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Summing sinusoids of different frequency

  1. Sep 17, 2013 #1
    I'm curious if there is a way to combine sinusoids of multiple frequency into a single sinusoid?

    For example, I'm looking for a way to combine:

    Acos(2παt + φ) + Bcos(2πβt + χ) where A, α, φ, B, β, χ are scalar constants and α & β represent the frequency.

    I'd even be interested in a less complex solution to:
    cos(2παt) + cos(2πβt)
    if this is much simpler than the general form above.

    I found an incomplete explanation on here that does a transform and then recombines using complex numbers ending up with a cosine with an arctangent inside.

    However, explanations of the process are probably better than just a discrete example alone.
  2. jcsd
  3. Sep 17, 2013 #2


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    cos(a+ b)= cos(a)cos(b)- sin(a)sin(b)

    [itex]cos(2\pi\alpha t+\phi)= cos(2\pi\alpha t)cos(\phi)- sin(2\pi\alpha t)sin(\phi)[/itex] and
    [itex]cos(2\pi\beta t+\chi)= cos(2\pi\beta t)cos(\chi)- sin(2\pi\beta t)sin(\chi)[/itex]
  4. Sep 17, 2013 #3
    I don't think trig identities are going to be much help.

    Acos(2παt + φ) + Bcos(2πβt + χ) = A(cos(2παt)cos(φ) - sin(2παt)sin(φ)) + B(cos(2πβt)cos(χ) - sin(2πβt)sin(χ))

    Which has turned a sum of 2 sinusoids into 4 products and 3 summing operations on 8 sinusoids. This is kind of the opposite direction of where I was looking to go. I'm trying to figure out if theres a transformation I can use to get this all inside of a single sinusoid function, even if there is a trig function inside the sinusoid.
  5. Sep 17, 2013 #4


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    Summing sines of different frequencies can yield very NON sinusoidal shapes. Why do you expect to use a single sine function to describe non sine shaped functions?

    i.e. The sum of all odd harmonics yields a square wave.
  6. Sep 17, 2013 #5
    I don't expect to, I asked if it was possible.

    I know it is possible to write any repeating function as a sum of sinusoids (Fourier?) but I was just curious if there is a process to combine a sum of 2 into 1. I'm mainly interested for representing purposes, I just wanted to see if there was a way to represent the sum mathematically without both sinusoids needing to be shown. Maybe it could be represented as a series?
  7. Sep 18, 2013 #6


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    Functions that are called linearly independent cannot be summed into a single one.

    Sines of different frequencies are no different in this regard than trying to sum together, say, the functions Bz and Bz^2 into a SINGLE power in z.
  8. Dec 5, 2015 #7
    What about the period of the sum of sinusoids of different periods? What would that be?
  9. Dec 6, 2015 #8


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    It would be a repeating waveform, of period equal to the LCM of their periods. Try some sketches and discover for yourself.
  10. Dec 6, 2015 #9


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    To give an example, if the ratio of the periods is not a rational number, the sum will not be periodic.
    ##f(x)=\sin(x) + sin(\pi x)## is not periodic.
  11. Dec 6, 2015 #10


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    The short answer is no. Two different pure sine wave frequencies can not be summed to another pure sine wave frequency.
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