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There is a closed question

https://physics.stackexchange.com/questions/276667/what-is-the-maximum-distance-for-eavesdropping-sound-waves

on Physics Stack Exchange and I thought this might fit better in engineering (also as one of the comments there has been suggesting). And my question here is related to that but a bit different.

If we want a remote listening device which eavesdrops (collects) the sound signal and then transmits it to some receivers far away (so this process may require a type of electroacoustic transducer to convert the sound signal to electromagnetic waves, etc.）, typically how much power/energy would this entire device require, with respect to the distance of the transmission (receiver)? What are the limitations in its engineering and powering perspective?

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  • $\begingroup$ I would think the power would be dominated by the radio tranmissions rather than the soundpickup. $\endgroup$
    – DKNguyen
    Commented Oct 11, 2021 at 21:30
  • $\begingroup$ @DKNguyun Thanks for the comment and yes, that's also one of my major points here ("remote"). If the device were connected to a fixed power source (like a plug) then it would be an entirely different story. $\endgroup$
    – user271828
    Commented Oct 11, 2021 at 21:37
  • $\begingroup$ Mobile phone : all around the world. $\endgroup$
    – Solar Mike
    Commented Oct 12, 2021 at 3:14
  • $\begingroup$ Welcome to Engineering! This looks like a 'Naive design' question. Such questions are excessively broad and are therefore not a good fit for our format. See if you can edit your question to make it specific and answerable. $\endgroup$
    – Wasabi
    Commented Oct 12, 2021 at 15:18

1 Answer 1

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There is an upper limit for how loud a sound that can be produced in air on earth. I think that moonshot rockets were about as loud as a sound can be.

Now we have sound propagation. This will be affected by the ground, temperature, conditions, and a ton of other things. Realistically, to get sound traveling "the maximum" we will need conditions to support curving the sound around the earth.

We then have the inverse square law for sound - the intensity of the sound wave decreases with the distance from the source.

Lastly is the size of the receiver. Britain built huge concrete parabolic acoustic listening devices to detect bombers. These concentrate the sounds from a particular direction for improving sensitivity. I suppose there is an upper limit in size of what could possibly be built for this, practically it is probably around 500 to 1500 meters if resources weren't a constraint.

So none of this actually gives an answer. But a combination of loudest sound and largest receiver would likely allow reception on the opposite side of the earth, which is practically the largest distance we could achieve.

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  • $\begingroup$ Hi Tiger Guy, thanks for the answer. I wonder what if the sound signal is converted to other types (say radio wave) before transmitting, then how the maximal transmission distance would be related to the power, etc., given the constraints on engineering and physical laws/conditions. (Sorry if my question is not clear enough.) $\endgroup$
    – user271828
    Commented Oct 11, 2021 at 22:15
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    $\begingroup$ @user271828 This is going to get closed to because it's too broad. It's like asking what's the biggest swimming pool we could build? $\endgroup$
    – DKNguyen
    Commented Oct 11, 2021 at 22:44

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