Explaining Sound Ratings
The four types of sound ratings referenced on this site are STC, OITC, IIC, and Delta IIC (ΔIIC). Below is a summary of each of these sound transmission classes and how they might relate to your project.
STC stands for sound transmission class. This is the most common rating used in North America for determining airborne sound transmission loss between 125 Hz and 4,000 Hz. This range covers the majority of common noises we hear including speech, television, music, dogs barking, and other similar annoyances. A higher STC rating often shows improved performance. However, the rating is essentially an average over the 16 frequency points tested.
Because of this, a product can perform exceptionally well in one range, poorly in another, and still end up with a better STC rating than a competing product that may have performed better in a frequency range more relevant to your project requirements. This is a pretty common occurrence that should not be ignored when comparing products. Sort through the misleading ratings by gaining access to actual sound test data showing the STC contour line on the transmission loss graph. The performance in relation to the STC contour line will reveal the consistency of the product over the most common frequencies.
The STC rating for a basic insulated 2×4 wall with a single layer of drywall on each side is ST 35-38. This rating increases to STC 42-45 with steel studs. The STC rating for a basic wood structure floor/ceiling is STC 40-43. The STC rating for a six inch concrete sub-floor is 52-55.
A good level of isolation for walls and ceilings is STC 50 plus. A high level of isolation for walls is STC 60 plus with ceilings at STC 50 plus.
OITC stands for outside inside transmission class. The original purpose of the OITC rating was to determine performance of products in relation to exterior noise, which is often heavy in low frequencies. We also use the OITC rating to determine a product’s value for use in low frequency sensitive areas such as home theaters and recording studios. The OITC rating represents transmission loss results from 80 to 4,000 Hz using a different mathematical equation than the STC rating. The results express in decibels as opposed to the point system used to determine STC ratings and with weighting more towards low frequency performance. A higher number shows better performance. As a result of this method of calculating, OITC numbers are typically much lower than STC numbers. These two tests have to be used together and cannot be compared to each other.
When planning an isolation project, the OITC rating is often more crucial than the STC rating. This is because of the inclusion of the 80 and 100 Hz frequencies in the OITC calculation. These two frequencies are now more commonplace thanks to high quality speakers and sub-woofers. Mass and decoupling combined are recommended for increasing the OITC rating of any assembly.
IIC stands for impact insulation class. Acoustic labs conduct the IIC test using a tapping machine with steel faced hammers. These hammers strike a test floor material generating sounds between 125 Hz to 4,000 Hz. The impact creates vibrations that travel through the floor into the receiving side (the room below). The engineer plots the results of each tap on a graph, compares the results to the reference assembly, and determines the IIC rating from comparing these two tests. A higher number shows better performance. The IIC rating does not account for any squeaking or rattling caused by loose wood frame construction. Nor does it account for low frequency footfall noise or structural deflection.
The IIC ratings for basic concrete sub-floor with no resilient underlayment is around 28 IIC to 35 IIC. The IIC ratings for basic wood structure with no resilient underlayment is around 40 IIC to 45 IIC. Without the aid of sound control products, the IIC rating of basic wood structures will be noticeably higher than concrete structures as they are inherently more resilient.
An IIC rating of 50 and above is most common in building code and HOA requirements.
Delta IIC (ΔIIC)
The Delta IIC rating shows what the product adds to the assembly in terms of isolating impact footfall noise. The Delta IIC test starts by testing a full assembly, typically six to eight inches of concrete (this is not a test conducted for wood structures), with nothing above or below the concrete. Then an underlayment installs directly to the concrete, and the same test repeats. The Delta IIC rating is the performance gain between the first and second test. A higher number shows better performance.
The Delta IIC rating is the best rating to consider when comparing the performance of different types of underlayment. It keeps the manufacturers from promoting misleading results obtained by using materials or methods of isolation that the average assembly does not use. This test is only relevant for concrete structures. Products with high Delta IIC ratings won't necessarily perform well in wood structures and vice versa for products that perform well in wood structures.