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Comment by sjburt | Hacker News Reader

Couldn't they obtain a 7khz tone by mixing any two tones seperated by 7khz and applying a low-pass filter? I don't see how this proves anything.

And the 155dB number cited by the Canadian government would require considerable (hundreds of watts) of power to produce at close range.

This whole case defies logic. The early reports were of mysterious symptoms not associated with any obvious exposure. Once ultrasound/infrasound began being mentioned, people began reporting hearing mysterious sounds.

The recording makes even less sense: If the sound levels were really sufficient to cause physical trauma, surely they would overwhelm the phone's microphone?

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A low-pass filter would filter everything out assuming the threshold is below the two frequency. The beat frequency of the two tones would indeed by 7kHz, but if you think of the resulting waveform, the contingent parts are smaller, faster oscillations that that would be suppressed by the filter.

> 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...?

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.

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.

mixing = multiplying, not superimposing

I know, it's confusing ...

That only works if you rectify the beat pattern, so you need some source of nonlinearity in the system.

Well, I do remember something anecdotal about a sound frequency that can make you poop. Apparently, it stimulates the bowel muscles.

That was "busted" in a Mythbusters episode.

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