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In California, audio recording without express notice or consent of recording of a confidential conversation generally can only be carried out if an recorder warning beep-tone is audible for each party to hear at the beginning of the conversation.

And while case law does not specify requirement of such a recorder warning tone to be compliant with the federal standard or even reference it to establish that such a standard is the basis of such a case law, out of an abundance of predation, it might be reasonable to use such version that is federal standard compliant, too.

So far, I found a few lines on the matter on Wikipedia and therein the federal standard definition of a recording warning tonehere.

However, I have not found any examples of such a half-second, 1400 Hz burst that would meet this federal standard definition.

Any legal audio library that may contain it?

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    It's trivial to generate such a tone with standard audio software. For instance with sox you can do sox -n tone.wav synth 0.5 sine 1400. Nov 6, 2021 at 2:23
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    Or play a high F on your piano or favorite instrument. Or ask a coloratura soprano to sing the high note from the "Queen of the Night" aria Nov 6, 2021 at 3:49
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    I wonder if this would be on topic on out Open Data sister site. I suppose it depends whether sounds are "data". I would ask there & see, but you have already accepted an answer. Nov 6, 2021 at 8:16
  • Thank you, I didn’t know there existed such a site!
    – HJay
    Nov 8, 2021 at 21:11

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I could not point to a "legal audio library", per se, but based on @Nate Eldredge's tips, I found an online tone generator in case any other options would be less accessible:

www.wavtones.com

Here you follow the same advice as above (1400 Hz etc.), and choose the option "burst", then "on time" 0.5 seconds, with "off time" 3 seconds. When done, there is a less than conspicuous "Download .wav file" button where you can get a copy based on the presets. BUT THIS WILL STILL NOT SUFFICE.

From here, you will need to use an audio editor to elongate the "off time" by generating silence or copying silence sufficiently to 14.5 seconds. That would create one instance of the beep and the period of silence in an audio file which would need to be kept playing looped without recording-party intervention or ability to interfere every 15 seconds without any pause.

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