
An Engine that Harnesses Sound Waves - hoag
http://www.technologyreview.com/energy/32267/?p1=A1&a=f
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hoag
I had never heard of Sterling engines before, so I found this quite
fascinating.

I have a question though to whomever may offer an explanation. Towards the
bottom of page 1, the article explains that "[w]hen pressurized helium on the
top side of the metal plate is heated, sound waves traveling through the gas
are amplified, causing the plate to vibrate...."

When the pressurized He is _heated_ it causes the sound to be _amplified_? But
heating the He should cause the density to _decrease_ , of course, and
therefore _reduce_ the power of the sound waves propagating through the He,
no?

I'm not aware of any relationship between frequency and amplitude, just
V=(freq)(wavelength). What's going on? The only thing I can think of is that
perhaps there's some sort of resonance going on in the chamber? As the
pressure drops, the speed of sound should drop. But this shouldn't necessarily
affect the frequency... so I'm totally confused. Maybe there's a resonance
effect as the waves propagate slower and slower and interfere with one
another? I'm totally lost. I just don't see how _decreasing_ pressure (by
increasing temperature) _increases_ amplitude.

It's been quite a few years since studying E&M as an undergrad, so I'm a bit
rusty and just genuinely curious what I'm missing here.

Thanks!

