Recording Rooms

A series of articles on how to design and plan the acoustic treatment of rooms for recording music and for critcal listening.

Recording Rooms

Postby Scott R. Foster » Fri May 09, 2008 3:13 pm

Recording Rooms  

Tracking rooms and booths are two names for rooms in which one records sound, the difference being simply size [booths are very small, and tracking rooms can be anything bigger than a booth – some are quite large].  A reverberant field is desirable in a recording room because it can create a pleasing sense of space, and a richness of after-tones which compliment the sounds of the music made therein. A well balanced reverberant field can enhance the recording experience for musicians, and thus can increase their ability to produce their best performances. In order for the reverberant field of a room to be musically pleasant, it must have a high modal density and its modes must be reasonably well spaced. Tracking rooms which are above about 5,000 cubic feet [on the order of 20x25x10’ in size] in volume are of sufficient size to begin considering a full fledged reverberant field. Rooms over this size can benefit from reverberant fields if modal problems are avoided / controlled.

“High Modal Density” means having many frequencies at which the room will resonate, and this comes from a room being large enough that it contains many resonate frequencies all the way down to the lowest frequencies of importance.   Larger rooms have lower modes… and as the resonances of the room multiply and overlay each other going up the the harmonic chain of the audible band the modal density increases [more resonate frequencies per octave].  Thus modal density is greater in a large room versus a small room at any given frequency.

Because of the greater density of modes, the reverberant field of large room is more balanced and thus more pleasing to the ear… the reverberant field is less apt to have hot spots, or dead spots where a particular tone has an annoyingly loud or long echo, or have large gaps where all resonance is absent [troublesome null spots]. Small rooms are fraught with such inconsistencies low in the band – so much so that it is statistically invalid to peg a number to the average reverb time for a small room because the reverb time will be much too dependant on measuring position and the nature of the sound source.  Large room’s transcend these limitations [comparatively speaking] because they can support a diffuse reverberant field, where echoes are likely to come from one direction as any other, and can occur at many frequencies all the across the band [lows, mids, and highs].  In other words, the reverb field in a large room is diverse enough to take on a random element and sound “natural”.

Consider your preconceptions as to what you expect hear in a large hall which exhibits a strong reverberant field versus a small room.  You might reasonably expect a stone walled church to have a rich and luxurious reverb field – while conversely you might expect the field in a tiled bathroom to have a thin almost nasal character. The depth [extension into the range of very low frequencies] of the large chamber’s modes [and the resulting modal density] are the cause for these preconceptions [and the underlying real world experiences that cause them].

Mathematically and physically a couple of things start to happen as rooms get bigger. One is that the frequency ranges at which different acoustic paradigms of conception and treatment become valid are lowered. Just as it statistically invalid to speak of the average reverb time [RT60] for a small room, there is a frequency range below which it is useless to attempt to conceive of sound waves as rays that bounce around the room like billiard balls. Lower still, there is a frequency range below which it is useless to attempt to treat acoustic problems through diffusion [instead, below this range, absorption is the only solution – in other words, these ranges are inversely proportional to room volume = the smaller the room, the larger the block of low frequencies that can’t be conceived as behaving like billiards balls with predictable bounce patterns, and can’t be effectively treated with diffusion.

Young’s formula for calculating RT60 is room volume in cubic feet times .049 divided by the average Sabins per square foot for the room surfaces. Everest states that the approximate practicable lower limit for the use of diffusion is (the square root of (RT60 divided by the room’s volume in square feet)) times 11250. He also states that the approximate practicable lower limit for the use of a ray model for acoustic conception is the previous result times 4.

Small rooms are capable of having only modest amount of reverb content without suffering from unpleasant levels of coloration (the unpleasantness starts in low frequencies regions where modal densities are lower). For purposes of this article, let’s say that small rooms range in size from about 1,000 cubic feet to 7,000 cubic feet. The matters discussed in this section apply, to a greater or lesser degree, in such rooms dependant upon what the frequency range and amplitude of the sound source, the volume of the room, and on how evenly its modal content is spaced. The smaller the room the less reverb is desired, especially low frequency reverb. The louder the sounds are in a room the more unbalanced modes and insufficient absorptive treatments become a problem.

A room below 2,000 cubic feet [on the order of 14x18x8’ in size] is a very small room acoustically, and most project studios, fit this category. Small tracking rooms generally cannot tolerate much strength in their reverberant fields. Very small rooms with surfaces reflective to sound tend to have modes which are very strong in one part of the sound spectrum and audibly weaker (or non-existent) in others (“coloration”). Small rooms are not able to support modal densities sufficient to provide an uncolored reverberant field and colored reverb artifacts become troublesome even at modest volumes. There just isn’t enough space in a small room for a large enough variety of low frequency waves to be reflected by the limited square footage of surfaces available.

These issues reach criticality with booths. The reverberant field in closet sized small enclosures will have very low modal densities and will be highly colored. and this means that a large percentage of the surfaces inside a recording booth should be rendered acoustically absorptive. Recording for the spoken word require shorter reverb times than booths used for music (including singing) in order to preserve intelligibility.

This does not mean that small rooms cannot be used as recording rooms. For purposes associated with recording, a room can be “acoustically dead”, meaning that it has little or no reverberant field. Dead rooms have greatly diminished modal content because they contain so much absorbent material that sound energy made in such a room cannot persist for long, but is instead quickly captured by the absorbent materials and turned into heat. This is not all bad news because a room without a strong reverberant field has some positive attributes.  For example, such a room cannot possibly be muddy, and generally you can mic just about anything in just about any position and ignore the room’s effect on the results. Unfortunately however, when mic’ing instruments in such rooms, the resulting recordings sound unnatural because they do not convey a normal sense of space or contain a normal amount of decay products. Nonetheless, music recorded in a dead room is perfectly usable, but it must be augmented through exposure to a reverberant field during mix down in order to sound “real”. This is accomplished either by sending the signal through a reverb chamber and then folding it back into the mix, or the use of an electronic reverb unit. Dead rooms are a viable alternative for small studios but are generally not considered “best practice” for one’s main recording room. One of the primary square footage / budgetary decisions in the design of a studio space is to take this issue into account – namely will the studio have a room with a “live” reverberant field.

Bottom line is that the smaller the room, the more you have to reduce the reverb field to insure clarity and avoid recording room coloration along with the target sound of the instruments; the smaller the room, the less reverb can be enjoyed without causing problems.

See the Optimizing Room Size / Mode Calculation portion of this FAQ for more on modes and how they come about.
SRF
Scott R. Foster
 
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