"Security researchers have discovered a way to covertly hijack Siri and other smartphone digital assistants using ultrasonic waves, sounds that cannot be normally heard by humans. "
These sound waves are inaudible by definition. This is like saying human skin may sustain damage when exposed to ultraviolet light, that is not normally visible to the human eye.
So we don't know if the ultrasonics just wake up the device or if the phones' microphone is responsive directly to these sound waves. Most camera sensors [and snake eyes] have a different spectrum of response than the human eye. See, for example, infrared pictures taken with commercial camera sensors. It makes sense that most microphones respond to a different sound spectrum than the human ear. We know dogs can hear higher frequencies than humans.
By the layman definition it is, but that definition isn't complete as it will vary from person to person. The quoted sentence is correct, because the frequencies are not normally heard by humans (but there definitely people that can hear sounds we usually attribute with ultrasonic frequencies.
From a scientific standpoint it would probably be better if we define ultrasonic specifically from 20,000 Hertz to either infinity or some upper level where another term gets used. Note that medical ultrasounds usually operate at around 10,000 Hertz.
This is incorrect- medical ultrasounds are in the 1 - 20 MHz range.
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