Acoustics 101

Life is a noisy business. Virtually all human activity causes noise of some kind, and life in general seems to get noisier all the time. Noise is even recognized as a form of pollution, a detrimental influence to be managed and minimized.

In recent decades, noise has become a big issue in the building industry. As more and more of us live in mutli-family towers and work in high rise office towers, we're increasingly surrounded by ever more sources of noise, much of it human-generated. So architects and builders try to design buildings that reduce interior noise. They use materials and methods specially designed to minimize sound transmission between walls, and between floors.

Naturally, professionals need to be able to measure a material's capacity to reduce noise, so they can make informed choices about which materials to specify on a given project. They do that by referring to sound ratings, based on universally-recognized ASTM tests. Which sounds straightforward but, in practice, it's not. With this short overview, we'll try to demystify the acoustics ratings process, and show you how to assess the accuracy of ratings information.


Interior sounds are classed into two groups.

Sound Transmission Class (STC) refers to air-borne sounds - sounds that travel through the air, like voices, TV or music. These are the sounds you would hear through the wall you share with your neighbor.

Impact Insulation Class (IIC) are structure-borne sounds - sound generated by something impacting the actual building structure, like footsteps on a hard floor, which you would hear in the room below. In our business, this is the class we're most interested in.

Materials are tested to measure their sound-reduction properties, both STC and IIC, using ASTM tests. This is very definitely a job for trained professionals, but here's how it works in a nutshell.


Imagine two closed spaces, side by side for STC, or one above the other for an IIC test. In one space, put a machine that generates noise. If measuring STC, it will just emit a sound. If IIC, it will have a small hammer striking the floor to create an impact noise. (The same process is followed whether measuring STC or IIC, so we'll only describe one of them. Since IIC is more relevant to flooring, we'll go with that.)

In the adjacent space, a specialised microphone picks up the sound.


The test is run first with nothing on the shared wall or floor area. Data is recorded.


Then, the material to be tested is applied to the common wall or floor area, and the test is run again. Data is recorded. Then, after a great deal of number crunching, results are produced.


The first test in the empty space produced an IIC reading of 33. The second, with the added material on the floor, gave a reading of 57. So the tested material may be labelled as having an IIC rating of 57.

If the tests are performed in an actual building rather than a lab or testing facility, an 'A' will be added to the rating: AIIC 57. (The A stands for apparent.)

This is the number that will show up on job specifications. A project will require a floor/ceiling assembly that achieves a certain IIC or AIIC rating.

The problem is, these ratings can be very easily gamed. Here's how.



Let's go back to the testing site. This time, we're using the same material on the floor, but in the space below we added a dropped ceiling, and 12 inches of insulation. This test gives an IIC result of 69 … for the very same product that previously scored 57. So which is accurate?

Many times you'll see an acoustic underlay product boasting an unbelievable IIC rating; if it seems unbelievable, it probably is. A little due diligence will probably reveal that the rating was obtained using some questionable testing practices like those described above.

The big takeaway is that whenever you see acoustic ratings, it's very important to read the fine print: what was the testing scenario? Was it a realistic situation, or one stage-managed to maximize the reading?

To avoid this issue, many in the industry now use a newer term: Delta IIC – sometimes written with the Greek letter, like this: Δ.

The Delta reference is very simple: it isolates the value of the tested material from the overall score. Delta = value after testing – value before testing, or, in our test case, 57-33 = 24.

Delta values give a more transparent and accurate assessment of individual materials and assemblies, and are becoming the industry standard. That's a very quick overview of the acoustic ratings system and testing procedure. Here's how it works in practice.



In many modern buildings, especially multi-family dwellings, architects will specify acoustic ratings for various parts of the structure, including floors. In many cases, achieving the specified rating may require a custom 'assembly' – a combination of materials specially selected to achieve the target rating. Here's how a typical flooring assembly works.


This is a standard concrete high-rise, with an 8 inch slab between floors. We know the IIC – actually AIIC, because we're on location, not in the lab – is 33. When we put our flooring in place, the AIIC climbs to 57. But the specification is 65.

We can't add dropped ceilings and insulation, like in the questionable test lab scenario. Instead, we use an underlay specially designed to reduce noise underneath the floor.



The combination of slab, underlay and flooring material used together delivers an AIIC of 68, which more than meets the spec.

Just about any underlay will have some noise reducing qualities, but some products are designed specifically for this purpose. A 2 to 5 millimeter layer can do a pretty amazing job of reducing noise. So if you're looking at a noise control issue, look for an underlay specifically designed for the job. Its Delta rating will tell you just how good it is.

Acoustic underlays can be used with any hard surface flooring – wood, tile, laminate, stone, even vinyl – but some are very specific to a certain product category. Always make sure that the acoustic underlay you're using is designed for the type of flooring you're installing!

Designing flooring assemblies that meet specific performance criteria is a job for the pros. There are hundreds of different acoustic flooring products on the market and knowing which product is best for which application can be very challenging.

At Metro, our Architect and Design team specialize in this kind of work, and we routinely design custom flooring assemblies for commercial clients. If you're faced with a project requiring some custom solutions, ask your Metro rep for help. We're sure to have a solution to the problem.

For more information on acoustic underlays, check out Module 8 of Metro U in the Metroport section of the website. Dealer log in required.