This article will not be about sabines, decay times, phase cancellation, standing waves, or contain any formulas with square roots. You can get plenty of that from acoustics consultants and professional studio designers. Like good scientists they quantify and measure everything so that you know exactly what you are getting. Precision is valuable but not crucial to building a decent sound studio. This article is aimed mostly at the do-it-yourself person building a studio in their basement, or building a small educational or industrial audio studio where you couldn't afford to reach "professional standards" anyway. If you plan to do moderately demanding work on a tight budget, read on.
Sound conditioning versus soundproofing -
A lot of people think you can soundproof a room by putting egg
crates on the walls and maybe hanging a few drapes. These steps
are useful but only solve half your sound problems. In order to
control the noise in a room, you must insulate yourself from outside
noise (soundproofing) and then control the noise and echoes created
inside the room (sound conditioning).
First, soundproofing. An audio studio should be quiet. You don't want to be recording narration with busses, sirens, telephone rings, or squabbling children in the background. Soundproofing is the art of keeping these noises from entering your studio. The process of sealing out noise is fairly straightforward:
1. Seal all air leaks.
2. Keep the walls from vibrating.
Sound travels two ways: through the air and through hard substances (ie. floors, walls, etc.). Anything that lets air into your studio has the potential of letting sound in.
For the moment let's forget that we have to breathe or cool the air; we'll deal with this in a minute. Sealing a room means chalking every crack, corner, and gap. If Superman were to suck up all the air in the room, where would new air infiltrate from? Find that place and seal it up. If you have a hung ceiling, go above the ceiling and seal the areas between the walls and roof that let air infiltrate. Anything that penetrates a wall and doesn't need to move (i.e. water or waste pipes), should be chalked.
Now for the doors. They need to move, (unless you intend to work really long hours). When the door closes, it should be air-tight. A tight sealing door will fit snuggly around the top and sides of the casing. The bottom or the threshold is hard to seal. You either need to create a raised threshold that meets the door when it closes, or get a special door with a gate that drops as soon as the door latches closed (professionally made soundproof doors do this). If you are working with a really tight budget, simply install a well-built basement type door and door jam assembly. The whole works is designed to be airtight. If the weatherproof door threshold doesn't seal the door adequately, roll up a damp towel to cover the gap. Not very professional looking, but it does the job.
Outdoor windows have no place in an audio studio. It requires several layers of glass with air spaces between in order to seal out sound. Also, windows that open often leave tiny cracks for air (and sound) to get through. Outside of installing soundproof windows, the best bet is to seal up the window space with bricks and plywood, or install thick, decorative glass blocks to seal out sound while letting in light.
Once we've kept the sound from infiltrating through the air, we now attempt to seal out vibrations through solid surfaces. The laws of physics tell us that there are two ways to do this:
1. Mass
2. Air space
Massive walls don't vibrate easily. The reinforced concrete or concrete block walls in your basement will do an excellent job of sealing out sound. If you are scouting your business, school, or home for a good audio room, select a basement, preferably underground where the whole earth becomes part of your wall.
Wood frame houses, metal frame schools and businesses with walls covered with sheetrock lack the mass needed to seal out sound. Theoretically, you should have concrete block or solid cement walls composing all six surfaces of your room. This becomes a little expensive and crazy, especially when it comes time to build the door. We thus make exceptions and try to keep them to a minimum. If we cannot make the ceiling out of something heavy, (ie a concrete floor from the story above) then maybe we can select an area where the upstairs is very quiet, such as a rarely used storeroom. Avoid top floor rooms with a corrugated steel roof above a tile ceiling. Have you ever heard rain or hail on one of those things. Basements excel because they usually have concrete floors, cement walls, and only the top surface to worry about.
If you cannot go the cement and brick route, there is still hope. Nailing an extra layer of thick sheetrock to the walls will add mass, reducing vibrations through the walls. If starting from scratch, put up your studs and cover them with thick or double-ply sheetrock (both sides of the wall frame). Next, add a second complete wall just like the first, maybe 5" from it. The second wall should be independently supported so that the first wall doesn't touch the second. Your objective here is to put an air space between two independent walls, and not connect the walls in a way that would telegraph sound from the first wall to the second.
Building this second wall is a bit tricky; how do you nail up sheetrock when you only have a few inches space to work? The answer is to build the wall framework on the floor, attach the inside layer of sheetrock, and then flip the wall into position. Once anchored (by the exterior edges of the framework), you add the final layer of sheetrock. In the end, you get lots of mass from the sheetrock and several air spaces to weaken the sound that passes through. The single wall telegraphs some sound because the sheetrock is nailed to studs, thus sound which vibrates the first layer of sheetrock would vibrate the stud and then vibrate the next layer of sheetrock. With two walls however, the sound that makes it through has to traverse this independent air pocket before it can vibrate the next layer of sheetrock. By the time the sound gets through all the layers, it is pretty weak. Only the lowest, deepest rumbles of busses, trackers, bass drums, bass speakers, or the thud of falling furniture would get through these walls.
