Soundproofing a Room or House

Noise Reduction Techniques for Better Sound and Quiet Living

When "Almost" Soundproofing a Room Will Do

Soundproofing a Room brings up images of recording studios and torture chambers, but this article will deal more with the average person who simply wants a quieter life. That is why I have subtitled this "Almost" soundproofing a room.

Sometime there is a need to improve the quality of the sound in your home.  This might be to prevent outside noise from coming in, or to keep inside noise from disturbing the neighbors or to improve the acoustics internally.

All of these will rely on the same laws of physics and therefore the same basic strategies can be used in all three cases.

My Story - Why I Became Interested in Soundproofing a Room or a House

There was a time, for several months, when I suffered from tinnitus.  This condition is a continual ringing in your ears.  In my case it was accompanied by extreme sensitivity to sound, where loud noises where painful to the point of tears.  

At this point my interest in good sound quality went to a whole new level.  I have always had a strong distaste for clatter in noisy restaurants and gymnasiums, but not now I discovered that noise can be debilitating.

My story is not unusual, though the particulars vary.  There are people who need exceptional levels of quiet, whether because they are sick, or simply so they can work at night and sleep while the world bustles around them.  “Almost Soundproofing a Room” is for all who have a need for quiet in their domestic life.

The Physics of Sound - Necessary Information for Soundproofing a Room

Sound travels in waves through a medium.  The energy in a sound wave travels from molecule-to-molecule, whether in air or in something solid, like the beams of your house.  How well it travels through these media will vary depending upon the physical properties of the media.  

This is good.  It means by the proper choice of materials we can regulate how well the sound is transmitted.  That also means we can limit the transmission of sound.

As a general rule stiff, hard materials are good sound transmitters.  Soft, pliable materials are not.

Sound will travel from a hard material to another hard material with little energy lost in transmission.  That means that sound can go from your wood floor to your wood beams and on into other rooms.

The transmission of sound between a fluid (such as air) and a solid (such as your wall) will be incomplete.  Much of the sound will bounce off of the wall.  That is good if you are trying to keep sound on one side of the wall.  It may not be good if you don’t want sound bouncing back at you.  

Since sound transmission between a fluid and a solid is incomplete, most of the sound will stay within the fluid, in this case air.  This means that the biggest culprit for allowing outside noise in or inside noise out is a direct air-to-air transfer.  Gaps under doors, around poorly caulked windows and open ducts will allow your neighbors to hear your private conversations.  

The sound waves in the air, since they mostly stay within the air, bounce back into the room.  Some are bouncing off of the ceiling walls and floor and coming directly back at you.  Some will bounce off several walls at a shallow angle, essentially travelling along the wall.  A wave created from the same sound will have the same amplitude and frequency, and it will travel out in all directions, but the wave it creates will hit the walls at different times and bounce around at different angles.  

Waves bouncing off the different surfaces will have the same amplitude and frequency but will now be out of phase.  These out-of-phase waves tend to cancel each other out, with the crest of one wave striking the trough of another.  That is why our voices doesn’t just keep talking after we are done.  The sound travels so fast that the waves hit the wall and each other and get cancelled out fairly quickly.

However, some waves will not get cancelled.  They will line up in phase and the crest and the trough of the waves essentially get doubled in amplitude.  This is often the case where sound waves strike at an inside corner.  The waves come together in phase and the sound seems to actually get louder.  

This has been a very basic explanation of how sound travels within your home.  We will now use this knowledge to develop some strategies for noise abatement, acoustic improvement and soundproofing a room.

Separate Hard and Hard

If I am in a room and I am talking only a small portion of the sound will get transmitted into the walls of floor and travel on into the next room.  However, a solid noise source such as a speaker, or a refrigerator can transmit vibrations directly to the floor without relying on the air as a media.  That means the relatively quiet refrigerator humming might travel further than the louder conversation.

One strategy to reduce the transmission of sound is to separate hard surfaces from other hard surfaces with either air or something pliable in between.  As a practical matter we aren’t going to rebuild our house and put rubber pads between the joists and the beams, but we can  elevate our speakers, even on something as simple as a cardboard box, and put rubber mats under appliances.

Absorb the Sound

Another strategy is to put soft materials along walls or ceilings to absorb the sound waves. This can be very intentional, by putting up sound proofing insulation along the wall of a sound room, or it can be more natural, through the use of carpeting and drapes and soft, stuffed furniture.

For the average room everyday material will work. For problem rooms you may need to rely on special acoustic materials.These are available from specialty catalogs, and can get quite pricey.  

Acoustic insulation, for instance, will cost you much more than the insulation you buy at the hardware store. It is the same material, so you might be tempted to go with the cheaper product. The difference is in the density. The acoustic insulation is compressed. There is more fiber and less air. This gives it better sound trapping properties.

