Homework Help Thread ;D

AndThen?

AndThen?

pull the trigger.
Jun 11, 2007
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Okay, so I think this thread died in the crash and I could really use it right now.

Uhmm, here's my question:

Two small loudspeakers, A and B, are driven by the same amplifier (X) and emit pure sinusoidal waves in phase, The speakers are 3.00m from each other and point P is 4.00m from a line joining the two speakers as shown in the diagram below. Assume the speed of sound is 350m/s.


A ----2.00m---X--1.00m--B
....................|
....................|
.................4.00 m
....................|
....................|
....................P

Determine the frequencies that
i) constructive interference occurs at point P, and
ii) destructive interference occurs at point P.

This question's due in one and a half hours, so I doubt I'll get a reply, but this is revision stuff and it annoys me that I can't remember how to do it. So yeah, help would be great thankssss...


Btw, this thread is for anyone who needs homework help, not just me.
DISCLAIMER: Remember, this is "Homework Help Thread ;D" not "DO MY HOMMWORKZ FOR ME NAO THRED". Thanks for appreciating the difference. ;)
 
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What actually is the question?

This just seems like a description of the scenario to me:

Dave said:
Two small loudspeakers, A and B, are driven by the same amplifier (X) and emit pure sinusoidal waves in phase, The speakers are 3.00m from each other and point P is 4.00m from a line joining the two speakers as shown in the diagram below. Assume the speed of sound is 350m/s.
Click to expand...

It could be that I haven't done sound for a little while and I'm just not seeing it.

Edit: Looks like you just added the question in, I'll have a look. :p
 
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i phuckin hate u ths thred was my idea and i wanted the credit! ffs i baggsed in the thnking thread:mad5:

;)

For maths whip out Lew or the King;)
 
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  • #4
Physics, Foxy. ;D

And yeah Pete, just edited it in there... forgot at first.. haha thought no one would notice. xD
 
AndThen? said:
Physics, Foxy. ;D

And yeah Pete, just edited it in there... forgot at first.. haha thought no one would notice. xD
Click to expand...

I saw your diagram and that's all I saw, I jumped to con- ah forget it:frown2:
 
First I'd work out the path difference.

Distance from A to P = Sqrt (2^2 + 4^2) ≈ 4.5 m

Distance from B to P = Sqrt (1 + 16) ≈ 4.1 m

These seem like untidy numbers, so could be doing it totally wrong.

Thus path difference ≈ 0.4 m.

You'll have constructive interference when the path difference is a multiple of the wavelength or zero.

So you'll have constructive interference when λ = 0.4 m, 0.2 m, 0.1333 m, 0.1 m etc. [Using your speed of sound.]

Thus constructive frequencies are: f = v / λ : 875 Hz, 1750 Hz, 2625 Hz, 3500Hz etc.

I'm not sure if this is right, I haven't done sound for a while. What do you think?

If this is right, you'll have the destructive interference when the path difference is a multiple of λ/2.
 
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  • Thread Starter
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  • #9
Aw, mannn... Anyway, there's plenty of physics to go around! I really don't remember how to do sound-in-a-tube questions, and they always appear in the exams.

A tube that is open at both ends is placed in a deep tank of water. A tuning fork of frequency 256 Hz is sounded continuously above the tube. The tube is slowly raised out of the water and, at a position of the tube, a maximum loudness of sound is heard.

The tube is gradually raised from a position of maximum loudness until the next position of maximum loudness is heard. The length of the tube above the water surface is increased by 65.0 cm. Calculate the speed of sound in the tube.

Care to help? I'd like to know how to do it, not just the answer pl0x. ;)

EDIT:
Petey, that looks alright to me, but it says the lowest frequency that the destructive/constructive interference occurs... I know I didn't say that earlier but neither did the quiz.
 
Spotted a mistake in my answer post, all fixed up now. The correct equation is: f = v / λ.
Dave said:
Petey, that looks alright to me, but it says the lowest frequency that the destructive/constructive interference occurs... I know I didn't say that earlier but neither did the quiz.
Click to expand...
Well that's ok, the lowest frequency refers to the longest wavelength. So just use the path difference as the wavelength. So you'll get 875 Hz as the lowest frequency. Are you getting what I'm doing?

Want to try ii] yourself?

Disclaimer: I may be giving you the wrong answers, it happens occasionally. ;)
 
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  • Thread Starter
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  • #11
Okee, well I got an answer, I guess we'll wait 'til tomorrow to see if you've led me astray. :p
 
So my answer for i] is 875 Hz.

My answer for ii] is {if you want to check yours} :

λ = 0.8 m, so f = v / λ = 350/0.8 = 437.5 Hz

I can't really be assed with your other question tonight, might have a look at it tomorrow.
 
  • Thread Starter
  • Thread starter
  • #13
Yeah, I got the same for ii)... don't worry about the other one, we're most likely going through it all tomorrow anyway. Unless you're keen. Whatever floats your boat.
 
Edit: Problem solved, genius aunty came online and BAM. Sorted =]
 
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