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Sound Transmission Class (STC) Rating Explained!

Sound Transmission Class or STC is the rating system by which the ability of a partition to attenuate, (reduce the effect of) airborne sound is measured.

Materials are tested over multiple sound frequencies, from bass levels of 125Hz to treble levels of 4000Hz. They are then assigned STC ratings, a United States standard since 1961.

The partitions tested include:

Both commercial and residential construction rely on these ratings as a way of reducing airborne sound transmission between rooms.

Airborne noise is a result of a person or object making a noise. The noise generates energy in the form of vibrations or sound-waves. These sound waves are picked up by a medium, like air.

airborne noise transmission

The soundwaves will travel through the air until they collide with a solid object or structure like a wall, ceiling, window, divide, etc. The sound then vibrates and is able to be heard in the adjoining space.

Examples of airborne noise include: people talking, music, and the television.

Sound Transmission Basics

Basic knowledge of the defining factors of STC ratings allows for a greater understanding of how ratings are achieved and how they relate to soundproofing techniques.

What is a Decibel?

The unit that describes the intensity of sound. Decibel means one-tenth of a Bel and is shown as the abbreviation dB.

The scale is unusual as our ears are exceptionally sensitive and the scale would be inordinate if numbered 1 by 1.

Instead, it relies on a logarithmic scale. Simply shown, near-total silence registers as 0dB and a sound 100 times more powerful is 20dB and 1000 times more powerful is 30dB.

To give this a little perspective;

  • A whisper: 15dB
  • General conversation: 60dB
  • Lawnmower: 90dB
  • Live concert: 120dB
  • Fireworks: 140dB

What is Frequency?

Frequency is a useful measurement of musical sounds and is expressed in Hertz (Hz).

sound frequency

Music has strong repeated sound waves and the speed with which they vibrate determines the pitch of the sound. The number of wave cycles that occur in 1 second denotes the frequency.

What is Transmission Loss?

Measuring volume from both sides of a dividing wall will give you different results. This is due to sound vibrations decreasing from one side of the wall to the other, the sound waves lose their potency during travel.

transmission loss
Image Credit

The difference between the two figures is the TL or Transmission Loss coefficient.

For instance:

Loud music on one side of a wall may record 90dB and only 65dB in the neighboring room. Therefore, there is a 25 dB TL.

Frequency of the sound can alter the TL figure greatly, some higher and lower frequencies (pitched) music can have a very similar dB figure with little transmission loss. The sound can be heard almost equally on both sides.

How STC Ratings are Calculated

Measuring an STC rating is a complicated process. Multiple differentials are considered, these are called 'deficiencies' when calculating the overall rating.

This is the layman's explanation:

  1. Place a speaker in a corner of the test room along with a microphone in the adjacent corner. A second speaker is placed diagonally opposite the first, on the other side of the wall in the receptor room.
  2. Measure the volume difference in the 2 adjoining spaces and workout the Transmission Loss in dB.
  3. Test both values independently against 18 of the most commonly used frequencies. These fall between 125Hz and 4000Hz.
  4. Plot all of the points on a graph. The results should form a curve. Those results will be compared against Industry Standard curves.

Your curve will be assigned the STC rating from the industry contour that closest resembles yours.

What is Considered a Good STC Rating?

The higher the STC number denotes the greater the ability to reduce sound transmission through a medium.

The necessity for high-quality is contextual, dependent upon what the space will be used for, and how important the blocking of sound transmission is.

For instance, Government buildings, board rooms, and honeymoon suites require an STC rating of 55+. The same applies to multi-unit living, where airborne noise comes from multiple sources and directions.

In some offices and commercial buildings, even small family homes, a mid-range STC rating is sufficient.

As explained above, the scale is sensitive and dramatic differences in sound transmission, and volume might result in a minor rating difference.

The following scale shows the designated STC rating of a full height, internal wall with the conversation taking place on one side, and overheard on the other.

