David has taught Honors Physics, AP Physics, IB Physics and general science courses. He has a Masters in Education, and a Bachelors in Physics.
What is Sound and Matter?
Matter is all around us. And so is sound. It's hard to ever go anywhere that's truly in silence. Despite these things surrounding you every day, do you really know what they are?
Matter is the stuff in the universe: the physical material which takes up space and has mass. So what is mass? Mass is the measure of how much of that stuff you have, and it can be figured out by putting an object on a scale.
Sound is what happens when you transmit vibrations of energy through that matter. Sound is made up of wave vibrations. When you talk aloud, the motion of your vocal cords causes the air to vibrate. Air is made up of molecules, and as those air molecules vibrate, they hit each other. Because of these collisions between air molecules, a sound will spread out from one side of the room to the other very quickly.
You might find this hard to believe, but that's all sound is: vibrations and collisions of air molecules spreading around a space. When those vibrations reach the inside of your ear, they make your ear drum vibrate, and that allows our brains to detect and make sense of the sound. That's how we hear things!
So, sound moves through some matter in the form of vibrations, but one sound wave can also collide and interact with other matter. What happens when one of these vibrations in the air reaches a solid surface? Let's talk about the two possibilities: absorption and reflection.
Absorption of Sound
When a sound vibration reaches a new material: an object, one thing that can happen is that the sound can be absorbed. This is often called sound absorption. Sometimes this is a good thing, and sometimes it's bad. If you're recording a new hit single in a recording studio, this absorption is probably good.
When you sing into a microphone in one of these studios, the walls are usually covered in soft, rubbery shapes. These materials and shapes are designed specially to 'soak up' the sound because people want to record a pure sound with the microphone. They don't want the sound to bounce off the walls first and then go into the microphone, because that will sound muddier and not as nice. Using the concept of sound absorption is one of the ways music artists create such pristine, clear recordings.
Lower frequencies tend to go through materials more easily without being absorbed, which is why it's so easy to hear when a neighbor is playing music with a heavy beat, or a deep man's voice more than a higher woman's voice.
Echoes and Reflection of Sound
Absorption can be useful, but sometimes absorption isn't so good. You wouldn't want to create a space with a lot of absorption in a music hall, for example, because without a full studio to mix layers of sound together, it would sound weird and unnatural. The opposite of absorption, called sound reflection can be a good thing because it can make a sound come across as natural or rich. Many music halls and performance venues are designed to let the sound bounce off the walls and fill the hall. Most of the time, in our everyday lives, sound bounces all over the place. This sounds very natural to us humans, and is the basis for acoustics, or the quality of sound in a particular room.
There are some more extreme examples of sound reflection, and echoes are about as extreme as it gets. If you go into a deep cave, and shout, 'Hello!' then a second later you'll hear a 'Hello!' coming back. There's nobody else there, though. It's just your voice - the sound waves you produced - bouncing (or reflecting) off the walls and coming back to your ear. In a cave, there is a lot of sound reflection, and very little sound absorption. Bats use this to their advantage, and can see in the dark by sensing how the sound waves they produce reflect off the cave's walls. This is called echolocation.
Matter is the stuff in the universe, and sound is a vibration of energy moving through that stuff. When we talk or make a sound, we cause the air molecules to vibrate and hit each other. These vibrations eventually reach our ears and vibrate our ear drums, allowing us to hear a sound and make sense of it in our brains.
When one of these matter vibrations reaches a new material, like a solid wall, two things can happen: reflection or absorption. If the sound is absorbed, the vibration peters out and you don't hear it again. This can be useful in recording studios. If the sound reflects strongly, you might hear echoes or after-sounds. In a music hall, that can make the sound seem rich and natural. In a cave, it can cause echoes.
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