STUDIO BASICS IN BRIEF Studio isolation depends on several things, some obvious, some not. To state the first obvious factor, instruments are directional, mikes are directional, and instrument amplifiers are directional. Again obviously, the directionality of these three can be used in an intelligently designed setup to keep the sound of one instrument out of another's mike. The second gimmie is room treatment. There are three ways to go wrong on this, and two of them are common. The most prevalent treatment problem is the stuff used to deaden the room. So many studios have put cheap carpet on every available surface that it has come to be regarded as a good thing to do. In fact, it is a terrible thing to do. Carpet, drapes, acoustical tile, and thin Fiberglas all share the same characteristic. They don't absorb bass. They do absorb top end. Some better than others. Carpet and drapes absorb about one-fifth as well as Fiberglas, and are many times more expensive. Cheap acoustical tile absorbs very well, but only in the middle of the spectrum. Fiberglas and Sonex absorb to the maximum, but except in suspended ceilings, thin fuzz drops dead below 400 to 500 HZ and Sonex is unthinkably expensive for ceilings. So unless the studio walls are so flimsy as to vibrate with bass and send it outside, the result of using a lot of thin studio treatment is a working space that ranges from boomy at bass frequencies to an absolute grave on top. The sound of the room is reflected in the microphone isolation characteristics, with the low end sound falling through every mike in the place, and a dry, sterile top end. Needless to say, it's very hard for musicians to work in a room like that as they can't hear each other bouncing off the walls. All boom, no tinkle. Overtreated, dead rooms are a frequent problem, and a near inevitable result of using thin acoustical treatment, as too much of it is usually laid on in an attempt to pull down the room bounce at medium frequencies. The third standard problem is, of course, insufficient treatment and a room that is too live to yield adequate isolation. Which brings up the subject of how much treatment is needed in a studio. Interesting, as not only is there no simple answer, there has been virtually no valid material published on the matter. The difficulty here lies in the fact that the amount of treatment for a studio varies, but not directly, with the size of the room, so how much is a complex calculation. However, the result of the treatment can be stated in reasonably simple terms, and the amount can be arrived at by experiment. First, the result. A vocalist needs a minimum of about 26 db isolation on mike to get decent results, as normal limiting eats 10 to 12 db of the available margin. Other instruments should therefore die away by at least that much before hitting the vocal mike. Simple distance won't do it, as a performance room must be at least somewhat live so the players can hear each other. Anechoic chambers are out for recording. If there is sound bouncing around in a room, the bounce will at some point be as loud as the sound source that created it. That point is called the critical distance, and the Dc of a sound source is a statement of how far it travels in a room before it goes constant volume. Dc varies with a number of factors including the directionality of the source but, as rhythm section instruments have about the same directionality as voice, a general treatment can be made for rhythm and vocal isolation. The treatment needs to yield 26 db or better of die off before the source goes constant volume. Sound dies off by 6 db per distance doubling until the Dc is reached, with the last figure a 3 db point. Therefore, assuming a mike to mouth distance of six inches, 1ft=6db, 2ft=12db; 4ft=18db; 8ft=24db, and as the last figure adds only 3db, a voice Dc of 16 feet adds 3 db more for 27 db of acoustical loss at that figure. You can live with less, as little as 10, but it's not quite satisfactory. 12 is definitly OK for general work, and usually used, but 16 feet is just bloody wonderful, as you can put a vocal anywhere in the room, and allows the mixer to do anything he(she) wants instead of the room's dictating all kinds of weird stuff to keep garbage out of the mikes. That's 16 acoustical feet, not straight line, and unless you have a low live ceiling, pretty easy to set up. To find the amount of treatment necessary for a given room, acoustical math is handy if you have it, and available either in a book titled SOUND SYSTEM ENGINEERING by Don and Caroline Davis published by Howard W.Sams, or in the program on this site. Otherwise, just measure the room's Dc for a human voice. This is done by walking toward someone in the middle of the room while chanting one one one one, test, oom, or whatever turns you on as long as it's constant in volume and tone. The listener will hear the talker at constant volume until the room's Dc is reached, at which point the talker will become suddenly and obviously louder. Passing thru the Dc a few times in each direction will nail it down pretty good, and the trick of using a voice for this is better than it might seem at first blush. For one thing the equipment (none) is readily available. For another the system works in any language, and the results can be 'phoned in. Mostly,however, it checks the room at about 100 Hz, where bad rooms go live, and yields remarkably accurate results as a result. Clapping one's hands while looking wise and murmuring hmm and aha is much more impressive, but as handclaps and the like normally trigger the ear well up into the midrange, it's not uncommon to clap out a room at one second and find that it's three or more at voice frequencies. Finally, it is a simple fact that rhythm instruments were designed to work with the human voice, and it is rational to set up a room around that centerpiece. Studio design per se is dealt with in some detail elsewhere in these texts but in passing it should be pointed out that isolation is mostly a function of Dc, and getting a voice Dc of 12 to 16 feet in a room with less than 10,000 cubic feet of volume produces a room so dead as to be damn near impossible for musicians to play in. Generally, small rooms have big problems and vice-versa. Some less obvious isolation factors include proximity of sound sources to walls and instrumental volume. The first is easy. Don't build or set up a studio with instruments within 4 to 5 feet of a wall. The wall will reinforce the sound just like any theater back wall, and you'll hear it on every mike in the room. Taking the above one at time, room volume, in cubic feet, puts an absolute limit on how much instrumental volume, in db/spl, the room will hold without creating problems. This limitation can produce some astonishing situations. Some years ago, the writer tried to record an operatic soprano in a livish 9000 cubic foot room. And failed. The lady produced so much sound on her loud notes that she loaded the room, and every loud note distorted the mike, as it heard her voice from three or four directions at once. She turned a nice little studio into a horrible echo chamber. The session was moved to a bigger room, worked fine, and the lesson remembered. There is probably a firm rule for ceiling height, but I haven't seen it. So to make a rough estimate, for standard instruments and normal seating, a live session studio should have a ceiling height in the area of 14 to 20 feet. It does not follow that a low ceiling ruins the sound but to get a reasonable volume of 12 thousand cubic feet or more, you need a whole lot of floor space with a low ceiling. Low ceilings work fine, but place a lower limit on the number of people that can work in the room. Additionally, the room volume works against total instrumental volume, so one can pack strings in like sardines, but 20 brass or three 200 watt amps can be a disaster. The room volume versus instrumental volume effect may account for the fact that small garage studios seem to have less trouble than small basement studios.