Auralex Acoustics

Auralex Acoustics

What’s an STC?

STC stands for ”Sound Transmission Class.” This is a single-number rating that can be used to compare the acoustical isolation of different barrier materials or partition constructions. The method used to determine the STC of a material or partition type is complicated. The following is a basic description of the methods covered in much more detail in ASTM E413:

The method to determine STC is conducted using two test rooms: a ”source” room and a ”receiver” room. The source room will contain a full-range test loudspeaker. The receiver room will contain a microphone, which is connected to sound-measuring devices. There is a nominal opening between the two rooms – usually about 9′ wide by 8′ high, but can vary in accordance with the standard.

The first step is to measure the sound transmitted from one room into the other through the opening. The sound is measured in decibels (dB) in 1/3-octave bands from 125 Hz to 4000 Hz.

The next step is to ”plug” the opening with the material or partition construction. This could be a single layer of barrier (such as plywood or SheetBlok), or a complete wall with as many materials, layers, air gaps, etc. that can fit in the opening. The edges are completely sealed and sound transmission between the rooms is measured again.

The sound level from the ”after” test is subtracted from the sound level ”before” plugging the opening. The resulting difference is the transmission loss or ”TL.”

Next, the TL is plotted on a graph of 1/3-octave band center frequency versus level (in dB). Now this is where it can get confusing. To get the STC, the measured curve is compared to a reference STC curve. (For an exact definition of the reference curve, see ASTM E413 or the third edition of the Handbook for Sound Engineers, edited by Glen Ballou – Chapter 3, ”Acoustical Noise Control.”) Two criteria are used to ”match” the curves:

  1. The reference curve shall not exceed the measured TL by more than 8 dB in any 1/3 octave band, and
  2. The sum of all the ”negative discrepancies” shall not exceed 32.

This actually sounds more complicated than it is. A simple spreadsheet can be used to calculate the STC for any range of TL values.

Once the two above criteria are met, the value of the reference curve at 500 Hz is read as the ”STC” of the material or partition type.

So, what does the STC actually tell us about a material or partition? Very little. Just as NRCs don’t tell much about the specific absorption properties of a material, STC tells very little about the isolation properties. It is always better to compare the actual TL values in different octave or 1/3-octave bands to get a better idea of the performance of one barrier or partition versus another. Just as different materials with the same NRC can have different performances, different materials with the same STC can behave very differently.

Here is a table of two sets of TL data that yield the same STC:

125 Hz 19 29
250 Hz 22 29
500 Hz 25 29
1000 Hz 28 30
2000 Hz 31 29
4000 Hz 34 29
STC 24 24


As you can see, the performance at low frequencies is dramatically different, yet the two materials or partitions have identical STCs.

A direct result of this is that many building materials have similar STCs – ½” drywall, ½” plywood, SheetBlok, etc. are all around STC-26 or STC-27. However, the performance of the SheetBlok – a ”limp-mass” barrier is significantly better at low frequencies. This is something you cannot compare using STCs – you have to look at the performance in octave bands.

Anyway, the STCs for a multitude of materials (barrier, as well as doors, windows, etc.) and constructions types (walls, ceilings and floors) have been tested over the years. If you cannot find the STC of a construction type, contact us to find out what you should build to get the isolation you need.

One important thing to note about STCs is that they do not add. At all. In theory, the STCs of two barriers that are identical in every way can be combined and see an increase of 6 in STC. (This is an extension of the mass law to STCs – doubling the mass theoretically improves TL by 6dB.) But this is rarely what ”really happens.” Two dissimilar barriers or partitions would have to be tested together to absolutely determine the new STC. Spreadsheets can be used to predict the performance of combinations using the TL data from the two separate materials or partitions. However, since these predictions often use the mass law, the predictions are rarely realized in the field.

To finish this section off, we’ll leave you with some STCs and their ”subjective” ratings. (Note: Some people are more sensitive than others. It is completely possible that an STC-50 partition may not be enough for someone who is hypersensitive to noise in their environment.)

STC 25-35 Conversation can generally be heard and understood through material/partition. Examples: Hollow-core wood door, one layer of drywall on each side of studs with no insulation.

STC 35-45 Conversation can generally be heard, but not understood through material/partition. Examples: Solid-core wood door, one layer of drywall on each side of studs with insulation.

STC 45-55 Conversation can neither be heard nor understood. Loud sources such as stereos or machinery can still be heard. (Minimum acceptable STC for studio partitions.) Examples: Acoustical doors (Industrial Acoustics, Overly), staggered stud construction, RC8 Resilient Channel on single stud partitions, 6” concrete block (painted).

STC 55-65 Most noise sources rendered inaudible. Loud sources may still be sensed. (Recommended for studios.) Examples: Double door ”airlocks”, combination concrete block and stud/drywall partitions (double walls), double stud walls with decoupled air cavities.

STC 65+ Considered ”silent” by most observers. Desirable design criteria for severely noise sensitive spaces (e.g., studios). Examples: Double concrete block walls, massive double stud walls with larger (6”+) air cavities.

Note: STCs are laboratory measurements. Some losses take place when the material or construction is implemented in the field. Due to these losses, the ”Field Sound Transmission Class” or ”FSTC” aka the ”Noise Isolation Class” or ”NIC,” is usually 5 – 10 lower than the laboratory STC for the same material or construction. This should be noted when choosing a construction or material for implementation in the field.