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I understand that my question is not desireable for ham radio communication at all, but bear with me.

I want to use long range walkie talkies whose microphone is attached to an underground acoustic sensor and bury it into the ground, it transmitting their acoustics on a specified channel constantly, in order to "baby monitor" a certain area.

Obviously a number of these devices would need to work in parallel to be effective. I'm just wondering if a number of these devices are transmitting on a specific channel, will a receiver walkie talkie be able to hear all the signals coming from the ground sensors?

If not is there a radio setup/circuit that would allow for long range acoustic monitoring of all these sensors concurrently?

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if a number of these devices are transmitting on a specific channel, will a receiver walkie talkie be able to hear all the signals coming from the ground sensors?

The exact result depends on the modulation used, but in no case will it be all that useful.

  • If they use FM (almost certain for any modern off-the-shelf equipment), you will hear mostly the strongest signal, and if two signals are of similar strength you will hear interference sounds as well as some of the other signal. This is known as the capture effect.
  • If they use AM, you will hear the audio from all of them, but mixed in proportion to the RF signal strength (not the audio volume). So the one closest to the receiver will be much more audible.
  • If they use a digital voice modulation, you will probably hear the strongest one and other transmissions will be entirely inaudible except as interference; degradation and drop-outs of the signal, not any of the other audio.

In order to get multiple audio signals through, you must use multiple channels (frequencies), or use a system that is intended to share a single channel (e.g. as cell phones do).

If not is there a radio setup/circuit that would allow for long range acoustic monitoring of all these sensors concurrently?

What comes to mind is some kind of digital mesh network. Since you're intending to spread sensors across an area of land, a mesh means that each sensor node can forward data from others so that you get your monitoring done with less total power needed. Using a suitable audio codec, or a simple noise gate, you can have the nodes which are not hearing any significant sound not send any data, thus saving power (again) and bandwidth.

However, I'm not familiar with the field to recommend equipment, and it would probably require a bit of configuration and programming to set up the monitoring (though it is probably easier to make an inefficient prototype than something fit to run off batteries/solar permanently).

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    $\begingroup$ Re, "If they use AM..." Fun fact: Aircraft VHF radios use AM precisely because it allows everybody listening on a channel to know when two operators try to speak at the same time. If FM were used, then, for example, an air traffic controller might be completely unaware of an urgent message from a pilot because the receiver "captured" the signal from some closer pilot instead. $\endgroup$
    – Solomon Slow
    Jul 12, 2019 at 21:47
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    $\begingroup$ Re: If they use AM, you will hear the audio from all of them - yes, but you also hear a full-volume whistle at the audio frequency of the difference between the carriers. Without special effort, two AM radios are unlikely to be less than 100 Hz apart. You hear this effect listening to air traffic control. $\endgroup$
    – tomnexus
    Jul 13, 2019 at 5:01
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If it is a small number of sensors and they are relatively far apart, you could consider using a separate receiver for each transmitter, each receiver having a directional antenna pointing at its transmitter. This still uses one channel but has multiple monitors. You could use any number of summing methods to combine everything if you like.

There are several problems with this approach depending on how the sensors are laid out, the biggest being that "directional antennas" are not perfect and if the transmitters are close together (really this means small angular separation as seen at the receivers) this could have significant challenges. But with a little imagination I think you can see how to address it.

The "multiple receiver" thing adds some flexibility; receivers do not all need to be at a central location, they just need to be somewhere with alternate communication infrastructure (like an internet, for example). SDR receivers are inexpensive and fun --fun is important when monitoring underground pressure sensors :-) @KevinReidAG6YO 's observation that FM will only decode the dominant signal is a feature in this scenario.

One other suggestion... Depending on the bandwidth you are monitoring, you could consider using an acoustic frequency shifting technique so each transmitter sends a unique acoustic band on that transmitter frequency. Then AM would make more sense than FM and there would be no need of multiple receivers.

Good luck with your project!

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Modern digital cell phones share channels -- a cell site can service dozens of phones on a single channel. Whether this is practical for your application is questionable, however.

For digital cell communications, the phone digitizes the speech and transmits the data in short bursts, tens of times a second. The return transmission to the phone is handled similarly.

Each burst contains either the data since the last burst, or enough data to last until the next, when the sound is re-rendered to real time. Since the transmission burst is a tiny fraction of the real time duration of the sound it encodes, this allows multiplexing on a time-sharing basis of, in the maximum case, many more than a dozen cell phones on a single tower channel.

The technology to do this, unfortunately, is expensive, and represents decades of research in digital voice transmission. Further, parts of it are still covered by patents, so you can't just buy hardware that does this job (at least not at this level).

One way you might be able to emulate this, however, would be to use VOIP software. Skype is a well known example that handles conferencing well (especially if you don't try to conference video, but voice only). You'd need one VOIP device for each sensor site, and a continuous connection (ethernet, wifi, or connect through the internet cloud -- some software will work without a central server, some requires it), then set up a conference. How well this would work over the long term (say, more than an hour or two) is a question left to the experimenter...

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  • $\begingroup$ more 20 to 100 times per second than once or twice :) and "more than a dozen" is a bit of a understatement :) $\endgroup$
    – Marcus Müller
    Jul 13, 2019 at 21:22
  • $\begingroup$ @MarcusMüller When I used to hear them (in 2002 or so), digital phones were brand new, and it was closer to the speed of a slow watch tick (probably than closer to 4-6 per second than 2), rather than something I'd hear as a buzz. It might well be a lot faster now. $\endgroup$
    – Zeiss Ikon
    Jul 14, 2019 at 0:10
  • $\begingroup$ I can assure you that the GSM standard isn't that slow. On the contrary, the classical GSM audio codec encodes batches of 160 samples at once, and packs them into GSM frames, which works out to 20 ms frames. That roughly coincides with the period of spectrum allocation in GSM. What you might have been hearing might be specific mixing products of the control channel and other periodic things! $\endgroup$
    – Marcus Müller
    Jul 14, 2019 at 10:03
  • $\begingroup$ Modern digital mobile phone technology requires a very-expensive cell tower to coordinate activity and power levels in the channel. $\endgroup$
    – rclocher3
    Jul 15, 2019 at 15:32

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