25 Soft speech can be understood
30 Normal speech can be understood
35 Loud speech can be understood
40 Loud speech is heard, not intelligible
45 The point where privacy begins
50 Normal speech can't be heard
55 Loud speech can't be heard
60 Very loud speech not audible
65 Superior soundproofing

A rough guide when planning noise reduction solutions is as follows:

  • Poor: 30 -39
  • Good: 40-49
  • Better: 50-59
  • Excellent: 60-67

How to Improve STC Ratings

When assembling thin stud walls, it is a challenge to supply satisfactory soundproofing without taking up too much space.

It might not always be a straight-forward task either, there may be structural and thermal alterations to consider. There is also the aesthetics, adding a layer to a wall means a lot of furniture removal and redecorating.

Adding mass to a wall is the easiest way of improving an STC rating, although this may come at the cost of losing valuable floor space.

Increasing STC ratings on Walls During Construction

Soundproofing is usually thought of after construction. However, the following are some of the best ways to increase the STC rating of a wall during the construction process.

Stud Arrangements

Alternative stud arrangements such as staggered-stud or double-stud construction is an expensive way to raise the rating of an interior wall during construction.

Resilient Channels

The use of resilient channels are also very effective. The channels are fitted to both wood and metal struts with drywall panels screwed on top.

As long as the installation has been carried out correctly and the drywall is isolated from the struts, noise and vibration have difficulty traveling to the outer wall, resulting in transmission loss.

Increasing STC ratings on Existing Walls

gypsum drywall

When looking to increase STC scores to existing walls as a retrofit, resilient channels aren't recommended. There is insufficient space between the wall and the new layer of gypsum for them to be effective.

The addition of fiberboard-based products is cheap and simple. As a stand-alone solution, the boards offer very little in the way of soundproofing, however, when used beneath a layer of drywall, ratings increase by 3 to 5 points.

Metal sound clips work in much the same way as resilient channels, although they can be fitted retrospectively. They rely on a similar method of panel isolation and achieve good STC rating increases. Installation is labor-intensive and is comparatively expensive though.

Overall, the best way to improve STC scores on existing walls is with the addition of single or dual layer of drywall.

Results vary based on multiple factors such as manufacturing process and material used during production and the thickness of each sheet.

It is very important to recognize that STC ratings of mediums cannot simply be added.

James Burkett

To reiterate the above, an STC rating is a logarithmic value, similar to equations.

For example, a wall may have an STC rating of 34 and a single sheet of drywall may have a rating of 20.

When combined, you will not achieve an overall rating of 54, instead, you may see an increase in STC rating of 2 or 3 points.

When installing drywall to a thin stud wall, ensure all gaps are sealed. The smallest gaps of just an inch can result in huge TL loss, as much as 20 STC points.

Some examples of STC improvements of a thin stud wall using sheets of drywall:

  • With 2-sheets of 1/2” drywall: STC 33
  • 2-sheets of 5/8” drywall and standard insulation: STC 34
  • 2 x 4 wooden studs, 2 x 5/8” drywall, plus insulation STC: 34 -39
  • 5/8” metal studs, 2 x 5/8” drywall, no insulation STC: 38 – 40
  • 5/8” metal studs, 2 x 5/8” drywall, with insulation STC: 43 – 48

STC rating and Low-Frequency Noise

Although STC ratings are a solid guide of a material's soundproofing abilities, they shouldn't be solely relied upon.

High-rated walls may still underperform with very low-frequency noise. For sound of 125Hz or below, a more effective soundproofing solution should be employed.

Final Thoughts

It is a given that people live and work together more harmoniously in quieter environments. Interior walls with low STC ratings are no longer acceptable. Especially when STC ratings in the 50s and 60s are comfortably achievable.

Now the next time you look at a soundproofing material, you'll have a better understanding of what Sound Transmission Class or STC actually means.

James Burkett
James is a self-employed writer and editor and has the privilege of working from home. However, he quickly discovered how unwanted noise can disturb both work and sleep. Over the years he's effectively soundproofed his entire work space and a number of rooms in his house. Needless to say he can work and sleep much more peacefully.
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