STC Rating

The Good and the Bad of an STC Rating

The Good and the Bad of an STC Rating

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.

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.

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Popular Materials with a High STC Rating


Luxury Liner Pro™ Automotive Noise Barrier
Luxury Liner™ Mass Loaded Vinyl Sheets (1 lb / 2 lb)
Luxury Liner™ Rolls of Mass Loaded Vinyl (1 lb / 2 lb)

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 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.

STC rating measures the amount of sound blocked by the wall

One other problem is that STC doesn’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. Let’s get into what it takes to block sound.

How to Create an Effective Noise Barrier

MASS

Sound is really nothing more than vibrations. Sound travels from room to room by sending vibrations through the wall. The more mass something has, the more difficult it is to vibrate, therefore, by adding mass to a given material, you can increase its ability to block 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.

2lb mass loaded vinyl

Luxury Liner 2 lb mass loaded vinyl adds mass at a relatively thin total thickness (1/4")

Minimal Leakage

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 should aim for no gaps and seal any edges with acoustic caulk. 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 your average door.

Transmission Loss % of Area Open
13 dB loss
5% open
17 dB loss
2% open
20 dB loss
1% open
30 dB loss
0.1% open

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. When dealing with automotive soundproofing you can use soft decoupling jute and foam materials. In homes, you can use double stud walls or a staggered wall stud and then stuff batt insulation material in the air gap.

Practical Applications of SC 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
25
Normal speech easily understood
30
Loud speed easily understood
35
Loud speech heard, but not understood
40
Loud speech heard as a murmur
45
Loud speech barely able to be heard
50
Loud speech not heard
60+
You're soundproof

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
Almost imperceptible
+/- 3 STC points
Just perceptible
+/- 5 STC points
Clearly noticeable
+/- 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.