What are STC and OITC ratings?

What is STC?

The Sound Transmission Class (STC ) is a single-number rating of how well a material or building partition attenuates airborne sound at frequencies: 125 Hz, 250 Hz, 500 Hz, 1k Hz, 2k Hz, and 4k Hz. STC ratings are widely used to rate interior partitions, ceilings and floors. Other materials that can use STC as a rating system are doors, windows, and specialty soundproofing products.

An STC rating is selected by first measuring the partition for it’s transmission loss at each frequency. These transmission loss values are then plotted on a sound pressure level graph and the resulting curve is compared to a standard reference contour. This curve is then matched to the appropriate predefined Transmission Loss Curve to determine an STC rating. This measurement system is decent for speech sounds, but not as much for loud music, mechanical equipment noise, transportation noise or any sound with low-frequency content below 125 Hz. Think pounding on a door or explosion sound in a home theater. The transmission loss of a wall will vary depending upon the frequency content of the sound, therefore the STC rating system falls short when it comes to covering a large range of frequency. In short the STC rating system should only be used as a rough guideline and should not be completely relied upon for real world applications.

Article Glossary:

  • Frequency (Hz) is the speed of the vibration, and this determines the pitch of the sound. Frequency is measured as the number of wave cycles that occur in one second.  The unit of frequency measurement is Hertz (Hz for short). A frequency of 1 Hz means one wave cycle per second and a frequency of 1k Hz means a thousand wave cycles per second. Frequencies that are exactly half or double the Hz of another frequency are considered to be one full octave apart.

  • Decibel (dB) is a measurement of how loud a sound is. Think of dB as the volume knob on your receiver. 50 dB is quiet while 140 dB is so loud it can be damaging to your hearing.

  • Transmission Loss (TL) - Transmission loss is a measurement of the decibel (volume) or dB difference from one side of a wall to the other. 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.

What is OITC?

Developed in 1990, the OITC rating System was designed to measure the transmission of street sounds (such as car horns, sirens,construction, and low-flying airplanes) through exterior walls, Windows and façade elements. Exterior noise tends to be a lower frequency than interior noise (such as voices), so the OITC rating System emphasizes low frequency sounds in its calculations. Professionals use the OITC rating System less frequently than the STC rating System.

Today, the OITC rating System is used to assess any barrier meant to block out low frequency sound. For example, if your neighbor has a subwoofer-equipped sound System, you may want to consider OITC ratings for all of your barriers, interior and exterior.

How is an OITC Rating measured?
Like the STC rating System, OITC measures sound intensity loss in decibels. If a 105-decibel above ground subway only registers as 80 decibels after traveling through a Window or wall, the sound experiences a 25-decibel deficit. The ability of a barrier to create a specific decibel deficit varies according to the frequency of the sound passing through it: in general, very high and very low frequency sounds are more difficult to block.

A barrier’s OITC rating is measured using data gathered over an 80 to 4000 hertz frequency range. Once the data is collected, professionals calculate a barrier’s OITC rating in accordance with standards laid out by the American Society for Testing and Materials (ASTM). In general, a barriers OITC rating is lower than its STC rating.

How does OITC help?
Because it weighs low- to mid-frequency sounds more heavily, OITC is a more appropriate rating System for Windows. The blares and beeps of heavy traffic, the roar of airplanes, and the rattle of trains all produce a low-frequency cacophony.

However, for the average homeowner, the STC ratings for windows are sufficient—and preferable. The OITC rating System is still in its infancy, and has yet to diffuse throughout the Soundproof Window industry. While perhaps a better measure of Soundproofing, you’re generally better off asking for a Window’s STC rating.

In addition to being more widely used, the STC rating System has agreed upon benchmarks that make it easy for laypeople to understand how the rating will impact noise transmission. When in doubt, ask for the window STC rating on your Soundproof Windows and barriers. It’s a clearer and more widespread measure of sound transmission.