How is sound activated and triggered recording work?

I'm thinking of getting a surveillance camera. probably recording Sound activated. How strong is it has to be sound. For example, if only it was someone moving on the start recording when the sound has to be stronger. On the other hand, would be passing cars outside the house to activate the recording.

The microphone heard constantly and when the volume reaches dB predetermined level begins recording. Generally, you can set the level of what you want. Motion sensor can be a trigger for better surveillance camera.

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