More Best Practices
How do you define comfort inside your home? Typically, thermal comfort is the biggest consideration—not being too hot or too cold. Having the right indoor humidity is another—not too damp nor too dry—which can affect not only comfort, but also human health and the structure’s durability. There’s also the visual aspect of comfort—it’s always nice to be in a room that “feels good” because of the way it was designed and how it is finished. Finally, there’s noise-related comfort, by which I mean the sounds experienced and how the pollution of sound waves can decrease comfort levels.
“Acoustical comfort” results from limiting or restricting the level of noise—whether from inside or outside—that enters a space. Sound moves on airwaves, which are rapid fluctuations in air pressure above and below atmospheric pressure. These fluctuations in air pressure can contact and reverberate through some surfaces. Air pressure differences caused by noise can also enter a house through holes in the air barrier and through openings between spaces—for example, under doors.
Sound is measured in decibels (dB). A decibel is a unit of sound pressure or noise intensity. The smaller the number, the quieter the noise. For instance, 60 dB is the level of normal conversation. An impact wrench produces 103 dB, and an ambulance siren is about 120 dB. The loudest possible tone is 194 dB.
Sound transmission ratings
Sound Transmission Class (STC) is a commonly used rating system that is based on a noise spectrum focused on speech sounds. STC is the difference in noise when measured in dB inside the space where the noise originates compared to outside the source room. For example, you’re in a room with a radio playing your favorite building science podcast and the conversation is measured at 75…
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24 Comments
Randy,
Our building code also has a good set of tables showing both sound and fire ratings for wall and ceiling assemblies.
https://free.bcpublications.ca/civix/document/id/public/bcbc2018/bcbc_2018dbp9rt
Thanks Malcolm, very inclusive list of details and options.
Randy,
Framing it as Acoustical Comfort seems like a really useful approach. I'm definitely going to use that idea. Thanks.
A great primer on a subject that's been badly underrepresented here on GBA, epecially with shared wall housing being inherently far more sustainable than detached single family homes where it isn't as big of a concern.
Its also a fascinating subject given the mechanics of acoustics not following "common sense" rules, for instance the Triple Leaf Effect https://www.soundproofingcompany.com/soundproofing_101/triple-leaf-effect where adding a layer of drywall in an assembly gives a lower STC rating.
bcade,
"Its also a fascinating subject given the mechanics of acoustics not following "common sense" rules".
It really is a counter-intuitive science - which is why it's always best to select a complete, tested assembly.
I was hoping to see more information on conventional assemblies. For example for a single family residence what is a reasonably priced & easily constructed 2nd floor assembly, say bedrooms over living space with hardwood floor? Is cork underlayment, resilient ceiling channels, and 3" batt of acoustic insulation good enough? There is much more published information for separating different units in multi-family.
dan_saa,
That's the beauty of the links both Randy and I have posted. They give you lots of options that use the conventional materials used in single family dwellings, that yield predicable results if you use the whole assemblies. They answer exactly the question you asked.
Sometimes it's really nice to reduce the intensity of the noise during a party. One method that was startling when walking through a show home involved its dining room. Above its chair rail the designer had installed batting (like in a bed comforter) covered by cloth that was complementary to the window covering. It was held in place by vertical strips of 1/4" wood stapled through the drywall to the 16" on center studs. While walking and talking into the dining room the reduction of sound bounce was absolutely amazing.
I, too, notice a lot of houses with highly reverberant interiors, including my own. I installed 1/2" fiberglass duct liner (Linacoustic) on one wall in my house, with stretched fabric over that (Fabricmate), which looked pretty good. That treatment did decrease the reverberance. (I measured the reverb with free software from Room EQ Wizard.) The wall is up high so I haven't tried to hang any pictures or other things on it.
Is it possible for a home to be too quiet?
I am a retired septuagenarian. I kinda pay attention to ordinary neighborhood happenings such as birds singing around 5am, the neighbors kids coming out to play, the mail carrier stopping at my box. Less desirable is the louder sounds of delivery trucks, trash trucks, fire sirens and trains in the distant rail yard.
Is profound isolation necessarily desirable?
iconoclast2222,
I agree - the goal isn't the absence of sound. That's where I think the concept of Acoustical Comfort is useful. You tailor the response to try and provide the most pleasurable environment, eliminating possible irritants - sounds through party walls, or between rooms, and those you find troubling from outside - while still maintaining a link to those you wish to enjoy.
One of my recent projects had the occupants in their new, higher performance home sleep through a severe storm where they lost several trees in the yard. They had no idea a storm had moved through until the next morning. It may be a good idea to have a weather radio or set a cell phone up to alert when severe weather is in the area. The effects of sound proofing can be a double-edged sword.
The verbs “damp” vs “dampen”
The internet, in all its wisdom, varies on this. I was trained that “to damp” is to lessen or decrease, notably to damp the draft on a fire. “To dampen” is to add moisture, as when my grandmother would dampen the laundry prior to ironing. (Yeah, who does that anymore??!!)
My parochial school education has “dun me good” concerning my use of the English language. Pardon my “nit picking” (Who remembers the origin of this cliche?).
You are technically right (the best kind of right?) -- Green Glue is a "damping" material, not a "dampening" one.
