Microphones, from those in smartphones to hearing aids, are built specifically to hear the human voice — humans can’t hear at levels higher than 20 kHz, and microphones max out at around 24 kHz, meaning that microphones only capture the sound humans hear.

The sound’s frequency is transmitted from ultrasonic speakers. It is completely inaudible to humans, but able to be recorded by microphones.

Researchers have designed a sound that is completely inaudible to humans (40 kHz or above), yet is audible to any microphone. The sound combines multiple tones that, when interacting with the microphone’s mechanics, create what researchers call a “shadow” — a sound that the microphones can detect.

When conversing, the inaudible signal translates to white noise in the microphone to prevent unauthorized microphones from recording voices. Because it’s inaudible, it would not interfere with the conversation. Military and government officials, for example, could secure private and confidential meetings from electronic eavesdropping. Cinemas and concert venues could prevent unauthorized recording of movies and live performances.

The signal can also be used to send communication between Internet of Things (IoT) devices, such as an Amazon Echo or Google Home, which would reduce the growing load on Bluetooth — the primary way IoT devices communicate. The signal could protect users from unauthorized recording when communicating with voice-activated systems.

The sound’s frequency is designed and transmitted from ultrasonic speakers, but the microphone — the receiver of the signal — is not altered in any way. Off-the-shelf microphones will react in the same way to the signal. Since the signal can be received without modifying the microphone, the technique could be readily available to interact with smart-phones and other devices.

There may be ways to misuse this technology, however. Inaudible sound jammers could affect someone wearing a hearing aid because the internal microphone would pick up that sound. Also, the sound could be used to jam all phones trying to make calls. Like all techniques, inaudible sounds can be used in different ways, but with the knowledge of how it can be used negatively, the researchers can develop strategies to prevent it.

For more information, contact Romit Roy Choudhury at This email address is being protected from spambots. You need JavaScript enabled to view it.; 217-300-7577.


Tech Briefs Magazine

This article first appeared in the April, 2018 issue of Tech Briefs Magazine.

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