Table shows sound transmission classs (STC) for common building materials.

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Sound transmission class for common building materials#

Material	STC [dB]
Single pane glass window (dual pane glass window range is 26-32″)	27
Single layer of 1/2″ drywall, glued to 6″ lightweight concrete block wall, painted both sides	33
Single layer of 1/2″ drywall on each side, wood studs, fiberglass insulation	39
Hollow CMU (Concrete Masonry Unit) 4″	44
Double layer of 1/2″ drywall on each side, wood studs, batt insulation in wall	45
Single layer of 1/2″ drywall, glued to 6″ lightweight concrete block wall, painted both sides	46
Hollow CMU (Concrete Masonry Unit) 6″	46
Hollow CMU (Concrete Masonry Unit) 8″	48
Hollow CMU (Concrete Masonry Unit) 10″	50
Hollow CMU (Concrete Masonry Unit) 8″ + Z-Bars 2″ + drywall 1/2″ on each side	52
Single layer of 1/2″ drywall, glued to 8″ dense concrete block wall, painted both sides	54
Hollow CMU (Concrete Masonry Unit) 8″ + 1 1/2″ wood furring + 1 1/2″ fiberglass insulation + 1/2″ drywall on each side	54
Double layer of 1/2″ drywall on each side + staggered wood stud wall + batt insulation in wall	55
Double layer of 1/2″ drywall on each side + wood stud wall on resilient channels on one side + batt insulation	59
Double layer of 1/2″ drywall on each side, on double wood/metal stud walls spaced 1″ apart + double batt insulation	63
Hollow CMU (Concrete Masonry Unit) 8″ + steel studs 3″ + fiberglass insulation + drywall on each side	64
concrete block wall 8″, painted + drywall on independent steel stud walls 1/2″ each side + insulation in cavities	72

Sound insulation#

When a sound wave moves through the air meets a barrier in the form of a wall, part of the acoustic energy is reflected, part is absorbed inside the wall (converted to heat), and the another part is transmited out (on the other side of the wall). We can write it mathematically as follows:

$\alpha + \beta + \tau = 1$
where:
- $\alpha$ - absorption coefficient (determines the part of the energy that was absorbed inside the wall),
- $\beta$ - reflection coefficient (defines the part of the energy remaining in the first room),
- $\tau$ - transmission coefficient (defines the part of the energy that was emitted to the second room).
The transmission coefficient can be used as a measure of the acoustic insulation, because it determines the sound intensity ratio on both sides of the wall:

$\tau = \frac{I_t}{I_0}$
where:
- $I_t$ - intensity of the wave on the other side of the wall (sound intensity level audible in the second room),
- $I_0$ - incident wave intensity (sound intensity level audible in the first room).
In practice, the transmission factor is most often given in the logarithmic scale. In this way, we obtain a decrease in sound intensity given in decibels, so-called transmission loss:

$\Delta TL = -10 ~ log (\tau) = 10 ~ log \left(\frac{1}{\tau}\right)$

Sound insulation ratings#

Sound insulation depends, among other, on the frequency of the acoustic wave. This means that the complete information on the insulation properties of a given material should contain serie of numbers for different frequencies.
The weighted sound reduction index determines the partition insulation in relation to the model curve. By applying a predefined formula to the curve measured, we can simplify it to the single number.
Both Rw and STC describe the insulation against typical air noise.
The method of measuring the Rw coefficient is defined by the ISO 10140-1: 2016 standard.
The method of measuring the sound transmission class (STC) is defined by the ASTM E413-87 standard.
Ther Rw is more common in Europe, when STC is more common in USA.
To reflect the fact that insulation is different at different frequencies the ISO standard defines two additional coefficients with different frequency curve:
- RA1 - more sensitive in the range of medium and high frequencies, better reflects the insulation in the case of noise sources such as:
  - conversation, music, radio, TV,
  - children's fun,
  - rail traffic with medium and high speed,
  - traffic on a fast road> 80 km/h,
  - jets at close range,
  - industrial plants emitting mainly medium and high frequency noise,
  - etc.
- RA2 - more sensitive in the range of low and high frequencies, better reflects the insulation in case of noise sources such as:
  - city traffic,
  - rail traffic at low speeds,
  - helicopters,
  - long-range jets,
  - disco music,
  - industrial plants emitting mainly low and / or high frequency noise,
  - etc.