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
|
|