Register to reply 
Fourier series 
Share this thread: 
#1
Apr212, 07:27 AM

P: 36

In fourier series we have small waves on the top of big waves (the function seems like that),
but the small waves do not have the same amplitude. Does somebody know how to get a function with waves and small waves on the top but with the same amplitude. 


#2
Apr212, 01:23 PM

Emeritus
Sci Advisor
PF Gold
P: 4,500

What is the definition of a small wave if it doesn't have anything to do with amplitude?



#3
Apr212, 02:22 PM

P: 36

Can you see now, small waves on the top of big wave are not the same (equal amplitude) {click on the picture to see it better} 


#4
Apr312, 08:45 AM

PF Gold
P: 302

Fourier series
If you're talking about the changing amplitude of the Fourier Series approximation then the answer is that you really can't. Due to the Gibbs Phenomenon, you'll have an overshoot at any discontinuity, of which the amplitude doesn't diminish.



#5
Apr312, 09:57 AM

P: 36

Do you know how to get a function (any kind of function) with "small" waves on the top of "big" waves, but for the same amplitude of all small waves?



#6
Apr312, 11:26 AM

Emeritus
Sci Advisor
PF Gold
P: 4,500

Is small referring to the wavelength of the wave? Just take something like cos(x)+cos(100x)
http://www.wolframalpha.com/input/?i...Bcos%28100x%29 Why would you want such a thing? 


#7
Apr312, 01:02 PM

P: 36

I mean something like that:
Scanned at 3.4.2012 1958.pdf Can you get a function something like that? (rotate the picture) 


#8
Apr312, 01:59 PM

Math
Emeritus
Sci Advisor
Thanks
PF Gold
P: 39,310

The Fourier series is of the form
[tex]\sum A_n cos(nx)+ B_n sin(nx)[/tex] It looks to me like your series happens to have only two nonzero terms, one with a period of about 6 and amplitude 3000, the other with period about .6 and amplitude about 100. In other words, something like [tex]3000 cos(2\pi x/60)+ 100 cos(10(2\pi/60))[/tex] 


#9
Apr412, 12:31 PM

P: 36

do you have better idea? 


#10
Apr412, 01:23 PM

Emeritus
Sci Advisor
PF Gold
P: 4,500

The 60s should be 6s to get a period of 6 (although changing the period of both won't change the local structure), and he forgot an x in the second cosine
Here's the reworked version graphed in wolfram alpha http://www.wolframalpha.com/input/?i...%3D0+to+x%3D12 


#11
Apr1412, 10:47 AM

P: 36

I found a function: sin(1cos(x))
But there are only 2 "small" waves on every wave (put the function in wolfram) How to get 3, 4, 5, ... or n "small" waves? 


Register to reply 
Related Discussions  
When to use fourier integral instead of fourier series expansion  Classical Physics  2  
Need help to find application of the Fourier series and Fourier Transforms!  Introductory Physics Homework  8  
Complex Fourier Series & Full Fourier Series  Calculus & Beyond Homework  5  
What is the point of Fourier Series if you can do the Fourier Transform?  General Math  9  
Fourier Series / Fourier Transform Question  Electrical Engineering  6 