SoundChapter 16

April 18, 2011

Create a vocabulary square for –

SoundChapter 16, Sec. 1

The Nature of Sound

Sound is a longitudinal mechanical wave.◦Like all waves, sound begins with a vibration.◦The vibrations cause air particles to vibrate.◦The air particles transfer the vibration from

particle to particle until the vibrations reach your ear drum.

◦The vibrations are then carried from your ear drum through your ear until the vibrations become nerve impulses that are transferred to the brain which allows you to hear the sound.

How Sound Travels

Sound waves not only travel through air particles, but they also travel through many different mediums.

For example, you can hear the sound waves that travel through wood when you knock on a wooden door.

The knock causes the particles in the door to vibrate. The door particles transfer the sound vibrations until they reach the other side of the door.

Then the air next to the door begins to vibrate, and the air particles carry the sound waves to ears that hear the sound.

Interactions of Sound Waves

Sound waves have the same wave interactions as other mechanical waves- Reflection, Diffraction, and Interference.

Reflection:◦Sound waves may reflect when they hit a hard

surface. Sound waves that reflect are called an Echo.

◦Most echoes occur when a material’s surface is very hard and smooth. You probably will not hear an echo if there are soft

materials that absorb most of the waves.

April 19, 2011

Create a vocabulary square for:

ElasticityChapter 16, sec. 1

Diffraction of sound waves

Sound waves also diffract.Sound waves diffract when they bend

around barriers or spread out at the end of narrow paths.◦Example – sounds travel through door ways

and spread out in the room so that everyone in the room can hear them – no matter where they are.

Interference of Sound Waves

Sound waves interfere with other sound waves all the time.

Sound waves can have constructive or destructive interference.

Remember:◦Constructive interference is when two waves meet

and their amplitudes combine to form a wave with a larger amplitude.

◦Destructive interference is when two waves meet and actually cause a decrease in overall amplitude.

The Speed of Sound

The speed of sound depends on the elasticity, density, and temperature of the medium the sound is travelling through.◦Elasticity – the ability of a medium to bounce

back after being disturbed.◦Density – how much matter there is in a given

volume.◦Temperature is well – temperature – how hot or

cold a medium is.

The Speed of Sound

Elasticity The more elastic a medium, the faster

sound travel through it.Solids are more elastic than liquids or

gases so sound travels more quickly through solid mediums than it does in gas or liquid mediums.

The Speed of Sound

DensitySound travels more slowly in dense

materials than it does in less dense materials.

The particles in a more dense material do not move as quickly as the particles of a less dense material.◦This means the particles that are transferring

the sound vibration can not transfer it as quickly.

The Speed of Sound

TemperatureSound travels more slowly at lower

temperatures than it does at higher temperatures.

Particles at lower temperatures move more slowly than particles at higher temperatures which means the particles can’t transfer the sound waves very quickly.

Captain Chuck Yeager

Captain Chuck Yeager was the first person to break the sound barrier on Oct. 14, 1947.

This means that he was the first person to fly a plane faster than the speed of sound.◦The speed of sound at 20 degrees is 343 m/s.◦The speed of sound at 0 degrees is only 330

m/s.Read about ‘Chuck’ on p.545.

April 20, 2011

Create a vocabulary square for:

◦Loudness

Properties of Sound

Sound waves have 3 properties: ◦Loudness◦Pitch◦The Doppler Effect

Loudness

The loudness of sound depends on:◦The amount of energy it takes to make the sound◦The distance from the source of the sound

Energy of a sound source –◦The greater the energy to create the sound – the

louder the sound.◦ If you pull back really hard on a guitar string, when

you release it, the wave in the string will have a larger amplitude than if you barely pulled it.

◦A sound wave with a lot of amplitude will be louder than a wave with a smaller amplitude.

Loudness

Distance from a sound source:◦The closer you are from the source of a sound,

the louder the sound will be.◦For example:

If your friend is whispering and you cannot hear what he or she is saying because the whisper is not loud enough, you can simply move closer to your friend, and her/his words will become louder. You will then be able to hear without your friend having to speak any louder.

Measuring Loudness

Loudness is measured in a unit called decibels (db).◦Sound you can barely hear = 10-db◦Soft music = 30-db◦Whisper = 20-db◦Rustling leaves = 10-db◦Sounds louder than 100-db can damage your

ears, especially when you listen to them for long amounts of time.

Read p.548

Pitch

The pitch of a sound depends on the frequency of the sound wave.◦Sounds with a high frequency have a high pitch.

Example – a soprano sings at a frequency of about 1,000 Hertz. (Hz is the unit for frequency)

◦Sounds with a low frequency have a low pitch. Example – a bass singer will sing at about 80 Hz.

Humans can hear frequencies between 20 and 20,000 Hz.◦Frequencies above 20,000 Hz = ultrasound◦Frequencies below 20 Hz = infrasound

Pitch

How does pitch change? You are able to change the pitch of your voice with you

vocal cords – located in your larynx. As air passes through your windpipe (trachea), it passes

over the vocal cords and causes them to vibrate. You use your throat muscles to tighten and loosen your

vocal cords.◦ Tight cords create a higher frequency which creates a higher

pitch.◦ Loose cords create a lower frequency which creates a lower

pitch. Instruments vary in how they change the pitch based on

the instrument.◦ Guitars use the same technique – tightening and loosening the

strings.

The Doppler Effect

The Doppler effect is a change in frequency of a wave as its source moves.◦This creates the ‘who-who’ sound of sirens as

they move toward and away from you.◦The pitch of the siren on a fire truck sounds the

same to the firefighters in the truck, but different to people on the street as the truck moves toward and past them.

◦When the source moves toward someone, the frequency is higher than it would be if the source were sitting still.

◦See figure 12 on page 550.

Shock Waves

At high speeds, the Doppler Effect causes something called Shock Waves.

The waves in front of the fast moving vehicle (jet) pile up in front of the plane because the plane is moving faster than the sound waves coming from the plane.

The pile-up of sound waves is called the sound barrier.

As the plane moves through the barrier, a shock wave forms as the sound waves overlap.

The shock wave releases huge amounts of energy which people on the ground hear as a loud noise called a sonic boom.