Excellent article -- all good stuff!
Sound attenuation is a topic that is near and dear to my heart due to my home theater passion so it's great to see an article that is accurate, when so many frustrate me with their exaggerations and misrepresentations.
The one very mild nit-pick I'd have is regarding using mineral wool for absorption rather than fiberglass. While it is technically true that mineral wool absorbs a wider frequency range and at a greater amount than fiberglass, the amount of both is so minuscule that it simply doesn't matter. This is exacerbated by the fact that "absorption" plays a minor role in sound attention, especially compared to the overwhelmingly larger effects made by "decoupling" and "mass" (and even "damping" in certain cases). Quite a bit of the (already limited) effect of the absorption layer is largely negated the instant you encase it in drywall.
The end result is that in a real-world scenario, mineral wool will either have zero difference compared to fiberglass or it will be so slight that it takes sensitive measurements to discover. If you are using mineral wool for many of its other wonderful properties, then by all means spend that extra money and reap the acoustical benefits as well. But if you're incorporating mineral wool exclusively as a sound absorption layer inside of an enclosed wall, then IMO you're wasting your money when fiberglass would have been much less money for the same effective result.
Kurtgranroth,
I has suspected that as the STC rated assemblies in our building code don't distinguish what sort of batts are used, they just call out the presence of "absorptive material" in the cavities.
Kurtgranroth,
I agree that there isn't a lot of performance increase between fiberglass and mineral or stone wool types of insulation. My issue is with the quality of the install. The photo of the poorly installed fiberglass insulation, which was installed by an insulating contractor, is commonly what I see in new construction projects. The mineral wool insulation tends to stay in place better. I've seen where kraft faced insulation was used for sound, it does stay in place better than plain fiberglass batts. I've had better luck with Rockwool, just my opinion.
>Quite a bit of the (already limited) effect of the absorption layer is largely negated the instant you encase it in drywall
From past conversations with acoustic consultant, the benefit of the absorption layeris reducing reverberation inside the wall or ceiling cavity which would result in increased noise transmission without it. They also confirmed the Triple Leaf Effect mentioned by bcade above (though not by that name).
In my experience, air-sealing between rooms or apartments is absolutely critical to a soundproofing installation. Having an unsealed electrical box or other small penetration will allow sound to bypass all manner of decoupling or damping. This can turn a 95% done job into something that is only 20% effective, so rigor is required to make the effort worthwhile.
Sheets, or pads, of acoustical putty are an effective and inexpensive wrap for electrical boxes and can even be used to damp duct registers as well as air-seal them. It is billed as “non-hardening”, so it can be a tidy alternative to acoustical caulk.
Having access to only side of a wall or ceiling makes installation harder, but not impossible, to seal a penetration from behind. Using more than one layer of Sheetrock, with or without Green Glue or other damping adhesive, will allow you to leave off an access strip through which you can access the back of the box you want to seal. Staggered seams in the top layer will minimize the effect of un-muddled seams in the base layer.
Although I haven’t tested the theory, it’s suggested that multiple layers of Sheetrock be different thicknesses to better block different frequencies of sound. On floors, I’ve found even a thin layer of self-leveling cement will seal and damp to noticeable effect. Rubber feet or mounts for isolating vibration sources, like dryers or other motorized equipment can also produce big improvements.
Regardless of the combination of methods and materials used, good sound reduction conveys a very high quality impression, as any luxury car will demonstrate. Tenants are more likely to enjoy their space and inhabit it longer when they are not subjected to the sounds of their immediate neighbors, so there is a good economic incentive for investing in soundproofing.
Very useful article, references and comments. thanks. Perhaps in a follow-up article you could cover the theory and application of sound damping in public spaces when noise generated within the space affects the sound quality of the same space. As an example, I am working with our local county government on reducing the internal noise of a large steel building used as an exhibit hall and meeting space when large crowds are present. I have in mind insulating the exposed steel frame (1" vinyl FF in a 8" cavity) with damp spray cellulose, then covering with coarse fabric. I haven't yet found examples or technical articles that address this type of application.
Another useful discussion, would be how to make the noise levels from generators softer or at least directional. In my particular case it would be the portable generators used for camping and especially by subs at jobsites. Neighbors can get pretty upset when generators wake them up in the morning. I've been told it's possible to build a "sound box" that will muffle the noise of the generator .for about 270 degrees and send it in just one direction. Does anyone have any experience in this area?
A significant amount of generator noise can be blocked/redirected simply by leaning a sheet of plywood against the generator. I've built boxes, but they didn't seem worth the additional effort for a temporary job site application. I have seen an automotive muffler adapted to a portable generator which made a big difference.
I remember, that years ago, Chicago's Museum of Science & Industry had a display of 2 big elliptical shaped pieces of plexiglass that apparently took sound waves and sent them in one direction. sort of like TV satellite collect/ concentrate radio waves.
Article mentions using a double layer of drywall. I often use 5/8" drywall (70 lbs /sheet) throughout. Not as effective as two layers of drywall, but it does add more mass with less labor, less complication with electrical boxes and extension jambs. Probably also worth considering specifying standard weight 1/2" drywall (51 lbs) vs. the lightweight drywall (39 lbs).
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