Sound Isolation and Noise Control
"A chain is only as strong as its weakest link."

This is a very appropriate saying with regard to sound isolation. We are often asked questions like: "What can I do to this wall to stop the sound going through to the bedroom on the other side?" It's almost incomprehensible to people that the wall may not be (and probably isn't) the only part that is leaking sound to that bedroom. The other parts might well be the floor, the ceiling joists and other shared walls. You could make changes and increase the STC (Sound Transmission Coefficient) dramatically for that wall but the result might be marginal because the majority of the sound is getting through elsewhere.

In order to deal with sound control, one should understand how sound travels. In residential environments, it will either be airborne or structure-borne. Airborne sound is pretty simple - this is what we hear within the room. Combinations of airborne and structure-borne approaches need to be considered for sound isolation. One may ask, "But if the room is sealed, isn't all the airborne sound contained?" Up to a point the answer is yes but a ½ inch layer of gypsum is not going to stop 50Hz, just slow it down (i.e. attenuate it) and it could become both an airborne wave and a structure-borne vibration in the next room.

Let's look at structure-borne sound. Have you ever been in a room on a concrete slab where someone was bouncing a golf ball 2 or 3 rooms away? If you are sharing the same concrete slab with no breaks in it, you will hear that golf ball almost as if you were in the same room. You are not getting any airborne sound transmission - this is all structure-borne. Many people think that high mass will stop all sound but actually, sound travels faster in dense material than in air. The golf ball experiment shows us that mass doesn't stop the sound at all. Rather, it transmits it to other parts of the house - quite efficiently too.

So what do we do if we want to isolate sound? The answer is quite simple. Only two things stop sound - mass and space. You need mass to contain the airborne sound but then you also need space (an air gap or similar unobstructed area) so that the structure-borne sound cannot be transmitted. You may have heard of sound isolation techniques such as staggered stud walls or resilient channels. These work on those principles - there is a high-mass wall, an air gap and then another wall to make sound transmission difficult.

Simple, right? Well, in principle yes but the devil is in the details. You have to figure out how to actually execute a plan that attacks the weakest link effectively. Let's say you're designing a recording studio and there's a train track outside the studio. Of course it might be smart to consider finding a new location but if that fails, what are you going to do? You're going to need a lot of mass to stop the noise from a train, like a concrete block building. Still, structure-borne vibration is going to get through so you will need an air space and a second high-mass wall. How much air space and what type of wall? This is where acoustical engineers come into play. They can measure the problem, its effective frequencies and then calculate what size air gap and what mass (walls etc) are needed. Say you've built that great wall. You should be fine, right? Hold it - not so fast. What about the ceiling? You have to use a similar technique there. Don't forget the doors and windows either. You will need sound locks to ensure that sound does not penetrate there. So now you've taken care of the walls, the doors, windows and ceiling. You have a concrete bunker that a nuclear warhead could land next to and everyone would be safe inside. Can you hear the train inside your bunker? You bet! What did you do to the floor? Nothing. The vibration from the train goes through the ground into the concrete slab and right into our studio. You've just built the most expensive fallout shelter known to man because it's worthless as a recording studio. Your weak link was the floor. With all the right intentions elsewhere -- but nothing was done about that floor -- a fix is going to be very expensive.

Fortunately, this was fictional. I don't think anyone has built a serious recording studio next to a railroad track but there have been many studios built near highways and floor vibration is very real and very important.

When considering sound isolation, there are really two aspects: Keeping unwanted sound out of the listening environment and retaining wanted sound within the listening environment. You'd think that if you had one you would necessarily have the other but that's not always the case. First consider the level and source of noise that might get into the listening environment. Is there a busy road outside? Is there a hard surface floor above the room where people walking will be heard? What about the HVAC (heat and air conditioning system)? Is it a duct system and will sound travel through those ducts? How about the noise generated by the HVAC system itself? Now we have to determine what types of noise problems are probable with each of these. Are they structure-borne such as a person walking in the room above or are they air-borne, such as HVAC-generated noise?

Structure-born noise is often the hardest to deal with. You need, as stated above, a combination of high mass and an air space to interrupt positive structural contact. In the case of the hard floor above the listening environment, you will need to decouple the floor joists from the ceiling joists. The most common and effective way to do this is with spring-loaded isolation hangers. Companies such as Mason Industries and Kinetics make these isolation hangers, which are suitable for creating a suspended high-mass dry wall ceiling (see below). That is not to be confused with the typical suspended ceiling, which has very little mass and will not stop sound from exchanging into or out of the room very effectively.

Now let's look at another type of structural noise - the busy road outside. You might think this is air-borne and it could very well be. That noise will penetrate windows quite effectively but if you are in a basement on a concrete slab, there may be more noise that comes in via the structure. In recording studios, this is particularly important and needs to be dealt with. Kinetics makes a product called a RIM Floating Floor system (see below). This system rolls out onto the sub floor and then you build a standard floor above it. It has a resonance frequency of around 4Hz so anything in the audible bandwidth will not penetrate if it is installed correctly. A sub floor (2 x ¾" plywood layers or more), the walls and the final finished floor can all be built onto this floor isolation system.