Similar stories exist for all the other acoustic materials. You might find, through trial-and-error, that some of the normal products work well-enough for your needs. If you have a bad acoustic problem the special sound-proofing materials are probably worth the extra cost.

Disperse the Sound

A sound wave hitting a wall will bounce off as a wave, but it it hits a series of surfaces it will bounce off as a bunch of small waves.  Having things in the room, hard or soft, alters the acoustic because it disperses the sound waves, breaking them up into smaller waves.

Having a lot of smaller waves means there are more opportunities for the waves to interfere with each other, which means they cancel each other out more quickly.

Pictures on walls, lamps, bookcases, all have some effect.  This isn’t exactly soundproofing a room but it is improving the acoustics of a room.

On a smaller but wider scale, adding texture to walls and ceilings does something similar.  Any kind of interference to the sound waves on a wall has a double effect.  It not only disrupts the wave bouncing off the wall, it disrupts the waves travelling along the walls.  Which leads us to our next topic for soundproofing a room.

Focus on Problem Areas

Flat surfaces form natural pathways for all the waves that strike at a low angle. Since there is a concentration along the walls, disrupting this flow will have a disproportionate effect.  Any kind of material that sticks out from the flat surface will work, but the soft, acoustic materials will work best, as it will not only disperse the sound but absorb it.

Drapes on a wall are great, even when they cover a very small area.  Wall-hangings and soft-sculptures can also be employed this way, and will look quite natural.

Corners are another problem area. Here three hard surfaces join and the waves tend to amplify. The amplified waves will tend to be aimed at certain spots so that you end up with certain locations in your room being noisier than others.  

You could eliminate the corners. This is extreme, but effective.  With some creative use of wallboard you can convert every corner into a complex collection of flat surfaces coming together at odd angles, or even make corners completely rounded.

A far easier strategy is to place acoustic foam in the corners.  This will look odd from the interior design standpoint, but will be very effective acoustically. Acoustic material catalogs offer products for this specific purpose. Making such a product look like it is part of your decor will be difficult, though perhaps easier for those who opt for an ultra-modern look.

Acoustic foam is one of those acoustic materials that make almost soundproofing a room possible.  Like sound proof insulation part of its secret is a greater density.

Somewhat less of a concern than the ceiling corner is the corner where two surfaces meet. A similar strategy can be employed here.  

I will close with a quick discussion on traps.  These are usually placed in the problem areas, so it is a good place to touch upon these.

There are frequency-specific traps, where you only trap sound within a very limited range of frequencies, and there regular traps.  Here is the difference.  A frequency-specific trap is a bit like a woodwind in reverse.  It relies on a large hollow object with a small opening.  The size of the opening and the size of the object will determine the frequency it traps.  You can google Helmholz Resonators if you need to size one.

A regular trap can take various forms but it is usually box or tube or board placed in the corner.  There is an opening along the length of the trap and it is filled with dense, acoustic material.  Sound travelling along the wall goes in but doesn’t come out.  

I am not sure how effective it is compared to simply having the corner filled with acoustic foam, but it is perhaps more visually appealing.  For instance you could simply have a wide board in the corner, but with a gap between it and the wall.  Painted the same as the wall it would blend in, without having a serious effect on the visual appeal of the room.

By focusing on the problem areas soundproofing a room becomes a less daunting task, and the corresponding costs are reduced.  It may result in a partial solution, but often that is all you need.  My final story will provide an example of how a simple application of acoustic materials made an unfriendly room a much nicer place.


My Final Story - Almost Soundproofing a Room Saves My Hearing

My church built a large, multi-purpose room.  It was gymnasium-sized but you could also have banquets and it had an upper balcony area that was great for games, like ping pong and air hockey.  I couldn’t stand to be in that room.

Like many gymnasiums it had horrible acoustics.  It had some padding and some carpeting along the lower wall area, but the rest was hard surfaces.  It had a strange reverberation when you talked.  The big walls made big waves and it took several seconds for the sound to dissipate.  With hundreds of people in it you had to shout to have a conversation, and with hundreds of people shouting to converse you rapidly developed a headache.

I started researching ways of soundproofing a room, which is where I gained all of this knowledge.  I tried to sell the church on installing various soundproofing materials and products.  The hang up was that the powers that be feared the rambunctious ball players would damage just about anything I proposed.

Finally, in an example of act first, get permission later, one of the ladies in the church bought hundreds of sheets of two-inch-thick sound insulation and installed them along the top of the walls right below the ceiling.  Wow.  It was simple.  I won’t say it was cheap, but cheaper than a lot of solutions.  And it was effective.

The room still needs improvement.  If I had my way I would make a lot of changes to that room, but now I can stand to be in it.  Almost soundproofing a room may not be the best way of describing this project, so maybe I’ll just say that we repaired an acoustic disaster. Either way, it made a huge difference to me and many others who use that room.

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