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Whether or not your speakers are efficient at transforming electrical power into acoustic power is another story. If you can lift your speakers by hand, your efficiency is maybe 1% if you're lucky.

Not quite 'sickness', but my aunt lives on the side of a large hill overlooking a pretty valley... Her balcony used to be a nice place to sit and relax. Now her down-hill neighbor (approximately 2km away) has a wind turbine in his yard and the low frequency periodic noise from it has transformed her balcony into an annoying place to be and she can no longer sleep with the windows open. She's not claiming sickness, she's merely claiming annoyance..

Okay so let's say that from right up in front of the thing you experience 105 dB of sound. Now let's use some basic math to compute what 105 dB at 0.5 meters away sounds like when you're 2,000 meters away. 32.958 dB should be the intense ear splitting result at the balcony. Does your neighbor have some super noisy form of wind turbine or does your aunt go insane inside a kitchen when the refrigerator is running? Does she have to turn her air conditioning and refrigerator off in order to sleep? Because according to every resource out there, physics put that noise at sub 40 dB. Even if we bump it up to rock concert levels (120 dB) it should be 48 dB at 2 km and that's about as loud as an AC unit.

Now, how loud is acceptable at the edge of someone's property before you think the authorities should be involved? And think carefully about people who like to use air condition/compressors, mow their lawns, have yard parties with music, drive motorcycles and do any good patriotic non-save-the-rainforest stuff before you answer.

dry air: 0.6 dB/m at 50 kHz, 1.8 dB/m at 100 kHz

No. Sound is not so linear as that. You cannot take a chart that says sound is attenuated by 1800dB at 1km and simply divide by 1000 to get the attenuation at 1m.

Remember inverse-square law: Check it out. (And more here.)

All that aside: The simplified rule of thumb for sound at audible frequencies, for a spherical waveform (such as that emitted by a phone), is that sound falls off at a rate of 6dB for each doubling of distance.

So, if you're making noise that measures 80dB@10cm, you get the following results at these increasing distances:

74dB@20cm
68dB@40cm
62dB@80cm

etc.

And we only care about frequencies in the audible range, despite the implication in TFS, or it will be completely unable to work with existing phones (which is the main point of the thing to begin with). To wit: Combine Nyquist theory with the shitty analog electronics and 48KHz (at best!) ADC/DAC in a phone, and the resultant system must be either audible to a sufficiently-close non-damaged human ear, or else be completely non-functional.

So, there's no point in even discussing how well the thing might behave at 50 or 100KHz, because that's never going to work with existing phones.

And the whole argument is moot, anyway: The transport layer for this sort of payment system, whether RFID or barcodes or acoustic signalling or Bluetooth or avian carrier, will be recordable by a sufficiently-motivated and clever person. It therefore must have strong security (whether cryptographic or otherwise), or it will fail and be exploited. And if it does have strong security, it doesn't matter if it's recordable or not, since any recovered data will be useless to the eavesdropping party.

it'd have to be lower... it said "half the wavelength", in TFA, assuming 2 meters for a person (thats a bit over), This site (http://www.sengpielaudio.com/calculator-wavelengt h.htm) suggests that 85.75hz is what is needed. Using 6 feet people (12 foot wavelengths), we get 93.8hz

Not very practical for a building or even a room, except for a special EMI testing room.

Or maybe I'm completely missing something. Maybe it doesn't take that much containment to actually stop 2-way communications at those frequencies

-S

That calculator is intended for airplanes.

Indeed. That's why I found a better calculator. (Yes, I'm still being lazy. ;-))

The center of the thermosphere can get as hot as 2,000C

Which only makes the problem worse in the other direction. 2,000C in "air" would produce a speed of 2,138 mph. Thus the Shuttle would only be travelling ~Mach 8 as it passes through the hotter parts of the thermosphere.

but where the shuttle is, it will probably just be a few hundred degrees kelvin on average (depending on solar conditions).

At 200K, the calculator gives a result of ~634 mph. Of course, one has to be cognizant of the different gasses, but that's probably not too bad as an estimation. The "Mach 25" number might be correct if the atmosphere were right around 240 Kelvins. Which it may be on occasion. But it certainly isn't stable, and the Space Shuttle orbits at a variety of altitudes. Like I said, the "Mach 25" number is just silly. :-)

But yes, representing the Shuttle's speed in relation to mach number is pretty silly ;) I prefer good 'ol km/s.

Glad we agree. :-D

For example, the Mount St. Helens volcano released energy in just one day (18 May 1980) equivalent to 400 million tons of TNT - about 20,000 Hiroshima bombs.

Although I think your criticism of the parent is probably correct, this isn't a very helpful comparison. It's also true that a barrel (42 gallons) of oil contains more energy than a ton of TNT, and a ton of coal contains around 7 times as much. (see this table). A ton of TNT makes a lot bigger hole in the ground, though. What needs to be considered here is not just energy content, but rate. How fast was the energy liberated? Mount Saint Helens was not equivalent in explosive power to 20,000 Hiroshimas, so that bit of trivia is a little misleading.