2. SUMMARY
• Constructive and destructive interference
• Spherical waves oscillate in time and space
• s(r,t)= sm(r)cos(kr-wt+ϕ)

3. CONSTRUCTIVE
• Amplitudes add
• When the wavelengths are a multiple of an integer they are in phase
• Δd=pλ where p is an interger

4. DESTRUCTIVE
• When sources are out of phase destructive interference occurs
• Path difference is half of an interger multiple.
• Δd=(n+1/2)λ where n is an interger

5. APPLICATION
Concerts and Theatres
Must be designed in order to reduce destructive interference.
Question:
What are some ways that you could reduce the destructive interference?

6. ANSWER
Some examples include:
Design of walls and ceilings
Why? If the material used can absorb the sound then less sound will be
reflected. This means less opportunity for destructive interference.

7. APPLICATION
Question:
How do noise cancelling headphones work?
Answer:
Through computer programming the headphones create a noise that is half
a phase out of the surrounding noise resulting in destructive
interference.