Beat frequency question (1 Viewer)

[erucibon]

The question asks:
Consider the diagram which shows one soundwave (drawn as a transverse wave X) and the resultant wave, Z, formed as this wave is superimposed on another, lower pitched wave (not shown). The frequency of X is 120Hz. What is the beat frequency of the resultant wave?
(The answer is 118Hz)
The question gives a wave graph of X and Z but i'm not sure how to use this to do the question.
Any help is appreciated
Thanks

 

[Drdusk]

Can you attach the diagram?

 

[erucibon]

Can you attach the diagram?

 

[Drdusk]

View attachment 26884

Hi sorry for the late reply, been at uni all day.
There's two possibilities here, either you've accidentally looked at the wrong answer or the answer is just wrong. We know that



If 118 Hz is the correct beat frequency, then it would mean Hz. This is impossible. Beats only occur when the two frequencies of waves interfering are very close together. This has a difference of 118 Hz!, impossible to get beats with a difference that large.

I think its a typo, because 118 should instead be the frequency of Y, so beats can be observed.

And just to make sure, I went into my Lecture Notes for Physics, and yes it says this phenomenon only happens when the two waves have slightly different frequencies..

 

[erucibon]

Hi sorry for the late reply, been at uni all day.
There's two possibilities here, either you've accidentally looked at the wrong answer or the answer is just wrong. We know that



If 118 Hz is the correct beat frequency, then it would mean Hz. This is impossible. Beats only occur when the two frequencies of waves interfering are very close together. This has a difference of 118 Hz!, impossible to get beats with a difference that large.

I think its a typo, because 118 should instead be the frequency of Y, so beats can be observed.

And just to make sure, I went into my Lecture Notes for Physics, and yes it says this phenomenon only happens when the two waves have slightly different frequencies..

Sorry, I read the answer wrong. It is 2Hz, but how do you get this value?

 

[Drdusk]

Sorry, I read the answer wrong. It is 2Hz, but how do you get this value?

Wait I'm so confused, then what was 118Hz given for?

Was that given as the frequency of Y? If that's the case just use the beat formula I gave you above.

 

[jazz519]

Maybe measure the axes and make some type of scale.

Because it says 200 hz frequency for the first graph. Recall that period is inverse of frequency

So T = 1/f = 1/200 seconds

Then think about definition of period (one wavelength on a time axes)

Then I guess that tells you that is 1/200 seconds and then maybe measure the axes for the beats graph period and make some type of scale between them

That’s just a suggestion as to where the 118 maybe came

 

[Drdusk]

Maybe measure the axes and make some type of scale.

Because it says 200 hz frequency for the first graph. Recall that period is inverse of frequency

So T = 1/f = 1/200 seconds

Then think about definition of period (one wavelength on a time axes)

Then I guess that tells you that is 1/200 seconds and then maybe measure the axes for the beats graph period and make some type of scale between them

That’s just a suggestion as to where the 118 maybe came

Yeah I thought of that too, because it doesn't say "not to scale", but since he doesn't know how to do the question how did he end up with the number 118Hz anyway.. because if 2Hz is the actual answer, then 118Hz should be the frequency of Y. So Idk if its just a coincidence or what. You get me?

 

[jazz519]

Yeah I thought of that too, because it doesn't say "not to scale", but since he doesn't know how to do the question how did he end up with the number 118Hz anyway.. because if 2Hz is the actual answer, then 118Hz should be the frequency of Y. So Idk if its just a coincidence or what. You get me?

Yeah I get what you mean

 

[erucibon]

Yeah I thought of that too, because it doesn't say "not to scale", but since he doesn't know how to do the question how did he end up with the number 118Hz anyway.. because if 2Hz is the actual answer, then 118Hz should be the frequency of Y. So Idk if its just a coincidence or what. You get me?

The question after that asks to calculate the frequency of Y, which is 118Hz

 

[Drdusk]

The question after that asks to calculate the frequency of Y, which is 118Hz

Ah okay then you would just do what Jazz said. In that case we know . Now period is defined as the Time between successive Wavelengths. We can use this to develop a scale for the two waves. Using that definition for period and your ruler by measuring the wavelength of X in cm, you can develop a scale for the two graphs, something like Then just measure the wavelength in Cm for the Beat Graph and using that scale find the Period of the beat frequency wave. Then use to find its frequency.

I must say this is a very annoying and crap question. Like seriously measuring the actual scale...