Sound and Noise
Generation Propagation and Reduction
What is Sound and How Do We Hear It?
Noise is simply unwanted sound.
So first we might want to look at sound and how it is generated.
Sound is actually changing air pressure. That is, a generator of sound must move the air back and forth, creating ‘‘sound waves’’ that can be heard (presumably by humans).
One way to picture this is thinking of a large concert bass drum. When the mallet strikes the drum head, the head begins to move back and forth, or vibrate. As it does, it ‘‘pulls’’ some air in front of it towards itself, and then as the head moves out it pushes that same air away from it again. In doing so, the drum head creates small changes in air pressure that move (or propagate) through the air. These ripples in the air move out in all directions (acoustically, though not always equally), eventually striking our eardrum.
The human eardrum is like a very small drum head itself that can be moved by these minor changes in air pressure. As it moves back and forth, we perceive sound.
In the simple example above, we can imagine the bass drum head moving in and out, and we might be able to imagine the waves in the air moving towards us, eventually striking our own ear drum, which moves in harmony with the waves, and our brain ‘‘hears’’ the sound the bass drum made.
In real life however, it can get somewhat more complex, even though following the same basic principles.
