How can I determine the STC (Sound Transmission Class) rating for my condo walls and ceiling?

Question

I work out of my condominium and generally like to have music playing throughout the day. A few nights every week I work late, sometimes until after midnight, so I'm thinking about soundproofing my office to ensure my neighbors won't hear my music.

My music is rarely annoyingly loud, but for a few albums every week I do like to turn it up more than normal, and for my own peace of mind I'd like to make 100% sure my neighbors aren't hearing anything.

Before I make any decisions, I'm trying to figure out the STC (Sound Transmission Class) rating for my building/unit so I have some idea for the level of soundproofing that is needed. I live in Redondo Beach, CA and these units were built in 1970. I know there are current minimum STC ratings required for new construction but I'm assuming that these have changed over time. I've been trying to find information about STC rating requirements for my area in 1970, but no luck so far.

Is there any way to determine my unit's STC rating without paying a specialist to test it? Or can I find the minimum STC rating required for buildings in Redondo Beach during 1970? Is there a website out there that would have this information?

Answer 1

I lived in Southern Calif. in the '70s, I'm pretty sure there was no STC standard for multi-family residences at the time. The only requirement was for certain party walls to be fire rated, and not necessarily every party wall. There are publications available that show the STC ratings for various constructed assemblies. You can find an assembly that is similar and get some idea of the rating for your assembly. However, you cannot claim your assembly achieves a certain STC rating unless it is built EXACTLY as the published example. If there is any variation, you cannot claim any STC value at all without having it tested.

Still, reviewing the various assemblies will give you an idea of what needs to be done to achieve a reasonable rating. BTW, much like water proofing, there is no such thing as soundproof, only degrees of sound resistance. Note it is called a Sound Transmission Coefficient. The implication being some sort of sound will be transmitted in all cases.

Perhaps the biggest issue with applying sound treatment to an existing building is your options for eliminating flanking paths is very limited. Even if you build a very effective sound wall, unless you address how the sound can travel through the adjoining floor, ceiling, and sidewalls, it may have only minimal effect.

That said, I applaud your consideration of your neighbors. Doing something is certainly better than doing nothing. But do not think you can blast your audio system with impunity just because your sound treated the one common wall. Effective sound treatment requires addressing ALL the related construction adjoining the wall in common.

Answer 2

The number won't mean much. Sound transmits two ways to your neighbor. One, by the air pressures being transferred to the building materials via vibration then into the next room by the building transferring the vibration back into the air to be heard by your neighbor. This is what is being talked about in the rating you speak of. The other way is for the sound vibrations to find a path into your neighbors while staying in the air, I.e. by air leaks through building openings and other accidental openings. This is probably the larger part of your problem and should be addressed first. You need to create air tight seals around doors and windows and make sure that there are no air leaks around outlets and other building features that could accidentally provide a path for the air to leak. In my building, a condo rehab of an old building, ther are many air leaks between units even though there is supposed to be 2 hour fire walls between units. Sound transitted via vibration will be mostly bass, thump thump thump. Sound transmitted via the air would have all it's frequencies intact. If the problem you are having is bass thumping that will be very hard to prevent because it would require isolating the sound source from the building, essentially building a room within a room with vibration isolating mounts or supports. Just insulating the wall will not stop this kind of transmission. It goes through every building element. It can be achieved by layering materials of different weight and vibration qualities. Wood, lead, wood or drywall, air, drywall. The sound gets diminished with each transfer and moreso if the materials are different densities. The problem is the entire room has to be isolated this way since sound will transmit through structural elements if they are not isolated. As you can see it is a serious problem that requires some experience and science and a budget. Focus on the air leaks for the most bang for your buck, but don't expect to get rid of the thumping. Minimally, covering the wall, ceiling and floor with a layer of rubber and another layer of finish material would help somewhat, provided you had taken care of the air leaks too. Putting another layer of Sheetrock on the adjoining wall with sound channel between the layers will give some relief but you will likely feel it is not enough as it is intended to reduce transmission of conversational levels of sound and specifically to reduce the comprehensibilty of speech when heard through the wall.

Answer 3

Even if you knew the STC requirements of the 70s, your question assumes that everything was built to code when your place was contructed. Since you can't really know that for sure, you can only conduct tests from the adjoining units. If that's not an option (and it probably isn't), you're going to have to look into how you can ensure that your walls meet a specific STC rating. This means that you will need to research various soundproofing options, the most popular of which are 1) floating a wall on RSI clips, 2) soundproofing compounds like Green Glue, or 3) specially-designed acoustical drywall like QuietRock. There's also MLV (mass-loaded vinyl), but the material and shipping costs of that are outrageous which renders the return investment in soundproofing quality pretty minimal. You also need to know that low-frequency sounds are very costly to dampen and that the STC rating system is pretty much designed for frequencies at or above 125 hz (I'd like someone to correct me on this one if I am wrong).

As bcworkz also pointed out, you need to consider flanking paths like ceiling and floor, especially if you're in a multi-floor, townhouse-style unit. If the room you're soundproofing is on the top floor, you need to consider your flooring since the sound waves can travel diagonally through your floor and right into the first-floor living area of your neighbor's unit. They can also flank which means they vibrate your subfloor which vibrates the joists beneath which will end up vibrating your neighbors joists and then your neighbor's floor. Also consider attic space above your room which can serve as a potential top-level flanking path for sound much like your floor.

One of your best bets is to introduce yourself to your neighbors. Let them know that you are conscious of your noise production and that you want them to alert you of any disturbing noise entering their unit that originates from yours. They will most likely deeply appreciate this gesture and gladly comply. This means that you will have a good chance in knowing whether or not your noise requires dampening and, if so, to what degree (see aformentioned paragraph about soundproofing options).

In the end, you're going to be left with taking a stab in the dark. Even if you read on some website about how adding an extra layer of drywall with Green Glue in between will give your wall an STC of 56, there could be a good number of other unaccountable factors can totally throw of your initial STC calculations. This can result in lower-than-expected results and will require additional soundproofing application on top of what you've already done. Expect to spend several thousands of dollars to get a satisfactory result.