Description: Industrial wind turbine infrasound is not the best weapon, but it is a weapon. This German video documents the harmful effects of the infrasound produced by industrial-sized wind turbines. The dangers of infrasound have been known since the 1980s when the U.S. military heavily invested in infrasound (below 20 Hz) as a weapon.
HT/Fernando
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All this talk of beat frequencies has me puzzled. Do two frequencies differing by 10Hz actually produce any energy at 10Hz?
Roger at March 20, 2019 at 12:24 pm asked: “Do two frequencies differing by 10Hz actually produce any energy at 10Hz?”
Absolutely NOT! But the situation is a bit more complex than some think:
http://electronotes.netfirms.com/EN213.pdf
Consider the trig identity (one of several equivalent forms):
Sin(A) + Sin(B) = 2 Sin[ (A+B)/2) ] Cos[ (A-B)/2 ]
Consider the case where B is slightly less than A. The right-hand side says that the superposition is basically the AVERAGE frequency (A+B)/2 multiplied by HALF the difference: (A-B)/2.
Traditionally we have heard that the “beat rate” IS the difference frequency, not half the difference. This is because the Cosine term has TWO peaks (one positive and one negative) during each of its cycles. Typically here A-B is sub-audio – the ”thumping”.
Now to the point: If you ask what the SPECTRUM is, it is just the LEFT side of the equation: the SUM as you wrote it down to describe (linear) superposition. NOTHING NEW – the sum (resulting spectrum) is just the sum (the process).
To be complete, you may, and often will, PERCEIVE something recurring at a 10 Hz rate (the “envelope”), but any proper spectral analysis will show no energy there, (absent any secondary non-linear effects).