Physics 11

Traveling Waves and Sound

Types of WavesSoundThe Doppler Effect

Types of Waves

Mechanical Waves ⇒ waves that involve the motion of a substance (the medium) through which the wave moves

Electromagnetic Waves ⇒ a transverse wave consisting of oscillating electric and magnetic fields (no medium is required)

Matter Waves ⇒ at the atomic level, particles such as electrons and atoms must be treated as waves called matter waves

An Electromagnetic Wave

⇒ An electromagnetic wave is a transverse wave consisting of oscillating electric and magnetic fields.

disturbance

direction of travel

Transverse Wave

Longitudinal Wave

direction of travel

disturbance

Sound is a longitudinal wave

A wave in which the disturbance is parallel to the direction of travel of the wave.

direction of travel

disturbance

⇒ In sound waves, air molecules are disturbed in the samedirection as the wave.

⇒ However, an individual air molecule does not travel with the wave. It only oscillates back and forth.

Sound

⇒ All sound is produced by an object that oscillates back and forth.

Sound

⇒ A speaker produces sound by a diaphragm which oscillates back and forth.

⇒ The diaphragm produces successive regions of increased pressure (condensations) and decreased pressure (rarefactions).

Sound

Regions of increased pressure are called condensations.

Regions of decreased pressure are called rarefactions.

Sound

Sound

condensation

⇒ Your eardrum then detects the differences in air pressure.

Sound

Sound An Electromagnetic Wave

⇒ An electromagnetic wave is a transverse wave consisting of oscillating electric and magnetic fields.

The Visible Spectrum

⇒ Imagine if you can the world view of little creatures who could see only a very tiny portion of the visible spectrum, creatures who are color blind to all other parts. Their world view would be very limited.

⇒ Guess what? We are like those little creatures, in that the spectrum of colors we can see are a tiny portion of the electromagnetic spectrum!

The Electromagnetic Spectrum

⇒ The electromagnetic spectrum consists of electromagnetic waves of all frequencies.

The Electromagnetic Spectrum

⇒ Visible light makes up a very small portion (less than one millionth of 1%) of the measured electromagnetic spectrum.

A stationary sound source

What happens if the sound source is moving?

If the sound source is moving toward an observer, the observer will hear a sound of higher frequency.

If the sound source is moving away from an observer, the observer will hear a sound of lower frequency.

I hear a lower frequency

I hear a higher frequency

⇒ The frequency of sound (pitch) increases when a source moves towards you.

⇒ The frequency of sound (pitch) decreases when a source moves away from you.

The Doppler Effect

⇒When an observer is moving towards a stationary source, the frequency is greater (and the wavelength is smaller).

⇒When an observer is moving away from a stationary source, the frequency is smaller (and the wavelength is bigger).

The Doppler EffectThe Doppler Effect:

Formal definition: the change in frequency or pitch of the sound detected by an observer because the sound source and the observer have different velocities with respect to the medium of sound propagation.

What it means: if either you or the source of sound is moving, you will hear a sound of a different frequency than is being emitted.

An object traveling at the speed of sound

A shock wave is formed!

bow wave ⇒ V-shaped wave produced by an object moving on a liquid surface faster than the wave speed.

Bow Waves and Shock Waves

⇒ A similar thing happens when a plane is traveling faster than the speed of sound. A sonic boom is heard when the shock wave reaches listeners on the ground.

Breaking the Sound Barrier

sin θ = s

vv

v = speed of sound

vs = speed of object

We could figure out the speed of the jet by measuringthe angle of the Mach cone!