3A Acoustical control in buildings PDF

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ACOUSTICAL CONTROL IN BUILDINGS 3A – Group 8

Acoustics is a science that deals with the production, control, transmission, reception, and effects of sound. This includes the minimization of noise transmission from one space to another and the control of the characteristics of sound within spaces themselves. Building acoustics is important for the consideration of factors that affect the occupants and the functionality of the building and it must be studied in order for the sounds created within spaces would not have negative effects. Proper design of the building acoustics ensures that any potential problems regarding sound and noise control are addressed such as privacy, health-related concerns, safety, and many more. While the systems of any structure are essential, acoustical control must also be taken into consideration especially during the planning phase. A building with good aesthetics and fully functional systems but does not have good noise control may hinder its functionality. We know that noise often distracts, disturbs, or even causes confusion; there may even be instances where unwanted sounds might cause stress and anxiety and we need to make sure that the building is comfortable for the occupants. Sound • • •

Sound is caused by the vibrations which transmit through a medium such as air and reach the ear or any receiver that can detect it. Sound intensity is measured in Decibels (dB) Sound pitch is measured in Hertz (Hz)

Reverberation time – the time that it takes a sound made in a room to diminish to one millionth of its original intensity Sound Absorption – is the loss of sound energy due to the sound waves being absorbed by materials capable of doing such. Sound Insulation – reduces the transmission of sound

Sound is the feeling caused by the vibration of the air or other mediums. But when the sound is not pleasing, it became unwanted which we refer as noise. Acoustic and noise plays an important role in the construction of buildings and other structures. Sound Generation Sound can be generated from single or multiple sources but is typically characterized by three characteristics.

Frequency (Pitch): written as Hertz or Hz, is the measurement of the tone or musical note of a sound. It might have a high pitch like a flute might make (2000 Hz) or a low pitch as from a tuba (as low as 29 Hz). Wavelength: The wavelength of a sound wave is the distance between the start and end of a sound wave cycle or the distance between two successive sound wave pressure peaks. It is equal to the speed of sound in the material, such as air divided by the frequency of the sound wave. Amplitude (Loudness): The amplitude of a sound wave is visually its height from bottom to top but is in fact indicative of the strength of the energy in that wave. It is measured in decibels (dB), such that the higher the dB rating, the more energy, and hence the louder the sound that we hear.

SOUND TRANSMISSION CLASS (STC) Sound Transmission Sound transmission is a common acoustic issue for it can be both airborne or structure borne vibration. Airborne sound travels through the air and it can transmit through different materials. Also, it can pass under doorways, through ventilation, under, over, through and around obstructions. The sound becomes a noise when it reaches a room where it is unwanted. Noise from the outside surroundings can transmit through the exterior structure of the building, such as that from automobiles, trains and airplanes. On the other hand, noise from the interior surroundings such as mechanical equipment or speech can transmit from one room within a building to its adjacent place or space. Sound transmission can cause noise control, confidentiality, and privacy issues. Most of the offices such that of the counselors’, lawyers’, or human resource departments can’t function in a space where sound will transmit through the walls and into its adjacent spaces. If the sound transmission in a room or place is not properly controlled, it will not provide privacy for the users.

Transmission Loss is a measurement of a partition's ability to block sound at a given frequency, or the number of decibels that sound of a given frequency is reduced in passing through a partition. The basis for determining partitions Sound Transmission Class is measuring transmission loss over a range of 16 different frequencies between 125-4000 Hz. 125-4000 Hz is consistent with the frequency range of speech. The STC rating does not assess the low frequency sound transfer. Special consideration must be given to spaces where the noise transfer concern is other than speech, such as mechanical equipment or music. The Sound Transmission Class (STC), which is determined by ASTM E90, is a singlenumber rating of a material's or an assembly's ability to resist airborne sound transfer at the frequencies 125-4000 Hz. Generally, higher STC rating blocks more noise from transmitting through a partition. But even with a high STC, penetration, air-gap, or flanking path can cause a serious damage to the isolation quality of a wall. Flanking path are the means for the sound to travel from a place to another other than through the walls. Sound can flank over, under or around the walls. Also, it can travel through common ductwork, plumbing or corridors. STC is highly dependent on the construction of the partition. A partition's STC can be increased by: • • •

