Chapter 11 Acoustics

Question 1

Sound Properties

Answer 1

Sound travels in waves that consist of a high-pressure front followed by a low-pressure front; when the ear perceives a series of these pressure fronts of equal spacing, a tone is heard

Question 2

Sound waves have four basic qualities:

Answer 2

frequency, velocity, wavelength, and power

Question 3

Frequency:

Answer 3

• is the number of successive pressure fronts that pass a given point in one second.
• Measured in Hertz (Hz)
• Sounds perceived as high notes have high frequencies; low notes (bass) have low frequencies

Question 4

Velocity:

Answer 4

depends on medium in which sound travels, as well as medium’s temperature

Question 5

Wavelength:

Answer 5

distance between pressure fronts measured in feet and inches (Direct relationship between frequency and wavelength based on the speed of sound)

Question 6

Power :

Answer 6

is the quality of acoustic energy as measured in Watts (Power is quality people perceive as loudness)

Question 7

Loudness

Answer 7

• Human ear can perceive sound power in a very wide range
• Sensation of hearing is proportional to logarithm of source intensity
• unit of loudness is Decibel (dB)
• 0dB is threshold of human hearing; 130 dB is pain threshold

Question 8

Transmission Loss and Noise Reduction

Answer 8

• Reducing sound transmission from one space to another to an acceptable level is one of the primary considerations in selecting construction elements and detailing barrier assemblies
• Transmission through barriers is primarily retarded by mass
• Barrier with less stiffness will perform better than stiffer barrier, if equal in weight & are

Question 9

Two concepts in noise reduction

Answer 9

• preventing or minimizing the transmission of sound from one space to the other
• reducing noise within a space

Question 10

Transmission loss:

Answer 10

difference in decibels (dB) between sound power incident on barrier in source room and sound power radiated into receiving room

Question 11

Noise reduction:

Answer 11

arithmetic difference in decibels between intensity levels in two rooms separated by a barrier of a given transmission loss; also dependent on area of barrier, absorption of surfaces in receiving room

Question 12

STC

Answer 12

• Sound Transmission Class (STC)

- Higher STC ratings result in better performance in stopping sound

Question 13

Noise Criteria

Answer 13

• All normally occupied spaces have some amount of background noise
• Acceptable amounts vary with type of space, sound frequency; people tend to be more tolerant of noise in public libraries vs. bedrooms

Question 14

Fundamentals of Sound Absorption

Answer 14

-Used to reduce intensity level of sound within spaces, controlling unwanted sound reflections, improve speech privacy, and decreasing reverberation
-Coefficient of absorption is the ratio of sound intensity absorbed by a material to the total intensity reaching the material
-Coefficients below .2 are considered sound reflective; above .2 considered sound absorptive
Three types of materials/construction components for sound absorption: porous materials, volume resonation, and vibrating panels

Question 15

Three types of materials/construction components for sound absorption:

Answer 15

porous materials, volume resonation, and vibrating panels

Question 16

Porous materials are the most common types of sound absorbers, some characteristics are…

Answer 16

• Acoustic panels & ceiling tiles

- Sound energy is converted to heat by friction as it travels among material’s fibers

Question 17

Volume resonators (also called Helmholtz resonators)

Answer 17

• are construction elements that have a small slot leading to a larger chamber
• Designed to absorb low-frequency sound by converting energy to frictional heat at opening, and by reducing its energy by bouncing sound around in cavity

Question 18

Vibrating panels

Answer 18

absorb low-frequency sound by converting sound energy into vibrational energy

Question 19

Noise Reduction Coefficient

Answer 19

Coefficient is the average of a material’s absorption coefficients at four frequencies (250, 500, 1000, and 2000 Hz)

Question 20

Noise Reduction Strategies

Answer 20

• Avoid designing rooms with hard, reflective surfaces on walls, floor and ceiling to prevent overly ‘live’ (noisy) space
• Average absorption coefficient of room should be at least .20; above .50 usually not desirable/cost effective. Lower values o.k. for large rooms, higher values needed in smaller rooms
• Each doubling of absorption amount results in noise reduction of 3dB, which is hardly noticeable. In order to have noticeable effect (5dB), absorption must be tripled
• Absorption materials more effective at ceiling in large rooms, on walls in small rooms
• Absorption increases with an increase in thickness of porous absorbers, except for low-frequency sounds
• If corridors are higher than their width, some absorptive material on walls as well as ceiling is recommended, especially if floor is hard (reflective) material
• Absorption of porous materials depends on material thickness, density, porosity, and orientation of material’s fibers

Question 21

Reverberation

Answer 21

-Reverberation is the prolongation of sound as it repeatedly bounces off hard surfaces.
-Affects intelligibility of speech, quality of music
Defined as time it takes sound level to decrease 60dB after a source stops producing sound
-Desirable quality if appropriate to use of space; recommended time for offices is .3-.6 second, while auditoriums are best with 1.5-1.8 second times
-Can be controlled by modifying amount of absorptive and reflective finishes
-Each doubling of absorption reduces reverberation time by half

