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# Doppler Siren

Level-pitch Emergency Sirens
 (+4, -2) [vote for, against]

Vary the pitch of the siren sounds emitted by a travelling emergency vehicle with respect to that vehicle's velocity and its audience*.

Caveat: *Does Require the emergency vehicle knowing the position and velocity of every member of its potential audience as well as uni-directional speakers to target the correct Doppler-shift-compensated signal to each.

 — Dub, Sep 06 2012

Shepard tone http://en.wikipedia.org/wiki/Shepard_tone
Plays audio example. [doctorremulac3, Sep 07 2012]

 It would probably be effective enough for practical purposes to have the low-frequency speakers pointing forward and the high frequency ones rearward, the frequency shift being calculated on the basis of the vehicle's forward airspeed (not groundspeed).

 That way, the tones wouldn't change.

[+]
 — 8th of 7, Sep 06 2012

 You could have the front speakers pitching down and the rear speakers pitching up, the speed determining how fast it changed it's pitch.

 You'd play what's called a "Shepard tone" going either up or down. (see link)

 It would only be totally flat for people standing still relative to the motion of the siren but those moving would have the pitch sound somewhat flatter. If other cars were going the same speed the effect would obviously be lost.

 For a science museum exhibit this would be amazing. Just have a track with a little car with speakers on the front and back playing their respective tones.

Very cool idea. [+]
 — doctorremulac3, Sep 07 2012

I believe, to some extent, the doppler effect is desirable, since it helps those ahead of the vehicle realise it is approaching them.
 — MechE, Sep 07 2012

 I thought the whole charm of the post was there being no mention of why you'd want to get rid of the doppler shift, just that you could.

 Which is reason enough for me.

But yea, doppler shift does help you know when the thing's coming at you.
 — doctorremulac3, Sep 07 2012

[doctorremulac3] Wow, I like that Shepard Tone. Is it the same technique that ducks use to avoid echoes?
 — Dub, Sep 07 2012

Qua?
 — gnomethang, Sep 07 2012

 [MechE] {checks the URI.} OK, I'll make an adjustment. The tone is varied for all observers *except* the target of the emergency call. They hear an unmodified version, so they can guage the approachig vehicle's speed and distance.

Qua/ Indeed!
 — Dub, Sep 07 2012

ducks avoid echoes? qua(ck)!
 — po, Sep 08 2012

[+], with [8th of 7]'s correction.
 — Inyuki, Sep 09 2012

 I still think Doppler Siren sounds like someone out of a 1950's B movie, the one with the giant asparagus menacing mankind..

 probably the one with a leather bomber jacket and fag always in his mouth, the one who says "So, Doc, whaddya gonna do now".

Doc being either that complete loon out of the original "the thing" or Pierce Brosnan out of "Mars Attack"...at at at at..at at..
 — not_morrison_rm, Sep 10 2012

Presumably the vehicle will play the time honoured obligatory strains of Colonel Bogey as it disappears into the gloom.
 — Lesser Spotted Kiwi, Sep 11 2012

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