OITC Rating

What is an OITC Rating?

OITC stands for Outdoor-Indoor Transmission Class. It is a relatively new (around 1990) system developed to measure how products perform in keeping outside sounds from penetrating into your living space, or office space. The window and door industry consider OITC ratings at least as important as STC (Sound Transmission Class) ratings.

Why is OITC Important?

Outdoor-Indoor Transmission Class determines a product’s performance in eliminating or suppressing exterior noise. OITC measures lower frequency noise such as construction noises, car horns, and the sound of airplanes. Other low frequency noise comes from high quality speakers playing bass-heavy music. 

How is OITC Measured?

Before I try to explain this measurement, you (and I) need to know a little about some common units of measurement. A little–except for real sound nerds–because I do not want your eyes to roll back in your head.

• Frequency – This can be defined as the musical note (tone) of a sound. It is the number of sound waves (vibrations) that pass by a given point per second. It is measured in units called Hertz (Hz). One Hz is equal to one cycle per second. (Note: Hertz is named after a German physicist–not the car rental firm.)
• Decibel – This is a measurement of the loudness of a sound. The Sound Pressure Scale begins at zero decibels (dB), which is the point at which humans hear sounds, and goes up from there. The cute chart below will give you an idea of the dB level of common situations. If you would like more information please link to these sites: Wikipedia, YouTube Pro Tool Reviews. (You are way tougher than I am if you wade through the complete Wikipedia article.)

• Transmission Loss – This is defined as ‘the decrease of sound intensity that is reduced by a wall or other structure at a given frequency’. Measured in decibels. For instance–the firecracker in the above chart registers 125 dB outside. If it registers 80 dB in your bedroom, the transmission loss is 45 dB, which gives you a reading of the efficiency of your soundproofing. The        BAFX Decibel Meter can give you the readings you need. Rumor has it that there is an App for that. (I do not know because getting my  cell phone turned on is a win in my world.)

Outdoor-Indoor Transmission Class ratings are a measurement of the sound intensity lost through any type of barrier–such as walls, windows, doors, even a fence–and measured in decibels. The success of any barrier in increasing the decibel deficit is dependent on the frequency of the sound getting through. Low and high frequencies are the hardest to block.

OITC ratings are calculated using the frequency range of 80 Hz to 4000 Hz. The lower frequencies are given more weight in all OITC calculations. The results are plotted on a graph and compared with standards set by the American Society for Testing and Materials (ASTM). The product–quite often windows and doors–is then given its OITC rating. A higher rated product will work better to block, and/or reduce, the lower frequency noise.

Is OITC Better than STC?

The short answer is ‘No’. The long answer is ‘In some cases’. Sound Transmission Class (STC) rating system measures sound transmission loss between 125 Hz and 4000 Hz. It is the average of 16 frequency test points. It is also the most used soundproofing rating system. Professionals tend to trust it more than other systems. (If you have read some of my other articles on this site, you have noticed that I use it almost exclusively.) Which does not mean you should ignore other ratings like OITC.

When to Use OITC Ratings

The Outdoor-Indoor Transmission Class rating system was developed to measure the amount of street sound noise travelling through walls, windows, doors, etc. Outside noises tend to emit lower frequencies. OITC measures sound transmission loss between 80 Hz and 4000 Hz (weighted towards the lower end). Therefore, if you live close to a busy road, or a construction site of some kind, you should consider both OITC and STC whenever you are soundproofing walls, or changing windows and doors. 

Another instance where OITC will probably be more important than STC is in the soundproofing of recording studios and entertainment rooms or home theaters inside the house. Assuming you will be using high-quality speakers and sub-woofers (whatever they are) along with a lot of bass, you are going to want to use product that gives you the highest OITC rating possible. Adding mass to the walls, and decoupling will increase your rating. 

OITC ratings are generally lower than STC ratings because the results are in decibels rather than the STC point system. Higher numbers in both are better. The two rating systems are not interchangeable. They have to be used together. Using both will give you a more complete picture of the product you are planning to install. This window chart will give you a comparison of rating numbers–just to give you some idea of the number difference.

End Notes 

For some information on adding mass and decoupling please check out the soundproofing wall section of my article How to Soundproof a Garage. You will find sections on Mass Loaded Vinyl, Green Glue, and Resilient Channel.