What is wrong with the ‘Bracelet of Silence’

Moral questions on Wearable Microphone Jamming and Frequency Jamming in general

6 min readMar 3, 2020

“We engineered a wearable microphone jammer that is capable of disabling microphones in its user’s surroundings, including hidden microphones. Our device is based on a recent exploit that leverages the fact that when exposed to ultrasonic noise, commodity microphones will leak the noise into the audible range. Moreover, our device exploits a synergy between ultrasonic jamming and the naturally occurring movements that users induce on their wearable devices (e.g., bracelets) as they gesture or walk. We demonstrate that these movements can blur jamming blind spots and increase jamming coverage. Lastly, our wearable bracelet is built in a ring-layout that allows it to jam in multiple directions. This is beneficial in that it allows our jammer to protect against microphones hidden out of sight. “— Yuxin Chen, Huiying Li, Shan-Yuan Teng, Steven Nagels, Zhijing Li, Pedro Lopes, Ben Y. Zhao, and Haitao Zheng

This provides a gist of the research. Let’s break down each line one by one.
“We engineered a wearable microphone jammer …”, they have built a cool device, as expected, next. “Our work is based on a recent exploit” I think they mean various recent papers published by researchers at University of Illinois at Urbana-Champaign (UIUC). Just an assumption.

One of them is called BackDoor: Making Microphones Hear Inaudible Sounds, in this paper they used jamming microphones for covert data transmission. In the section 3.1 Measurements and Validation, it is said,

“For the above idea to work with unmodified off-the-shelf microphones, two assumptions need validation.
(1) The diaphragm of the microphone should exhibit some sensitivity at the high-end frequencies (> 30kHz). If the diaphragm does not vibrate at such frequencies, there is no opportunity for non-linear mixing of signals.
(2) The second order coefficient A2 needs to be adequately high to achieve a meaningful signal-to-noise ratio (SNR) for the shadow signal, while the third and fourth order coefficients (A3, A4) should be negligibly weak. We verify these next.”

Before moving forward, I’ll like to point out that all the related research works are very interesting because they seem to be exploiting innate property of audio systems. Before reading the papers, I was confidently saying “hah, some audio signal processing and we’re done”.

Me ready to build a counter proof-of-concept

After started reading, I understood that this won’t work. The ultrasonic input is saturating the microphone. In this situation, no amount of digital filters can recover the desired signal. Then I thought if there was a physical low pass filter (LPF circuitry) that was placed over the microphone or (maybe) fix this by either installing an analog filter so the ultrasonic noise can never reach the ADC or sample the whole range (up to 44.1KHz is a good start) and filter digitally, that can? work. At the same time, after reading the paragraph which seemed (to me) that devices affected were only MEMS microphones. A larger dynamic or condenser microphone which are inherently insensitive to ultrasonic frequencies will render this attack useless, BUT, after re-reading, I noticed something, in the start of the paper, they have stated,

“In the simplest case, BackDoor plays two tones at say 40kHz and 50kHz. When these tones arrive together at the microphone’s power amplifier, they are amplified as expected, but also multiplied due to fundamental non-linearities in the system. Multiplication of frequencies f1 and f2 result in frequency components at (f1 −f2) and (f1 +f2). Given that (f1 −f2) is 10kHz in this case, well within the microphone’s range, the signal passes unaltered through the low pass filter (LPF). Human ears, on the other hand, do not exhibit such non-linearities and completely filter out the 40kHz and 50kHz sounds.”

Non-linearity in audio systems is also known as distortion, and nonlinear mixing is impossible to eliminate entirely because every system, whether electrical mechanical or even digital, goes nonlinear when it reaches its amplitude limits. Obviously, some more gracefully than others.

Okay, let’s move forward.

By saying, human movements “can blur jamming blind spots and increase jamming coverage”, they mean, by making the jammer a wearable, they allow the device to move along randomly and counteract the nulls caused by the multiple speakers. Random movement causes the nulls to not stay in one place for very long. As we know that the sound is highly directional, by not making it a wearable and placing it somewhere can lead the creation of ‘jamming blind spots’, which are nothing but simply the places where the signals from two or more transducers cancel each other out.

*Simulations depicting how different transducer layouts radiate around the simulated device, when moving in space, a wearable jammer outperforms stationary jammers.*

Bonus: There is another paper which I encountered while making this blog was called DolphinAttack, again done by researchers at UIUC where they attempted to use the ultrasonic audio band as an inaudible attack vector to play an ultrasonic noise that no one can hear except for the smart assistant.

With great power comes great responsibility

While this and all related research around “Jamming for Privacy” may sound great, I think it is not the solution, at least until we get hit by a dystopian future.

If still invested in the very idea of dystopian future, this place is for you —
Mass surveillance in popular culture

Jamming Audio as well as many (most?) other frequency bands such as WiFi, GSM, 3G, LTE, etc. are very simple and very illegal.

What’s the problem in jamming?

Do we thought how will this research impact our surroundings?

Let’s say somebody is wearing this device in a public space then what will happen to the callers in surrounding area?

During a phone interview, Mr. Lopes turned on the bracelet, resulting in static-like white noise for the listener on the other end. — nytimes

Do we thought what will happen to people attached with accessibility devices, say, cochlear implants, hearing aids etc.?

Yes, a lot of confused hearing aid users. The wearable jammer will hinder the devices which are critical for them to properly function.

Any other concerns?

Likely, the device will confuse the dogs, most likely hinder bats as they use echolocation of their prey and many other animals. Maybe, the only positive thing will be the less likeliness of rodents to nest near us.

So we should not go around shouting “What could go wrong with blasting out ultrasonic noise in every direction (Besides a lot of confused dogs…)”

It would be a loss if due to a critical need to protect privacy we end up inadvertently harming a group of people that are already at a disadvantage. For me that trade-off is not worth it.

Simply put, using this device publicly is insanely irresponsible and inconsiderate.