What is Soundproofing?

There are 5 basic principles of soundproofing:

  1. Mass
  2. Absorption
  3. Conduction
  4. Mechanical Decoupling
  5. Resonance.
soundproofing basics

Mass: This one is pretty straightforward. More density = more sound absorption. Thing is, you need it to be REALLY dense to make a big difference, so simply adding a layer of drywall to an existing wall will only give you a slight decrease in sound transmission.

Absorption: Any kind of insulation within a wall’s cavity will help absorb some sound, although it will not trap any low frequencies. This is most effective when the walls are decoupled so if there are studs connecting the two walls, the effect of the insulation will be minimal. Still, putting something in the wall is better than nothing.

Conduction: This is the transmission of sound through vibration of solid objects connected to each other. Conduction is a very efficient way to transmit sound, and as such any wall that has drywall directly connected to the studs on either side will never perform well. Sound will also travel through indirect routes largely via conduction, and this is called flanking noise.

Mechanical Decoupling/Isolation: This is one of the most effective ways to combat conduction, and is why the most effective soundproof walls are completely decoupled, so there is no direct path for the sound to travel. If not paired with other soundproofing principles, decoupling a wall will help with transmission of mid-high frequency sound but will increase resonance and sound transmission at lower frequencies, so it must be done in combination with other principles to be truly effective.

Resonance: Despite best efforts on all of the above principles, sound will still resonate a well insulated, decoupled wall if it hits the right frequency to vibrate the components of the wall. This isn’t common in high frequencies, but is a challenge for low frequencies (imagine how strong bass rumbles certain objects at particular frequencies; therefore a decoupled wall will rumble x2 when the right frequency is hit for the mass of the wall).  This can be offset in two ways:

  1. Lowering the resonant frequency of the wall: one can lower the frequency at which the wall wants to resonate by making sure there is plenty of mass in each wall (tougher to vibrate), and by adding insulation / plenty of air in between (absorption). This can help push the resonance point low enough that only the deepest frequencies will vibrate through the wall. Sadly this is why your subwoofer will almost always annoy the neighbors, as these methods can only push the resonant frequency so low.
  2. Damping: this refers to any means of reducing resonance altogether by absorbing or redirecting sound, and can go a long way to combat resonance and conduction. By far the most effective damping agent out there is green glue. It’s one of the most effective soundproofing aspects of building a soundproofed wall by spreading the compound to cover the area of the wall and by applying acoustic caulk to seal all the gaps and fill the seams.

Understanding Sound Transmission Basics

When soundproofing a room, “Sound transmission class” ratings indicate a wall’s (or floor’s) ability to block sound. STC ratings are determined in a laboratory under ideal conditions. There are all kinds of variables in the real world that STC ratings don’t take into account. (These include walls that contain plumbing lines and sound traveling around walls, through ducts and via other pathways.) Still, STC ratings are useful for comparing sound control methods and materials.

A typical wall made from 2×4 studs and covered with ½ inch drywall on both sides carries an STC rating of 34. You’d be able to hear, and partially understand, a loud conversation taking place on the other side of this standard wall. Put fiberglass insulation in this wall and its rating rises to 39. You’ll still hear the voices on the other side, but they’d be muffled and unintelligible. Cover one side of the insulated wall with resilient channel and 5/8 inch drywall and the STC jumps to about 50. At this point you wouldn’t hear the conversation at all—unless they started shouting.

sound levels chart