Physics Grade 11

Unit 4: Waves and Sound

Note 4: Interference, Superposition


Reference: Chapter 9.1, 9.2 (part)


Even though sound waves are longitudinal, we often represent them as transverse waves to make them easier to study.



Interference is the result of two or more waves meeting each other at a particular point in space at the same time.

Constructive vs. Destructive (addressed earlier in Note 1)

Principle of Superposition: the net wave that forms at the point of interference has an amplitude that is equal to the sum of all the amplitudes that come together at that point.

Example: Draw the resultant wave that would be formed by wave A and wave B meeting.









Sound Waves & Matter:

Like all waves, when sound waves encounter a new medium they may be

1) Absorbed

2) Transmitted

3) Reflected



When sound passes from one medium to another it either speeds up or slows down. This changes the wavelength but the frequency remains the same.



Reflection: Demo with Bell Wave Machine

Pulses reflected from a fixed end are inverted (i.e. a crest would be reflected as a trough and vice versa). Phase shifted by 1/2 wavelength.

Pulses reflected from a free end are NOT inverted.

Students draw both cases in their notes.

When a wave enters a new medium the speed and wavelength change, and partial reflection occurs. For the partially reflected wave no speed change occurs (since its not changing medium), only the partially transmitted wave has the change.

Standing Waves
Superposition Questions

Pg 419 Practice Questions #1

Pg 419 #1,2,3