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SOUND TRANSMISSION

Some sounds are more intense than others. The difference in intensity between the faintest sounds we hear and the loudest we hear is like the difference between the weight of a paperclip and a battleship. But don’t worry about having to rate sounds on the basis of intensities. Below is an easy-to-use scale for comparison, Scale One. The column on the left, “Sound Intensity”, lists the intensity of the sound based on a rating of one for a barely audible sound. The corresponding figure in the right column shows the sound pressure level. Sound pressure levels are a convenient method of classifying or rating sound intensities. They are expressed in decibels, usually abbreviated “dB”. Note on the chart that whenever the sound pressure level increases to 10 dB, the sound intensity increases 10-fold. It’s a logarithmic scale, and it also lists sound pressure levels for some very common sounds.

SCALE ONE

Sound Intensity

dB

Typical Sound

1,000,000,000,000

120

Thunder Clap

100,000,000,000

110

Nearby Riveter

10,000,000,000

100

Boiler Factory/Subway

1,000,000,000

90

Noisy Factory

100,000,000

80

Noisy Office

10,000,000

70

Average Street Noise

1,000,000

60

Average Radio

100,000

50

Average Conversation

10,000

40

Quiet Radio

1,000

30

Average Auditorium

100

20

Whisper

10

10

“Soundproof” Room

1

0

Threshold of Audibility

In basic technology, noise is defined as any unwanted sound. How annoying or disturbing an unwanted sound is can be expressed in terms of how loud it is. “Loudness” is defined as the intensive attribute of an auditory sensation in terms of which sounds may be ordered on a scale extending from “soft” to “loud”. Loudness is chiefly a function of the sound pressure, but it is also dependent upon frequency.

• Under typical conditions the human ear cannot detect a change of 1 or 2 dB.

• The human ear can notice a change of 3 dB if there is no time lapse between the two sounds and the sounds are of moderate or low intensity.

• A 5 dB change can be noticed if the sounds are sufficiently loud.

• A 7 dB change can always be noticed regardless of loudness.

• The human ear perceives a 10 dB change in sound level a “twice as loud” or “half as loud”.

The actual measurement of the reductions in sound transmitted through windows is usually undertaken in acoustical laboratories. For the average person, only two basic points need to be understood.

Engineers think in terms of sound frequencies. For each frequency, the reduction in sound produced by a sound barrier is called the sound transmission loss at that frequency. This is commonly abbreviated STL or TL.

2. In determining the effectiveness of a window or wall in reducing outdoor sound (traffic, Train, aircraft), individual sound transmission loss values ranging from 125 to 400 Hz are measured.

The acoustical properties of a window are frequency dependent and since there are 16 transmission loss values to describe these properties, it is desirable to reduce this amount of data to a single number. In the case of transmission loss properties this single number rating is called the Sound Transmission Class (STC). The STC rating is used to compare the sound insulating properties of walls, floors, ceilings, windows or doors. The higher the STC number, the better the sound insulating properties are. The most popular STC ratings seem to fall in the 40 range, which most residential windows cannot meet without significant glazing upgrades and/or exterior mounted secondary windows or sash, such as storm windows. Local codes vary, so check with your local building inspector before you buy products or start your project. 


Here are some important points to consider in making your glass selection:

• Sound reduction will increase with increased glass thickness (weight per square foot).

• Sound reduction will decrease somewhat with increasingly larger glass area, but not enough to make much difference in the majority of residential glass sizes.

• Laminated glass is particularly effective in reducing sound transmission. This is because air pressure changes from sound bow or bend laminated glass easier than equally thick plate glass.

• Dual glazing less than one inch in overall thickness does not work as well as laminated or commercial glass for noise reduction. These units reduce sound about as well as the individual lites of glass from which it was fabricated.

• If dual glazed units are used, they work best to reduce sound when they include laminated glass and different thicknesses of glass.


 Tables Two and Three give approximate STC ratings for various thicknesses of glass.

TABLE TWO

Glass, Inches

STC Rating

SSB (3/32”)

26

DSB (1/8”)

28

3/16”

29

1/4”

31

3/8”

33

1/2”

36


TABLE THREE

OAT, Inches

Configuration

STC Rating

1/4”

⅛ x .030 x ⅛

35

3/8”

3/16x.030x3/16

36

1/2”

� x .030 x �

38

1/2”

� x .060 x �

39

3/4”

� x .060 x �

41


Most window products today are dual-glazed for increased thermal performance. Table Four gives some examples of glass/air space combinations and their STC values. The STC values shown are nominal values achieved by operable windows of different types of substrates. Actual STC values may vary slightly due to window size, type, and construction.

