Why Isolation (Soundproofing) is Important

Isolation (also known as soundproofing) is important for a variety of reasons.  The most commonly recognized reason is to stop sound from bothering other people in the house or other people outside the room and the house/building.

A more important reason is to stop sound from getting INTO the room.  Why is this important?  In a recording room setting, it’s critical to stop sound from contaminating what’s being recorded.  In the control/mixing room, home theater or listening room, it’s important to keep the noise floor down to a minimum so it’s not masking details and limiting the dynamic range of the room / equipment “system.:

Let’s look at an example:

Let’s say you have a home theater system which is capable of 100 dB peak output.  In a normal room, the noise floor will usually be say 50 dB.  Things like HVAC fans, dishwashers running, people walking, sound coming through windows, etc. all contribute to this.  In this case, the maximum dynamic range the room/system can achieve is 50 dB (100 dB max minus 50 dB noise floor where sound will be masked by the noise).

Now, if we build a room and pay attention to the isolation / soundproofing, we can hopefully get the noise floor down to say 20-30 dB.  If its 30 dB and we have the same equipment, we now have a maximum dynamic range of 70 dB (100 dB max minus the 30 dB noise floor).  This results in more dynamic range, less need to turn things up as loud to hear small details and harmonics, etc.

Sound travels between rooms in 2 ways:

  • Through the air
  • Through the structure

Sound traveling through the air is relatively easy to understand.  Any holes you have in the room are sound leaks.  Outlets, switches, can lights, gaps under doors, HVAC ducting, etc. are all excellent sound transmission paths.

Sound moving through the structure is much less understood.  Sound vibrates your walls, your ceiling, your floor, your tin ducting, etc.  All of those things are connected to other parts of the building.  When they are caused to vibrate by the sound in your room, the other side or what they’re connected to also vibrate and recreate that sound in other parts of the building.  Even solid concrete basement floors are a flanking path for sound to get to the rest of the structure. (Flanking is a term used to describe a path by which sound ‘goes around’ the boundary of the space.)

So, how do we deal with these issues of soundproofing?  The answer is “It depends.”  It depends on your budget, your situation, whether the room is already built or not, what your required level of performance is, etc.  Let’s look at a scenario:

Existing room to be used for a listening room or home theater in a home.  Bedrooms are not directly next to this room but are close enough that sound transmission is an issue.

Level 1 Soundproofing

  • Insulate the walls.  This is not expensive to do and will give a nice performance gain.  The walls now become absorbers in the bass and also do not transmit mid and high frequencies in either direction.  The walls also now do not ring like a big drum.
  • Replace the door to the room with a solid core wood door and add seals to the door.  The door is usually the biggest sound leak in any room.  Hollow core interior doors are horrid blockers of bass energy.
  • Remove the baseboards and use a 50 year Latex caulk or acoustic caulk to caulk the gaps under the drywall (between the drywall and floor) and also caulk the wall to the floor to stop air penetration.

Level 2 Soundproofing

  • Level 1 plus
  • Remove the existing outlets and switches and build an MDF “backer box” behind each of them. Have a hole in the top of the box only big enough for the Romex to enter and caulk that.  Replace with old work boxes.  Think about how many holes there are in your room with switches and outlets.  If you can’t take down the drywall to build the boxes, at a minimum, use putty pads on the rear and sides of the plastic box to add some mass and seal it up tightly
  • Repeat the above for any can lights in the room.  Make sure to use IC rated cans if you’re going to box around them.
  • If you have access, replace any tin ductwork with flex tubing and isolate in an MDF box with 90 degree bends in it. This not only stops sound from getting out but also makes the room much quieter. Consider larger vent diffusers.  HVAC is usually ignored but is extremely important.  Remember, it goes all through the building

Level 3 Soundproofing

  • Levels 1 and 2 plus
  • Add a second layer of drywall to the room and use Green Glue between the layers.  This gives tremendous gains in isolation across the spectrum down into the deep bass range.  The additional mass of the drywall stops a good amount of sound by itself.  The Green Glue provides what’s called a ‘visco-elastic damping layer’ between the sheets.
  • Consider building a plug for any windows in the room.  Make the front something that has mass to it like MDF.  Fill the rear with insulation.  If you have windows recessed into a wall, make the plug small enough to fit in while using some ¾” foam weatherstripping for a seal.  If you don’t, then make a 2×2 frame around the window trim and slide the plug around the outside.

Level 4 Soundproofing

  • Levels 1-3 plus
  • Work on the floor.  You can either float a floor using Dri-Cor or you can use a specifically designed rubber dampening layer and then another layer of subfloor. You can also add a 3rd layer of gypcrete between the existing floor and the rubber matting.  Remember that the floor joists under you are likely shared with the rest of the structure.  Even with a concrete floor, bass frequencies will transmit through to the other walls of the structure and through the building.
  • Add a separate HVAC system for the room.   Systems called Mini-Splits are relatively cost effective ways to go.  The good ones have running noise levels of 25 dB.
  • Consider adding a 3rd layer of drywall.
  • If you’re removing the drywall to accomplish some of this, consider adding RSIC-1 isolation clips and hat channel before replacing the drywall.  This will cost you a couple of inches in room size but will really ratchet up the isolation.  Remember that the other side of those studs is many times drywalled and in another room.  Even with all of our efforts, there will still be some movement.  That entire wall acts like a big woofer.