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yborg
Inaudible to most human beings. Your dog won't like this.
johnhenry
Just because a dog can hear something doesn't mean that it will be bothered by it.
TeMPOraL
And even if it would be, the attacker is not likely to care about your dog's happiness.
jaclaz
Well, you could train your dog to attack anyone that emits an ultrasound "Ok, Google", then the attacker will care. ;)
gnicholas
True. But isn't this basically like a dog whistle (which is at high frequencies and is used as a deterrent)? If this were super low frequencies, I'd be less likely to make the leap from [inaudible to humans but audible to dogs] -> [annoying to dogs]. But if we already use high frequencies to annoy/discourage/train dogs, isn't that evidence of what dogs think of these frequencies?
johnhenry
It's noise from the whistle itself that drives dogs crazy, not it's pitch. The reason this isn't basically a dog whistle is because a normal whistle produces a bunch of random annoying noise. If you continually blow a normal whistle, most things that can hear it will be annoyed. So, a dog whistle is simply a normal annoying whistle that humans can't hear. This, on the other hand, isn't a bunch of random annoying noise. If humans could hear it, it would just sound like someone speaking. There's no reason to believe that dogs would be especially annoyed.
logfromblammo
I'm pretty annoyed by Munchkins, and enraged by Alvin and the Chipmunks, so maybe there is a reason.
DannyB2
You're thinking ultrasonic TV remotes from a different millennium, long ago swept away in the sands of time.

Dogs aren't bothered by the pitch, but by the loudness. Ultrasonic remotes were amazingly LOUD. Loud enough for some humans to perceive.

Upvoter33
Thus begins the wave of security attacks on machine learning. Buckle up - we're going to see a lot of papers like this in the next months and years.
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powrtoch
I'm surprised that this works. I always assumed Siri etc. would do some bare-minimum pre-processing of the audio input, if only to reduce noise, of which the simplest kind would be cutting frequencies that cannot be produced or registered by humans. Any insight into why this isn't already the case?
virgil_disgr4ce
This was my question. Seems bizarre to not bandpass everything as the first stage!
TeMPOraL
From skimming the paper it seems they're making the signal demodulate itself directly on the microphone - by the time it hits a low-pass filter and ADC, the audible frequencies are already injected.
anonymfus
This attack exploits non-linearity in microphones and amplifiers. ADC receives signal already demodulated to an audible frequency.
bsenftner
This is like the altered eyeglass frames that make some facial recognition systems misidentify. Carefully constructed 'inaudible' could also mean 'transparent to our perception' for this class of exploit. I could envision an exploit that monitors the ambient noise in a location and mimics A/C startups, vacuum running, blender, whatever could be used to cover almost anything.
sjbase
Seems easy to defeat with frequency bounds checks.

But presumably there exist adversarial sounds that to a human seem like music, or gibberish, or a different phrase, but sound like "pay joe 1000 dollars" to the machine.

The sonic equivalent to these: https://arxiv.org/abs/1703.08603

TeMPOraL
Not sure what you mean by "frequency bounds check", but if you mean a bandpass filter, then it is not going to save you. The attack, as I understand the paper, works by having the AM-encoded ultrasound demodulate to the audible frequencies at the microphone, before frequency filtering and ADC. This would imply you could use this method to inject voice even to simple recording software (or even analog hardware!) - it's not limited to machine-learning voice recognition systems!

The paper describes possible ways to mitigate the attack, though. One of them is to let the whole signal pass, and then try to detect ultrasonic AM in software, demodulate it and subtract from the audible frequencies. This is pretty much correcting for the physical structure of the microphone in software.

sjbase
Wow, yeah you're right. I gave the feasibility analysis a more thorough read and it seems like they're exploiting the amplifier inside the mic itself to down-convert the frequency on portions of the signal. I don't fully understand what nonlinearity is being taken advantage of, but to as an acoustics layman this seems pretty impressive!
AstralStorm
Which means they're not using the membrane but the crummy amplifier electronics. Fixed with one or two capacitors.

Alternatively they're using almost deadly signal levels to overdrive a microphone at a distance.

AstralStorm
I'd say it would require extremely good placement of the emitter and knowledge of exact microphone mechanics to pull it off anywhere else than in the lab.

I'd be impressed if it even worked in a lab.

sailfast
Can this level of signal be broadcast over TVs? Could this be embedded in an advertisement that triggers a bunch of devices to read from somewhere?

I too, eagerly await the pre-processing software that blocks all inaudible frequencies. Not sure why this was not done in the first place.

TeMPOraL
> I too, eagerly await the pre-processing software that blocks all inaudible frequencies. Not sure why this was not done in the first place.

This won't save you. By the time the signal leaves the microphone and reaches the ADC, it's already in the audible range.

morsch
For most of the tests, they used a dedicated ultrasonic speaker. However, they also tested it with a Samsung S6 Edge controlling an Apple watch; it worked. So you don't strictly need specialized hardware.

Whether or not it works for TV depends on the broadcast signal (processing, audio codec) and the playback device speaker. The latter should work out somehow, given that a smartphone speaker can do it. Most audio codecs go up to 44 or even 96 kHz, so the range should be there. But digital TV signals carry lossy audio, designed to save space by throwing away data not discernible by humans. I'm not sure how high the fidelity of AAC is beyond 20 kHz. Should be easy enough to find out.

lunchables
How about a completely inaudible: "Alexa, buy me a leather mask".
anfractuosity
I guess you could also use https://en.wikipedia.org/wiki/Sound_from_ultrasound, which would mean unless you're in it's line of sight you shouldn't hear it.

I'm specifically thinking of the parametric array type.

I'd love to see how directional that is sometime, as there are cheapish implementations of hardware for it.

Edit: Originally I thought a human might be able to hear their sound, but reading a bit more I don't think that's the case since it's exploiting non-linearity of the microphone

Also it seems ultrasound can be used to affect other MEMS devices:

https://arstechnica.co.uk/gadgets/2017/07/sounds-bad-researc...

I wonder if there's any acoustic metamaterial that you could place over the mic etc. to filter ultrasound out before it reaches it.

Animats
Those sound-from-ultrasound systems have been around for a while. The principle is similar - both exploit nonlinearities in the receiving medium. The sound-from-ultrasound people seem to have finally brought up the audio quality to an acceptable level for music. The nonlinear process by which two ultrasonic signals combine in air to produce an audible signal introduces distortion. So the signal-generating software has to compensate somehow.

(Why is backspace so slow in this text box? Firefox 54, Ubuntu 16.04.)

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greesil
If they are not using a strong enough anti-aliasing filter the emitted audio can appear in-band looking just like normal speech.
acd
Clever attack! Then the phone AI voice command system must start to recognize that the speakers voice is the original speaker via user finger printing. Then the attacker will counter attack with another new system that emulates the original speakers voice and speech rate via deep learning. You will then as a counter measure have to press a hardware button to use voice commands. Seems like microphone proximity sensing and frequency filtering are easy first starts as counter measure as already mentioned in this thread.