How do you control what you can’t see? This is often the challenge with sound: it behaves strangely and can be very complicated to track. As a result, the techniques and instruments used to track sound give complicated results. It’s all fine and dandy for an expert to point to a waveform and talk about decibels, but it still makes little sense to everyone else.
Fortunately a series of ratings have been developed to help make sense of sound insulation. Though these do not replace the insight of an acoustics professional, they give us a better grasp of what we want to know when selecting the right acoustic partition for our sound insulation needs or deciding on the best treatment for that troublesome noise.
(You can also learn more about sound in our post 4 Beginners’ Tips To Acoustic Treatment)
Isolation, Diffusion and some Sound 101
If you go back a sentence, you’ll note we’re talking about sound ‘insulation’, not sound ‘diffusion’. This is key when it comes to the two ratings we are discussing here.
Diffusion is how sound is scattered and is what you look at to make a room sound good. If you have a noisy restaurant and you want the sound of customer voices to not dominate the room, that is a diffusion challenge. Insulation is all about keeping the wrong sounds out of a room, such as a busy canteen next to a boardroom.
This is where sound ratings come in. They determine the sound absorption of material. So a partition with a higher rating will be better at absorbing sound than a partition with a lower rating.
Ratings are determined in highly scientific laboratory tests, but we can focus on two critical elements that are always looked at: frequency, which is measured in hertz (Hz), and decibels (dB). Frequency could be called the pitch of the sound and is determined by how frequently sound waves follow on each other. You can visualise it as sea waves rolling after each other in quick succession.
Decibels are how loud the sound is. This is worthy of an entire post on its own, but to keep it simple: more decibels equal more volume. In terms of our wave analogy, decibels are determined by how big or tall the wave is.
So bass music at an event would be tall waves that hit the shore relatively slowly, while a whistle would be tall waves hit the shore in quick succession. Diffusion is when you scatter the wave and absorption is when you reduce its size or frequency.
(You can learn more about frequency and more in our handy post 4 Elements of Basic Acoustics.)
When looking for materials that absorb or block sounds, there are two ratings systems you will encounter:
STC or Sound Transmission Class
The first is the STC rating. This is a United States standard scale: the higher the number, the better that material can reduce sound by reducing the frequency and decibels.
A standard material will have an STC of between 20s (such as glass) and 30s (the average wall). Effective sound insulation appear at STCs around the 50s. STC testing standards are always updated, so an STC rating from two decades ago won’t be the same as that same rating today.
There is one catch with STC: it doesn’t cover low frequencies very well. A material can have a high STC number, but not really block sounds such as rumbling traffic, reverberating construction or the droning hubbub of office voices.
Rw or Weighted Sound Reduction Index
Rw is the alternative to STC and used by most of the world. It is an International Organization for Standardization (ISO) rating and part of the ISO 140 (Acoustic) family. Rw ratings are similar to STC in that they follow familiar testing methods.
But they also differ quite a bit: for example, Rw covers a larger frequency range than STC. You can’t simply compare an Rw rating to an STC rating. Some professionals prefer Rw because it corresponds to the decibel scale. So an Rw 50 rating means you could expect the noise you want to ‘block’ to be reduced by 50 decibels.
Which should I choose?
STC is often quoted in online literature around acoustic solutions. But this has more to do with the overwhelming presence of US-based companies online, not due to any perceived superiority. In fact, STC can be a bit hard to comprehend at face value. Rw on the other hand works in tandem with the Decibel scale, so if you can determine the decibels of the sound you wish to reduce, you can get a good sense of what Rw level you want in an acoustic partition. The international standards that dictate Rw also makes it easier to be in line with global compliance and green credentials.
In summary: STC values are around 3-4 dB higher for the equivalent sound insulation in Rw. (47dB STC = 44dB Rw)