Soundproofing downsides?

Discussion in 'Listening Room / Home Theater Build Projects' started by Eric SVL, Aug 2, 2018.

  1. Eric SVL

    Eric SVL Member
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    All,

    I'm not building my own theater (yet!), but I'd like to get some input on something I ran across recently. I see talk of soundproofing, but very little of its downsides, except the extra headache to achieve it. The best way for me to explain this is to link to what I read. Start here, particularly the comments in paragraph 4:

    https://www.avsforum.com/forum/19-d...-required-2017-home-theater.html#post50684857

    And this post on the following page: https://www.avsforum.com/forum/19-d...equired-2017-home-theater-2.html#post51003913

    What I take away from that is, a well-sealed room can be very problematic for bass, and one that "leaks" more would generally have less ringing/faster decay and smoother response. What do you guys think?
     
  2. Wayne A. Pflughaupt

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    We have some folks here with soundproofed rooms, so maybe they will weigh in.

    That said, I’d expand on the bold section of the quote below from your first link...

    ... to say that any dedicated room with shoebox dimensions will sound worse than a standard living room. These rooms have a bass “dead zone” in the dead center of the room, and the bass level increases as you move from that point towards any wall. Thus, the bass sounds different at every seat in the room. People spend all sorts of money to overcome this problem, one of the popular options being the use of multiple subwoofers spread around the room.

    Contrast that to the last house I lived in, that had a living room with a cathedral ceiling open to the dining room and kitchen, and upstairs landing. On other words, about as irregular a room as you can get. But, one subwoofer, properly equalized, was sufficient to deliver bass that sounded the same anywhere in the room.

    So, unless you have a compelling reason to build out a dedicated room, especially a soundproof one, just stick with the family room.

    Regards,
    Wayne
     
    #2 Wayne A. Pflughaupt, Aug 3, 2018
    Last edited by a moderator: Aug 5, 2018
  3. Tony V.

    Tony V. Moderator
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    The challenge with soundproofing a room is that the room acoustically becomes dead and has alot less reflective surfaces. That can be repaired somewhat by keeping some walls sound reflective. Sound causes vibration naturally in walls and this in its self is actually a good thing. When you sound proof a room you eliminate that as much as possible to avoid transfer of that vibration to the outside of the room. This in turn also makes a room far less optimal for particularly music.
     
  4. Todd Anderson

    Todd Anderson News Editor / Reviewer/ Senior Admin
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    I'd be curious to read what @Sonnie and @Matthew J Poes have to say about this....

    Actually sound proofing a room (no sound leakage) can be tough to achieve in most home settings, require quite a few specialized building materials... as for completely sealing a room versus having leakage, obviously too much leakage just means you'll need to compensate with more sub energy. Not sure if a totally sealed room means low end perfection, tho.
     
  5. Sonnie

    Sonnie Senior Admin
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    Get in a car that has a lot of subwoofering in it... play it loud with all the windows rolled, then crack the window, the bass just got more pronounced/louder. However, that's what Dirac Live is for in a sealed room. Stopping deep bass is nearly impossible inside of a house, short of 8-10" thick walls AND the same in a ceiling, then you still will have that thunder rumble thru the house. As Tony mentioned, you have walls that vibrate and cause sound... so to completely soundproof the room, it would have to pretty much detached.

    Our room has a hallway between our main house and the room... stagger studded walls, two sheets of 5/8" drywall with green glue between the layers on the hall side... tightly blown insulation on the inside wall, 3/4" plywood, green glue, 5/8" drywall on the inside, then a thin layer of sheetrock mud that's painted. The wall is right at 9" thick. The ceiling is 1/2" drywall, 3/4" plywood, green glue, 5/8" sheetrock, and a pile of thicker heavy dense blown insulation on top of it in the attic. The entry is double solid wood core doors with two thresholds for sealing each door. Others here that have been to my home several times can tell you that when you walk into my room, shut the first door and then go to shut the second door, you have to press pretty hard to shut it, I have to literally lean up against it with my body to get it shut because of the seal of the first door. It's all very very tight in that room. The ONLY thing you can hear in the house is what seems like faint thunder off in the distance when I'm cranking up something with super heavy bass, like Cloverfield, etc. It's rare you hear anything outside of the room... and I have zero issues with a superb bass response. :T
     
  6. Matthew J Poes

    Matthew J Poes Staff Writer
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    I think that soundproofing is really misunderstood, unfortunately. When I took the time to study the topic and learn what I could, I actually went back to school, literally. Of course, there is no such thing as soundproofing 501, but I met a professor who taught a course on sound transmission loss covered in a physics course. I took that and some online available architectural acoustics courses.

    I won't bore you with everything I learned, but I say that to make the point that there is a lot of bad information on the web, even expressed by experts who i think attempted to reduce a complex topic into simple terms to the point of inaccuracy.

