Table shows spectral contour used to measure sound insulation in various standards such as STC or Rw.

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Sound insulation countours#

Frequency [Hz]	STC [dB]	Rw [dB]
100	not defined	-19
125	-16	-16
160	-13	-13
200	-10	-10
250	-7	-7
315	-4	-4
400	-1	-1
500	0	0
630	1	1
800	2	2
1000	4	3
1250	4	4
1600	4	4
2000	4	4
2500	4	4
3150	4	4
4000	4	not defined

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.