How is sound controlled?
A sound wave can be controlled in one of three different ways – it can be reflected, diffused or absorbed. Each of these reactions will depend entirely on the nature and composition of the material it comes into contact with, and each can be used to some extent in soundproofing. Below is a brief explanation of what happens in each case.
Sound is bounced off a surface. This usually occurs on flat, rigid surfaces with a lot of mass like concrete or brick walls. Because the sound wave can't penetrate very far into the surface, the wave is turned back on itself like a ricochet. The sound bouncing back off the surface creates an echo.
When a sound wave hits an irregular surface like foam or carpet, the vibration breaks up and travels along many much smaller paths. This divides the energy of the wave, sending it in many different directions which depletes its energy faster.
When a sound wave hit a particular surface, the kinetic energy driving it is converted into a small amount of heat energy which dissipates, leeching power from the sound wave and causing it to decaying faster. This is the kind of sound insulation provided by things like foam and rubber. How well a material absorbs sound depends on a number of different factors, including material density and how porous it is.
Which is the most effective method of soundproofing?
is often used to redirect noise from outside - consider highway barriers, which reflect traffic noise into the sky. If you can always control the way sound is reflected then this type of soundproofing can be effective. Reflective barriers are a good way to block out exterior noise.
is a great way to prevent echoes, dispersing the sound wave in all directions when it hits an irregular surface. Think about how much of a difference carpet or a wall rug can make in a brick or concrete room. This method is very effective for high to medium frequencies, as the vibration strength is less than that of a low frequency sound, and therefore easier to disperse.
performance varies a lot based on the frequency of sound and the absorptive capabilities of the material. A commonly used sound absorber is the underlay in carpet; this works to draw energy from the sound wave and convert it into a tiny amount of heat, creating that ‘deadening’ of a sound. Absorption works best in mid to high frequencies - lower frequency sounds can push through with more force.