Science of Sound waves and acoustics and the Melbourne Recital Centre by Mialeah.

Science of Sound

waves and acoustics and the Melbourne Recital Centre by Mialeah

What is sound?!

Think of a guitar string. In order to make a noise you pluck the string, so that it vibrates. But why does it make a sound?

When you pluck the guitar string, it moves from one side to the other side, doesn’t it? That is the string vibrating.Air is everywhere right? So, when there is space, the air rushes to fill it. It’s like being in a water tank, water gets in everywhere (except water-tight bottles etc- the air fills them instead).Anyway, you pluck the string, and it bounces back to one side, let’s say the left, leaving space for the air to fill on the right side. So the air stretches to fill the space, or rather rushes to fill the space. This is called Expansion.

soundwaves

But then the string snaps back the other way, the right, and so the air that has stretched now needs to “compress” on the right side, and the left side of the string needs to be filled with air. And so both sides of the string keep getting filled up and pushed back, the air keeps getting stretched and compressed. And like dominoes, the stretching and compressing continues through the air, because the air is pushing on itself. This is how a sound wave is formed. (the animation makes more sense.)It may also help to think of soundwaves like a water wave. When you push water, it makes a wave. It is the same with air. When you push air, you make an air wave. Our ears record the air waves, and we interpret them as sound. That is a soundwave.

Imagine, in this animation, that the red line is the guitar string, and the black dots are particles of air.

soundwaves

Sound cannot travel in a vacuum because there are no particles in which to make a wave.Click on this link to see how; http://www.ltscotland.org.uk/5to14/resources/science/sound/belljar.asp

How do we hear?!

Here is a picture of an ear.

Sound waves are collected by the outer ear, the round flappy bit. It is round and curved so that it will collect soundwaves. If you cup your hands behind your ears, you will be able to hear more clearly, because you are collecting even more soundwaves.

The soundwaves travel into the ear and into the ear canal. They then hit the ear drum, which is much like a normal drum, and the soundwaves bump against it. Some little bones behind the ear drum move as the ear drum vibrates, so that they carry the vibration patterns to a tube filled with fluid called the cochlea. In the cochlea, there are several tiny hairs called cilia which are attached to auditory nerves. The nerves pick up the cilia vibrations in the fluid and send signals to the brain.

the ear

Why do sounds sound different?!

pitches and volume

The way a sound sounds depends on how fast the waves are and how big they are. These are called Frequency and Amplitude. Frequency is the measure of how many cycles/waves there are in a certain period of time, and Amplitude is how big the waves are.

The louder a sound, the bigger the wave and the bigger the amplitude. The higher the sound, the more waves/cycles in a period of time.

Click on this link to see what happens when adjusting the volume (amplitude) and adjusting the pitch (frequency); http://www.ltscotland.org.uk/5to14/resources/science/oscilloscope.asp

The diagram below shows a standing wave. It is a standing wave because it reflects back on itself like a circle.

This slinky can be an example of how waves are made. It stretches and contracts, just like air does when making a sound wave.

pitches and volume

On this diagram, are soundwaves. The bottom wave has more waves in a certain period of time, and so is high pitched. The wave above that is medium pitched and the one above that wave is lower pitched, because it has less waves in a certain period of time. They are all pure tones.

The top wave is a noise, a combination of the three below. When waves interact they either plus together or take away from each other, depending whether they are in phase or not. When they are in phase, they will plus to make a louder sound, or have a higher amplitude. When they are not in phase, they will take away from each other and cancel out or make no sound at all. They deaden.

Anechoic chambers?!

Here is a picture of an Anechoic Chamber.

anechoic chambers

Anechoic chambers are one of the quietest places on Earth. In space you can’t hear anything because there is no medium to carry the soundwaves. The next closest thing to the quietest place on earth is outside in an open plane. It is very quiet there because there is no object that can reflect the soundwave back to you. In this case soundwaves behave similar to light.

When you produce sound, you know you have produced it because it is reflected back to you and your ears pick it up. Having no reflection is the essential aspect of an anechoic chamber.

Soundwaves aren’t necessarily all absorbed, but also reflected onto itself, so that it cancels itself out. Because of the angles in the wall, the soundwaves reflect off in a particular angle to the wall, and travel in that direction. Because there are many other triangular prisms in the wall, another soundwave may hit the triangle side above and be reflected down onto the other soundwave. The soundwaves then cancel each other out. In a sense the soundwaves are trapped in the wall, bouncing back and forth, until they hit another soundwave and cancel out.

anechoic chambers

This picture is a close up of the wall of an anechoic chamber. As you can see, the foam bits start with a small point and get larger as they go out. This is to ensure that all sizes of soundwaves (frequencies) can be absorbed by the wall. The material is holy which also helps to absorb the sound energy. The idea is to drag most of the sound away from the room and to reflect the rest back on itself, flipped around, as you can see in the explanation below.

Here is an example of different waves against each other. They are in phase, (they go up and down at the same time) so they plus together to make bigger waves.

This one has some waves in phase and some out of phase. When waves are out of phase, they go up and down at opposite times. Because of that, they cancel each other out. It’s like maths, you plus nine, you take away nine, you get 0.

This one has all the waves out of phase. This means there is no sound at all.

The Melbourne Recital Centre?!

the walls

The Melbourne Recital Centre has been specially designed so that music from the stage will be heard perfectly from all the seats, even from the balconies.

In order to do this, they have specially designed walls that reflect or absorb the sound in a predicted way.

As you can see in the close up of the wall, there are all sorts of angles which soundwaves can be reflected in from the squares in the wall. They have all been carefully calculated so that the soundwaves are reflected and spread out all over the audience. But the trouble is to avoid cancelling sounds out, like the slide before. The walls have been carefully calculated so that the waves aren’t out of phase, but rather in phase and plus to make a higher amplitude and spread at angles to fill the whole theatre.

outside the building

Sounds are made by vibrations. Outside the building there are lots of noisy sounds;

trams, cars, buses, people talking, building equipment, etc. So the building needs to block out those sounds. How does it do that?

NOISE VS MELBOURNE RECITAL CENTREWell, sound can only be carried through a medium.

The mediums the building is facing are air, and the ground. Yes! Sound can travel through the ground.

Sit on the ground and hit the ground with your palms. Can you feel the vibrations?

Anyway, so to deal with the air, the walls must reflect soundwaves. The concrete walls are thick and dense, so most of the sound waves are reflected and only a few pass through which eventually die out anyway because the wall is so dense, so it takes longer to pass through, therefore giving more time for the soundwaves to die out. So that’s the air dealt with.

Now, the ground. Did you know that there are springs under the building? So the sound travels through the ground, or the vibrations travel through the ground and are absorbed by the springs under the building, so that the vibrations don’t travel to the building, but all the sound energy is collected by the springs.

MELBOURNE RECITAL CENTRE – 1NOISE - NIL

inside the theatre

These pictures show the inside of the theatre. The grooves on the walls aren’t just pretty, they also help reflect or direct the soundwaves in the right direction.

As you can see, the room is shaped so that the soundwaves are reflected up and onto the audience. No sound escapes and no sounds cancel out. The audience can hear everything.