Mechanical Waves vs. Electromagnetic Waves

• Mechanical – require a medium to travel– ex: water, sound, rope

• Electromagnetic – can travel through space– ex: light, microwaves, radio waves

The Science of Sound

Tuning forks create sound by

compressing the air around them.

These compressions vibrate air molecules as they travel away

from the fork.

Sound requires molecules to vibrate.

No molecules no sound.

What’s wrong with this?

So spaceships are silent in

space.

Sound moves through air as

compressed air molecules.

Compressions enter our ears, move things

around, and we hear.

There are several parts to the human ear.

Ear Lobe

Ear Hole

The ear lobe serves as a reflector to bounce sound waves into the ear hole.

The ear hole is how compressions enter the ear.

The ear canal directs sound to the inner ear.

The ear drum is a thin tissue. Compressions cause it to vibrate.

The hammer is connected to the ear drum. When the drum moves, the hammer moves.

When the hammer moves, it bumps the anvil.

The anvil is connected to the stirrup so it vibrates with the anvil.

The stirrup is then connected to the cochlea which turns vibrations into a signal for your brain.

Does sound travel faster in liquids and solids or in gases?

In general, liquids and solids

Can sound move in a vacuum?

No way…

Sound travels faster in warm air than in

cooler air. On average, it moves at

about 343 m/s.

That’s about as fast as a jet plane.

Lets say you come across a dark, deep well and you

want to know how deep it is. What would you do to find

this out?

With a thermometer (to know the temp.

of the air) and a stop watch you can

calculate how far your echo travels.

Let’s say it’s an average

temperature day, and it takes 3

seconds for your echo to return. How

deep is the well?

Speed of sound = 343 m/s

Time = 3 s

d = vt

So the sound travels for…

1,029 m

Is that how deep the well is?

It’s 1,029 m to the well and back! So the

distance to the well is 515 m!

Two things we always notice about

sound waves are their amplitude and

their frequency.

The amplitude of a sound wave, or the amount of energy it

has, is also known as its volume.

We can say that sounds with high

volume are loud, and sounds with low

volume are quiet.

Or that sounds with high volume have a

bigger amplitude than sounds with low

volume.

LOUDNESS

Amplitude

Too much amplitude

, or volume is

a dangerous

thing.

We measure volume in an SI unit called the

decibel (dB).

About 35 – 40 dB is an

average speaking volume.

About 10 dB is a whisper, and anything over 80 dB, like a dance club or gun shot, can

cause hearing damage.

The other thing we notice about sound waves is

their frequency. Musicians call frequency

pitch, or notes.

When the frequency increases, the pitch, or

note, gets higher.

When the frequency decreases, the pitch, or

note, gets lower.

PITCH

Frequency

Sound Test

An interesting thing happens to the frequency

of a sound when the source of the sound is

moving.

What do you think is making this sound? Why

do you think the frequency is changing?

The compressions in the direction the source is

moving bunch up.

Meaning that the wavelength is less, so the

frequency, or pitch, is higher.

The waves behind get more spread out, making

the frequency lower.

Describe the motion of these cars.

This is called the Doppler Effect.

It looks like this.

If the sound waves get more bunched up when the source moves faster, is it possible to make it so that they are all on top of

each other?

Or maybe even to move so fast the they can’t

keep up?

Yes. Jets that fly faster than 350 m/s travel

faster than the sound they produce. So we say

they ‘break the sound barrier’.

It looks like this.

The Doppler Effect

s

dsd vv

vvff

Key Points

1. ALWAYS establish a coordinate system.

2. Make sure that the positive direction is from the source to the detector.

A train moving toward a sound detector at 31.0 m/s blows a 305 Hz whistle. What frequency is detected on each of the following?

A stationary train

A train moving toward the first train at 21.0 m/s

Now the train is moving away from the detector at 31.0 m/s and blows the same 305 Hz whistle. What frequency is now detected on each of the following?

A stationary train

A train moving away the first train at 21.0 m/s

Example:• Jordan, a trumpet player, sounds C

above middle C (524 Hz) while traveling in a convertible at 24.6 m/s. If the car is coming toward you, what frequency would you hear? Assume the speed of sound is 343 m/s. (Start with a picture!)

• Answer: 564 Hz• Make sure you think logically about

what the magnitude of the frequency should be.