Is Wall Insulation an Effective Way to Block out Low Frequency Noise?
Isnt it amazing how much sound were bombarded with everyday? We love to find a quiet moment where we are not acoustically assaulted by people, equipment, traffic, stereos, neighbors, cell phones, and on and on. Its hard to find that quiet moment, and perhaps even harder to find that soundproof place. Sound levels are so intense that many walls and building materials just dont stop the sound satisfactorily.
Think of it. How often are you bothered by noise from the person in the room next door, even though the doors shut? Have a music student in the house? Dad like to watch movies late at night? How about the neighbors having yet another party?
How in the world do we cope? Its no wonder that Ben Affleck slept in a soundproof sensory deprivation chamber in the movie Davedevil. But of all the noise that we retreat from theres one thats the worst of all. Bass. Its the worst because its the hardest to stop. In the acoustics industry, bass is known as low frequency sound. Its the boom-boom music sound you hear coming up behind you while driving. You hear it long before you see the car thats creating it. Low frequencies are the jets flying overhead if youre near a major airport.
This is the worst sound of all because it assaults from long distance. It penetrates normal building walls and ceilings. Low frequencies are the largest and generally the most powerful sound waves we come up against daily. They are hard to escape. Paradoxically, the low frequency bass is what is being sought more and more in home theaters, bands, and music systems. So our dilemma is only going to get worse.
The volume level of sound is measured in Decibels (dB). The frequency of sound (bass, midrange, treble, low notes, high notes, etc) is measured in Hertz (Hz). The sounds that are most problematic when we retreat to a place of soundproof solitude are the low frequencies below 125Hz. This is the realm of the worst sound of all.
There would seem to be a need to set standards for healthy living that describe how to insulate yourself from such noise. In fact, there is a standard that measures a structures ability to hold back sound. It is called STC, which stands for Sound Transmission Coefficient, and is the most recognized and discussed sound isolation term in North America.
Building codes will dictate what STC level a building needs to have. A new condo, town home or apartment will have to comply to these building codes and build walls, ceilings and floors that meet these STC requirements. If this is so, how come I can hear my neighbor? Why is my new condo so noisy? After all, the structure is built to code.
The reason that new structures are still noisy is because of the STC standard itself. STC does not measure any of the worst sound of all. It stops measuring at 125Hz exactly where the worst sound starts. So you have a high STC wall that meets code, but sounds terrible. Great.
There is a need to change this STC standard to include these low frequencies. They do this already in Europe and Australia. In the meantime, what can you do?
Look for building techniques that have been tested below 125 Hz. Good performance below 125Hz. will help you a lot. Be aware of products like resilient channel, sound boards, foams, tapes, clips and especially mass loaded vinyl (MLV). These products wont help you in these critical low frequencies. Consider materials that damp the sound vibration. These types of products are generally known as visco-elastics and are most effective with the worst sound of all.
Insulation as a solution?
One of the oldest and most established methods for improving the sound isolation of walls is the use of insulation. Its fast, reasonably inexpensive and easy to install. You probably have some interior walls in your own home filled with insulation. But how effective is it, anyway? Does it stop the type of sound that is most annoying you?
It might be best to first consider how sound moves from one side of a wall to the other. A typical wall will have a sheet of drywall / stud / drywall. The wall will have air spaces in between the studs. The drywall is very rigidly connected to the studs.
Sound waves will hit that wall, and attempt to vibrate through to the room next door (or floor below, etc). The sound can vibrate into the next room through one of two paths:
Through the rigid drywall-stud-drywall connection path Through the open drywallairspacedrywall path
The insulation will only be able to affect the vibration traveling through the airspace between the studs. The rigid drywallstuddrywall connection is not reduced with insulation, obviously, and will conduct vibration just as effectively whether insulated or uninsulated.
This has been an extensively laboratory tested scenario, fortunately. The detail of such lab tests are conclusive, and one finds that the low frequencies are not stopped by the addition of insulation. Further, the high frequencies are not reduced significantly. Mid frequencies are reduced somewhat, however, allowing the overall rating of an insulated wall to be 2-3 STC (measurement of sound isolation) points higher than an un-insulated wall. This seems like a good thing. At least the insulation reduces the middle frequencies.
The problem, however, is that most sound isolation problems are in the low frequencies that insulation does not affect. This means that for many of us, insulation alone wont stop the sounds that bother us the most.
Does this mean insulation is useless for soundproofing? Not at all. It simply points to the fact that insulation shouldnt be relied on as the sole solution for soundproofing. If you are building a home or an addition or remodeling, you just cant rely on insulation alone to soundproof.
So where does insulation play a role in sound isolation? Insulation is a great synergist with any of the following other sound isolation techniques:
De-couple the walls with staggered stud or double stud construction (very effective) Increase the mass of the wall (moderately effective) by adding drywall.
Damp the wall with a field applied damping compound (very effective)
By deploying a combined technique approach, you will have much greater success in your soundproofing efforts.