From a quick research, it seems that iPhones are not able to play ultrasonic sound: "iPhone 4s is rated for 20Hz - 20,000Hz, which means it cannot generate ultrasonic frequencies." - http://stackoverflow.com/questions/10034346/ios-how-to-produ...
No it doesn't. All speaker drivers act as bandpass filters. On the ends of their frequency ranges those +/-3dB points are generally their -3dB points. For most speaker drivers their low frequency cutoff will be caused by the resonance of their motor, dropping off rapidly after that point. The upper frequency limit will be caused by the inductance of their motor's coil with a similar drop off above it.
It's unlikely that the iPhone speaker could reach 48kHz. If you look at expensive high frequency units (tweeters) they'll generally top out at 20-30k. A lot of headphone drivers (which are more similar to what's in the iPhone) barely make it to 20k.
> You can still make some noise higher or lower than that, it will just be more distorted.
Again, nothing about distortion here. A speaker driver will output sound outside of its -3dB points, but, obviously it's going to be much quieter (-3dB is already half as quiet).
https://itunes.apple.com/us/app/acoustic-picture-transmitter...
Fun to play around with and think of the possibilities of using sensors in unintended ways.
That's not to say it would work on an iPhone.
In this example, at ~19 kHz, a properly configured high-pass filter could easily remove any human-talking sounds.