by Paul Bergman
If youre not interested in this topic, theres a better than even chance your neighbors are. A man who has spent time in soundproof rooms explains the basics.
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There has always been a good deal of popular confusion between soundproofing and acoustical conditioning. The latter, of great interest to sound professionals and audiophiles alike, has to do with controlling reverberation within a room. The former, also of great interest to both professionals and audiophiles, has to do with keeping sound out (recording studios for instance) or keeping sound in (listening rooms with nearby neighbors) The two functions require quite different techniques, and in some cases could even conflict.
For sound vibrations to travel, there must be a medium (in space, as the cliché has it, no one hears you scream). A vacuum, therefore, cannot transmit sound. We know that air transmits sound with reasonable effectiveness, and because of its low mass it does so over a large band of frequencies.
Even so, it has limitations, being less effective at high frequencies than at lower frequencies. This is the reason that a loudspeaker placed far from you will seem to have somewhat rolled off highs, and that the voice of someone shouting at you from a long way off will seem muffled.
All solid objects encountered in your room will have much more mass than the air, and they will for that reason be less efficient at conveying vibrations with short wavelengths, namely highs. This is of course the reason that, if you hear your neighbors stereo system or TV, the offending sound will seem to be composed almost entirely of very low frequencies. Therein lies a clue. In fact, we have two clues.
1) Soundproofing is not a problem at high frequencies, but only at lower frequencies, and:
2) Sound will not be transmitted from one space to another unless there is a continuous transmission medium from one space to the other.
In nearly all cases, the transmitting medium will not be air (unless there is literally a hole in the wall) but some more massive matter, such as wood, drywall, etc. Such materials are very efficient conductors of long wavelength vibrations, and this is why most walls and floors cannot be considered soundproof. It appears evident that what we must do is find a way to interrupt the path of the vibrations. Fortunately, there are techniques for doing this.
We established earlier that sound will not travel through a vacuum, and possibly you are wondering whether we could use a vacuum to isolate a room acoustically. In actual fact that is not practical, for a reason that will become evident. Let us suppose that we build a wall panel whose inner and outer surfaces are separated by an evacuated space:
The vacuum itself will achieve (if it is total) 100% blockage of sound at all frequencies. However such a panel is not practical for a reason that becomes evident if you look closely. The materials separating the two panel surfaces, top and bottom, are made of solid materials. Acoustic energy will be transmitted through those materials, bypassing the evacuated space.
Here we have another principle of soundproofing: isolation is of little use if it is not total. A truly soundproof wall with even a small leak in it is not soundproof at all.
As you can see, stopping the transmission of sound is not going to be easy.
Actually setting up a vacuum inside a wall would, in any case, be quite expensive, and, what is worse, the vacuum panel would be fragile, prone to failure from the smallest leak. Fortunately, there are alternatives that are at once more economical and more stable, and yet do not sacrifice effectiveness. To see how such a panel might work, let us look at another phenomenon affecting the transmission of sound. Consider a sound wave striking a wall boundary:
As we can see, a certain amount of the energy travels through the wall and presumably out the other side. This is where soundproofing and acoustical treatment come into conflict. For soundproofing purposes, we would rather that most of the energy stay in the room, but that would make the room unpleasantly reverberant.
You would probably expect that the walls hard surface is the reason that so much energy bounces back, but the explanation is somewhat more complex. That hard boundary represents an abrupt change of mass between the highly compliant air and the much less compliant drywall. This sharp boundary makes the transfer of energy relatively inefficient, with the result that only a fraction of the energy penetrates the wall.
What happens to the energy that does not penetrate the wall? It cannot, certainly, simply vanish, for the First Law of Thermodynamics says that (in non-nuclear events), energy is neither created nor destroyed. There is nothing to absorb it. It can, therefore, only bounce back.
I might add parenthetically that the conflict between soundproofing and acoustical treatment is quite real. Let us suppose that a soft material, such as an acoustic tile has been placed on that wall. The tile is denser than the air but much less dense than drywall. The change in density is therefore less abrupt, being achieved in two steps (air to tile, and tile to wall). Less energy will bounce back, some high frequency energy will be absorbed, but more energy will be transmitted.
In order to do this, we can use what weve learned about the inefficiency of sound transmission across an abrupt change of mass. Remembering this, which of these two wall structures do you think would do a better job of stopping sound transmission?
Although the two structures have the same mass, the one on the right will be superior, simply because it contains more abrupt changes: air to drywall to air to drywall again.
Incidentally, we can make that structure even more effective by filling the air cavity between the drywall panels with a fibrous material such as mineral wool or fiberglass insulation. Thats because the cavity between the drywall sheets will resonate, and friction of the vibrating air against the fibres will produce heat. We cant actually destroy the energy, but we can turn it into non-acoustic form.
Incidentally, if your goal were to control reverberation in the room, you would make the first panel much thinner, to maximize the energy that would enter the wool-filled cavity and be transformed into heat. I described the actual construction techniques some years ago in an article I wrote in this very magazine (UHF No. 36).
