The Hidden Sound You Can’t Hear (But Your Phone Might)
How Ultrasonic Beacons and Modern “Acoustic Analytics” Quietly Connect Your Devices
After our last post about whether your phone is listening, a number of readers reached out asking about one particular advertising tracking technique: ultrasonic beacons. That post is available here:
Most had never heard of ultrasonic beacons, which is not surprising. They’re one of the lesser-known tools in the ad-tracking toolbox. While they’re not widely used today, they’re a fascinating (and unsettling) example of how creative advertisers can get in trying to connect the dots about you.
Having spent 15+ years working with Big Tech and major marketing teams from the legal side, we’ve seen firsthand how far companies are willing to go to track behavior, and how quietly those lines move over time.
Let’s unpack how they work, where they’ve been used, and what their modern versions look like today.
What Are Ultrasonic Beacons?
Ultrasonic beacons are inaudible sound signals embedded in digital media, such as a TV commercial, a website ad, or even an app.
Your ears can’t hear them, but your phone’s microphone can. Sort of like a dog whistle.
When a participating app on your device “hears” that unique signal, it can confirm that your phone (and therefore, you) were present when the ad played.
In short, it’s a way to link your behavior across devices (TV, phone, tablet) without relying on cookies, location data, or login credentials.
Why Advertisers Used Them
Advertisers love connections. They want to know that the person who saw a TV ad also picked up their phone and searched for the product or visited the store’s website.
Ultrasonic beacons briefly looked like the perfect bridge between the physical and digital worlds.
Here’s how it worked in practice:
A TV commercial would emit a short ultrasonic signal during the ad.
An app on your phone (one that had microphone access) would “hear” that sound and quietly report back that your device was nearby.
This data helped advertisers measure “cross-device engagement” (did the ad influence your later behavior?) and fine-tune future targeting.
When It Crossed The Line
Researchers at a German university first documented large-scale use of ultrasonic tracking between 2015 and 2017. They found the technique embedded in hundreds of Android apps and dozens of retail and media campaigns. (source)
One ad tech company behind the trend was SilverPush, which marketed its software as a way to connect offline and online ad exposure.
By 2017, regulators stepped in. The Federal Trade Commission (FTC) and Google Play both warned developers that they had to disclose ultrasonic tracking or risk removal from the store. Google eventually banned apps using the technique entirely (a rare privacy friendly move by Google).
While ultrasonic beacons never became mainstream, they revealed just how far companies were willing to go to follow users across screens and devices.
Is Ultrasonic Tracking Still Happening?
Today, ultrasonic tracking is rare, but it’s not extinct.
Most major platforms, including Apple and Google, now ban apps that secretly listen for ultrasonic beacons. That doesn’t mean the underlying idea disappeared. Rather, it’s just evolved and been rebranded in some cases.
The modern buzzwords are “proximity marketing” and “acoustic analytics.”
Proximity marketing uses nearby signals (like Bluetooth, Wi-Fi, or even ultrasonic sound) to deliver hyper-local messages. A retail store might trigger an in-app coupon the moment your phone detects an in-store signal. When done transparently, it can be convenient. When done quietly, it can reveal your location history and shopping patterns without your consent.
Acoustic analytics takes it a step further. It uses ambient sound, such as crowd noise, background music, or even the echo of a room, to learn about environments and user behavior. Some analytics firms claim they can estimate foot traffic, identify the type of venue you’re in, or match your device to a TV ad that’s playing nearby. This is all accomplished through sound data.
These systems don’t always use ultrasonic frequencies anymore, but they operate on the same principle: using your microphone as a sensor to connect digital and physical behavior.
And this approach fits into a much larger trend: collecting behavioral biometrics. That includes subtle signals like how you type, scroll, move your mouse, or hold your phone. Together, these data points form a unique behavioral fingerprint, one that can be used to track or identify you even if you block cookies or use a VPN.
Ultrasonic tracking listens to your environment. Behavioral biometrics listens to you. Both show how the line between “useful analytics” and “intrusive surveillance” keeps moving outward.
Not all proximity or acoustic tracking is inherently bad. For example, some museums, sports venues, and public events use it for interactive experiences. The problem is it becomes invasive when users aren’t told what’s being collected or how it’s being used.
That’s the blurry line the privacy community keeps an eye on.
How To Protect Yourself
You don’t need to panic about ultrasonic or acoustic tracking, but it’s smart to make sure your devices aren’t open microphones for marketers.
Start by checking which apps have microphone access and why. Voice assistants like Siri or Alexa make sense if you’re already comfortable using those devices. On the other hand, a weather or coupon app doesn’t make sense. If an app’s purpose doesn’t involve sound, it shouldn’t need to listen.
Keeping your operating system up to date helps too. Both Android and iOS now require apps to clearly disclose when they’re using the microphone in the background. That little orange or green indicator light on your screen is there for a reason. If the light is on and you’re not recording, dig deeper.
You can also use tools that limit how apps communicate when you’re not using them. On Android, NetGuard lets you block network activity on a per-app basis. On iPhone, Lockdown Privacy can do the same. Even switching to a privacy-focused browser like Brave or DuckDuckGo reduces the number of third parties listening in on your digital life.
If you want to go further, there are experimental apps like SoniControl and SilverDog that can detect ultrasonic signals in your environment. Most people don’t need them every day, but they’re useful reminders that these techniques exist and are measurable.
In short, awareness beats paranoia. Take a few minutes to review your permissions, block background data you don’t need, and be intentional about what your microphone can access. Those small steps make a big difference in making you a harder target for both advertisers and analytics companies that push the limits of what’s acceptable.
The Takeaway
Ultrasonic beacons might sound like science fiction, but they’re a real example of how quietly your devices can work against you.
They may not be common today, but they’re a reminder that advertisers will exploit any available channel to track behavior: visual, digital, or even acoustic.
And behavioral biometrics shows that the next wave of tracking might not rely on sound at all. It might come from the way you move, type, or interact.
Greater awareness and control is the solution.
Every time you review app permissions, limit data sharing, or block a hidden tracking channel, you make yourself a harder target.
And that’s the kind of privacy win that keeps stacking over time.
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Disclaimer: None of the above is to be deemed legal advice of any kind. These are *opinions* written by a privacy and tech attorney with years of working for, with and against Big Tech and Big Data. And this post is for informational purposes only and is not intended for use in furtherance of any unlawful activity. This post may also contain affiliate links, which means that at no additional cost to you, we earn a commission if you click through and make a purchase.
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I truly wish that you had more followers.
Thank you for your efforts.
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