Adding mass Increasing or adding air space Adding absorptive material within the partition

Adding Mass The weight or thickness of a partition is the major factor in its ability to block sound. For example, a thick concrete wall will block more sound than a thin gypsum/2x4 wall. Mass is commonly added to existing walls by adding additional layers of gypsum. When the mass of a barrier is doubled, the isolation quality (or STC rating) increases by approximately 5 dB, which is clearly noticeable. Increasing or Adding Air Space An air space within a partition can also help to increase sound isolation. This, in effect creates two independent walls. However, the STC will be much less than the sum of the STC for the individual walls. The airspace can be increased or added to an existing partition. A common way to add an airspace is with resilient channels and a layer of gypsum. An airspace of 1 ½" will improve the STC by approximately 3 dB. An air space of 3" will improve the STC by approximately 6 dB. An airspace of 6" will improve the STC by approximately 8 dB. Adding Absorptive Material in the Partition Sound absorptive material can be installed inside of a partition's air space to further increase its STC rating. Installing insulation within a wall or floor/ceiling cavity will improve the STC rating by about 4-6 dB, which is clearly noticeable. It is important to note that often times, specialty insulations do not perform any better than standard batt insulation.

RULES OF THUMB General rules of thumb for controlling noise between spaces: • • • • • • •

A wall must extend to the structural deck in order to achieve optimal isolation. Walls extending only to a dropped ceiling will result in inadequate isolation. Sound will travel through the weakest structural elements, which, many times, are doors, windows or electrical outlets. When the mass of a barrier is doubled, the isolation quality (or STC rating) increases by approximately 5 dB, which is clearly noticeable. Installing insulation within a wall or floor/ceiling cavity will improve the STC rating by about 4-6 dB, which is clearly noticeable. Often times, specialty insulations do not perform any better than standard batt insulation. Metal studs perform better than wood studs. Staggering the studs or using dual studs can provide a substantial increase in isolation. Increasing air space in a wall or window assembly will improve isolation.

Changes in STC/Changes in Apparent Loudness: Changes in STC Rating

Changes in Apparent Loudness

+/- 1

Almost imperceptible

+/- 3

Just perceptible

+/- 5

Clearly noticeable

+/- 10

Twice (or half) as loud

Recommended Ratings In general, loud speech can be understood fairly well through an STC 30 wall but should not be audible through an STC 60 wall. An STC of 50 is a common building standard and blocks approximately 50 dB from transmitting through the partition. However, occupants could still be subject to awareness, if not understanding, of loud speech. Constructions with a higher STC (as much as 10dB better - STC 60) should be specified in sensitive areas where sound transmission is a concern. The Uniform Building Code (UBC) contains requirements for sound isolation for dwelling units in Group-R occupancies (including hotels, motels, apartments, condominiums, monasteries and convents). UBC requirements for walls: STC rating of 50 (if tested in a laboratory) or 45 (if tested in the field*). UBC requirements for floor/ceiling assemblies: STC ratings of 50 (if tested in a laboratory) or 45 (if tested in the field*). * The field test evaluates the dwelling's actual construction and includes all sound paths.

An assembly rated at STC 50 will satisfy the building code requirement. However, as mentioned above, occupants could still be subject to awareness, if not understanding, of loud speech. Therefore, it is typically argued that luxury accommodations require a more stringent design goal. Weaknesses The STC rating is based on performance with frequencies from 125 to 4000 Hertz (the speech frequencies). The rating provides no evaluation of the barrier's ability to block low frequency noise, such as the bass in music or the noise of some mechanical equipment. The STC rating is a lab test that does not take into consideration weak points, penetrations, or flanking paths. The field test however, does evaluate the entire assembly and includes all sound paths. STC RATINGS FOR VARIOUS WALL ASSEMBLIES Below are the STC ratings of various wall assemblies, each presented to help illustrate concepts, improvements and rules of thumb. The estimated ratings are based on laboratory test results from various compendiums of STC ratings. It is recommended to consult a professional acoustician for more detailed information or to analyze the specifics of your project/assembly.

