[tossaway1]

> Couldn't they obtain a 7khz tone by mixing any two tones seperated by 7khz and applying a low-pass filter?

I don't understand this comment. Are you implying that a low-pass filter would shift the frequencies of part of the signal rather than (mostly) filtering some of the frequencies away...?

[kaoD]

https://en.m.wikipedia.org/wiki/Heterodyne#Mixer

> In the most common application, two signals at frequencies f1 and f2 are mixed, creating two new signals, one at the sum f1 + f2 of the two frequencies, and the other at the difference f1 − f2.

Then the low pass filter is used to remove the original signals and the sum, leaving only the difference.

[sjburt]

When they are mixed they produce a beat frequency. When it is sampled at a lower frequency only the beat frequency remains.

I may be wrong but I think a filter would have the same effect in this case.

My point is that any number of inputs could produce the observation, so their claim of reverse engineering is rather questionable. In fact they have no evidence of ultrasound at all.

[amelius]

mixing = multiplying, not superimposing

I know, it's confusing ...