I have an occupancy sensor connected to a lighting control system. It uses a 40Khz ultrasonic mic to detect sound to determine if people are still occupying a room.
I'm wondering whether the mic could be used to record normal conversation?
Electrical – Can a 40Khz ultrasonic microphone record normal voice speach
audiomicrophoneproximity-sensorsensor
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Especially for a home hobby project I'd probably start with PIR (Passive InfraRed) sensors. They are cheap and very effective at detecting something warm like a human body moving around.
However, PIR sensors will not detect static warm objects like someone sleeping or sitting still on the couch. With enough PIR sensors around the place, you can probably infer where people are motionless by where you know there was movement and in what direction. PIR sensors don't inherently give you direction, but enough of them activated in sequence does. For example three sensors triggered in sequence in a hallway is a strong clue someone is walking down the hall in that direction. If you saw motion of someone entering a room and then motion in the room, but nothing at the doorway, then you can make a good guess the person that entered is still inside but motionless.
This system isn't foolproof, but PIR sensors are cheap and remarkably sensitive, so with enough of them I think you can get to quite a useable level.
One thing to keep in mind is that other warm moving things will trigger PIR sensors too, like pets moving about. If you have a dog, then aiming the sensors so they only see motion a few feet off the floor helps. Cats jump around a lot, but are smaller, so maybe there is a way to not trigger on cats. This system will be a lot easier if you know the only warm moving things are humans though.
I can only relate my experiences: -
If you want to detect a normally-not-powered passive type tag at extreme distances you have to power to that tag from a significantly bigger magnetic field. Making your magnetic field stronger is the only way I can know (and can recommend). Making your tag more efficient in recovering a fraction of this power is also part of the deal. Making the energy needed by the tag smaller is also part of the deal.
Once the "passive" tag is receiving sufficient energy from that magnetic field, it can transmit an RF signal to announce its presence - because it is only very weakly powered it may not be able to transmit more than a few hundred microwatts. This transmission should not have to do-battle with the prevailing magnetic field that powers it - it should be on a carrier frequency that is unconnected with the power magnetic field for this to work most effectively. This will require that the stationary object that generates the power magnetic field is capable of receiving this RF signal.
So now you have two transmissions - the transmission that powers the tag and the transmission from the tag containing ID data - neither are at the same frequency if you want maximum distance.
At about 4 inches (maybe 5 inches if I pushed it), a system I developed could detect the presence of a normally unpowered device. However, I needed to transmit about 1 watt across the gap because the device was doing other things that needed the power - it was rotating on a shaft and wires wouldn't work. The FM transmitter it used was at 80MHz and transmitted at about 1mW. The receiver could detect this at about 1m but it wasn't particularly designed to detect it more than 4 inches. The magnetic field it generated was quite large and the coil it used was wound from Litz wire - I reckon it was about 3 uH and had about 400 volts peak to peak across it at 600kHz (work out the current for yourself!!). Operating the magnetic field at 13MHz could be better but it starts to become a trade-off because, in your situation you want the "detection area" to be large - this means a large diameter coil and you want maximum current through it to produce the bigger and more far-reaching field you are fighting against the inductance of the coil. You need current in that coil to produce a magnetic field and the more the better.
To get that current, I used 250 strand Litz wire and parallel tuning to make the circulating current in the coil much much bigger than the drive current from the generator. This makes it easier to design the generator of course.
In short, if you want to power the tag at distance, think big coil and think litz wire and think parallel tuning for maximum efficency. The power receive coil was also very low loss and highly tuned to get as much voltage as possible when set at the maximum distance. This is what you should focus on in my opinion.
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Best Answer
It really depends on 1. the microphone and 2. the circuit they've designed.
To take two randomly selected ultrasonic mic's from DigiKey:
Knowles SPU0410LR5H:
Invensense ICS-41350:
As you can see, the Knowles part's response to 40kHz is only ~2db less than the 1kHz response. This means that the microphone is as good at picking up "normal sounds" as ultrasonic sounds.
The Invensense part, on the other hand, is much better at picking up ultrasonic sounds. The chart only shows to 20kHz, but the datasheet talks about higher frequencies. Note that, even though the "normal sounds" are attenuated, the mic can still pick them up.
However, this is only the microphone. Even if the mic can hear the sounds, it doesn't mean that the circuitry can. For example, your sensor is likely filtering out lower-frequency sounds to make it easier to use ultrasound to determine occupancy. You can't really tell without looking at the design and the code.
Having designed purpose-built sound sensors, I can tell you that recording and transmitting speech is much, much more resource-intensive than only looking for occupancy. "Occupancy" is a simple "yes/no" signal. Speech would need to be sampled, recorded, buffered, and streamed.