1. Insulation will noticeably improve the STC rating of an assembly. Estimated STC Description Wall Assembly Rating 3 5/8" metal studs, 5/8" gyp (2 layers total), No insulation

38 – 40

3 5/8" metal studs, 5/8" gyp (2 layers total), Batt insulation

43 – 44

2. Staggered or double stud walls are higher rated than single stud walls. Estimated STC Description Wall Assembly Rating 2x4 stud, 5/8" gyp (2 layers total), Batt insulation

34 – 39

Staggered studs, 5/8" gyp (2 layers total), Batt insulation

46 – 47

2x4 studs, 5/8" gyp (2 layers total), Batt insulation

56 – 59

3. Metal stud walls perform better than wood stud walls. (NOTE: This only applies to single stud assemblies. For double stud assemblies, there is virtually no difference.) Estimated STC Description Wall Assembly Rating 2x4 stud, 5/8" gyp (2 layers total), Batt insulation

34 – 39

3 5/8" metal studs, 5/8" gyp (2 layers total), Batt insulation

43 – 44

4. Resilient channel can improve the STC rating of an assembly. (NOTE: These ratings are based on laboratory tests. Because of the special care required when installing resilient channels, actual results could be substantially lower.) Description

Estimated STC Rating

2x4 stud, 5/8" gyp (2 layers total), Batt insulation

34 – 39

2x4 stud, 5/8" gyp (2 layers total), Resilient Channel, Batt insulation

45 – 52

Wall Assembly

5. Adding additional layers of drywall can improve the STC rating of an assembly. Estimated STC Description Wall Assembly Rating 2x4 stud, 5/8" gyp (2 layers total), Batt insulation

34 – 39

2x4 stud, 5/8" gyp (3 layers total), Batt insulation

39 – 40

2x4 stud, 5/8" gyp (4 layers total), Batt insulation

43 – 45

6. Drywall between double studs can dramatically reduce the STC rating of an assembly. Estimated STC Description Wall Assembly Rating

2x4 studs, 5/8" gyp (4 layers total), Batt insulation

44 – 45

2x4 studs, 5/8" gyp (2 layers total), Batt insulation

56 – 59

2x4 studs,5/8" gyp (3 layers total), Batt insulation

59 – 60

2x4 studs, 5/8" gyp (4 layers total), Batt insulation

58 – 63

STC Ratings for Masonry Walls STC ratings for masonry/CMU walls is based on weight of the block and whether the cells are filled or not and what material it is filled with. Estimated STC Ratings for CMU Walls Hollow Units Wall Thickness, in. Weight STC

Grout Filled

Sand Filled

Weight

STC

Weight

STC

4

20

44

38

47

32

46

6

32

46

63

51

50

49

8

42

48

86

55

68

52

10

53

50

109

60

86

55

The STC rating of a CMU wall can be estimated based on its weight using the following formula: STC = 0.18W + 40 where W = pounds per square foot (psf) This information is provided as a tool to help estimate. The estimate could easily be off by as much as +/- 4 dB.

NOISE REDUCTION COEFFICIENT Noise Reduction Coefficient (NRC) - is a single-number index determined in a lab test and used for rating how absorptive a particular material is. NRC rating ranges from 0.00 (perfectly reflective) to 1.00 (perfectly absorptive), and it can be viewed as a percentage for how well a material absorbs sound. It is simply the average of the mid-frequency sound absorption coefficients (250, 500, 1000 and 2000 Hertz) rounded to the nearest 5%. For example, a thick, polished marble floor may have an NRC of 0.00 (absorbs very little sound), while a 2” thick piece of SilentFiber can have an NRC of 0.95-1.00 (very absorbent). While NRC is commonly used and accepted, it can still be misused or misunderstood. The following are the things to be considered before specifying a particular material based on NRC: •

The NRC rating is an average of how absorptive a material is at four frequencies (250, 500, 1000 and 2000 Hz). This rating is appropriate for assessing how well a material absorbs sound within the speech frequencies, but can be inadequate for sound generated by music, mechanical equipment or other low-frequency sounds.

