Why the heck wouldn't they have the wake-word listener confine itself to human-audible frequency ranges? Seems like that would be a really simple fix with zero loss of real-world functionality....
It might be that although the sound is inaudible to humans, it gets distorted around and inside the device, and that distorted sound is at audible frequencies that the alexa microphone picks up. Something like how these ultrasonic directional speakers work: https://www.holosonics.com/what-makes-a-sound-source-directi...
Yeah, it's weird. There was already a scare about ultrasonics being used for ad tracking in 2016 [1] so it's not like this is an unknown attack vector, and I thought the subsequent patch efforts had already added filters that stopped the phones listening on ultrasonics.
I also remember seeing a presentation on the first gen Echo which went into its noise cancelling tech, making sure that stuff coming out of the speaker wasn't received by the mic, so the success of the speaker-to-mic attack vector also seems totally bizarre.
That's on purpose. They created a specific inaudible frequency that the Alexa listens for which causes it to ignore the wake word. This is how they keep from annoying everyone and also blowing up their own servers if, for example, they want to run an Alexa commercial during the Superbowl.
> Reddit user aspyhackr may have figured out the trick Amazon uses here. Apparently, the Alexa commercials are intentionally muted in the 3,000Hz to 6,000Hz range of the audio spectrum, which apparently tips off the system that the “Alexa” phrase being spoken isn’t in fact a real command and should be ignored.
Seems to be the inverse - if the wake word _lacks_ these frequencies, then Echos ignore it.
Yes, I had misremembered it. That does leave a mystery of why they are listening for inaudible frequency. I wonder if it might have to do with their plans of having devices that can connect to other connected devices when they aren't connected to the user's wifi.