The same applies to ceilings and floors; you need massive layers with air spaces between. Professionals use sheets of lead on some ceilings and walls because it is heavy, easy to work with, and is easy to attach to odd shaped surfaces. Try this if you cannot get sheetrock to cover odd shapes; just cover the areas with lead sheets.
Back to doors; professional soundproof doors are made of heavy steal with a sandwich of foam between the layers. You can get close to the same effect by picking doors designed for home basements. They are usually made of steel, are strong and heavy, and because they are exterior doors, are equipped with airproof seals.
One door may not be enough. Since you may have two walls, install two doors, perhaps 5" apart. Make sure the door handles are offset so that they don't bump each other when the doors close.
Another solution is to use sliding patio doors as shown in the February article on studio design. The double paned thermal windows are heavy duty, have lots of mass, have a built-in air buffer (the double pane), and have an air tight seal. Installing two such doors multiplies the soundproofing effect.
Sliding glass doors are convenient because they don't sweep a wide space that has to be kept clear, can be left open to ease 99 trips to the studio with your arms full, and add an aesthetically pleasing openness to the room. Visitors can look in to see what you are doing. You can look out and remain aware that there is a world passing you by as you record the 97th tongue twisting take of Frisbane's Fabulous Floral Festival.
Air conditioning -
If you are just building a sound booth to add a three minute narration
to something once a week, you can probably live with stale air.
Or you could air condition the room vigorously before production
begins and then shut down the air conditioning, perhaps even covering
the vent with a heavy door, during the production. During breaks,
you can open the door and reactivate the air conditioner to make
the room livable again. On the other lung, if you or other clients
plan to spend much time in the room, you'll need continuous air
conditioning. The problem with ventilation is that it makes a
woosh sound and also ruins the airtightness of the room. Let's
attack these problems one at a time.
You hear a woosh sound when air moves quickly. To slow down the air, you need larger air ducts and larger exit grates with widely spaced louvers.
HVAC experts can make the calculations of how many cubic feet of air you will get with a certain fan and a certain duct size. A less precise but adequate design method is to discuss with your contractor the duct specifications he is using elsewhere in your building and ask for the next size higher. The same is true in your home; where normal ducts may be 9" square, you may se;ect ducts 12" or 14" square.
Remember that a room is not a balloon. For air to enter a room, other air must leave the room. Normally this air escapes above the ceiling or under doors or through windows or louvered ducts in the walls. These are all sources for sound leakage, so you have to duct the old air out as carefully as you did the new air into the room. Here are some ideas on how to get your ducts in a row:
You don't want noise from the air conditioning compressors or fans coming into your room through the ducts. Air conditioners and blowers should be placed as far away from the studio as possible. The studio roof is not a good place because the air conditioner will vibrate the roof. Next door is also not a good place for the same reason. Placing the HVAC compressor outdoors or two rooms away will probably do the trick.
As the air passes through the ducts, it will still carry some of the sound of the fans. The inside of the ducts should have baffles and sound deadening insulation so that only air reaches you, not sound. To a lesser degree, the return air ducts also need some baffling and insulation. This will keep sound from traveling backwards into your studio from the exhaust air shafts.
Ducts are made of thin metal and tend to oil-can and vibrate easily. This problem is rather minor if the duct terminates at your studio wall. It is more serious if the duct has to traverse part of your studio before the air can exit; there is more vibrating surface exposed to the room. Various kinds of sound dampening materials can be glued to the outside or inside of the ducts to quiet them. In the absence of the professional materials that they use on dishwashers and automobile hoods, you could try gluing carpet to the duct surfaces. Make sure that you are not breaking any fire regulations.
Sound conditioning -
Sound conditioning is the art of quelling reflected sound in a
room. You don't want echoes, echoes, echoes in your recordings.
Step one in designing your studio is to be sure that the length, width, and height of your room are not the same or multiples of each other. The worse studio in the world would be 8' x 8' x 8'. A room 8' x 16' x 8' is also troublesome. The reason is that waves bounce back and forth between the walls, ceiling, and floor and combine with each other to strengthen certain frequencies in certain locations. When sound vibrations fortify each other like this, they are called standing waves. The room may have a hollow tone in one place and have strong reverberations in another. Odd sizes such as 8' x 11' x 9' make better sounding rooms.
Another way to thwart standing waves is to build oblique walls. Instead of being orthogonal (90 degree angle between walls) the walls are skewed. The secret to this odd-ball solution lies in how sound waves reflect: like a ball ricocheting off the walls of a corridor, sound can bounce back and forth between parallel walls. If the walls are oblique, the sound, like the ball, gets deflected elsewhere and the sound breaks into random patterns.
A real-world problem with the above plan: Many contractors are unable to build an oblique wall without screwing up the job. If traditional right angle walls is all they know, goodness knows what off-the-wall angles one may end up with. If you are building your own studio and you are as skilled a carpenter as I am, building an oblique wall is easy; making things straight and even is next to impossible.
Although one could conceivably tilt the walls in or outward or tilt the ceiling or floor, this construction may be hard to do and maybe end up looking like the set from "The Cabinet of Dr. Caligary".