•

Because this rating is an average, two materials with the same rating might not perform the same in identical applications.

•

The NRC is based on lab tests. Because the lab is a near perfect environment that is rarely duplicated in everyday applications, some products will not test the same in the field. Certain factors, such as installation variables, are not accounted for in the lab. A product that receives high ratings in the lab may not perform as well in the field.

•

Make sure the mounting procedure used in the tests is consistent with your intended installation if you expect the same results.

•

NRC does not have anything to do with the material's barrier effect (STC).

•

Communication of NRC ratings by manufacturers can be misleading and sometimes deceitful for the following reasons: o Some manufacturers will quote absorption at the more-desirable higher frequencies. NRC should be based only on the absorptive characteristics at 250, 500, 1000, and 2000 Hz. Make sure the product data you're reviewing is at these frequencies. o A manufacturer of a wall carpet product could provide an NRC rating of .80, which is extremely good. But, if there is fine print be sure to read it, you may see that this rating was achieved while the carpet was installed over fiberglass. In this installation configuration, the fiberglass, not the carpet, acts as the sound absorber. Without the acoustic material behind, the wall carpet will probably only achieve an NRC of .20.

NRC Ratings of Common Building Materials It is important to remember that certain absorptive properties of materials may differ drastically based on numerous variables. For example, a concrete wall could be 0% to 35% absorptive. If available, it is always best to get the tested NRC rating from the manufacturer. It is advisable to seek an un-biased third party, such as an acoustical consultant, for NRC confirmation on a particular product and installation.

Noise Reduction Coefficients (NRC) for Common Building Materials:

(Adapted from Architectural Acoustics, M. David Egan and Sound Analysis and Noise Control, John E.K. Foreman)

These ratings may not reflect the specific NRC for your particular material, depending on variables such as type, size, thickness, installation methods, etc.

NRC vs. STC

NRC -single number index for rating how absorptive a material is -A material with an NRC of .80 will absorb 80% of the sound that comes into contact with it and will reflect 20% of the sound back into the space. -average of the mid-frequency sound absorption coefficients (250, 500, 1000, and 2000 Hertz) rounded to the nearest 5% -gives no information as to how absorptive a material is in the low and high frequencies.

STC -single number rating of a material’s or assembly’s barrier effect -A higher STC rating blocks more noise from transmitting through a partition.

-based on performance with frequencies from 125 to 4000 Hertz (the speech frequencies) -does not take into consideration weak points, penetrations, or flanking paths

NRC and STC are completely exclusive of one another. A material with a high NRC will help to absorb sound, stopping it from reflecting back into the room. The same material may (and probably will) have a low STC and allow a large amount of sound to pass through it and into an adjacent room. In much plainer language, NRC is a measurement of how well a product makes the room you are in quieter, while STC measures how well a product keeps sound from escaping the room. If you are in a noisy room like gymnasium, you want a product with a higher NRC on the walls or ceiling to make the room less noisy. If you are in a conference room where you are talking about sensitive topics that you don’t want people on the other side of the wall to hear. You need a product on the wall with a high STC. It is best to consider building acoustics during the design planning phase instead of after the completion of the building. Once the occupants experience noise within a space with no acoustical consideration, additional acoustical solutions may be installed or it may even be a call to redesign or reconstruct the space and will cost more. It is best to have a acoustical consultant that may be approached for acoustical control solutions for buildings.

REFERENCES Acoustics.com nrcratings.com stcratings.com Merriam-Webster. (n.d.). Reverberation time. In Merriam-Webster.com dictionary. Retrieved December 18, 2020, from https://www.merriamwebster.com/dictionary/reverberation%20time Merriam-Webster. (n.d.). Acoustics. In Merriam-Webster.com dictionary. Retrieved December 20, 2020, from https://www.merriam-webster.com/dictionary/acoustics Arsenault, P. J. (n.d.) Acoustical control in buildings. Continuing Education. Center. https://continuingeducation.bnpmedia.com/courses/certainteed-insulation/acousticalcontrol-in-buildings/1/ Soundproofing Company (n...