How We Test Sound Blocking Products
Sound Blocking (Soundproofing) and Sound Absorption: What's the Difference
Sound is like water. It doesn't have a shape or form, but molds itself to its surroundings. And like water, sound can be absorbed by some materials and contained by others. Click here to learn more sound basics.
Sound absorption products soak up sound. They absorb sound waves and prevent them from bouncing off the walls. They improve the quality of the sound within a room. Click here to learn more sound absorption basics.
Soundproofing products, on the other hand, trap the sound. They contain the sound within a space, making it impossible for the sound to move to other parts of the building. They also stop unwanted sound from entering the room. Click here to learn more soundproofing basics.
What's STC?
To be able to compare the performance of different soundproofing products, we rate them by how much sound they can block. This rating is called STC, which stands for Sound Transmission Class.
Generally, the higher the STC rating, the better the material is at blocking sound.
How is STC determined?
To determine a STC rating for a sound blocking product, two rooms are assembled: the source room (where the noise is located) and the receiver room (where the noise is detected).
Sound blocking material is installed on the wall that joins the two rooms.
A large speaker in the source room emits a sound at a variety of frequencies (pitches) and at certain dB (volume) levels.
The receiver room has a microphone and a diffuser. The diffuser is used to distribute the sound energy evenly within the receiver room. The microphone is used to measure the amount of sound that is detected.
The sound travels from the speaker in the source room, through the sound blocking material on the shared wall, and into the receiver room. The microphone measures the dB (volume) for each frequency (pitch).
Frequencies Play a Role
One soundproofing product is tested at a variety of frequencies. A product can have different results for each frequency.
It's important to verify the STC rating advertised is for the frequency you're trying to treat. Some products with low STC ratings perform better than those with higher STC ratings for certain frequencies.
If you're not sure the STC rating is for the frequency you're trying to eliminate, request a copy of the Transmission Loss data. The Transmission Loss data provides details on the sound blocking product's performance at each frequency level tested.
Sample STC Rating at Varying Frequencies
| Freq. (Hz) | Specimen Transmission Loss (dB) |
|---|---|
| 80 | 24 |
| 100 | 26 |
| 125 | 33 |
| 160 | 33 |
| 200 | 37 |
| 250 | 42 |
| 315 | 45 |
| 400 | 46 |
| 500 | 49 |
| 630 | 52 |
| 800 | 55 |
| 1000 | 57 |
| 1250 | 60 |
| 1600 | 60 |
| 2000 | 56 |
| 2500 | 56 |
| 3150 | 62 |
| 4000 | 67 |
| 5000 | 72 |
Defining STC Ratings in a Common Way
The table below defines STC ratings by what the receiver would hear through a wall that has been treated with soundproofing materials. Please note: these descriptions are based on subjective definitions and may vary by the person hearing the sound.
| STC Rating of the Soundproofing Material | What the Receiver Would Hear through the Wall (How Effective the Product Is) |
|---|---|
| 30 - 35 | most sentences heard clearly |
| 36 - 40 | words and phrases heard occasionally |
| 41 - 45 | Loud speech audible, music and TV's heard |
| 51 - 55 | Speech barley audible, loud music and TV audible |
| 56 - 60 | Loud speech inaudible, loud music faintly heard but bass is still audible |
| 61 - 65 | Music faintly heard, bass still audible, power tools heard |
| 66 - 70 | Music heard if very loud, power tools still audible |
| 71+ | most airborne noise is blocked |
Can You Hear Me Now? STC Ratings in Practical Applications
Each time a structure is treated with soundproofing materials, the amount of sound that is transferred through the structure is reduced. Each time the STC Rating of a structure is increased by 10, the sound transfer is reduced by 50%.
For example: a wall had an STC of 35. A sound barrier material was added and it improved the wall to an STC of 45. This means that sound leakage was decreased by up to 50%.
The chart below illustrates how changes in STC impact the actual sound transmission property of a wall. Please note: these descriptions are based on subjective definitions and may vary by the person hearing the sound.
| Changes in STC Rating | Changes in Perceived Loudness |
|---|---|
| +/- 1 | Almost imperceptible |
| +/- 3 | Just perceptible |
| +/- 5 | Clearly noticeable |
| +/- 10 | Twice (or half) as loud |
Since STC and dB (volume) levels are directly related, the same is true for drops in Transmission Loss data, expressed in dB. For example, a +/- 5 dB change is also clearly noticeable, just like a +/- 5 STC change is clearly noticeable.
STC Depends on the Install
It's important to note the STC rating is based on testing a product that has been installed in a specific way. To achieve the same results, your product needs to have been installed using the same construction method. If your product has not been installed properly, it will not be as effective (have as high of an STC rating).
Before you purchase any sound blocking product, be sure to check the STC reports to determine if your construction method will achieve the results you want.