    First, let's get the concept straight. Soundproof generally means stopping sound from entering or exiting a room. That can be achieved through blocking or absorbing. The techniques we use tend to do a little of both. Floyd Toole once made a point about soundproof rooms being counterproductive, that they are quite bad for bass and sound bad (as bass smoothness is a big part of our perception of overall sound quality). He's right, but one thing I think is missing from that point is conditions under which that is true, and if it is always true absolutely. A soundproof room is bad for bass when it contains all of the bass and doesn't provide any damping for it. That is sometimes true but not always true. A concrete wall, like the foundation in our basement, is a barrier, its rigid, dense, and thick. Add the dirt wall behind it and realistically little to no sound gets out. On the other hand, two layers of drywall with some damping and a decoupling mechanism doesn't really blow LF's all that much. Certainly more than a single layer did as both mass and rigidity went up, but not anything like that cement wall. On top of that a wall structure absorbs LF's. It acts like a panel trap. If you build it like a soundproof wall, you actually increase LF absorption by quite a bit. Models are not perfectly accurate but I'll include some because it at least makes the point. Actual absorption at LF's is quite a bit higher and thus reflection is quite a bit lower. They dont' accurately account for the resilience of the mounting method used nor have I been able to accurately model the effects of the damping on the drywall. While I've added a custom drywall material to approximate this, it still doesn't match real measurements (and I've been told by experts that this modeling problem is well known).
    Absorption of walls.PNG

    Red is a normal wall and blue is a CLD wall. Note that both are terminated walls in which one side is this drywall and the other side is going to be the cement foundation. The wall behaves differently if it has drywall on both sides.

    Reflection of walls.PNG
    And this is the reflection.

    As you can see, creating a soundproof room actually does little to contain the bass over what the concrete wall of a basement already does, but quite a bit to improve LF absorption.

    Now let's look at the partition wall, which is fairly similar to what a wall looks like if it is above ground as well. Now we don't have this solid rigid cement anymore so less sound is contained. We also have added absorption from the other panel. Let's compare the CLD wall to the non-CLD wall.
    partition wall.PNG

    I'm not going to bother to include absorption, but I'm sure you get the picture. If it isn't being reflected, then its probably being absorbed or passed. In this case, its being absorbed. At very low frequencies the transmission loss of the CLD wall is only slightly higher than that of a regular wall under these simulated conditions. In practice, it is higher because the re-transmittance of sound through the direct vibration of the structure is drastically reduced. Something that isn't being modeled here.

    Basically, theaters in basements suck for bass. The modes are much worse than they would otherwise be above ground. That isn't the soundproof wall constructions fault if anything that helps, its the cement foundation and thick earth surround it. Soundproof rooms using decoupled walls and CLD construction provide more (not less) LF damping and thus should provide better bass. There are other benefits to adding soundproofing to a theater though. These techniques provide a better noise floor for better sound. My own theater has a very low noise floor such that my equipment is my limiting factor at all frequencies. For movies that make my projector a problem, but for music, it really means that my noise floor nearly as low as is practical. I can hear everything that is in the recordings, even the quietest sounds, without having to turn it up to unnaturally loud levels or needing to apply some kind of compression effect to make it tolerable(like the Loudness effects from Audyssey and Dolby).

    For me, soundproofing is an important part of room design if we are talking about CLD walls. It is a nightmare if we are talking about true soundproofing in the sense of concrete foundations, Brick walls, etc.
     
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  7. Todd Anderson

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  8. Matthew J Poes

    Matthew J Poes Staff Writer
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    Hey Tony, worth noting, I think you are describing sound absorption. Soundproofing means the blocking of sound typically. Technically you could block all sound by absorbing it all, but it's just not reasonable. If you took a wall of normal construction and added 2" of acoustic insulation to every surface it would reduce sound by about 7-10dB's but only above 1khz. The soundproof wall as I described below with two layers of 5/8" drywall with green glue, decoupled mounting, and core insulation would provide closer to 60dB's of additional transmission loss at 1khz and more than 100dB's as you get higher in frequency. In fact, my model says you would see a 130dB difference at 7khz, which I believe is probably not even possible to measure. It's really just another way of saying that no sound gets out (in the test structure, the CLD wall has 160dB's of isolation at 7khz, the standard wall is 28dB's of isolation, and the acoustically treated wall is 38dB's.
     
  9. Matthew J Poes

    Matthew J Poes Staff Writer
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    One last point. Toole's measured scenario where an extra layer was installed doesn't match modern soundproofing. Adding green glue does more than just add damping. It also physically separates the two layers of drywall so that they don't couple and act as a single structure. As such the CLD wall is not really any stiffer than before. Where the new CLD wall might be about 1.2-1.5 times stiffer than a single layer, the wall Floyd tried is likely going to be 8 times stiffer.

    Modern soundproof structures are also often decoupled. Double stud walls are still fairly stiff, but hat channel walls are actually somewhat less stiff than wood stud walls. This matters too.

    From a purely sound blocking standpoint that is an unfortunate feature. Below the resonant frequency of the wall structure, typically around 60-80hz, the mass of the wall is mostly irrelevant to containing sound. It's the stiffness that dominates. In the case of bass quality, its a good thing. It means the wall still flexes and it lets both bass out and additional absorption to take place. If you are trying to contain a lot of bass you actually end up needing to do what Sonnie did, add a different stiffer material like plywood, chip board, OSB, etc. OSB stops LF's much better than drywall. Same with cement board. In fact, my approach to building a room that maximally contains LF's (where I can't otherwise use cinderblocks) is to use two damped layers of cement board on the receiver side and two layers of damped drywall on the source side.
     

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