Originally published in the July 1996 issue of:
How to be an Isolationist
By Barry McKinnon
Sound isolation is one of the most difficult and expensive acoustical design factors to consider in a residential environment, and is often overlooked, yet it is the one area that has a more insidious effect on the enjoyment of a home theatre.
The major problem is that it is somewhere between difficult and impossible to provide sound isolation in typical residential construction without some significant renovations being required.
Room acoustics is an issue that has been steadily moving torward its rightful place in the planning and sales of home theatre systems. There are more pre-packaged products available to make it easier to sell "acoustics" as some part of the home theatre package. Many home theatre contractors (to their credit) have discovered that the acoustical issues can sometimes degrade the performance of the hardware package when it moves from the showroom to the actual listening room, and that it is better to sell the room acoustics as part of the package rather than wait till they have to fix it later, at additional cost.
The past few years have seen a major increase in the amount of acoustical information being published relating to home theatre and home listening rooms. There's been ample opportunity for the installers to experiment with treatment placement, and find for themselves, the advantage of killing early reflections that blur and shift localization and introduce timbre changes, or the advantage of optimizing the surround speaker environment for the particular type of devices being employed. The more mundane acoustical issues, like controlling the fan noise of the projector or equipment rack have also been addressed.
There are home theatre and listening room acoustical issues that (quite literally) run deeper than this, and those are the ones that are often being ignored by contractors and the home owners. Sound isolation is one of the most difficult and expensive acoustical design factors to consider in a residential environment, and is often overlooked, yet it is the one area that has a more insidious effect on the enjoyment of a home theatre. That comes from the reaction generated in those people who are not participating in the home theatre experience by choice.
We've all had some opportunity to experience this in our own lives at some time, whether it was a low rent apartment we had with party animal neighbors, or even a steady stream of "boom" cars driving by while we're trying to read on our patio (or for that matter in a house with all the doors and windows closed). Noise is becoming a serious issue in many municipalities, especially those with increasing population densities, and no matter how much fidelity that noise may have to the source material, if it's being generated by someone else, and you're not participating by choice, it's noise.
The first level of the problem is the split between single family dwellings and multi-family dwellings. In a single family dwelling, there are two issues; can the neighbors hear your system when they don't want to, and can other family members in other parts of the house hear the system when they don't want to. This is important because the effect on the "victims" of noise are quite different. When it's your neighbor that's interrupting your activity, it generally incites thoughts about what an inconsiderate jerk they are for disturbing your peace (especially if it happens more than once), and because it requires confrontation at some level, and possible conflict, it triggers the fight or flight reflex, with all the adrenalin and anger that follows. When it's someone else in your own household (assuming a minimum of sociopaths in your home) who is creating the disturbance, there is usually the opportunity for negotiation, (or discipline if your home is equipped with teenagers) as a means of controlling the noise. The interruption may trigger annoyance, but there is generally a sense that it can be controlled "administratively," with less conflict. (And just because you haven't seen a dB or other technical term so far, don't skip past this, as all of this will relate to noise isolation ratings later.)
In a multi-family dwelling, the primary concern is that the home theater system not impact your neighbors, for all the reasons I mentioned previously. The big difference is that the noise intrusion will be roughly equivalent for both your neighbors and the other family members, whereas, in a single family dwelling, you would have to be having one wild party for it to be as loud inside your neighbor's house as it is inside your own.
