Sound to imitate a WWI machine gun

[Andres Rodriguez]

I have a large radio controlled model of the Fokker Eindecker(the gun was synchronized to the motor and shot through the propeller arc without hitting the blades). I am trying to simulate the sound of the machine gun. The sound of each round will be trigered with a Hall effect sensor and a magnet attached to the propeller shaft. My problem is that I can't figure out the paramenters I need to use on the sound command, i.e. note and duration.
Any help would be appreciated

 

[manuka]

Yikes!

Mmm- 400 rpm each gun? Twins? Synchronized firing? Forget tones as I'd say PULSOUT pin,duration will be your friend,organised in a tight loop & amplified to suit. The pulse duration (relating of course to the firing) is in 10µsec units, so ( as an example) PULSOUT 20 will give a 200 µs ( = 0.2ms) pulse. Since a typical 4MHz 08M loop (being the period between rounds) is only about 1 millisec you'll need a decent pause to reload the next round, although the H.E.S reading could provide that automatically. Of course 400 rounds per minute is ~7 per sec (thus ~140 ms between shots), meaning you have ample Picaxe time to get organised.

Is this all you'll be doing with a PICAXE on board your Fokker? Rather a waste! How about at least some pulsed LEDs in parallel to relate to the firing ? The overall effect could be so realistic that you'll probably need to provide trenches,tin hats & toilet paper (!) for the unwary too. Pseudo-code to try initially -

ww1:
(HES READADC routine about here)
pulsout 2,10 '100µs = .1ms at PICAXE pin2
pause 140 ' 140 ms pause if needed
goto ww1

 

[andrew_qld]

My understanding of syncronized machine guns was that a cam stopped the gun from firing while the propeller was in the way. The machine gun still fired at its normal cycle rate (430-440 rounds per minute for a Maxim LMG 08/15) whereas the propeller went at about 1000 RPM. The propeller could rotate several times between the gun firing.

Unless you are actually going to actually fire something from your model I'd just experiment with the sound command on the Picaxe until you had something that sounded right then repeat it 400 times a minute.

If you syncronize the sound to an RC model engine doing 10,000 RPM then it isn't going to sound very authentic.

Thats my 2 cents worth anyway..

 

[Jeremy Leach]

What I'd probably try is to make an amplified white noise source and then chop this sound rapidly on and off with the picaxe output. Can't gaurantee it would sound right though.

 

[Dippy]

I agree with Jezzer though I'd use white noise + sharp attack/decay envelope. Op-amp reading time. Isn't a specialised chip available for this?

 

[manuka]

Specialised AK47/bomb drop/explosion chip =splutter! They've been around since Bill Gates wore short pants & of course "drove" many a '80s video game. One shudders to picture engineers selling their souls designing such delights as the complex sound gene SN76477/88...

 

[Dippy]

Chirp, jangle, woosh, boom. That's what it says on the Data Sheet.
Job done then.

 

[jodicalhon]

If you'd like to try Jeremy's idea, there is a very simple white noise generator on this page (figure 7a):
http://www.uoguelph.ca/~antoon/tutorial/xtor/xtor6/xtor6.html

I haven't tried it, so can't attest to its usefulness.

There's a similar idea here:
http://www.maxim-ic.com/appnotes.cfm/appnote_number/3469

Jo

 

[manuka]

Given the piston engine an esteemed "SpeakJet" chip may be an teasing overkill? Although ~US$25 these have been all the rage since ~2005, & overdue hence for our PICAXE treatment. Anyone worked with them? See => http://www.robotshop.ca/home/suppliers/magnevation-en/magnevation-speakjet-ic.html

 

[hippy]

I believe that SOUND frequency parameters of 128 and above generate white noise and running at 8MHz might improve noise, although lower frequencies and under-clocking can also be used to good effect.

I wouldn't worry about synchronising to actual propeller as that seems to be a lot of effort for little gain, unless it were specifically to demonstrate how the gun only fired while not obscured by the blades. If the blades are turning and the guns are firing people will automatically fill in the blanks ( unintended bad pun ) and assume they are interleaved with the propeller. It's good enough for Hollywood where they just throw dagga-dagga-dagga on the sound track.

 

[Dippy]

Out of interest, how fast does the prop go round with your little Fokker?
Too fast and the sound would just get blurred.
Dagga-dagga-ratta-tat-tat sounds good to me.

And yes, hippy, that was a terrible pun.

 

[Michael 2727]

My $0.02c worth.
To get any decent sound you would need a large speaker
or it will just sound like a bug in a bottle.
Then you need an amp, etc.
I would look for a cardboard or steel can with a steel base.
Then use a small hobby motor with a flexible strip mounted
to the shaft plastic or rubber then a plastic or rubber hammer
mounted on the end of the strip, which when rotated
will hit the can base as a drum. This will be far louder than
a small amp/speaker. You can adjust the rpm to suit.
Some older good quality cassette tape motors had fairly
slow RPM, good torque and ran from 3v to 6v.

 

[Andres Rodriguez]

Wow, lots of help, THANK YOU Gentlemen

I posted this question late last night and went to bed. I just got up (I live in NY) and found all these suggestions.
A bit more information, about the project. The Eindecker's engine (Rotary Oberursel) turned 1600 RPM and the German guns fired 800 rounds per second. Some Eindeckers had 2 guns therefore they fired one round per revolution.
I am using a British engine (Laser V200). It is four cycle with two cylinders in V configuration and electronic ignition. I expect it will turn about 8000 RPM so that part will not be very realistic but that is understood by the judges.
The Picaxe will be used also to light a couple of LED inside the barrels of the guns. Another function will be to force missfiring on one of the cylinders, those planes did not have a throttle so to slow down they used a button to ground the ignition momentarily.
This plane will be used in competitions for Scale RC Airplanes. All mechanical features (i.e. retacts, flaps, guns, etc if appropriate for the type of plane) are demostrated with the plane on the ground in front of the judges. They have to see/hear that the sound occurs when the blades are out of the line of fire.

 

[hippy]

They have to see/hear that the sound occurs when the blades are out of the line of fire.

A solution to that may be to measure the speed of prop rotation ( or simply flag if the engine is on ) so you can run in two or more modes; the synchronised mode for judging and free-running modes when in flight where synchronisation would not be discernible and the resultant effect more realistic.

 

[boriz]

"a magnet attached to the propeller shaft" ?

Surely a lot of effort is put into balancing these engines and props!. I would go for an optical system, or even synchronise from the electronic ignition.

 

[sedeap]

Gun Fire

So... IF i understand correctly... 8000 RPM....

60.000µs/8000=7.5

Mmmm... 7.5µs per turn? Holy shit! too fast !!!

Probabily need 3 or 4 turns to "shoot" one round, or maybe sound like buzzer.
and surely don't want mess with the propeller balance attaching magnets on it...
so... the propeller body, have metallic "core" ?
or have screws?
maybe that can be magnetized instead of "put" external magnet...
or use one kind of "flywheel" to mantain balance... or somethig like this...

Not simply task... at that speed.