I recorded a video from some birds around my car while the car was off and its alarm system was on. After watching the video I noticed that there is an oscillating noise recorded by the phone near ultrasonic sensor of alarm system. The recorded noise pattern is exactly the same as sensor's LED blinks. Now the questions are: 1- Is iPhone 7 plus microphone capable of recording higher frequencies than 20 Khz 2- If answer to question 1 is "Yes" how does iphone convert that recorded ultrasound to audible?


closed as off-topic by Andy aka, PlasmaHH, PeterJ, Bence Kaulics, Wesley Lee Jul 24 '17 at 15:48

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  • "Questions on the use of electronic devices are off-topic as this site is intended specifically for questions on electronics design." – Andy aka, PlasmaHH, PeterJ, Bence Kaulics, Wesley Lee
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  • \$\begingroup\$ Contact apple. Read this: electronics.stackexchange.com/help/on-topic and note the off-topic questions. \$\endgroup\$ – Andy aka Jul 24 '17 at 11:33
  • \$\begingroup\$ Perhaps the it is an alias of the signal that you are hearing. Put the sound in Audacity and run an FFT. (or ask it on SP) \$\endgroup\$ – Jeroen3 Jul 24 '17 at 11:40
  • \$\begingroup\$ Read about "anti-aliasing" filters in digital audio systems. Crappy signal chains don't always get anti-aliasing right. Contact Apple and ask them if that applies to the Iphone 7. If you need to fix it, a suitable anti-aliasing filter between mic and input socket should work. \$\endgroup\$ – Brian Drummond Jul 24 '17 at 11:49
  • \$\begingroup\$ I guess this is a good question but badly framed. \$\endgroup\$ – pipe Jul 24 '17 at 11:49
  • \$\begingroup\$ sounds like you're pulling rf/emi from the led's pwn signal, not ultrasonic undertones... \$\endgroup\$ – dandavis Jul 24 '17 at 18:03

Disclaimer: These are comments on audio processing in general. Internals of Apple products are not well documented.
Ultrasonic transducers emit rather large amplitude signals that can propagate to a microphone just as audio waves do. While microphones generally accept audio signals up to 20 kHz, some can respond to ultrasonic waves (above 20 kHz). There are a few mechanisms that could produce audible artifacts in an audio recording:

  • Alias As Jeroen3 has suggested, the sampling rate of audio recording may transpose the inaudible ultrasonic frequency to an audible tone. An internal anti-aliasing filter could be unable to deal with very large out-of-band ultrasonic frequencies.

  • Overload Non-linearity of the microphone, or the following preamplifier can cause audio signal amplitude to be attenuated or distorted. Compression or clipping of an audio recording could result while the ultrasonic burst is active.


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