1. PHY138 – Waves, Lecture 4 Today’s overview
Sound as a pressure wave
The Doppler Effect
The Principle of Superposition
Constructive and Destructive Wave Interference

2. Reading Assignment
Please read the following from Serway and Jewett before class on Monday: Chapter 14, up to and including Section 14.6
A assignment is due this Friday at 5:00 PM. It is based on Chapter 13 material: mechanical waves and sound.

3. Last day’s lecture: some comments
Regarding quiz on swings: by standing up on a swing, you raise your centre of mass closer to the pivot point, thereby reducing L.
This increases the natural angular frequency, ω.
This reduces the period, T.
Medical ultrasound imagers use sound with frequencies in the range f =1-20 MHz.

4. Sound Waves
Sound is produced by an oscillating surface in a medium (ie a speaker in air)
Sound is a pressure wave:
Sound is also a longitudinal displacement wave of individual molecules in the medium :

5. Doppler Effect for sound
Creates a change in pitch or frequency
All speeds are measured relative to the medium (ie air)
f goes up if source and observer are approaching each other
f goes down if source and observer are moving away from each other.

6. Chapter 14: Principle of Superposition
If two or more waves combine at a given point, the resulting disturbance is the sum of the disturbances of the individual waves.
Two traveling waves can pass through each other without being destroyed or even altered.

7. Quiz
Two traveling sinusoidal waves passing the same point, each of wavelength λ, result in total destructive interference if the waves
1. are in phase.
2. are π/2 out of phase.
3. are π out of phase.
4. are traveling in opposite directions.
5. None of the above

8. Wave Interference
Two waves moving in the same direction with the same amplitude and same frequency form a new wave with amplitude 2Acos(ø/2), where ø is their phase difference.