A few moments of thought will lead you to the conclusion that the double drywall technique, with or without fill material, may not be truly effective. This is for the same reason that the vacuum chamber we considered earlier cannot work. The two sheets of drywall, in most structures, will be nailed to the same wall studs and sills, typically two-by-fours. The drywall sheets and the two-by-fours will form a solid entity, providing an acoustical path which low-frequency sounds will be able to follow with ease, totally defeating the purpose of the double wall.
For that reason, a soundproof wall must be composed of two independent wall sections, with distinct sills separated by several centimeters, and staggered studs, which are nailed each to their respective sills, and which are not opposite each other. It is best if the sills are sitting on a surface which will not readily transmit sound from one to the other. Concrete, being very dense, is especially effective.
I should add that it is very important that there not be any leaks through which acoustic energy can take a shortcut, otherwise your efforts will be fruitless.
We have seen that resonance (of the airspace between the drywall sheets) can help get rid of unwanted energy. However, resonance can also be an enemy. A drywall panel, or any other rigid material used in the wall, will have a frequency of resonance. The exact frequency will depend on the panels mass but also on its physical dimensions, or even on the dimensions of unsupported sections of the panel. If sound at the panels resonant frequency strikes the panel, then the panel itself will vibrate like a diaphragm, transmitting energy with little impediment, as though it had become transparent to sound. The same transparency, to a lesser degree, will occur at one or more multiples of the resonant frequency.
It is impossible to eliminate this phenomenon, though the deleterious effects of resonance may be reduced by damping, using, once again, fibrous materials. Damping prevents materials from continuing to vibrate once they have begun. If multiple barriers are used, it is useful to select the materials so that they dont have the same resonant frequency.
In some installations, such as recording studios, sheets of lead are used as one of the blocking materials. Not only is lead very dense (a 4 mm thick sheet has the stopping power of a 4 cm thick sheet of plywood) but it is so limp that it will not resonate at anything close to audible frequencies. However lead is expensive, and the slightest leak will allow sound to go around it, rendering it useless.
Important though walls are, floors are even more problematic in soundproofing, for reasons that are evident. Noise sources we wish to guard against are often in direct contact with the floor. Footsteps are the obvious example, and loudspeakers are typically sitting on the floor as well.
Once again, much depends on whether you wish to keep sound in or keep it out. If you have a listening room, and if you wish to keep from disturbing people elsewhere in the house, it is best for it to be on the bottom floor, perhaps the basement, with its dense concrete floor. If you are planning a recording studio, which must be isolated from external sound, it is better to be on the top floor, with no one above you.
In most cases, the floor carries a great deal of acoustical energy, and once vibration has entered the building structure there is really no way to keep it from propagating. It is best, therefore, to keep the energy out of the floor in the first place.
For footfalls, there is nothing better than thick carpeting, not because carpets have much sound-stopping power -- they do not -- but because a foot striking a carpet will not produce sound in the first place. This is the reason that some apartment leases contain a clause requiring that all floors be carpeted.
Speakers are another matter. No contact with the floor is required for speakers to produce sound, and we must simply minimize the transmission of the sound produced, especially at low frequencies, from entering the floor structure. It appears intuitively correct that placing the speaker on a rug or a pad would help, but in fact such soft materials are surprisingly ineffective at the low frequencies that are so problematic. Rather better are spikes or cones, two devices much in favor among audiophiles. Not only does a spike anchor the speaker, but the boundary between the tiny spike point and the large floor surface is another of those abrupt changes of mass unfavorable to the transmission of energy. With spikes, or better yet cones, much of the energy will bounce back into the speaker.
(I might add that, despite the popularity of spikes, their role is not understood by everyone. I have read white papers on loudspeaker design that say the opposite: that the role of the spike is to funnel energy into the floor so that it may be disposed of. This would not be a good thing even if it were to happen, but the hypothesis is demonstrably false, as anyone who has experimented with spikes or cones can confirm.)
A very expensive room may actually have a floating floor, a freestanding floor structure which is isolated from very dense subflooring by resilient supports, often made of Neoprene. This is not as effective as actually making the floor float above the structure, but the law of gravity being immutable, at least on this planet, we must settle for what we can get.
Real-life rooms, as opposed to purpose-built studios, have windows, which offer much opportunity for sound to leak around even the most heroic soundproofing materials. They also have doors. A studio door can easily cost thousands of dollars, and even so the window frame lets through at least some of the sound the expensive door is designed to stop.
Absolute soundproofing is not possible, and even very good soundproofing may be more expensive than most people will want to afford. Some major studios, whose owners can easily bill over $200 an hour, are built to very stringent standards: less than 30 dB noise in the studio even with a helicopter hovering 50 meters above the roof. Most audiophiles will settle for being to listen to a little night music without having the police come to the door.
What is important in planning a room is understanding how sound is transmitted through a structure, and becoming familiar with a few of the common techniques for limiting its transmission. Then, whether you do the work yourself or hire an architect, you can get the best results for your money.