The major problem is that it is somewhere between difficult and impossible to provide sound isolation in typical residential construction without some significant renovations being required. If there are already renovations contemplated as part of a home theatre installation, it is usually possible to pick up an additional 6-10db of midband isolation and slightly less improvement in the spectrum below 125Hz. That is on top of a fairly minimal isolation rating for standard interior and exterior walls. The isolation that is provided by building walls seems to be something that is not well understood, let's look at some common wall construction techniques to check out the numbers (Yes, at last, some dB's)
Consider a wood frame residential construction. The outside walls are either 2x4 or 2x6 wood studs with an exterior finish of some kind attached over exterior sheathing (usually 5/8" thick). There is almost always a gap left in the sheathing for ventilation and expansion. The wall will have batt insulation in it, then a vapor barrier, and then gypsum wall board on the interior. That wall is rated in the range of (sound transmission class) STC 45, and will likely measure 5-6dB poorer than that in field tests. You can expect 40dB of attenuation above 500Hz in an exterior wall with no windows in it. Most wood frame interior walls are simply a layer of gypsum board on either side of a 2x4 stud. A high end construction may have added batt insulation in the stud cavities. On a good day, this has a rating of STC 35, without penetrations for AC outlets. Floor construction is typically 2x10 or 2x12 wood joists, with a plywood or T&G subfloor, vinyl or carpet floor finish on top, and for most finished basements, there will be furring strips and a layer of 5/8" gypsum board mounted to the underside of the joists, but no batt insulation in most cases. In a floor with no penetrations for heating ducts or light fixtures, this construction may provide an STC 40, and measure at FSTC 35. This will provide about 35 dB of attenuation at 500 Hz. In most of these cases the low frequency attenuation of the wall or floor construction will be in the order of 8-10dB (if there are no windows in the walls). That's not 8-10 dB less attenuation, that's 8-10 dB of attenuation at 125Hz, and only 4-5dB at 30Hz.
Areas of the continent that use masonry block or brick construction for exterior walls will fare better in exterior isolation, especially in the low frequency range, but again, only without windows in the walls. These buildings will still have standard wood frame construction for interior walls and floors, so the same expectations for interior sound isolation will apply.
In areas that allow multi-family wood frame construction, there is usually a slightly greater sound isolation requirement for the demising wall between spaces, although that is as much driven by fire separation requirements as anything else. A demising wall will usually be required to meet a minimum rating of STC55, although this will vary in each municipality. The wall separating the living space from the hall way will usually have a lower isolation requirement. On a good day, with careful construction, you might expect to see 48-50dB of attenuation above 500Hz between adjacent apartments, or attached townhouses. This would likely translate to 12-15dB at 125Hz and as little as 7-8dB at 30 Hz. Even though the wall may be rated for better performance, most construction uses a common floor plate, or at the very least a floor plate that connects to the adjacent floor plate at the bearing walls. Even if the wall should have delivered better attenuation, it will be compromised by flanking paths in the floor and ceiling structure. This is worse in areas that have a requirement for seismic rated construction, which will prohibit saw cut floor sheathing, and will require seismic bracing inside of the demising walls (which are generally the bearing walls) compromising the isolation by a further 10dB.
The answer must be concrete construction, it's much more massive, and should have better isolation. In general that is true, as long as you can keep the sound out of the concrete structure, it provides much better isolation. However, once sound energy gets into the concrete, it happily travels through all of the connected structure. You've probably been in a concrete building and heard the sound of a hammer drill in use somewhere in the building as a distant, yet audible whirring with no discernible directional characteristics. Many concrete residential buildings still use wood or steel studs for demising wall construction to keep the building mass low, so there may be no improvement in sound isolation between adjacent spaces. Where fire code requirements demand block or concrete demising walls, there is usually gypsum wall board applied to the face of the block or concrete. Unless this is applied with an airspace between the studs and the concrete, the problem of exciting a masonry structure remains. Floor finish and ceiling finish will also play a role in determining the available isolation between floors. A marble tile or hardwood floor, sitting directly on concrete, may still transfer a substantial amount of sound energy directly into the structure, especially as a bending wave travelling along the concrete floor. This will propagate to the expansion joints, which could mean that several apartments on the same floor, and the one below, will share the massive subwoofer experience of T2.
FSTC is Field Sound Transmission Class rating, and these are the same construction methods, measured in place in the real world, with real contractors making real mistakes and compromises in construction.
The other glaring omission is the measurement bandwidth, by stopping at 125Hz, the STC rating says nothing about the expected isolation of any home entertainment generated sound, which will have their predominant energy levels below 125Hz.
So far I've referred to STC ratings of construction methods, it's time to take a closer look at what that means, and how it's derived. This is a laboratory measurement of a wall, floor or ceiling construction where the 6 foot or 8 foot square test panel is sealed into an opening between a source chamber and a receive chamber. A reverberant sound source room presents the test sample with a uniform sound field across its entire surface, and then many sound level measurements are made in the receiving room, statistically averaged to determine what the Sound Transmission Class rating of the construction will be. The measurements are made from 125Hz to 8000Hz, primarily to evaluate the speech privacy of the construction. FSTC is Field Sound Transmission Class rating, and these are the same construction methods, measured in place in the real world, with real contractors making real mistakes and compromises in construction. Typically the FSTC will measure 5-6dB lower than the STC rating. For some reason, likely the lack of extensive field measurements on all STC rated constructions, few people in the world of architecture are familiar with FSTC ratings, and still use STC ratings from the building code books.
