Acoustics: Resilient Channels and Sound Absorbency

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    Michael Ermann

    I was so happy to get the bat signal! I’ll chime in.

    Resilient channels act like a spring, decoupling one face of the partition from the other. They are made of floppy metal strips that run parallel to the studs or joists. One side is screwed into the stud and the other sides floats free of it. Then you screw the gypsum board into the free-floating side. They look like this and are installed like this.

    1. Hat channel is not resilient channel
    2. The metal should be floppy, if it is rigid than it’s not resilient channel
    3. If you use long screws when attaching the gypsum board to the channel, they will pass through the channel and attach rigidly to the stud behind it, short-circuiting the idea of the “spring” because you are in fact attaching to the stud directly.
    4. Resilient channels aren’t that useful in double stud, staggered stud, or floppy metal stud (24 gauge or thinner) construction because each of those support systems serve as a “spring” itself. Adding a resilient channel as a second spring doesn’t help.
    5. There’s a myth that the key to sound isolation involves fiberglass batt insulation in the cavity. Cavity insulation doesn’t help much in rigid assemblies because the sound short-circuits through the rigid structure, bypassing the cavity, but glass fiber helps a bit in these springy/decoupled assemblies: double studs, staggered studs, thin-gauge metal studs, or resilient channel

    You are correct that you want a high STC rating for the partition, but not really correct in your prescription for absorbing material in the adjacent room. In theory, if the source room was a steady noise, like a mechanical room, and the receiving room was more absorbent, you might benefit a little in sound isolation. . . but really the way to effectively keep sound out of the adjacent room is in the barrier itself, not in the receiving room sound absorption profile. Even better yet, don’t locate the mechanical room next to the conference room in the first place, so the wall won’t have to provide the sound isolation. Because even if you get the wall right, the flanking paths (through the ductwork, through the electrical receptacles, etc.) will likely control how much sound travels between the rooms anyway! Surround the noisy room with buffer zones of the corridors, locker room, bathroom, storage room, electrical room, and other spaces tolerant of noise that you need in your building anyway. Effective noise isolation happens in the PA phase, even though architects and NCARB may think of it as a PDD thing. —Michael Ermann, The Amber Book

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    Sofia Salvat Mere

    Thank you for your response, Michael! This really helped clarify things for me.

    So if the key in to a high STC value is in the barrier itself, and resilient channels act to basically dissolve sound within the barrier by decoupling, then does it matter if the resilient channel faces the sound or receiver room? 

    I am asking because I came across a practice question that asked "Which side of the wall controls sound?" and it has left me stumped!

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    Michael Ermann

    Whoops. . . I meant to answer that question but forgot to.

    It does not matter which side--source room or receiver room-- the resilient channel is on. Just don't put it on both sides, because it paradoxically becomes LESS effective. Did the practice question really say "Which side of the wall controls sound?" . . . if so, that is a bad question without a lot more information.

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    Janeen Minguillo

    I've seen this question, and typically answer is on the source side. 

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    Michael Ermann

    Labs have tested resilient channel looking for the answer to this question, and the consensus remains that the results are "reciprocal," meaning that the performance is the same whether the resilient channel is son the source side or the receiver side. Labs do, however find a benefit to positioning the RC on the side with more mass in an imbalanced condition. In other words, if one side of the assembly has one layer of gypsum board, and the other side has two layers (not uncommon in assemblies trying to reach some minimum STC). . . put the resilient channel on the side with two layers.

    It doesn't surprise me at all that you've seen this question in other study guides (or in an exam), but know that the answer--that the source side is better for RC--is not correct. --Michael Ermann, Amber Book creator.

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    Sofia Salvat Mere

    Michael Ermann I am happy to report that I passed my last exam, PPD on August 18th. Thank you for all of your help in this studying process. Your input and study resources were invaluable!

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