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Improving The Sound Of Your Room - Part 2

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This post is part of a series called Improving The Sound Of Your Room (Premium).
Improving The Sound Of Your Room - Part 1
Improving The Sound Of Your Room - Part 3

These days it seems that nearly everyone has some sort of recording ability in their home, but that doesn’t mean the space you use actually sounds good. In this series, we’ll look at how to improve your listening environment for a lot less money that you’d think was possible. In Part 2, we’ll look at the principles of isolation.

If you’re building any kind of studio, especially one at home, the first question that both musicians and engineers ask regarding acoustics is, “How can I make sure that my neighbors don’t hear us?” When it comes to soundproofing, there are some very basic principles that just about anyone can understand that will get you 90 percent of the way there. The problem is that every extra percent of isolation that you wring out of your space from that point costs exponentially more money.


Also available in this series:

  1. Improving The Sound Of Your Room - Part 1
  2. Improving The Sound Of Your Room - Part 2
  3. Improving The Sound Of Your Room - Part 3

What Won’t Work

Before we look at some accepted ways to improve your isolation, let’s look at all the things that won’t work. The following are various materials that you’ll often see attached to the walls of a space in hopes of increasing the isolation.

  • Mattresses - There are so many things wrong with this that it’s hard to know where to start. Sure mattresses are made up of a lot of soft material, but it’s not the right kind for sound absorption, won’t affect the low frequencies at all (which are what causes most of the the isolation problems), accumulates mold and moisture, and makes nice homes for rodents and other unwanted critters. Plus, it’s pretty difficult to get enough of them to cover a room, and they take up so much space for so little benefit in return.
  • Egg Crates - Egg crates are light porous cardboard and do absolutely nothing for soundproofing. They can act as a sound diffusor at higher frequencies, but the bandwidth is so limited that they’re virtually useless there as well. Plus, they're highly flammable! It’s difficult to find enough of them to cover a room, but frankly, even using one is too many.
  • Carpet - Carpet attached to the wall is another product that will affect the sound of the room yet do nothing in the way of soundproofing since it doesn’t affect the low frequencies, which are the ones that you’ve got to control for good isolation. Carpet has exactly the same problem as mattresses in that it will begin to smell over time. Old or new carpet makes no difference, except that older carpet will smell more.
  • Foam Rubber - Foam rubber does have some acoustical absorption properties, but once again will do very little for the low frequencies that will cause all of your problems with the neighbors. It’s can be as expensive as materials with real acoustic control properties, degrades over time, and will burn like crazy if given the chance.
  • Rubber - Floor matts, mouse pads, neoprene, or any other variation of rubber will do very little to stop sound coming or going from your room. Once again, it’s much cheaper to buy proper acoustic materials that are easier to work with, but they won’t solve your isolation problem either.
  • Wall Cellulose - Pumping cellulose insulation into walls can make a slight difference in your isolation, but it’s marginal since there are much more effective ways to improve the isolation that end up being much cheaper. It can be helpful if used along with some other isolation techniques, but isn’t particularly effective by itself.
  • Fiberglass Insulation - Common fiberglass insulation once again has little ability to stop enough of the low frequencies that bug your neighbors, although, like with blown cellulose, it can be useful in conjunction with other techniques. Just pinning it to the wall won’t help though, but it will affect the acoustics of the room. It’s also a skin and eye irritant, takes up a lot of space, and the dust can be hazardous to your lungs when it’s left exposed to the room.
  • Plywood Panels - It’s true that plywood panels provide mass and mass is what’s needed to stop sound transmission (especially the low frequencies), but the problem is that wood transfers sound too well so the construction technique used is crucial. Not only that, if the panels are too thin they’ll resonate and vibrate, causing an even bigger problem.
  • Particle Board - See plywood panels.
  • Bales Of Hay - Unless you live out in the country, it’s unlikely that hay bales are much of an option, but they actually do work. The problem is that they take up a lot of usable space, make a nice home for various small creatures, and are a major fire hazard. Definitely not recommended!
  • Acoustic Foam - Acoustic foam is helpful in controlling the acoustics within a room, but it does nothing to stop sound transmission and is expensive to boot. Acoustic foam doesn’t even begin to affect the offending low frequencies, and using too much just makes the room seem dead and uncomfortable. There are much cheaper ways to achieve a better result.