You do not have to move the walls much to make an improvement. If you started with an 8'x 10' room and shrank one of the sides down to 9-1/2 feet and another to 7-1/2 feet, that would be enough to scatter the sound.
Next you want to keep the sound from bouncing off the ceiling, floor, and walls to begin with. Carpeting will do a nice job on the floor. Acoustic ceiling tiles will be adequate for the ceiling. You can buy ceiling tiles with more or less acoustic absorption. Generally, the rougher the surface, the more sound absorption you get.
In fact this is how sound absorption works. Vibrations that hit a hard, flat surface bounce back with full force. Vibrations that hit an irregular surface scatter in all directions instead of returning intact with a repeat of the original sound (the true meaning of the word echo). They scatter randomly creating a less obtrusive after-sound (what is technically called reverberation).
If the wall surfaces are even more irregular, such as the eggcrate shapes found in professional acoustic wall treatments, vibrations get trapped in the valleys and never escape. You hear no echo, no reverberation. A truly anechoic is quite unsettling. You feel like you're going deaf. Such perfection is usually unnecessary.
The least expensive irregular wall covering is drapes. Thick, massive, heavy, floor-to-ceiling drapes gathered to create 4" folds will absorb a lot of sound and are inexpensive to install if you shop the sales. Professional studios use fiberglass or foam insulation covered with fabric or chicken wire to do the same thing. Sound passes through the fabric and gets trapped amongst the holes and fibers of the material. Professional sound absorbent materials are available through companies such as Markertek and Acoustical Solutions/Alpha Audio Acoustics (see end of article).
If funds are really tight, you can glue paper egg crates to your walls and they will work pretty well. For safety, you may need to spray them with fireproof paint. Use the crates that are made of paper mache in preference to the foam ones. Don't try to eat all the eggs yourself or your mikes will pick up the sound of your arteries hardening.
You do not have to cover the entire wall with this material to be effective; a third of the surface may do. Feel free to cut and glue the material into artistic shapes and place them like wall hangings hither and tither.
Multiple rooms and internal windows -
Professional sound stages and radio stations that have audio engineers
in one room and performers in another need windows between the
rooms to facilitate communication. There is nothing magical about
windows, a surveillance camera and TV monitor could do the job
just as well. Skip Tullen of Tullen Sound Studios in New Jersey,
for instance, has a performance studio in the basement of his
house and all of the sound mixing equipment in a separate building
that once was his garage. Mr. Tullen views his performers through
the camera/monitor setup and talks to them through a loudspeaker
system. His engineering room has plenty of space for 3001 gadgets
without subtracting floor space from the studio.
If you do use internal windows, they should be double or triple paned glass. The windows should be tilted downwards towards the floor so they reflect sound away from the microphones and don't add standing waves.
If you do your own engineering (knob twiddling) you may not need a window at all. You may be doing the voiceovers and operating the recorders and mixing console at the same time. Naturally, you cannot do this in two separate places. Most small operations I have seen, including my own, have superfluous windows leading to idle storage rooms; we built the windows because we thought we needed them. In reality, we do so many of our own voiceovers that we had might as well park ourselves in the same room as our equipment. Even if you are working with one other person, it is easier for you to be in the same room than it is to sequester them elsewhere.
This plan only works if your equipment operates quietly. If your audio recorder sounds like a lawnmower, it obviously cannot share the same space with your microphone. One solution may be to move the noisy, motor-driven equipment to another room and operate it by remote control. Now all you need is quiet buttons. Audio feedback is seldom a problem because most people record with headphones instead of speakers in the room anyway. Once the recording is finished, you may listen to the result over speakers to judge ambiance and sound quality. Beware that you may be using $400 monitor speakers while your audience is listening to $2 monaural speakers in plastic TV sets. You may want to check your sound under these conditions before it leaves your shop.
Enough sound advice. Some final quotes seem fitting:
Knowledge is the small part of ignorance that we arrange and classify --- Ambrose Bierce
A man learns to skate by staggering about making a fool of himself; indeed, he progresses in all things by making a fool of himself --- George Bernard Shaw
ACOUSTICAL WALL COVERINGS AND SOUND BARRIERS
Acoustical Solutions/Alpha Audio Acoustics
2720 Enterprise Parkway, Suite 101
Richmond, VA 23296
Telephone 800-782-5742
Fax 804-346-8808
Markets products such as:
Soundtex acoustical fabric
Alpha Pyramid acoustical foam
Alphasorb fabric covered fibreglass panels
Audio Seal sound barrier material and blankets
Markertek Video Supply
145 Ulster Avenue, Box 397
Saugerties, NY 12477
Telephone 800-522-2025
Fax 914-246-1757
Markets products such as:
Markerfoam 54" x 54" x 2" panels @ $20 each
Sonex 48" x 48" x 2" panels @ $37 each
Try also:
Sonex Acoustical Products
3800 Washington Avenue N.
Minneapolis, MN 55412
Telephone 800-662-0032
Fax 612-521-5639
Acoustic Systems
415 E St. Elmo Road
Austin, TX 78745
Telephone 800-749-1460
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