Sound Transmission Control in Recording Spaces

Sound control, or rather, sound transmission control, is a key factor when designing recording studios. It can be the difference between a successful project and a lawsuit.

In this post, we're specifically talking about sound transmission into a space; STC, or sound transmission classes. Later posts will deal with sound control in the space (reflection, reverberation, etc). But first a bit about sound in general:

Sound is simply vibrations traveling through a medium, like air, or water, or in our case walls, floors, ceilings, and doors. Sound requires a medium through which to travel. So next time you're watching Star Wars remember that explosions are completely silent in the vacuum of space; that doesn't make for great cinema, though.

Sound is generally measured in Decibels (dB), which is the magnitude of the "loudness" of the sound. Breathing is about 10dB, a garbage disposal about 80dB, and the ear drum rupturing jet takeoff is 150dB. Keep in mind that the decibel scale is logarithmic, which is more useful than using, say, the Pascal scale for noise. A logarithmic scale more closely approximates the relative discomfort of "louder" and "louder" sounds. In architecture, there's not much you can do for a jet taking off outside, but plenty you can do for the garbage disposal in the next room.

There is a much more important measurement which will make or break a project: Hertz (Hz). Hertz is a measurement of the frequency of vibration (vibrations per second). This can be the vibration of electromagnetic waves, light waves, or in our case: sound waves. It is critically important to understand the scale and how it impacts the design of a space. Humans can hear between about 20 to 20,000 Hertz, depending on a variety of factors, including age, where your higher frequencies tend to start dropping off.

Now, down to how all of this effects recording spaces.

Medium to high-range frequencies are most easily blocked before entering your space. New technologies such as specialty gypsum board which contain a layer of viscoelastic polymers are available from almost every major manufacturer. We won't get into the science, but there's plenty of research out there on how these help dampen vibrations. Acoustical sealant has also come a long way. When properly installed, these sealants can reduce noise transmission in necessary penetrations and construction joints. Speaking of penetrations, keep them small, and keep them away from each other. One back to back receptacle in your special (and expensive) new wall is all it takes.

The construction of the wall is also important. There are many special wall assemblies that give great STC ratings (again in mid-high frequencies). Consult manufacturer's literature for more information.

Okay, ready for the kicker? No amount of viscoelastic polymers or tubes of acoustical sealant are going to keep out low frequency noises. That's right. The ASTM test for STC classes only test for sounds above 125Hz. So low frequency bass sounds like trains and thunder easily make it through a highly rated STC assembly. (And unfortunately are harder to take out in post production editing) No, there's only one way to reduce or eliminate low frequency sound transmission: mass.

That's right, good old fashioned mass. Tap on your desk with a pencil, and then tap a concrete floor the same way to illustrate the effect mass has on dampening vibrations, and therefore sound. The vibrations have a much harder time displacing the mass of the floor, which absorbs the energy easily and therefore prevents transmission of the sound. So if the project can support it, use grouted CMU walls in addition to some specialty gypsum board to cover all frequency ranges. Better yet, use solid concrete walls with footings separated from the rest of the structure.

The last thing to keep in mind, now that you've built massive walls, is penetrations. Keep them to a minimum if possible. Use acoustical sealant like you own stock in it, and deal with large penetrations the best you can.

Ductwork is a tough challenge. Use large ducts (for slow moving air, of course) with extra, integral insulation, and put extra ells (bends) in the ductwork to make it harder for the vibrations to "turn the corner."

Doors are the other large penetration to deal with. A vestibule with moderately priced "sound control" doors is always a great idea if the project can support it, but space is often a limiting factor. Higher rated sound control doors are more expensive - but heavy, which is what we want, remember? These doors often come with cam-lift hinges and a door bottom that drops into place when closing, sealing off the space. This means you want to add thresholds, even in interior spaces, and make sure they're flat, not fluted. The frames should be specified with the door, and by the same manufacturer to ensure proper operation. Either grout the frames (mass, mass, mass) or pack them with mineral wool.

For successful reduction of sound transmission, go high tech, and low tech. Mass for low frequency, and fancy new materials for medium to high frequencies.