How Does Soundproofing Work?

Charles from Altoona, PA, has an older brother that plays the trumpet. His brother practices the trumpet in the bedroom next to his. He wrote in asking if it was possible to block out the sound so that he does hear the trumpet while he’s in his bedroom. Charles is essentially asking about soundproofing. While he probably won’t be able to block all the sound from his room, Charles can take steps to minimize the sound that he does hear.

Average volumes of people talking, television sounds, and music playing can often be heard through walls easily. This is due to the fact that sound is a series of vibrations that move surrounding particles. The series of vibrations, or sound waves, carries the noise from the noise source across the room to our ears. Since particles are required to carry the vibrations, sound cannot travel in a vacuum. The more densely packed those particles are, the better the sound moves through since the particles don’t have to move the surrounding particles much. When you’re in an open field, though, you would notice that sound will not carry as well since the particles are more spread out. The farther sound waves have to travel from one point to another, the fainter the sound will become. When sound waves collide with a solid surface (e.g., a bedroom wall), there are a few things that can happen. The surface will reflect some of those vibrations back toward the source, it will absorb some of the sound by converting the vibrations into heat energy or it will transmit the vibrations to the other side (i.e., into the bedroom).

There are two main things to consider when soundproofing: noise transmission and noise reception. The sound coming from the trumpet is a noise transmission issue, while the desire to block the sound out is a noise reception issue. Next the source of the noise should be considered: the indoor noise vibrations your body feels are structure-borne noise, while overhearing a conversation is airborne noise. Soundproofing can be achieved by considering space, mass, and dampening. Space increases the amount of air between your ears and the source, which diffuses the noise by taking away the vibration channels. Mass, like a bedroom wall, can act as a sound sponge that soaks in the sound waves. Dampening sound requires specific materials (like insulation) that will convert structure-borne sound waves to heat energy. Dampening can be expensive.

Without having to spend money, one step Charles can take while his brother is practicing the trumpet is to create more distance between himself and his brother. For example, Charles can plan to hang out in the basement or another room in the house that’s far away from his bedroom while his brother is practicing. Or Charles could ask his brother to practice in another room that’s far away from his bedroom.

Little Lion Experiment
You cannot see sound waves in the air, but you can see their effects. This experiment will help you see the effects of sound waves. You will need: 1 large cake or cookie tin, 1 sheet of plastic wrap, 1 long rubber band, 1 baking tray, 1 wooden spoon, and some fine sand.
Steps: 1) Make a drum by stretching a piece of plastic film over a large round tin; 2) Stretch the rubber band around the tin to hold the plastic taut; 3) Sprinkle a teaspoon of sand on to the top of the plastic drumskin; and 4) Hold a baking tray above your drum, and hit it sharply with a wooden spoon.

What did you observe? What do you think caused the sand to dance up and down on the drumskin? When you struck the baking tray, the metal continued to vibrate for a fraction of a second afterward. As it vibrated, the air around is also vibrated. These little vibrations in the air, the sound waves, quickly work their way out through the air in all directions. When the sound waves hit the drumskin, the drumskin is vibrated too, which causes the sand to dance up and down on the drumskin. The sound waves that reach your ear make you hear the bang.