Understand that all of these materials will have at least some affect on the sound of the room, but will do almost nothing by themselves to help improve your isolation.


Two Basic Isolation Principles

There’s really no secret to acoustic isolation, although everyone thinks there is. All it takes is adhering to the following principles during construction:

Isolation Principle #1: “All It Takes Is Mass”

If you want to increase your isolation, you’ve got to increase the mass of the walls and ceiling of the structure that you’re in.

The more mass your walls have between you and your neighbors (that includes walls made from cinder block, brick, wood, drywall, etc), the more you’ll keep the outside sound from getting in, and the inside sound from getting out.

One of the ways that most pro studios accomplish soundproofing is by building a room within a room, which is done by putting the floor on springs or rubber, and building double or triple walls with air spaces in between on top. Needless to say, this gets expensive and is impossible to do if you start out with a small space to begin with. But there are other ways to improve your isolation that can really be effective and completely acceptable (though never completely soundproofed) that are quite a bit cheaper. We’ll cover these in Part 3 of this series.

Add some mass to the walls and ceiling to increase your isolation. This could be as simple as adding another sheet of drywall to your existing wall, all the way up to building double studded walls with an air gap in between (see Figure 1). Before you go nailing up another sheet of drywall and expecting total isolation, you must be aware of some acoustic realities.

Figure 1: A double studded wall

Walls are subject to what’s known as a “mass law” that states that every time you double the mass of the entire wall, you get an extra 6dB of isolation, which you can hear but it’s not a huge difference (you can barely hear 3dB). The problem is, you need about 10dB of extra isolation for the sound to subjectively decrease it by half. This means that you need about four times the mass to get only half as loud, which is not nearly enough to isolate a moderately loud band.

To put it another way, if you add another sheet of 5/8 inch drywall to your single stud wall, have a listen and decide it’s not nearly enough isolation, you have to add six more layers (for a total of eight) in order to cut the sound in half. And that assumes that there are no leaks in the wall and the sound isn’t going through another path such as the ceiling, floor, side wall, or window, in which case NO amount of extra drywall will help.

You can see the limitations of just adding more and more drywall. First of all, there comes a point in time when the wall just gets too heavy for the underlying frame, and as you’ll see in Part 3, there are much more efficient methods to increase the isolation.

Unfortunately, there is just no easy or cheap way to isolate a room. The easiest way is to use what’s known as “mass-spring-mass” or MSM walls, which means you have a wall (the mass), then an air space (the spring), then another wall (the mass). That gives you a double stud wall, which is essentially a room within a room (see Figure 1 again).

Isolation can be increased slightly by sound damping products like Green Glue and resilient channel, but mass and MSM walls are still the best way to get some major isolation. This brings us to Principle #2.

Isolation Principle #2: “Leave No Air Gaps”

Isolation can be easily defeated by air gaps anywhere in the room.

Think of air like water. If you fill the room with water, any space between any construction joint would let the water (or air) leak out, so the idea is to make sure that there are no air leaks.

Leaks that allow the sound to violate the isolation are called “flanking transmission” and are a major cause of poor isolation. You can have four foot thick concrete and MSM walls but you can negate those benefits if there are air-gaps anywhere in the room. This is especially true for doors, which are the greatest culprits for acoustic leakage, but can also be true of windows and seams between drywall.

There can be NO air gaps if you want maximum isolation, it’s as simple as that. It’s best to use an acoustic sealant on these spaces because it doesn’t break down with age (see Figure 2), but any kind of caulking will work in a pinch. It’s also important to have a tight seal around any light fixtures and on-off switches, AC outlets, mic panels, wall junctures, and HVAC vents (see Figure 3.)

Figure 2: OSI SC-175 acoustic sealant

Figure 3: Acoustic putty around AC receptacles

Sometimes, just eliminating the flanking transmission can increase the isolation by more than you think, so make it a priority.

As you’ll see in upcoming posts, changing the acoustics of a room can be pretty easy and inexpensive, but that’s not the case with soundproofing. It takes lots of mass and excellent construction techniques to make a difference. In Part 3 we’ll look at a few of the ways to isolate your room.

Some of the above material comes from my book The Studio Builder’s Handbook (written with Dennis Moody). You can read excerpts at bobbyowsinski.com.

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