Finally, let's look at air-borne noise. This typically comes through thin walls, windows or as stated previously, the HVAC (one of the most commonly overlooked issues in a good home theater design). Ductwork is like an open channel to sound transmission, a flanking path in acoustical terms. Have you ever talked in one room and been heard quite clearly in another room halfway across your house? Were you close to a HVAC duct when this occurred? So what can you do about this type of noise? Actually, it's relatively simple to reduce noise of this type from various parts of the house simply by adding a baffle box and/or plenum that is lined with absorbing material to both the trunk and return of the HVAC unit. A baffle box is basically a large box that has at least two 180º turns or a combination of turns for the equivalent aggregate. A plenum is kind of like a muffler, not as effective as the baffle box but much better than nothing (see below). The second aspect of the HVAC system is the noise it generates by moving air through the ducts and in and out of a room, plus the noise of the motors and fans of the unit itself. Most households have relatively small ducts for the volume of air moved. It's fine for most residential applications but not for our critical sound areas. We need to oversize the ducts and more importantly, oversize the diffusers where the air exits into the room. Typically we like to have air velocity into the room of less than 225 CFM (cubic feet per minute). An HVAC contractor can calculate the load required for the room and the amount of air exchange needed, to determine the diffuser together with the duct size needed to achieve this. If your HVAC contractor looks at you with a blank stare, get another HVAC contractor. It's also a good idea to line the last several feet of ductwork with a sound absorbing material.

Now that we have some of the basics covered, let's look at additional aspects. First is the interior high-mass structure, namely the walls. How will these be constructed so that there is a high mass combined with an air gap between two elements of mass? There are many ways to accomplish this. First you need to create high mass. This can usually be accomplished by using 3 layers of material, generally 2 layers of gypsum with some interior sandwich layer. I say 'some' because many interior layers are suitable depending on the needs of the room. We won't go into details but in general, suitable materials can be broken down into two categories: Homasote or Celotex, a low-density sound board; or a damping layer such as ASC Iso blocks or Audio Alloy's Green Glue. The second aspect is the air gap, which can be accomplished either by a resilient channel or by creating a separate or staggered stud wall. The greater the air gap and the higher the mass, the better the isolation will be (see below)).

Are we done yet? Not quite. Windows and doors need to be considered. These are an equally important part of the sound envelope and, as such, can be major sources of sound leakage. They provide another flanking path for unwanted sound. Again, the best way to stop sound is to have mass+air gap+mass. With windows, the ideal would be two sets of windows with very thick or substantially laminated glass in each and a nice air gap between them. This is often impractical and we have to look at something more reasonable. A pre-manufactured thermopane window with laminate glass inserts can do a reasonably good job of sound isolation. The laminate glass is basically glass with a thin layer of acrylic bonded to it. This acrylic bonding element between two panes of glass reduces the ability of the glass panels to freely vibrate at their fundamental frequencies, thereby reducing both the ringing artifact from the glass and their ability to transmit sound. It's not a perfect solution by any stretch but reasonably priced and much better than standard glass.

Doors are another area of concern. Those hollow-core doors with their ¾ inch gaps at the bottom are just about worthless in terms of sound isolation. You might as well have a curtain hanging there. Sound isolation for doors parallels the cost of the doors. A solid-core door will provide more sound isolation but still leak quite a lot at the threshold. An exterior door with good weather striping and a threshold to seal it when closed is better. There are doors with a cam hinge closure that really seals them shut and offers terrific sound isolation and if you really want to go for broke, you can always buy a recording studio door that resembles something like a bank vault door (and costs nearly as much). The latter is really pretty impractical and virtually never required for residential applications, but I wanted to mention the possibilities just the same. One very clever way of dealing with the door issue is to have what's known as a sound lock. This basically is a small entry area with doors on either end. This provides the air gap that is effective at sound isolation. When we use solid-core doors with good-quality weather stripping and a threshold, this creates excellent sound isolation.

There - now we've covered everything. Ceiling, walls, floor, windows, doors, HVAC. Surely there can't be anything else. Wrong again. Here are still two more things to consider in the acoustical criteria of the design: Lighting and electrical. You have to get these elements into the room but you don't want to compromise the sound isolation. Electrical boxes should be sealed enclosures and not have another box on the opposite wall within the same joist. This is a common pitfall because every electrical contractor knows it's easiest to put electrical outlets for adjoining rooms on a common wall within the same joist bay. Less wire, less work, less cost - but don't let them do it this way for a home theater. It's another flanking path that will compromise a lot of the hard work and effort you put into sound isolation. Lighting is another caveat. We never use recessed cans in high-mass sound barriers - period. Sometimes we use them in soffits that are not part of the sound barrier. In fact, we sometimes create soffits for the sole purpose of holding necessary lighting fixtures and/or HVAC ducting, so as to avoid compromising the sound isolation envelope of the room. However, I cannot tell you how many times the home owner goes to the electrician and says, "You know, I think I'd rather have recessed lights here than track lighting." The electrician obliges and guess what? All that expense put into creating a sound barrier was just thrown away because it's been compromised by a bunch of 8-inch holes for lighting.

Okay, that's enough for today. But before I sign off, I would like to emphasize one aspect of the electrical and lighting issues. You can see that execution of a design is as important as the design itself. If it's not done properly, you may have started with the best of intentions and ended up with very poor results. Make sure your designer/architect or engineer is in close communication with the builder to ensure that the plans are carried out without any creative deviations. Without that critical communication, it's unlikely that the result will be very rewarding.

Richard Bird & Chris Huston

Rives Audio website