The limitations of wall ratings are likely glaringly obvious to most people in the audio business. Small rooms do not have a reverberant field, and there is little likelihood that the sound field will be uniform across a wall surface, or that a listener will be adequately distant from a wall that a statistical measurement of the receive level will be valid. Most loudspeakers or subwoofers are placed near a wall (certainly within the pressure zone in the low frequencies), and most of the noise "victims" will be at a specific spot in relation to that wall on the other side. A much more valid rating would be the Insertion Loss of the direct sound path between source and receive positions. People in the audio business already know about inverse square law, and the fact that a sound increases by about 6dB every time you halve the measurement distance. A speaker producing 90dB at one metre should be almost 20dB louder at one tenth that distance (luckily other factors mitigate that near field effect or speakers would be infinitely loud at the cone and just tear themselves to pieces). What we do know is that speakers are significantly louder as you get nearer to them, and that while the speaker construction may affect that variation in level a bit, the actual output level of a loudspeaker at a one foot distance from the back side of it will be quite high, even when the level at the listening position is quite reasonable. It is this very loud sound source, at a specific point near the wall, that will be getting through to the neighbors.
The other glaring omission is the measurement bandwidth, by stopping at 125Hz, the STC rating says nothing about the expected isolation of any home entertainment generated sound, which will have their predominant energy levels below 125Hz. In general, except for spurious resonance dips in attenuation, the low frequency attenuation falls off at a constant 12dB/octave slope below 250Hz (mass law), so that you have about 12dB less attenuation for each octave below 250Hz. There are a few wall constructions that have been rated for their (Music Transmission Class) MTC rating down to 30Hz, but it is very difficult to find data for this.
It is apparent that the STC rating method collapses completely with a source located close to the wall. An even bigger problem is produced when the device is mounted on, or in the wall or floor/ceiling construction. The loudspeaker is then structurally attached to the wall, and whatever isolation may have existed has been compromised completely by having a noise source placed inside the isolating element. It is the combination of wall material and the airspaces that provide the isolation.
This is becoming a more critical issue as more high-end high-density townhouses, condos and apartments are being equipped with home entertainment packages as part of the purchase package. Some of these are customized to the owner's preference and some are a standard package. This can be a very lucrative market for a home theatre contractor that can forge a relationship with a building developer to include these systems as part of each unit. In a large condo or apartment complex, this could result in a hundred or more system sales. Sometimes the home theatre contractor is called in at the design stage to layout the TV positions, speaker positions, equipment positions etc., as part of this partnering process.
All of this is very interesting, you say, but I don't build the room, I just install the systems in it. There is an issue of responsibility that every home theatre contractor must be aware of. Whether the system is included as part of the new construction, or is added as part of the owner's improvements, the home theatre contractor may well find himself the focal point of subsequent legal battles over noise isolation, or even worse problems involving fire regulations, if any of the wall or ceiling constructions that were rated as fire barriers have been penetrated by speaker mounts. A home theatre contractor can't assume that the noise isolation issues will be someone else's responsibility, as the building developer and unit owner will assume that the contractor is taking the responsibility for all related issues as the "expert" on site. In these instances, it is certainly in the contractor's interest to learn more about construction methods and ratings, as well as related code issues before charging after a lucrative contract. Even more important is the need to identify areas that the home theatre contractor won't accept responsibility for to both the developer and owner. But the contractor has to recognize that those problem areas exist before they can do that. Judging from the number of phone calls we receive that relate to these issues, many home theatre contractors don't understand sound isolation issues adequately. Those stellar profits will erode quickly once a lawyer is involved.
There are several worthwhile books that deal with these issues, including David Egan's book "Architectural Acoustics", or Michael Rettinger's books on "Noise Control". These are expensive books by audio book standards, but can save you many headaches. It is also worthwhile to research the copy of the building code that applies to your market area, likely in your local library, or municipal engineering department.
United Entertainment Media Inc.