### Soundproofing Walls and Rooms for Beginners

Soundproofing a room or house requires at least a fundamental knowledge of how sound travels as well as frequencies and oscillations. Different frequencies require different methods and materials to “block”. In general, low frequencies are more difficult to block than high ones. But unless you’re trying to make a drum studio, blocking the high and mid-to-low-range frequencies are probably enough.

Brace yourselves for a little preparatory science…

As a rule of thumb, the stiffer a material is, the faster sound will propagate through it. Stiffness is a material property that indicates its resistance to deformation, as in, steel is stiffer than rubber. What this means practically, is that softer, spongier materials are more conducive to blocking sound than stiff ones. For those who are more mathematically inclined, this equation may explain things better:

where c is the speed of sound, K is the material’s elastic modulus, and ρ (Greek letter rho) is its density. This equation, known as the Newton-Laplace equation, implies two things. – That the square of the speed of sound through a given material is proportional to its elastic modulus, and that it is inversely proportional to its density.

The speed of sound decreases with higher material density. It is a common misconception that the opposite is true, due to density being associated with stiffness. Air, water, and steel are often used as a material comparison for sound speed, with the respective speeds being approximately 343, 1484, and 5120 meters per second in a 20 degree Celsius environment.

From the above comparison it can be seen that although water has a bulk modulus (2.2 Gpa) that is about 20,000 times higher than air (100 Kpa), the speed of sound through water is only about 4.3 times faster. The difference in density tips the scale back to a more modest number. Steel has a fairly high density at about 8,000kg/m3 but has a bulk modulus of 160 Gpa, about 73 times higher than water.

So with a bulk modulus that’s 73 times higher than water but a density only 8 times higher, it’s easy to see why steel makes such a good conductor of sound. Diamond, having an even more impressive bulk modulus at 442 Gpa, about 2.7 times higher than steel, but having only 40% of its density (3,500kg/m3), makes an even better medium for sound (11,000 m/s). Looking at it like this, it’s all a lot easier to understand right?

Examine Your Windows and Doors First!

Before going any further, you should know that windows and doors are the first things you should examine when soundproofing, not your walls. Generally speaking, walls are already considerably more soundproof than windows, which makes windows your weak link. Beefing your walls up won’t do a lick a’ good if all the sound shoots right out the windows. Same thing goes for your doors.

Determine the element in your room that is the least soundproof and start there. Windows can be soundproofed a number of ways. Dual panes are an option but aren’t nearly as effective as a double window – two separate windows layered in the same frame socket. Other options include heavy curtains that are somehow secured to the frame so as to seal the window as much as possible, or simply boarding the window up.

Practical Steps

Once your weak links have been properly augmented, you can now move on to the wall. Depending on how far you want to go with this project, you can either build off from your existing wall (especially if the existing wall is sheet-rock already), or you can make things a bit more complicated. Building off from your existing wall is simpler, faster, and may be all that you need anyway.

However, if your needs are greater than what can be provided from the above, you’ll need to remove the existing interior wall surface. By rearranging the wall studs in a staggered pattern you can essentially de-couple the interior wall from the wall on the other side (see image). This will obviously require more time, materials and know-how, but it makes a significant difference.

Once the studs are properly aligned, they are ready to have sheet-rock screwed onto them. Filling in the gaps between the studs is optional, but insulation is not recommended as it takes the place of air, which is an important element in soundproofing. Although air has low density, it has an extremely low bulk modulus, which if you recall, greatly inhibits the travel of sound.

Sprayed-on cellulose is an exception to this due to its higher STC value (sound transmission class), albeit a costly one. By squirting a generous bead of caulking (blue in the image) down the wall studs before screwing on the sheet-rock, you create a “partial de-coupling” due to the high compressibility of the caulking (compressibility is the reciprocal property of bulk modulus).

Using this method you install at least 2 layers (most homes come with 1 layer). As reference, an average home with a half-inch layer of sheet-rock on either side of a wooden stud frame has an STC of about 33. As many of you probably know, they don’t seem to offer much resistance to sound. By adding a second layer this value will go up to about 40.

However, by de-coupling the same 2 layers of sheet-rock from the other side (additional 2 layers) through methods such as the above stud staggering, an STC of 60 or more can be achieved. This is all with the same number of sheet-rock layers. The difference in raw STC value between the 2 may not seem to be that much, but it represents an 88% sound reduction, or just 1/1000th of the sound energy transmitted.

Like I said, it makes a significant difference! Here’s a table of STC values and how they generally affect the average person.

Noise or sound can be blocked or otherwise reduced by way of 1 or a combination of 4 things (note that a combination of all 4 is recommended if possible.):

• Decoupling. As shown above, air has a rather slow sound speed in comparison with other liquids and solids due to its high compressibility. Creating a gap or air pocket between 2 walls essentially decouples them.
• Mass. Being that density is a measure of a material’s mass per unit volume, mass is extremely important in soundproofing. Here we should remember that mass is not the same as stiffness. Ideally, you should go for materials with high mass but low stiffness, such as lead. But being that materials with high mass are often costly and labor-intensive to install, you must achieve a balance.
• Damping. This is the most effective when it comes to low frequencies such as a booming bass drum from your favorite party song ;). Damping, depending on the amount, results in a partial or complete prevention of sound oscillation. This can be accomplished by a density gradient in the wall materials. The most common density gradient are rubber (or similar) mats or sheets fastened to walls or placed on floors as a sublayer before drywall (for walls) or plywood (for floors). Adding a second layer of drywall adds mass. Mass and damping go hand in hand, in that damping materials are generally high-density.
• Sound absorption. Sound absorption converts sound energy to small amounts of heat by absorbing it via material porosity and/or surface geometry, and results in a modest reduction in sound level. Sound absorption is important when things like reverberation need to be controlled. Required surface geometry and material porosity will vary depending on the frequencies that need to be cut.

High sound absorption results in a “dead room” as opposed to a “live room”. Sound absorption almost always refers to the lining of interior walls with absorbent materials. In other words, absorption-based sound proofing methods are utilized when sound sources are in enclosed interior spaces and escaping noise and/or reverberation within the room must be minimized.

Other Tidbits:

Sound redirection: This is pretty straight-forward, and is the redirection of sound waves via angled surfaces. Certain materials absorb sound and certain materials reflect it. Windows for example, being made of hard glass, will reflect certain sound frequencies. By angling reflective materials, you can redirect sound in a direction of your choosing, ie, up into the ceiling, down into the floor etc.

It goes without saying that this particular method is only useful for specialized situations and generally won’t be very helpful for residential home applications.

Room Within a Room (RWAR)

This is essentially decoupling taken to its limits. The room within a room method is very effective in terms of soundproofing, but as is obvious, it takes a significant amount of space and materials, not to mention technical know-how. The concept is based on the principle of completely decoupling the inner wall, floor and ceiling, from the outer one. Due to the fact that sound travels quickly through stiff materials, any bridging via wall studs will compromise the soundproofing integrity. One must also take care to not inadvertently create a resonance chamber within this air pocket.