1. (used with a singular verb) Physics. the branch of physics that deals with sound and sound waves.
2. (used with a plural verb) the qualities or characteristics of a room, auditorium, stadium, etc., that determine the audibility or fidelity of sounds in it.
As you can see from the above description, acoustics can be described as the behaviors of sound in itself as well as the behaviors of sound in an environment like a room. Sound travels at different speeds depending on what material it is being transmitted through and what state of condition that material is in. For example, sound travels through the air but at different speeds depending on the altitude, humidity, temperature, gasses present and so on. So if you were to ask what the speed of sound is through air, you will get different answers based on where it is. You will receive different measurements of speed of sound at 30,000 feet in the summer vs. at sea level in Alaska, 1,000 feet in a rain forest or at sea level in San Francisco where a large concentration of carbon dioxide emissions exist. Sound also travels through other mediums such as metal, wood, glass, plastic, or any other materials surrounding you. Sounds travels through these mediums and again, are measured in speed differently based on different materials, their conditions, strengths and properties.
In simple terms, sound travels by vibrating or impacting molecules it comes in contact with and can easily pass from one type of molecule to another. The more molecules that are capable being vibrated, the more those molecules will pass on the vibration to other mediums. This is why water transmits sound faster than air. It is also why walls and floors pass on sound much better than a mattress or pillow. Sound waves also continue to travel by bouncing off of hard surfaces such as walls, ceilings, hard surface flooring and so on. As an example of sound traveling through different mediums, you hear a loud motorcycle starting up outside of your home. The combustion of the motorcycles engine is caused by gasoline exploding and sending it’s exhaust out of the tailpipe. Well the sound of explosions in the engine also travel out of the tailpipe and begin impacting air molecules. Now the air is sending those sound waves towards your house and hits your windows. Your windows are now vibrating and the sound is then continuing on into your home bouncing off of walls, ceilings, floors, objects in the room and into your ears. So in this entire process, the sound that originated inside of the motorcycles engine has passed through metal and other materials the muffler is comprised of, then passed through the air, then passed through the glass of your window, then into your environment, all from one source of the sound. To get an idea of just how powerful sound waves are, you have probably stood next to a loud motorcycle and can feel the vibrations of the motor on your chest or other parts of your body. Those are sound waves hitting you and your body vibrating from the air that just hit your chest with each rumble of the motor.
Because sound travels through walls, ceilings, floors and other surfaces as well as air, you will hear sound in virtually any environment. And because sound can travel through different materials, it is difficult to stop it from travelling to areas you wish to have no noise. You don’t want total silence either, since that is almost as bad as having a noisy environment. Since soundproofing is such a difficult process, it is much easier to treat the sound by reducing the amount or level of sound entering or reflecting in the environment you wish to reduce the noise pollution. So when people speak of soundproofing, this often a misnomer since they are more likely talking about sound treatment and reducing noise to a comfortable level.