Silverdog is a free browser extension for Google Chrome -- it should work in other Chromium-based browsers as well, and maybe even Firefox (not tested) -- that acts as an ultrasound firewall in the browser.
In Talking Behind Your Back, security researchers Vasilios Mavroudis and Federico Maggi describe attacks and countermeasures of ultrasonic cross-device tracking.
Ultrasonic tracking is a relatively new form of user tracking that uses inaudible sounds for that. A basic example is a TV ad that contains ultrasonic sounds that get picked up by an application running on the user's smartphone. It could then push the same ad on the smartphone, or monitor user behavior to find out whether a product website is visited.
Another example is the so-called proximity marketing. This too works with an application that the user carries, and relies on ultrasound emitters placed in the store. Companies may use the information to study user behavior in the store, and provide real-time notifications for products in proximity of the user.
The main issues with ultrasound beacons -- high-frequency audio tags -- is that they are inaudible by humans, and that most speakers and microphones have no problems capturing or emitting them.
The spectrum is usually in the 18000Hz to 20000Hz range, but there is no one standard that companies follow. While you do need at least two devices for this to work -- one that sends the signal, another that captures it -- it is fair to say that the practice has become more common in recent time and is on the rise.
Silverdog is a sound firewall for Chrome that blocks ultrasound frequencies in the browser. This prevents ultrasound tracking when you use it, but does nothing when other devices do it.
The extension works fine out of the box. It is set to a frequency of 18000Hz by default and a certain filter type, gain and Q. You may want to read up on the various filters that it supports, as the extension itself offers no explanation on the differences between those filters.
The Chrome extensions works automatically once you have configured it. You can turn the firewall on or off with a click on the extension icon in the Chrome address bar.
One limitation of Silverdog is that it won't work with Flash, but only with HTML5 content. The researchers have created a patch for Android's permission system which gives users more control over the audio channel.
Both the Chrome extension and the AOSP patches can be downloaded from the developer website.
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