We get a lot of calls from customers in recently built Boston condos who are hearing too much noise from the unit next door. There are rules now in the building code about soundproofing in condominiums and apartment buildings, but the code doesn’t tell you how to build an assembly, it just tells you the rating you have to achieve.
Party walls, floors, and ceilings are rated according to a “Sound Transmission Class,” or “STC,” that indicates how well they impede the passage of sound. The higher the STC, the better the element performs (that is, the less sound is transmitted). By code, walls between multifamily dwelling units should have an STC of 50 (if measured in the laboratory) or 45 (if measured on site in the field).
For a price, you can test any space to find out if it complies, but if the building fails the test, it’s still up to you to figure out how to make it pass.
Our company can perform the tests to measure the noise in an existing building and find out if the dwelling passes the STC 45 rule. But I don’t push this, and here’s why.
First of all, the code-required STC, like everything else in the code, is only a minimum. STC 45 or 50 is not very good. In a condo that just passes the code, you can still hear loud conversations, loud televisions, or thumping and bumping from next door. And how much a person is bothered by any kind of noise is subjective; it depends as much on somebody’s sensitivity to noise as it does on the measurable sound.
Another problem is that the engineering to scientifically measure sound is expensive—testing takes all day and costs thousands of dollars. (A full description of the testing procedures is beyond the scope of this article, but you can get an idea of what’s involved by reading about ASTM E90 and ASTM E989 and related field testing, as well as RT-60 reverberation testing.) If the clients are heading to court with a lawsuit, at the end of the day, they’ll have an engineering document that they can take with them. But even if they were to win their lawsuit, it would probably only require bringing the house up to the minimum code, which may not make them happy. Finally, the testing isn’t that helpful for planning the construction—our experience tells us much more about solving the problem than expensive instruments can. So unless a customer is dead set on going to court, I don’t recommend the engineering tests.
Instead, when I visit a building where a customer has a noise complaint, I listen to get an idea of the problem. At the same time, I judge how sensitive the client is to noise. I’ve had people say, “There! You hear that?” when my decibel meter wasn’t reading a thing. With couples, sometimes a certain type of sound bothers one partner, but the other doesn’t even notice it. It’s important to understand the client’s perception of the noise, early on.
We also have to understand the nature of the noise. There are different kinds of sound, and they travel differently. Voices or amplified music travel through the air. To stop those, we use a combination of airsealing and sound-absorbing material.
Footsteps, on the other hand, are an example of “impact sounds” that travel through structural framing, not air. (Bass notes from a subwoofer also travel through framing.) Structure-borne noise is harder to stop because it requires the framing to be isolated from other framing or from the space in the room.
Sometimes, we can stop one type of sound but not the other. So I may have to tell a client, “Yes, I can quiet things down, but I can’t eliminate all the noise. You might not be completely satisfied.”
And of course, there’s the budget: It may be possible to eliminate the problem by tearing out walls or floors and reframing, but most clients don’t want to spend $30,000 to do it. So the trick is to understand the source of the noise, its mode of travel, and the occupant’s sensitivity, and to communicate with the client about what our methods can achieve given the budget.