Waves

• Mechanical Electromagnetic

• Medium

• Transfer of …

• Pulse Periodic

• Perpendicular Parallel

Mechanical waves / EM waves

Need mediumDo not need medium

Wave features

• Crests / troughs• Wavelength• Frequency• Period• Speed• Amplitude - energy

Wave interactions - INTERFERENCE

Constructive interference

The pulses continue on

Destructive Interference

Pulses continue on

Reflection of a pulse: free boundary

Reflection of a pulse: fixed boundary

Standing Wave

node

nodenode

antinode

antinode

Standing wave

Standing wave

General formula for the SW

If F=ma

• Then the motion of the particles in a string will depend on:

The speed of wave in a string

Things to consider:a) How is the wave transferred?b) Does the velocity of particles

change?c) What does that change depend

on?

Sound intensity

Standing waves

Reflection

Sound - wave• Mechanical• Longitudinal • Spherical

Human Hearing Range

Pitch

Relates to frequency

Is subjective

Doppler Effect

Sound intensity / Decibel level

24

PIntensity

r

12 20 1.0 10 ( / )dB W m

What is the intensity of a sound 60 dB?

Resonance

If an external vibration matches the natural frequency of an object, the vibration will resonate and the object will start vibrating along.

Resonance

If a force is applied in synch (in phase) with the existing vibration, the amplitude of the vibration will increase.

Acoustic body

Electric guitar

Decibels

• Each 10 fold in intensity = 10 dB• What’s 1.0x10-4 W/m2 in dB?• What’s I of 120 dB?

Doppler’s effect

• The switch of the pitch• If the object is approaching, its sound

appears___________________• If the object is moving away, the pitch is

____________________

Harmonics

2n

vf n

L

41,3,5

n

vf n

Ln