one layer of gypsum board, install sound attenuating insulation, and replace the wall finish with a sound deadening board and a finish layer of gypsum board.
D. Cover one side of the partition with an additional layer of gypsum board, and add two aditional layers of gypsum board to the other side.
A. Add resilient channels to one side of the wall and attach a single layer of gypsum board to the channels. Glue an additional layer of gypsum board to the other side
The best way of improving the transmission loss would be to add mass and resiliency to the partition. This can be accomplished economically by adding extra gypsum board and mounting one layer on the resilient channels. Sound absobing channels would not affect the transmission loss between two rooms. They would only affect the NR in the room on the side where the panels were isntalled. Removing the wall finish would not be the most economical method for the results obtained by adding insulation and then replacing new wallboard over sound deadening board. Adding the extra mass of three layers of gypsum board wold not be as effective as using resilient channels with two additional layers of gypsum board as in option A.
Sound absobing channels affects the transmission loss between two rooms. True or false.
False. They would only affect the NR in the room on the side where the panels were isntalled.
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.
What is the single number often used to evaluate the acoustic qualities of partitions?
Sound transmission Class (STC)
The STC gives the designer a quick way to evaluate tested partitions in the common frequency range.
Which of the following statements are false? (Choose 2)
A. Sensitivity to sounds varies between the sexes.
B. People are egenerally more sensitive to middle and high frequencies than to low frequencies for sounds of equal energy
C. Most healthy young people can hear sounds in the range of 15 Hz to 25 000 Hz
D. Practically all common sounds are made up of energy in a wide range of frequencies
E. Speech is composed of requencies in the range of 125 Hz to 8000 Hz
F. Sensitivity to sound varies according to age.
A. Sensitivity to sounds varies between the sexes.
C. Most healthy young people can hear sounds in the range of 15 Hz to 25 000 Hz
What is the low end of sensitivity to sound?
Between 20 Hz and 30 Hz
15 Hz is too low to be heard
What is the high end of sensitivity to sound?
The upper limit is 20,000 Hz
What acoustic variable expresses the maximum allowable intensity of background sounds?
Noise criteria curves
Noise criteria curves are used to specify the allowable sound pressure levels at octave band center frequences
Which of these factors affects the reverberation time?
A. decibel level
B. frequency
C. room volume
D. sound intensity
C. room volume
A room’s reverberation time is defined as the time it takes for sound to decay by 60 dB. Rverberation time is dependent on total room absorption and room volume. It is not affected by the sound’s decibel level frequency, or intensity.
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)
What are the factors that affect the total noise reduction within a space?
- Transmission Loss
- The area of the barrier
the total sound abosrption within the quiet space
What is the difference between TL and NR?
TL is the difference (in decibels) between the sound power incident on a barrier in a source room and the power radiated into a recieving room on hte opposite side of the barrier.
NR is the arithmetic difference (in decibels) between the intensity levels in two rooms reparated by a barrier having a given transmission loss level.
What’s the best way to improving transmission loss to a partition?
Adding mass and resiliency
In order for their benefits to be clearly noticeable, changes to a partition assembly must reduce the sound transmission by how many decibles minimum?
5 dB
Changes in loudness of 1 dB are almost imperceptible. Changes of 3 dB are just barely perceptible. Changes in 10 dB are perceived as twice or half as loud.
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
Sound Transmission Class (STC):
a single number average of how well a building partition reduces the force of airborne sound
Noise Criteria (NC) Curves
- People typically can tolerate higher levels of low frequency sounds than of high frequency sound
- Variables of sound have been consolidated into a set of curves used in specifying the maximum noise level in a given space under a given set of conditions
- Different curves on a chart are specified which dictate the maximum frequencies and decibels that are allowed
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:
Methods for improving Imapct Noise
- Adding carpet and resilient tile floors
- Suspending ceilings
- Concrete slab floated on compressed fiberboard insulation
Sabin:
a unit of sound absorption, 1 ft2 of 100% absorbing material is 1 sabin
Sound Masking:
a technique used to hid unwanted sounds by the addition of controlled
sounds (aka Pink/White Noise)
Coefficient of Absorption (a):
the ratio of the sound intensity absorbed by the material
to the total intensity reaching the material. Varies with the frequency of sound/material
Reverberation:
the persistence of sound in an enclosed space after the source has
stopped. Continuous reflection.
Reverberation Time:
the time it takes the sound level to decrease 60 dB after the
source has stopped by producing sound
Reflection:
the return of sound waves from a surface
Echo:
a sound wave that has been reflected with sufficient magnitude and delay so that
it’s heard as a sound distinct from that transmitted directly
Diffusion:
random distribution of sound from a surface that occurs when the surface dimension is equal to the wavelength of the sound striking it
Diffraction:
the bending of a sound wave around an object or through an opening.
Resonance:
tendency of a system to vibrate at increasing amplitude at certain
frequencies
Reverberation Time:
the time required for a sound to decay 60 dB in a space after the
source has stopped producing sound
Persistence:
result of multiple reflections in an enclosed space in a short period of time
Sound Insolation:
reduction of sound energy levels through the use of material that
absorb reverberant sound and block airborne sound because of their high sound transmission loss factor (eg: acoustical panels, foam curtains, fibrous material)
Sound Isolation:
minimizing sound transmission from one area of a building to another through building design (eg: location of mechanical rooms, partition detailing, using dampers/pads/insulating materials)
Free Field Room (Anechoic Room):
room whose boundaries absorb all of the sound
Room Resonance:
phenomenon which occurs when sounds in a room that are within a
narrow band of frequencies tend to sound louder than sound of other frequencies
Absorptive materials are usually better at reducing the transmission of high
frequencies or low frequencie?
HIgh Frequencies
The absorptivity per square foot of any given surface varies from …
0 (all sound is reflected) to 1.0 sabin (all sound is absorbed).
Articulation Class (AC):
gives a rating of system component performance and does not account for masking sound
Articulation Index (AI):
measure the performance of all the elements of a particular configuration working together:
Ceiling absorption
Space Dividers
Furniture
Light Fixtures
Wall Partitions
Background
Masking
Systems
HVAC Systems
Absorbent materials
• Absorption is normally higher at high frequencies than at low
• Absorption is not always proportional to thickness but depends on the type of material
being used an the method of instillation
• It’s possible to obtain a sabin greater than 1.0 by using very thick blocks
• Installation methods have a pronounced effect
• All other factors remaining constant, the thicker the absorbent material, the better its low frequency absorption characteristic
Noise Reduction Coefficient (NRC):
a scalar representation of the amount of sound
energy absorbed upon striking a particular surface. It’s an arithmetic average of the sound absorption capability of a product at only four frequencies: 250, 500, 1000, and 2000 hertz, which represent the center range of human speech.
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)
A normal human being can hear sound in the range of
20 to 20,000 Hz
What is the difference between echo and reverberation?
Echo is when sound reflections bounces back so a listener hears Hello hello hello
Reverberation is when sound reflections bounce back so a listener hear Hellloooo
What is a ray diagram?
Method used for analyzing reflected sound distribution
