What is OITC Rating?
When building a home, many homeowners fail to consider how window selection affects their soundproofing. Sure they look into all the other aspects of window setting from its location to its energy efficiency, but they usually end up forgetting about the acoustics.
We can blame this forgetfulness on the fact that a window's acoustical performance does not affect a home's appearance. However, this unaccounted for factor ultimately affects how comfortable and quiet your home is.
So, just how much noise are the windows letting into your home?
Sound waves are transmitted through air, liquid, and solids to end up perceived by the ears. These waves are vibrations that can vary in volume (loudness) measured in decibels and frequency (pitch) measured in hertz.
The volume and frequency of sound tend to change as the waves move through different mediums.
To understand OITC, we need to look at some units of measurements and important terms involved. These include frequency, decibels, and transmission loss.
Frequency: this is the measurement of a sounds musical note. Frequencies capture the number of sound waves that pass by per second. It is measured in hertz (Hz). Humans can hear frequencies ranging from 20Hz to 2000Hz, and as we get older, our hearing declines.
Decibels: this is the unit that measures the amplitude of sound; just how loud a noise is. The Sound Pressure Scale begins at zero decibels which is the threshold for audibility for humans. The higher the decibels, the louder the sound.
Noises that pass 130 decibels cause discomfort, pain and even injury. The average household maintains a decibel level of between 40-50 decibels.
To see examples of common household noises in decibels, see our interactive infographics here.
Transmission loss: this is one of the main components of OITC ratings. It is merely the measurement got by getting the difference of sound volume, measured in decibels, taken from either side of the barrier.
For example, a drummer is in one room partitioned by a wall beating drums and producing 100 decibels of sound, if you are on the other side of that wall you may only register 70 decibels.
The transmission loss, in that case, is 30 decibels, therefore capturing just how much the barrier absorbs.
The American Architectural Manufacturers Association (AAMA) and the American Society for Testing and Materials (ASTM) came up with sound ratings to standardize and determine the rating of sound transmission for building components, particularly window.
The sound ratings are calculated by two primary control systems: sound transmission class (STC) and outdoor-indoor transmission class (OITC).
The STC is the most common rating used in the window industry and is typically used to measure sound transmission loss between a frequency range from 125 to 4000 Hz.
It was initially developed and is more applicable for evaluating interior areas. This is because its range covers mid to high-frequency noises such as speech and sounds from the tv and radio.
The OITC is a newer rating system similar to STC that was designed to measure sound transmission loss between a more extensive frequency range from 80 to 4000Hz.
However, it was developed to evaluate sound transmissions through exterior walls to determine the performance of a product about exterior noises.
These noises are more substantial in low frequencies such as traffic, construction, and low-flying aeroplanes.
How is OITC measured?
OITC ratings measure sound intensity reduction in noise volume on exterior walls and are provided in decibels.
Since it is focused on blocking outside noises, the weight is more towards low-frequency performances. This is because external noise sources are usually farther away, and low-frequency sounds travel farther than high-frequency sounds.
An exterior walls OITC ratings are measured using data gathered over an 80 to 4000 Hz frequency range.
The data collected is then, and professionals calculate the barriers OITC rating following standards set by the American Society for Testing and Materials (ASTM).
The ability of a barrier to create specific decibel reduction varies depending on the frequency of the sound passing through it.
For example, if a 120-decibel traffic noise only registers as 80 decibels in the building after traveling through a wall, that sound experiences a 40-decibel deficit.
The higher the OITC ratings, the better the soundproofing properties of your external barriers. This means the more significant the difference between the sound transmitted from outside and the sound heard in the building, the better the attenuation of the wall.
An STC rating roughly works the same way as OITC only it is mainly used indoors. It is measured by setting up a sound transmitter and a receiver on either side of the partition being tested.
For example, if a 60 decibel sound on one side of a wall partition is reduced to 40 decibels, then that partition is said to have an STC of 20.
The human ear perceives a reduction of 10 decibels as roughly halving the volume. That is, a 50-decibel noise will subjectively seem half as loud as a 60 decibel one.
The decibel reduction in STC, which isn't indicated in decibel but points, does not apply equally across the entire range of frequencies measured.
Therefore, a wall partition with an STC of 30 subjectively reduces high-frequency noise by 40 decibels while minimizing low-frequency noise by only 10 decibels.
Low-frequency sounds travel farther than high frequency sounds so any partition will have less sound attenuation for low-frequency sounds.
Requirements to isolate low sound frequencies include extreme mass and decoupling. As a result, a barrier's OITC ratings tend to be smaller than its STC ratings.
When planning an isolation project, the OITC rating is often more crucial than the STC rating because it includes the 80 to 100Hz frequencies.
These frequencies that are only available for calculation on the OITC rating are becoming more popular thanks to high quality speakers.
Measuring low to mid-frequency sounds makes OITC a more appropriate rating system for windows.
Related: How to determine STC rating of a wall?
Differences between OITC and STC
When it comes to age, STC is much older than OITC. Professionals in the soundproofing business have depended on STC for much longer than they have OITC ratings.
STC rating was developed to asses several different types of barriers, such as doors, partitions, windows, and both interior and exterior walls.
In 1990, OITC was developed mainly to assess the ability of external sounds from the streets could travel through exterior barriers such as walls and windows.
The exterior sounds that this newer creation was developed to capture include blaring car horns, low flying airplanes, sirens, and construction noises.
These outside noises emit sound at low frequency because of their distance from the receiver and therefore were suitable for a rating tool that can capture these low frequencies.
The OITC rating system placed a particular interest in calculating the lower sound frequencies.
However, even with OITC being the newer creation, the STC rating system is still the preferred rating method among professionals in soundproofing.
The OITC rating system is mostly consulted when there are low-frequency sounds involved between 80 to 100 decibels that can't be calculated by the STC rating.
OITC and Soundproofing Windows
So you have learned all about OITC ratings and its predecessor STC rating but are still unsure about how to shop for soundproof windows? There are already several things you should consider.
The main question you need to ask yourself is, what level of noise frequency are you dealing with? That will help you determine your rating of choice.
Perhaps if you live in a neighborhood next to a busy road or airport, then you'll need to lean towards a product with OITC ratings. The traffic is bound to emit a lot of low-frequency sounds at all times of the day, which can prove a nuisance if left unchecked.
An average window has an STC rating of 25 to 30 which will typically allow you to hear a conversation going on outside. If you need better soundproofing, then consider consulting a professional on the best materials to use.
Related: How to Soundproof a Window
Factors that can affect the ratings of a window include glass thickness, insulating gas, spacer type, and some glass lite.
These factors, alone or combined, affect the calculations and transmission loss. It's not surprising that the acoustics performance of a home or building windows is influenced by the type of glass and general construction of framing members.
There are a lot of choices to be made when determining the best products for your home. We aren't saying you should neglect the aesthetics of the house; aesthetics are a big part of making a home comfortable.
What we're saying is you need to be careful on how you select these products and maybe consider products with high reviews.
However, when in doubt, settle for an STC rating for your windows. They are more widespread and efficient enough to meet average needs.
Having a quiet and comfortable house to come back to every evening is a homeowners dream. So investing in soundproofing products is the way to go.