When researching soundproofing materials you’ll probably come across something called an STC rating. STC (or Sound Transmission Class) is meant to measure how much sound a partition, like a wall, would stop. You'll see it as part of the technical specs for ceiling/floor assemblies, doors, soundproof windows, and walls. Although it’s a commonly used rating to tell you how effective a material is at soundproofing, it comes with some pretty serious caveats. But before we get into that, let’s talk about how sound transmission works.
When researching soundproofing materials you’ll probably come across something called an STC rating. STC or Sound Transmission Class is meant to measure how much sound a partition, like a wall, would stop. It’s generally used to rate things like ceilings and floors, doors, windows and exterior wall configurations. Although it’s a commonly used rating to tell you how effective a material is at soundproofing, it comes with some pretty serious caveats. But before we get into that, let’s talk about how sound transmission works.
The Basics of Sound Transmission
What is a Decibel?
A decibel (dB) is simply a measurement of how loud a noise is. Think of dB as the volume control on your stereo. A quiet home will register at about 40 dB compared to a jet engine at around 130 dB. Decibels are not measured on a linear scale. A good rule of thumb is:
- +3 dB: doubles the sound’s energy level
- +6 dB: doubles the sound’s pressure level (SPL)
- +10 dB: doubles the sound’s perceived loudness
What is sound Frequency?
Frequency is measured as Hertz or Hz, and is the measurement of the tone or musical note of a sound. It might have a high pitch like a flute (2000 Hz) or a low pitch as from a tuba (as low as 29 Hz).
What is Transmission Loss?
To find transmission loss, measure how many decibels a sound makes, then see how many decibels can be heard on the other side of a barrier or wall, the difference is the transmission loss. So if you have a sound that is 100 dB on one side of the wall, but only 70 dB on the other side, you have a 30 dB transmission loss. The higher the transmission loss, the more sound the material is blocking, and the less you hear on the other side.
Keep in mind that transmission loss is not a fixed measurement. It can change depending on the pitch of a sound. So while you might have a transmission loss of 25 dB when someone’s playing drums, there may only be a 4 dB transmission loss while there is a vacuum running.
Ok so now that you’ve got these terms down, let’s get into what makes up an STC rating, and why it’s a useful number... but also one you should be careful using.
Pitfalls to Watch for with an STC Rating
So let’s start with what an STC rating *is not*. STC does not measure how many decibels a material can block, so if you have a soundproof wall that has an STC rating of 40, that *does not* mean that it can block 40 dB of sound. STC ratings cannot be added together. If you have a material with an STC rating of 30, and another material with a rating of 20, adding them together does not give you a rating of 50. In fact, the STC rating would probably be closer to 35.
Ok, so with that out of the way, it’s time to see if you were paying attention earlier. STC ratings are calculated by measuring the transmission loss values of 16 - 18 different frequencies between 125 Hz and 4000 Hz and plotting those values as a curve on a graph. That curve is then compared to standard STC rating curves to determine a score.
Was that confusing and overcomplicated? Well... that’s not for us to say. It just is what it is. :)
But to break it down further, basically you test a bunch of sounds at different frequencies or tones and see how many decibels are lost on the other side of the material. Record the findings and create a chart. Then compare that chart to other standard charts to determine your rating.
So... yeah. STC rating is based off testing done in an idealized laboratory setting. It can be very hard to translate these values to the real world, when you're dealing with potential faulty installs and lots of new variables (like all the open air around a sound proof fence - but if you have the right materials, like our sound blankets, and the right guidance - you can get the job done).
Just to add one other problem to the heap. STC Ratings don't test low frequencies under 125 Hz. You know what sounds are clocked under 125 Hz? Musical instruments, stereo systems, planes, trains, automobiles, machinery, ghosts (allegedly), shall I go on? Basically STC doesn’t factor in many of the noises that you run into on a daily basis.
So that’s it. That’s the end of the article. STC is an imperfect metric, but it can’t be trusted and there’s nothing you can do about it. Good luck!
Just kidding, we wouldn’t leave you out to dry like that. STC is imperfect, but it's the best metric we have. Let’s get into some principles so you understand what it takes to block sound.
How to Create an Effective Noise Barrier
Sound is really nothing more than vibrations. Sound travels from room to room through air gaps pr by transmitting vibrational energy through the wall or ceiling. The more mass something has, the more difficult it is for sound waves to penetrate, therefore, by adding mass to a given material or structure, you can increase its ability to block airborne sound. Something as simple as drywall is often used to add mass. One very effective material relative to it’s thickness is mass loaded vinyl sound barrier. Both are commonly used, and we've written about extensively in our guide on how to soundproof a room.
Luxury Liner 2 lb mass loaded vinyl adds mass at a relatively thin total thickness (1/4")
Every hole and gap in a space will create less effective soundproofing. The table below shows how just a 5% opening will change a wall from stopping 40 dB to only stopping 13 dB. When soundproofing vehicles, you’ll have some gaps (and that’s ok), but with wall construction you have no gaps and seal any edges with acoustic caulk to ensure it's all air tight. Pay special attention to sealing vents, outlets, and doors. A typical door without seals has nearly a square foot of space around it. Imagine a square foot hole in your wall and think about how much sound that would let through. That's why people often want to know how to soundproof a door.
|Transmission Loss||% of Area Open|
13 dB loss
17 dB loss
20 dB loss
30 dB loss
Sound Isolation & Decoupling
In a standard studded wall, when sound moves through it, it vibrates one side of the wall, then the vibrations move through the stud into the other side of the wall. By decoupling, you are separating the two sides of a structure so that they vibrate independently and sound does not travel through them as easily. The same thing can happen in a ceiling, which is why people often complain about footfall noise in apartments and look into soundproofing existing ceiling methods. The best answer in both of these applications is our Sound Isolation Clips, which will decouple the drywall from the building's structure. Sound isolation clips are also great for improving the IIC rating of a floor/ceiling assembly. When dealing with automotive soundproofing you can use soft decoupling jute and foam materials. Foam is only useful for decoupling or acoustics, so technically soundproofing foam does not exist, but that's a common misunderstanding we run into.
Practical Applications of STC Ratings
STC is not perfect, but it is a good indicator of a material or structures general ability to keep sound out.
Use this guide to help you gauge the real world effects of an STC ratings. Remember, these are general guidelines and don't account for all types of noises.
|STC Rating||What Can Be Heard|
Normal speech easily understood
Loud speech easily understood
Loud speech heard, but not understood
Loud speech heard as a murmur
Loud speech barely able to be heard
Loud speech not heard
It’s also important to keep in mind that STC ratings are not linear or directly tied to a decibel number. So going from an STC rating of 26 to 28 may not be noticeable, but going from 30 to 40 would be a drastic change.
|Change in STC Rating||Change in Perceived Loudness|
+/- 1 STC point
+/- 3 STC points
+/- 5 STC points
+/- 10 STC points
Twice (or half) as loud
Second Skin is Here to Help
If you’ve made it this far you’re pretty much an expert in sound transmission now, but that doesn’t make your soundproofing project any less complex. But we do this for a living, give us a call and tell us about your project. We’ll help you find the right materials and application to meet your soundproofing needs.