Understanding STC and STC Ratings

IMPORTANT! The STC measurement is only a partial insight into real-world satisfactory performance. It is an incomplete measurement since STC ratings do not factor low-frequency bass, which is where most noise problems occur.

Please don’t rely on STC values alone when specifying a soundproofing system. We've learned that a high STC-rated wall may suffer terribly in the low frequencies and is therefore not a true solution.

An effective soundproofing system will perform below the 125Hz STC frequency cutoff.

What is measuring STC good for?

Sound vibration decreases as it travels from one side of a wall to the other. To understand the value of STC ratings, we need to quantify and measure this loss.

Understanding STC can be valuable, but what really matters is whether your choice of soundproofing solution actually works.

Expensive, exotic building materials don’t generally help much, though they sound great on paper. Mass-loaded vinyl, resilient channel, “soundproof drywall,” “sound board,” “soundproof insulation,” and fancy isolation clips all promise to solve even your worst soundproofing problems, but in and of themselves, they don't always.

For good soundproofing results every time, stick to the inexpensive basics:

  1. Essential Decoupling
  2. Mass added from inexpensive 5/8” drywall
  3. A competent Damping Compound

What is STC?

Sound Transmission Class (STC ) is an integer rating of how well a building partition attenuates airborne sound. In the USA, it is widely used to rate interior partitions, ceilings and floors, doors, windows and exterior wall configurations (see ASTM International Classification E413 and E90).

In short, STC gives you a rough idea of how much sound a wall, for example, might stop. STC is the most common sound reduction measurement in use. As common as this measurement is, it is quite limited and should not be totally relied upon for real-world soundproofing expectations.

Sound Transmission Basics

DECIBELS: A decibel (dB) is simply a measurement of how loud a noise is. Think of dB as the volume knob on your receiver. 50 dB is quiet while 140 dB is so loud it can immediately injure your ears.

TRANSMISSION LOSS is a measurement of the dB (volume) difference on either side of a wall. Let's say we have a 100 dB tone on one side of a wall. Pretty loud. We measure this same tone on the other side of the wall and find we have 75 dB. So we would say that at this tone or pitch, we have 25 dB Transmission Loss. 25 dB less sound energy made it through the wall to the other side.

FREQUENCY, written as Hertz or Hz, is the measurement of the tone or musical note of a sound. It might have a high pitch like a flute might make (2000 Hz) or a low pitch as from a tuba (as low as 29 Hz).

Most humans are born with the ability to hear frequencies from about 20 Hz (low) up to 20,000 Hz (high) but that range shrinks as we get older.

Interestingly, a test tone with a different pitch sent through that same wall might only see a 4 dB transmission loss. The performance of a wall will vary greatly, depending upon the tone (frequency) of the sound.

What are STC ratings?

In 1961, STC was introduced as the method for comparing various wall, ceiling, floor, door, and window assemblies. STC is calculated by taking the Transmission Loss (TL) values tested at 16 standard frequencies over the range of 125 Hz to 4000 Hz and plotted on a graph. Your curve (what you actually measured) is compared to standard STC reference curves (see appendix). If your wall graph is closest to a standard STC 35 curve, your wall is said to have an STC rating of 35.

Adding Green Glue to boths side of a wall.
  • The magenta line in that graph is the standard reference STC contour.
  • The green line is the performance of the wall.
  • To calculate Transmission Loss and STC, the performance data should be obtained from a certified laboratory.

Higher STC is generally better, though not always, as we will see below.

Caution: The largest problem with relying on an STC number alone is that STC only considers frequencies down to 125 Hz. This can be misleading because most sound isolation complaints are from noise sources that are below 125 Hz.

Examples of noise sources below 125 Hz

  • Most of the sound energy generated by the average home theater
  • Traffic noise from airplanes, trucks, and heavy equipment operation
  • Guitar, bass, drums
  • Industrial equipment, especially pump system

Comparing two walls STC (in this graph, higher is better)

The example above demonstrates the problem with not considering data below 125 Hz. Frankly, neither of these walls stop much sound. Both are mediocre, with a low-frequency problem near 125 Hz. However one wall is STC 32, the other is STC 42!

This is because, with the blue wall, the big problem occurs at 125 Hz, and is therefore measured by STC. The black wall has essentially the same problem; however, it occurs just below 125 Hz and is therefore not calculated.