# The Naked Scientists Forum

### Author Topic: Exam question help  (Read 2744 times)

#### thegers91

• Jr. Member
• Posts: 19
##### Exam question help
« on: 21/05/2008 09:33:57 »
Could someone help me with this physics question as my exam is very soon

Thanks

#### JP

• Neilep Level Member
• Posts: 3366
• Thanked: 2 times
##### Exam question help
« Reply #1 on: 21/05/2008 18:33:08 »
I'm going to assume you've covered waves and constructive/destructive interference (you should look over your textbook/lecture notes to refresh yourself if those aren't familiar to you).  You also need to know that intensity of sound is proportional to amplitude of the sound wave at that point.

In the example above, the sound from each speaker travels a different distance to get to point y, as indicated by the dashed lines.  Sound interferes destructively (has an amplitude/intensity minimum) when one wave has traveled n+1/2 wavelengths further than the other (n=0,1,2,...).

Hopefully that gives you enough to get started.

#### turnipsock

• Hero Member
• Posts: 586
• Beekeeper to the unsuspecting
##### Exam question help
« Reply #2 on: 21/05/2008 21:18:21 »
Is it B?

#### thegers91

• Jr. Member
• Posts: 19
##### Exam question help
« Reply #3 on: 23/05/2008 10:21:39 »
Yes it's B, but I can't come to that answer

#### graham.d

• Neilep Level Member
• Posts: 2208
##### Exam question help
« Reply #4 on: 23/05/2008 13:27:30 »
The difference in the two paths shown is .08m so you will get cancellation of the signal when this is equal to half the wavelength. Hence 0.16m wavelength will be the answer. For longer wavelengths the cancellation may occur at a larger value of XY but cannot be at a shorter XY because you have not got to the half wavelength difference value. For a wavelength of 0.08m there will be a minimum for a smaller XY value but the question asks for what wavelength will the first minimum occur at point Y. At point Y with 0.08m wavelength there is a whole wavelength difference so it will be a maximum in this case.

#### The Naked Scientists Forum

##### Exam question help
« Reply #4 on: 23/05/2008 13:27:30 »