Understanding Acoustical Rating Systems for Soundproofing: OITC vs. STC

Acoustical rating systems are critical for making the right decisions when you’re looking to acoustically treat a space correctly, but they can be confusing. Often I hear from customers looking to soundproof, but they are tripped up by the difference between rating systems, which bogs down their project timeline. For example, I recently had a customer looking to soundproof her home office but she couldn’t figure out the right product to purchase because she couldn’t find any that had what she thought was a good NRC rating.

I was able to help her instantly by clarifying that NRC ratings are for sound absorption, not soundproofing. She realized she had been browsing sound absorption products instead of soundproofing products, only because of a small confusion about ratings!

Let’s clear it up quickly:

The Noise Reduction Coefficient (NRC) rating determines how much sound is absorbed, so this rating is meant for sound absorption materials. Sound absorption is the process to make the acoustics better in your space.

A Sound Transmission Class (STC) rating determines how much sound transmits through, so this rating is meant for soundproofing materials. Soundproofing is how we block sounds so they cannot leak from your space, nor can sounds from adjacent rooms penetrate.

Often confused, soundproofing and sound absorption (also called sound control) are two different things, so they need two different rating systems. That said, there is another rating system alongside STC for soundproofing called Outdoor/Indoor Transmission Class, or OITC. In this article, I will break down the differences so you can understand and browse soundproofing products more effectively.

The Two Soundproofing Ratings: OITC and STC

Soundproofing is how we stop sound from leaving a space to enter another, or have noises from outside come in. Soundproofing, also known as sound blocking, has a number of useful applications, including things as simple as reducing street sounds coming through windows in the home to more complex projects like keeping the loud and heavy sounds of a movie contained and separate from the sonic experience in adjacent rooms.

Generally speaking, to block sound we must create tailored assemblies, like walls, ceilings, and floors, and/or utilize special materials that are more capable of blocking sound than traditional ones used in construction. I like to think about soundproofing like blocking an arrow with a shield. If your shield is a t-shirt, you’re in trouble! If your shield is made of wood, the arrowhead might poke through, but it will stop the arrow. If the shield is made of concrete, that arrow isn’t going anywhere. With soundproofing then, we want to find materials like the concrete shield so we can block the majority of the sound, or maybe we just need something that mostly blocks the sound, like the wood shield.

To determine how much sound a material or assembly can block, we test them and use the results to determine an STC and/or OITC rating. Let’s understand the difference.

Sound Transmission Class (STC)

Originally developed to determine the sound transmission between interior walls, STC is the original and most widely used and recognized rating system when it comes to soundproofing. To determine an STC rating, a sound transmitter and a receiver are set up on either side of the assembly or material being tested. A selection of frequencies between 125 Hz – 4000 Hz are tested and the difference in the initial sound level is recorded in decibels (dB). Generally speaking, this difference is the STC rating.

STC RATING CHART
STC	What can be heard at this level
25	Soft speech can heard and understood
30	Normal speech can heard and understood
35	Loud speech can heard and understood
40	Loud speech can heard, but not understood
45	The threshold at which privacy begins
50	Loud sound can be heard, but are very faint
60+	At this level, good sound proofing begins. Neighbors generally are not disturbed by very loud speech from inside

To be more precise, the results from each frequency tested is plotted on a curve and this curve can then be compared to STC reference curves to determine which rating your results match. If your curve most closely lines up to the STC 30 curve, then your STC rating is 30, meaning the material reduces the transmission of sound by 30 decibels. The higher the rating, the most effective the material or assembly is at blocking sound.

Outdoor/Indoor Transmission Class (OITC)

The OITC rating system was developed to determine the transmission of street sounds through exterior walls, cladding, and windows. OITC is a very new rating in the world of acoustics and not all products carry an OITC rating, as they may predate that testing being more commonplace. Although very similar, the OITC rating system is aimed at lower frequencies, such as construction work, rumbling engines, low-flying planes, and car horns, and it utilizes a wider frequency range in the low end for testing. The same type of testing is performed as with STC, but the frequency tests are widened to a range of 80 – 4000 Hz. Although 80 Hz doesn’t seem too different from 125 Hz with the STC test spectrum, this is a significant section of the frequency spectrum for low-end sounds.

Because of this difference, STC is sometimes colloquially called Speech Transmission Class, as it is designed for frequencies similar to speech, not very low frequencies.

OITC vs. STC

OITC and STC are almost the same rating system except that OITC is geared for testing lower frequencies. This is useful for my customers as I can quickly determine which rating system would be more ideal to reference depending on the application. If someone is trying to soundproof a room in their home from adjacent rooms, and the main noises of concern are voices, then relying on an STC rating will be perfect. If, however, a customer is looking to soundproof their walls from exterior street sounds, or wants to soundproof a room for a home movie theater where films will loudly play low frequencies, utilizing the OITC rating will be more precise and the soundproofing results will be more effective.

