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If I have a 20 MHz surface wave travelling along a material, and then introduce a 50 or 100 Hz wave source, will there be a noticeable difference to the output?

(Surface acoustic wave type device, for generating and detecting the waves, lower frequency wave with other mechanism)

If the surface wave velocity is 4 km/s, the 20 MHz has a wavelength of 200 um and a 100 Hz wavelength would be 40 m. So would I be right in thinking that these are not really comparable? And a set up to measure 20 MHz would not detect a surface wave with such a low frequency and no change would be detected in the output?

Thanks for any help.

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    $\begingroup$ To what type of waves are you referring? Meaning, are these acoustic waves in a solid material or electromagnetic waves excited on the surface? $\endgroup$ – honeste_vivere Feb 1 '18 at 13:34
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    $\begingroup$ Theoretically, the resulting wave is $20 MHz +/- 100 Hz$. Practically, how can you measure it? For example, Surface Waves transducers will filter out this low frequency. $\endgroup$ – npojo Feb 1 '18 at 14:07
  • $\begingroup$ They would be acoustic waves on a (piezoelectric) solid, honeste_vivere. And thank you npojo, that was my thinking with the filtering and I am not sure it could be measured practically. What if I could include a 5 MHz wave now, wavelength 800 um, I should also see the 5 MHz appear at the output, provided there are enough IDTs for the wavelength? $\endgroup$ – Dave Feb 1 '18 at 15:04
  • $\begingroup$ I tend to modify my previous response. IDT will not pass $100 Hz$ but actually will pass $20MHz +/- 100 Hz$. So the question is whether your spectrum analyser can resolve $10 Hz$ on top of $20MHz$. As for $5MHz$, it depends on the bandwidth of the IDT. I am not sure why you "retreat" to wavelength discussions. $\endgroup$ – npojo Feb 1 '18 at 17:14
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    $\begingroup$ By the way, you should tag a name from whom you want response. e.g. @npojo. $\endgroup$ – npojo Feb 1 '18 at 17:22

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