Ultrasonic Jammers: How They Work and What to Check Before You Buy
An ultrasonic jammer makes it hard to record a conversation on a phone or voice recorder. People can’t hear its signal, but microphones pick it up and turn it into noise on top of speech. Here’s how it works, how to choose the right model and how to test it on your own phone or recorder.
What Is an Ultrasonic Jammer?
“Ultrasonic jammer” and “ultrasonic microphone jammer” are two names for the same device. It produces interference above the threshold of human hearing and targets the microphone, not the whole phone, which keeps working as usual.
Ultrasound is sound with a frequency above 20 kHz, or more than 20,000 vibrations per second. The human ear can’t perceive it. Ultrasound scanners, echo sounders and parking sensors all run on ultrasound. Phone microphones are designed for the human voice, but their electronics respond to these frequencies too.
How Ultrasound Interferes with Voice Recording
A microphone jammer emits powerful ultrasound. The microphone’s electronics are designed for a voice at normal volume and can’t cope with this kind of interference. Part of the ultrasound “leaks” down into speech frequencies and lands on the recording as noise. The words behind it become harder to make out.
Researchers at the University of Chicago describe this as an inherent property of microphone hardware, not a defect in particular models. That’s why the jammer works on all mobile phones and smartphones.
How strong the effect is depends on the conditions. A smartphone usually has several microphones in different parts of its body, and voice recorders are built differently. The emitters send out a narrow beam, like a flashlight, and obstacles weaken the interference. Two phones on the same table can record the same conversation with different levels of clarity.
- The jammer emits ultrasoundIts frequency is above 20 kHz. People can’t hear it, but microphones pick it up
- The microphone distorts the interferenceThe microphone’s electronics turn part of the ultrasound into noise in the voice range
- Speech gets lost in the noiseWords in the recording are harder to make out. How much harder depends on the conditions
Suppressing Recording vs. Blocking Connectivity
There are two fundamentally different ways to keep conversations from being overheard through mobile devices: blocking connectivity and suppressing microphones. An ultrasonic jammer interferes with the microphone’s recording, while a Faraday Capsule cuts off the radio link the phone would use to transmit it. Inside the capsule, phones lose cellular, Wi-Fi and Bluetooth connections and go offline.
Without a network, though, a phone can still record sound. The built-in voice recorder doesn’t need the internet, and the file is saved to memory. An app can send it later, once the connection is back. A standalone voice recorder isn’t connected to any network at all.
It works the other way, too. A jammer leaves connectivity alone, so a phone next to it still receives calls and messages. One device doesn’t replace the other. They complement each other.
| Ultrasonic jammer | Faraday Capsule | |
|---|---|---|
| What it affects | The microphone: it interferes with sound recording | Radio communication: cellular, Wi-Fi, Bluetooth, GPS |
| What it doesn’t do | Doesn’t cut off connectivity, turn off the phone or delete existing recordings | Doesn’t stop the microphone from recording audio to the phone’s memory |
What to Check Before Choosing a Microphone Jammer
Find out five things before you buy.
- Has the model been lab-tested?Professional lab equipment measures a device’s real specifications with high accuracy. A test report lists the test conditions and the results: interference level and frequency, coverage angle and effective range. You can verify and compare figures like these, but not marketing claims such as “reliable protection.”
- Where is the model allowed to be used?Some jammers aren’t designed for civilian use, and others must not be switched on in crowded places. These restrictions are spelled out in the user manual, so check the manual, not the ads. For meetings, choose a model the manufacturer explicitly allows for use in an office or inside a shielded capsule.
- How should the devices be placed?The manual should show where the jammer and the phones go and which way the emitters point.
- How long does the battery last?Compare that time with how long your meetings run. Take the figure from the manufacturer’s exact technical specifications.
- How safe is the jammer for people, animals and electronics?People can’t hear ultrasound, but it still has a sound pressure level. Review the sound pressure test report and check the restrictions the manufacturer lists. Keep in mind that dogs and cats can hear ultrasound, and a jammer can interfere with hearing aids.
In 2023, the Velter Jammer was tested in an anechoic chamber at an independent laboratory. Based on the measurements, it was found safe for people and pets when used properly. Even so, the manufacturer advises against pointing the emitters at animals or turning the device on near people who wear hearing aids.
Using the Velter Jammer with the Faraday Capsule
The Velter Jammer is an ultrasonic microphone jammer designed to work inside the Faraday Capsule. Together, the two devices cover both channels. The capsule takes the phones offline, and the Jammer makes it hard for their microphones to record the conversation.
How to use the Velter Jammer:
- Press the button to turn on the Jammer. The LED indicator should light up.
- Attach the Jammer to the magnetic mount inside the capsule.
- Place the phones next to it.
- Close the lid tightly.
When you’re done, press the power button again to turn the Jammer off.
The capsule walls reflect ultrasound and keep the interference close to the phones. Using the Jammer outside the capsule isn’t recommended. Also, never hold a switched-on Jammer up to your ears, use it if you wear a hearing aid or point it at pets.
The Faraday Capsule and the Jammer are sold separately or together in the Velter Meeting Privacy set.
How to Test Microphone Suppression on Your Own Phone
The clearest test is to compare two recordings of the same phrase on the same device. Record one with the jammer off and one with it on. Here’s how it works with the Velter Jammer.
- Take the phone or voice recorder you want to keep from recording.Prepare a phrase of about 20 seconds that includes names and numbers. They’re harder to guess from context.
- Position the device the way the manufacturer’s diagram shows.For the Velter Jammer, that means inside the capsule with the lid closed. Start the built-in voice recorder and read the phrase with the jammer off.
- Turn the jammer on without changing anything else.Read the same phrase in the same voice from the same spot.
- Assess how much the interference drowns out the speech.Can you make anything out? If so, compare how many words are intelligible in each recording. For a more reliable result, have someone who doesn’t know the text listen to both.
More emitters ≠ better suppression. In 2020, University of Chicago researchers showed that signals from multiple emitters cancel each other out at certain points. For a typical multi-emitter jammer, these “blind spots” made up 17% of all locations within 1.2 meters (about 4 feet). A microphone in a blind spot receives only weak interference.
Facts First
An ultrasonic jammer is a specialized tool, but one that’s increasingly necessary for keeping conversations private. When you choose one, rely on verified facts: test reports, a clear and detailed manual and safety data for the device. Also keep in mind that a jammer interferes with the microphone’s recording but doesn’t cut off the phone’s connectivity. For confidential meetings, it makes sense to combine it with phone shielding.