Question 22

Space Planning for Acoustic Control

Answer 22

• Zone activities of similar noise levels and plan areas of similar use next to each other
• Use quiet utility spaces as buffers between noisy areas and quiet areas
• Use closets, bookshelves, etc. on common walls to help separate rooms
• Stagger doorways in halls and other areas to avoid providing straight-line paths for noise
• Minimize area of common walls between rooms where sound transmission reduction is desired
• Avoid room shapes that focus sound; barrel-vaulted hallways and circular rooms tend to produce focused sounds, and may deprive some listeners from useful reflections

Question 23

Three ways sound can be controlled within a space:

Answer 23

• reducing loudness of sound source,
• modifying absorption in space
• introducing nonintrusive background sound to mask unwanted sound

Question 24

Speech privacy is:

Answer 24

is regarded as condition where talking may be heard as general background sound, but not easily understood

• Ceilings must be highly absorptive
• Space dividers that help reduce transmission from one space to adjacent spaces; dividers should have combination of absorptive surfaces over solid liner
• Other surfaces must be designed to minimize sound reflections
• If possible, activities should be distanced to take advantage of normal attenuation of sound by distance
• Background masking system should be introduced after absorbing and reflective surfaces are properly installed, to maintain balance between speech sound and background masking sound

Question 25

sound transmission control depends on?

Answer 25

on barrier mass and on its stiffness

Question 26

Partitions

Answer 26

• Add mass to partition – use masonry, add layer of gypsum wallboard
• Add insulation to stud cavity, increasing absorption
• Add resilient channels on one side of partition, causing wallboard to ‘float’, dampening sound striking it, reducing transmission
• Gaps in partition must be sealed, as sound will pass through, over and around an otherwise well-built partition
• Penetrations must be avoided, and sealed carefully when unavoidable
• ***Electrical outlets should be staggered

Question 27

It is critical to make sure all cracks in a partition are sealed?

Answer 27

True

• A hairline crack decreases a partition’s transmission loss by 6dB
• A 1x1” opening in a 100s.f. wallboard partition can transmit as much sound as the entire partition
• A wall with 0.1% open area (from cracks, holes, undercut doors, etc.) can have only a maximum transmission loss of 30dB, no matter how solidly constructed
• A wall with 1% open area can only have a maximum transmission loss of 20dB

Question 28

Sound Control for Doors & Glazing

Answer 28

• Doors placed in otherwise well-built sound walls dramatically affect wall STC; a 9’hx15’w wall with STC 54 and a 3x7’ door with STC 29 and sealed around its perimeter results in overall STC of 37dB
• Perimeter should be completely sealed with acoustic seals at jamb and head, with an automatic door bottom at sill; ref. fig. 4.12-14
• Door should be as heavy as possible, preferably solid core wood; when higher STC ratings required, insulated steel doors with double seals should be used
• For higher ratings, special prepackaged, sound-rated door assemblies should be used
• Laminated glass has higher STC rating, resilient framing can be used; laminated glass provides higher mass, interlayer improves damping
• Double and triple-pane assemblies can be used for higher ratings

Question 29

Plenum Barriers

Answer 29

• Plenum spaces above ceilings, heating registers that pass from one room to the next, air conditioning ducts, pipes, and other penetrations all must be sealed
• In many commercial projects, partitions stop at ceiling, allowing sound to travel from one room to the next through the ceiling plenum; when partition cannot be extended to structural floor above, a plenum barrier

Question 30

Floor/Ceiling Assemblies

Answer 30

• Floor & ceiling assemblies with high STC ratings are insufficient to evaluate effectiveness of transmission loss
• Impact noise generally caused by footfalls, shuffled furniture, and dropped objects on hard-surfaced floors

Question 31

impact insulation class (IIC) number;

Answer 31

• higher IIC ratings reduce impact sounds better than lower ratings
• IIC highly dependent on floor finish; carpeting effective at improving value; can also be improved by providing resiliently supported ceiling for floor below, floating finished floor on resilient pads over structural floor, or by providing sound-absorbing material in air space between floor and finished ceiling below

Question 32

Structure-borne Noise

Answer 32

• Noise can be transmitted by source directly through building structure
• Can be caused by dropped objects and footfalls on hard-surfaced floors
• Most problematic are machines; HVAC systems, plumbing fixtures/water flowing in pipes

-Machines should be mounted on resilient pads/spring mounts
Pipes should be wrapped in insulation, clamped to supporting structure

• Ducts should be lined with insulation, flexible connections between vibrating components
• Insulated chase walls at plumbing helps with fixture noise

Question 33

Reflection:

Answer 33

:is the return of sound waves from a surface. Sometimes, reflection rather than absorption is the goal, such as in concert halls, lecture auditoriums, and large conference rooms

• Reflection depends on surface size, smoothness, and sound wavelengths striking surface
• Reflection is useful in reinforcing sound in lecture rooms; can be annoying if it produces echoes, which occur when reflected sound reaches listener later than 1/17sec. After direct sound

Question 34

Diffusion:

Answer 34

is random distribution of sound from a surface; occurs when surface dimension of reflector is equal to sound wavelength striking it, or when individual textures on a large surface are equal to wavelength

Question 35

Diffraction:

Answer 35

is the bending of sound waves around an object or through an opening. Occurs when a reflecting surface is small compared to sound wavelength striking surface. Diffraction explains why sounds can be heard around corners and why even small holes in partitions allow so much sound to be transmitted.