How to Use Acoustical Ratings When Selecting Soundproofing Products

Before looking at soundproofing product examples, I find it is important to understand the unit we use to measure loudness in order to fully understand STC and OITC ratings.

Understanding Decibels (dB)

To measure how loud a sound is, we use decibels (dB). The important thing to understand about dB is that it is a logarithmic unit. To keep this simple and not get into the math, a difference of 10 dB is actually double the loudness.

Here’s a quick example:

If you were comparing two soundproofing products, the difference between an STC30 and an STC20 rated material would be about 10-15% from the same starting point. An example would be a 70dB noise with a STC30 product would yield about a 40dB end result – or a perceived reduction of about 85%. A 70dB noise with a STC20 product would yield about a 50dB end result- or a perceived reduction of 75%. So, in this case the 10 STC points is only a slight change in the original sound. This is counterintuitive, but this is how a logarithmic scale works instead of a linear scale.

As noted in the chart above, STC 25 is when there is essentially no soundproofing capability, and over STC 60 is when true soundproofing begins. Most soundproofing products lie in the range of 35-55. As we will see below, the STC rating of an individual isn’t the only part of the calculation to achieve a soundproofing result.

Using STC/OITC Rating Systems when Selecting Products

When it comes to soundproofing, there are many variables at play, most importantly of which is your specific needs. Selecting the right product for reducing sound coming in from a window is a lot different than soundproofing a bedroom to be used as a podcasting studio, for example. Let’s use these two as examples on a product journey.

Soundproofing Example 1: Soundproofing a Window

Let’s say you have a window in your home office that too much outside noise makes its way through. If you weren’t concerned about the light, then you might consider a soundproofing blanket to simply cover the window, like our PrivacyShield^® ABSC-25 Barrier Septum Soundproofing Blanket. This would be as simple as hanging, but how effective would the soundproofing be? The PrivacyShield^® ABSC-25 Barrier Septum Soundproofing Blanket has an STC rating of 33. Looking at the chart, this doesn’t seem to be much, but we need to also consider the inherent STC rating of the window.

If the window was just a hole, then just the blanket would be a total STC 35. The window has its own STC rating though, typically averaging about 30. Therefore, the blanket is adding to the window’s inherent capabilities. This is why we need to understand the logarithmic scale of decibels. If the window has an STC rating of 30, the final STC with the addition of the blanket is not 65. You would have to do more serious calculations to find the right answer, or test the blanket + window assembly, but essentially you just need to know these two things:

1. The STC rating of a soundproofing product is the rating of the product alone. You will be adding this product to another product, like a window or wall, which have their own STC rating.
2. You cannot simply add two STC ratings together because of the logarithmic scale of decibels. That said, the addition of a soundproofing product will be more than just the window itself, so you will be making the window more soundproof.

To make this a bit easier to understand, let’s look at another great product, our PrivacyShield^® Window Seal Kit. This is a great option if you’d like to improve the soundproofing capabilities of a window, but still want the light to come through. Take a look at this chart that compares the inherent STC rating of common windows and how it is boosted by the addition of the PrivacyShield^® Window Seal Kit.

PrivacyShield® Window Seal Kit
Sound Transmission Loss
Window Type		STC
Double Hung	No seal	24
	With PrivacyShield® Window Seal	37
Triple Casement	No seal	27
	With PrivacyShield® Window Seal	44
Fixed	No seal	33
	With PrivacyShield® Window Seal	51

As you can see, the higher the STC rating of the window by itself, the more soundproofing the window seal kit can offer. If you are able to discover the STC rating of the window in question, that would be extremely valuable to knowing the best product to buy. If not, you can always test the window, or talk to an expert who can help you calculate what you might need depending on the type of window in question.

Soundproofing Example 2: Soundproofing Walls for a Podcasting Studio

With our window example, we essentially wanted to significantly reduce the sound coming from outside. If you’re looking to seriously soundproof and eliminate sounds coming in, then we have to go further. If we are looking to soundproof walls for our podcasting studio, we need to address the wall itself. One way to do this is to use Green Glue Noiseproofing Compound between two layers of drywall to lessen the vibrations of sound passing through. As you can see below in the chart, the amount of Green Glue that is used has a powerful impact on the ultimate STC rating of the assembly.

Work with Acoustical Solutions for the Best Soundproofing Products and Expertise

I love soundproofing because it has a dramatic result and the road to get there is often a creative one. That said, it can seem complicated, and relying on the STC or OITC rating of products may not be enough information to deduce exactly what you need. I love hearing about customer projects and helping fill the information gaps so we can choose the perfect product for you.

Give us a call today and we can analyze your project and achieve the soundproofing you are hoping for!