A long beam supports a converyor-belt-like conveyor belt type thing that runs along the top of the beam, around the end, and along the bottom, and around the other end. This belt is driven by a motor connected to a variable speed controller marked out in miles per hour.

On the conveyor belt style component is mounted an array of lots of loud loudspeakers. These don't point in any particular direction but are omnidirectional speakers.

Each speaker is wired to a set of sliding contacts which contact to a sliding contact contact strip mounted on the beam, such that each speaker speaks only when it is at a position on the conveyor belt such that it is at the top side of the beam but not on the bottom side or on the ends.

Now while the belt is running and audio is fed to the speakers via the sliding contacts, the moving sound source will provide a delicious doppler effect even though the unit as a whole is not moving.

Mount the whole rig to the roof of your electric car and feed engine noises to it, giving the illusion to passers by that you are approaching at 70 mph even when you're dawdling at a walking pace. Or turn the speed control knob to reverse, to broadcast perfectly undopplerised audio from a speeding vehicle - perfect for emergency sirens.
-- pocmloc, Jun 22 2015

now I watched who morecambe and wise are https://www.youtube...watch?v=myptvBfapls
[pashute, Sep 10 2021]

// broadcast perfectly undopplerised audio from a speeding vehicle - perfect for emergency sirens. //

Now, that would be spooky. Two sets of speakers, one forward-facing, the other rearward. Process the output to downshift the forward signal and upshift the rearward signal, proportionate to the vehicle's forward velocity.
-- 8th of 7, Jun 22 2015


//70 mph even when you're dawdling at a walking pace// But 70mph _is_ dawdling at a walking pace. Just try it on a motorway.
-- MaxwellBuchanan, Jun 22 2015


If you mount lamps alongside the speakers then you will get automatic direction-specific red-shift / blue-shift as well. Win-win.
-- pocmloc, Jun 23 2015


Perfect for emergency vehicles that don't want you to know whether they're approaching, I suppose, but I feel that is more like something a vandal would do to the vehicles, than something the drivers would want.

I think the reverse would be more useful, the emergency vehicle giving the impression of coming towards you even if it's had to show down for traffic, so you know you still might need to get out of the way.
-- caspian, Jun 25 2015


This could easily be incorporated into the chimes of ice-cream vans (which would permit usage of the old Morecambe & Wise joke: <ambulance siren sound zooms past> Eric: "He'll never sell any ice-creams going at that speed")
-- hippo, Jun 25 2015



random, halfbakery