Acoustic treatment for the home

By: Steve Manes , Contributing Writer
In: Home Improvement Tips, Technology

An apartment dwelling friend called me last week and complained about her upstairs neighbors who sounded like they were raising cattle. The clomping noise was driving her nuts and she wanted to know how much acoustic ceiling tile would cost her. I told her that ceiling tile would be virtually useless in this application. That’s not what it’s made for.

There are two basic types of noise control: one that alters the acoustics inside a room (absorption) and one that contains that noise and keeps it from invading neighboring spaces (transmission). The respective technical approaches are about as similar in function as a kitchen sponge and a sea wall. In other words, different.

Acoustic tile is an absorptive treatment. It’s also a limited one insofar as it’s mostly effective at the frequency range and sound pressure level (SPL) of ordinary speech. It’s intended to reduce the reverberation time, a/k/a echo, inside a room. It does very little to prevent sound transmission to the room next door.

My friend complained, “But I see it used in expensive hotels and I never hear anyone walking around in the room above me.” That’s because what she doesn’t see is the eight inches of concrete under that tile nor the thick carpet and padding on the floor above. Hotel builders know what they’re doing. That acoustic tile is mostly there to make the room feel cozier. And heavy carpet actually does a better job of it.

Sound transmission comes in two flavors: airborne noise and structurally borne noise. Airborne noise is like standing outside your neighbor’s front door and yelling, “Hey Joe! Get your car outta my driveway!” Structurally borne house is banging on his door with your fist. Airborne noise is much easier to deal with because air is a fairly poor conductor of sound.

Sound likes density as a medium — like the water around a sonar head or the the wood joists in your ceiling. It’s a bit more complicated than that but it’s the answer to why acoustic tile doesn’t work in this application. Acoustic tile works on airborne noise. Your neighbors upstairs sound like a team of Clydesdales because they’re creating structurally borne noise which travels through the floor joists to the building’s framing to which, of course, your apartment is rigidly attached.

So how do you fix it? The most doable solution is to buy your neighbors thick, fuzzy slippers. Or a plush hotel carpet. Beyond that, it gets complicated, expensive and crazy impractical.

I built a couple of commercial audio recording studios and the way we dealt with structural noise was to remove any pathways for sound to move through the structure. Commonly, this is done with box-within-a-box construction.

The control room is typically a very tight room built on a concrete slab which is floating on spring-and-neoprene vibration absorbers similar to those used to isolate vibrating heavy machinery. Then another air tight room is built around it with no rigid physical connection between them, not even a screw. Access is typically through a sound lock, which is a pair of very heavy, air tight solid core doors.

To kill airborne noise, the windows are double or triple glazed glass of different thicknesses to prevent harmonic resonance, with several inches of air space between them. Even the plumbing is isolated with vibration absorbing connectors. In fact, that was one of the most difficult jobs because FDNY inspectors weren’t very accommodating with regard to the floor’s fire sprinkling system.

These techniques work very well but you can see how something like this would never be practical as a retrofit for an apartment or a house.

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