Hey guys can our phone capture sounds below 20hz??(inaudible to human ears)...??? if possible we could have some cool apps...
say i have some mp3 file...i could encode it with some inaudible contents...n then when i play it a n900 user could capture n decode the secret message..

i dont know what frequenci supports the N900, but i think like the other phones..

14hz to 20 or 30Khz

Uh... I think some you guys have no clue how digital audio recording/playback works.
When a digital recorder records something, it uses a process known as "sampling" - that is, for a 44100 sample/second recording, it takes a "snapshot" of the input every 1/44100 second. When played back by taking those samples and sending them through a DAC(digital to analog converter), you then hear it as sound if you've got a speaker plugged in.

This means that /frequency doesn't matter/. At least, not to the recorder itself. The microphone and speakers may not he able to handle high/low frequencies, but the digital hardware can.

Now, the above is only true for uncompressed .wav files. Compression - like mp3 - may easily mangle stuff that's deemed "outside of human hearing" to save space, at least at low bit rates. So, you're just going to have to experiment.

I guess they meant microphone + speaker response. I think speakers could be inefficient to produce low frequency sounds, as every speaker of every cell phone out there. They reverberate and lose sound power. In the other side, it is possible we could use high frequency (+20Khz) instead of low frequency, and then we could also freak some dogs... (there are already "dog whistle" apps for some other devices). It's clear these little speakers work better at high frequencies.
For secret communications we can use SMS... or we can make an app (that works both on Symbian and Maemo, ie, QT4) that generates and reads this kind of sounds and translates 'em as text... It would be useful inside a room, not farther...

It'd be cool if N900 would respond to dog whistle, so that it can bark and run to me (one can only dream ^^), but I think dog whistle is on the high freq. spectrum above hearing range.

Let me guess, OP wants to make N900 a EVP recorder right? It'd be a very cool project, definitely something I'd turn on alongside with sleepanalyser at night XD

I've been meaning to try use the accelerometers for capturing vibrations (and sound is vibration) below 400Hz. I have a very loud (90-100dB) place I can go to, and hope some of that vibration would be felt by the N900 itself and be recognizable when accelerometer data is played back as sound..

Ahem...cough ***up to the nyquist frequency, in this case at 44.1kHz sampling, 22.05kHz*** cough.

Yes. And because of the aliasing that would be caused by frequencies above the nyquist limit, those freqs are (mostly) filtered out before digitizing the signal, either by the mic design or an analog lowpass filter.

Also, recording infrasound frequencies would reduce the dynamic range for the useful signal (the one we can actually hear), so if the mic/digitizer design are any good, those are also filtered out with similar methods before A/D conversion.

Bottom line: you will need special or customized equipment to capture sounds that are significantly outside of hearing range of normal people. Properly designed standard equipment is designed to ignore them.

I thought Chuck Norris could here any frequency....

why ? are you on a ghost hunt or something ?