TABLE FOUR

OAT

Inner Pane

Airspace

Outer Pane

STC

5/8

SSB

7/16

SSB

27

5/8

SSB

3/8

DSB

28

5/8

DSB

3/8

DSB

29

5/8

3/16

3/16

31

3/4

SSB

9/16

SSB

27

3/4

DSB

DSB

30

3/4

DSB

7/16

3/16

32

7/8

SSB

11/16

SSB

28

7/8

DSB

5/8

DSB

30

7/8

3/16

9/16

DSB

33

7/8

3/16

3/16

33

7/8

� Lam

DSB

35

1

DSB

DSB

32

1

3/16

5/8

3/16

32

1

1/4

33

1

� Lam

9/16

3/16

35


There are many specifications, especially those concerned with windows near airports, where higher STC ratings are required. To achieve these higher ratings, one way would be to use windows configured with much heavier glass and much larger airspaces. Table Five gives some examples of what can be achieved if this course is followed. 

TABLE FIVE

Inner Pane

Airspace

Outer Pane

STC

1/4

1 7/8

1/4

39

1/4

2 1/2

1/4

40

1/4

3

1/4

42

1/4 Laminated

3 1/2

1/2 Laminated

43

3/8 Laminated

3 1/2

1/2 Laminated

44

Laminated glass helps to dampen the vibration of glass panels at critical frequency. You can expect a 2 to 3 dB improvement in the transmission loss, if the window has low air infiltration and there are no significant flanking paths. Flanking paths are weak areas of the window frame and sash that have higher sound transmission than the glass itself. Laminated glass performs better in warmer temperatures. The STC rating can improve by as much as 4 dB over a temperature range of 70�F to 90�F. Therefore, in warmer climates, we would recommend laminated glass be placed on the outside to obtain better sound performance. 

The second route to achieve higher STC ratings is to use a storm sash in conjunction with a standard window. Table Six gives some examples of what can be achieved if this course is followed.

TABLE SIX

Primary Window Glazing

Storm/Prime Airspace

Storm Sash Pane

STC

1/2 IG DSB – 1/4 - DSB

1 1/2

1/4 Laminated

35

7/8 IG DSB – 11/16 – SSB

2 1/2

DSB

41

7/8 IG DSB – 1/2 - 1/4 Lam

3 1/2

DSB

44

1/2 IG DSB – 1/4 - DSB

3

1/4 Laminated

45

7/8 IG DSB – 1/2 - 1/4 Lam

3 1/2

5/32 Laminated

45

Changing from a 1/2” to 5/8” airspace could yield slightly better results, but we would not expect more than 1 dB. With larger airspaces, or argon filled airspace cavities, you can normally expect a 3 dB increase in the STC. Should testing be required, the sound transmission loss values from the ASTM E90 test are used to calculate the STC and OITC ratings in accordance with ASTM E413 and ASTM E1332, respectively. AAMA 1801 specifies ASTM E1425, which requires operating force, air leakage and sound transmission loss measurements. ASTM E1425 specifies using the ASTM E90 test method for the sound transmission loss test. Estimates per window specimen per test exceed several thousand dollars.

Question: What type of glazing is generally required to achieve an STC rating of 40?

Answer: An STC rating of 40 cannot be achieved on a window with conventional 1” insulating glass. To achieve these ratings and above, a dual window assembly with two sets of sash or a prime window with an exterior or interior storm panel mounted upon it is generally required. Table Seven outlines the typical range of ratings with different glazing options. These ratings may be lower if the window system has significant air leakage or if there are flanking problems between the primary and secondary (storm) sash.

TABLE SEVEN

     

Primary Window IG

Prime/Storm Airspace

Storm Glazing

STC

1/8 – 1/2 air – 1/8

None

None

27-28

1/4 - 1/2 air – 1/4

None

None

31-32

1/4  - 1/2 air  – 1/4 Laminated

None

None

34-35

1/4 Lam – 1/2 air  – 1/4 Laminated

None

None

37-38

1/8 – 1/2 air  – 1/8

2”

1/8

39-41

1/4 inner- 1/2 air – 1/8

2”

1/4

42-44

1/4 Laminated inner – 1/2 air – 1/8

2”

1/4

43-45

1/4 Laminated inner – 1/2 air – 1/8

2”

1/4 Laminated

44-46





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