I have external powered mics, and I agree with Fenderman, the mics are very sensitive. I can hear what people are saying as they approach my walk and front door. I hear crickets, bird chirps, rain falling, etc.
I've taken the path of obtaining "implied consent", which is the legal requirement in my city. So I have a sign on the foot of my driveway that states audio and video recording is in place on my property. Anyone who proceeds past the sign has provided "implied consent".
When researching the situation, I found potential liability exists for me in both civil and criminal courts. Even if someone trespasses, they have "rights". Analogy: if someone climbs over my fence, climbs up a ladder I left leaning against the house, and then falls, I have liability exposure. Or if I set a bear trap in my garage to stop burglars, I have exposure.
I didn't find past legal judgements in my city that covered homeowner security audio recording. I'm not sure how common lawsuits or criminal charges really are, practically speaking. So a simple sign, planted right next to my alarm company's "This property is covered by xyz Alarm" sign, I figured I'm covered.
Lastly, the audio is recorded on my NVR. If I were to ever download footage to turn over to authorities, I'd strip the audio. Also, my NVR has the capacity to store 6 days of recordings. If a subpoena arrived, the footage would already be overwritten by then. Only my downloaded video-only recording would remain.
This thread raised the prospect of registering a homeowner's system with the police. I wondered if this would inadvertently let the police learn audio recording was in use, somehow. Would "registering" let the cat out of the bag, and result in "exposure"?
Audio might be helpful to solve a crime, even if that audio wasn't admissible in court. The police gather lots of info to solve crimes, and don't use all of it in court (eg: confidential informants). So maybe the police could view footage in my house, (audio & video), and maybe look the other way regarding audio recording, in the interest solving a more serious crime than improper recording. I'm describing "thin ice" for sure.
Bottom line: You can see why I asked about audio recording details in my reply to
@cainrand and
@Kawboy12R regarding a cam volunteer database.
Now that I've stirred the pot a bit more, I'll step back a few feet. I expect some to point out I mangled some of the finer legal distinctions...
Fastb