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Imagine a perfect audio speaker creating a planar square wave - driven into a parabolic reflector.
The fundamental wavelength is around twice as wide than the parabolic reflectors diameter.

I've read that parabolic reflectors can only really concentrate wavelengths a multiple less than the radius of the reflector, with the gain relative to frequency.

However a square wave comprises a number of frequencies (harmonics of the fundamental).

If I am driving a speaker with the square wave as described, approximately what kind of waveform would be picked up at the focus microphone?

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Your waveform would be a square wave with the lowest frequencies (those lower than the dish's cutoff frequency) missing. On an oscilloscope it would resemble a steady train of sharp spikes, each of very short duration. The frequency of those spikes would be equal to the harmonic series required to construct the square wave.

You can simulate this scenario by sending a square wave signal into a filter network that removes the bass frequencies, including the fundamental, and displaying the result on the O-scope.

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  • $\begingroup$ Thank you. I would guess the pressure from the original wave would still be there at the receiver, (as that pressure will still pass the receiver), however the high frequencies would be amplified to create a spike train with a small dc offset. $\endgroup$ – JovialMike May 30 at 11:37
  • $\begingroup$ probably true. easily tested! $\endgroup$ – niels nielsen May 30 at 18:03

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