UltraSound Detector is the App that allows you to detect ultrasound (ultrasonic) acoustic signals above the user-defined frequency (above 18 KHz by default).
This detector can be useful in following cases:

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  To check out the work-ability of various numbers of Ultrasonic electronic pest and insect repellent devices as well as any ultrasound emission devices if they work in the appropriate frequency range.
  
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  To locate the leak detection in air condition and refrigeration systems. You are probably familiar with the hissing sound a large leak makes. Smaller leaks also emit sound, however the frequency is too high for our ears to detect it. An ultrasonic leak detector lets the ultrasonic hissing sound be detected. Some leaks can be detected from several feet away, therefore access to the leak is not always necessary.
  
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  To detect the presence of Ultrasonic Cross-Device Tracking technology in some public places. The technology embeds high-frequency tones that are inaudible to humans in advertisements, web pages, and even physical locations like retail stores. These "ultrasonic beacons" emit their audio sequences with speakers, and almost any device microphone—like those accessed by an app on a smartphone or tablet—can detect the signal and start to put together a picture of what ads you've seen, what sites you've perused, and even where you've been.
  
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  To find out the voice to skull devices (V2K technology). This device is a weapon use for transmitting voices with low or high frequencies. Voices can be for commands or harassment attacks that may look like the target individual's own voice. Voice to skull technology is sometimes referred to as “synthetic telepathy.”
  
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  To detect some kind of ultrasonic weapons, that cause physical pain without human detection.
  
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  To check out the quality of tremble sounds (high frequency) of your loudspeakers.