Most of the comments focus on the more common problem of removing/ignoring the noise so that the other sound can be extracted. You want to do the other thing around: detect air-blow sounds, rejecting all other sound.
First, your zero crossing method is not going to be very useful for this. Air-blow is close to pink noise in signal shape, with some "tint" to the spectrum depending on position of blower, position of microphone, manufacture of phone, etc.
Because you say you have FFT already, I would run repeated frames of 50ms or so, and look for the signature of blowing into the microphone. It will likely be a very wide spectrum distribution without sharp peaks. Also, it will have a duration greater than a single frame.
Other signals will often have more distinct peaks within the spectrum. Thus, you could calculate how well the spectrum you get compares to a wide, pink-noise-like distribution. Beware that the output of the FFT will not keep the frequency bins in increasing order, but rather the "butterfly" order, and ever other data value out of the FFT is phase, rather than amplitude, and thus is not interesting to this analysis.
When you have both a "blow" sound and "background" sound coming in, you will have a "noise floor" of the blow sound, and individual peaks from the other sounds. You have to remove the peaks, and detect the blow sound based on whatever profile you can "underlay" your spectrum and still fit the blow sound. There are various curve fitting/regression functions you can use here.
In the end, I think you'll still have problems with this approach, as different phones have different sonic characteristics. You may have to "train" the application on the particular phone the user is using for best result.
The 2.4GHz frequency is more solid international standard than 433 MHz.
(source: http://en.wikipedia.org/wiki/ISM_band).
Any of the carrier frequencies mentioned could transmit to that distance.
Generally, lower frequencies have a better ability to pass through obstacles than the higher frequencies. But, it's almost always a question of the power of the signal.
If you don't have the power of the signal, you need to rely of the manufacturers data.
Note that generally the transceiver distances are line of sight distances i.e. no obstacles.
The obstacles can reduce that distance considerably.
I would recommend a 2.4GHz transceiver with at least double line of sight transecting distance that what you need.
TI are good, another option is the Nordic nRF24L01+ chip.
Note that it could also be possible with some Bluetooth modules but at that signal strength they are probably illegal!
Good luck,
Data source - I'm an EE Engineer.
Best Answer
No.
It's not an ISM band in Europe.
"Short-range device" usage is very specific, that's not the same as "ISM band"; on the contrary. The European legislation that you're referring to leaves it up to the member states to define what an SRD is – and in European member states, those always (?) have been walkie-talkies, not things like machine-to-machine communications.
Maybe that will change, but it remains to be seen. In any case, you cannot assume this band to be currently available; that European guideline is pending national legislation.