RPM?

[Ian Inverarity]

Well, if it sounds fast it must be fast!

 

[hugh hargett]

so you think the little 20 or so dollar handheld audio rpm devices have some high dollar descrete audio components amounting to more than a condenser mike and some complimentary components?

what frequencies would you expect to see when trying to find rpm ? from zero to 1000 hz(0-60k) would be my guess id filter everthing else out on the top and bottom end.Even more so for the big blocks coz you wont expect to see 60k with that. Alot of the software ask for a range you expect to see. Im sure thats to filter out eronious frequencies and just look for what you want .

Jeff, to answer your question yes I think the new smart phones which have high definition audio as well as eq and sound tailoring, apps like "RTA's" and such can handle the job well enough. If there is a percentage difference consistency youd just factor it in.

As far as doppler is concerned I would think that the ear or mike would remain in the same position while only the source moved in this scenario but id simply take the measurement when the boat passes right in front. Its true that with the tach not being on the boat it will be subject to some doppler if you put the phone in the boat while running that would take care of doppler yes or no? I also think that all the audio tach devices have a +/- accuracy to some degree that acceptable for what were doing here.

Jeff please elaborate.

 

[RodneyPierce]

Well, if it sounds fast it must be fast!

haha, right!

Someday I hope to step out of the "kiddy pool" and into the "real pool". We will get there. Just takes WORK!!!

 

[hugh hargett]

Now I need an Iphone!!! This could be a huge step forward in testing tools for the normal guy. Being able to do that between runs, adjusting the pipe between each one,.. then checking RPM,.. this would be a GREAT advantage in testing!

I thought phils was a android app???? :huh: If you have any smart phone of any type id just do an app search for something similar. My link was for a windows based so if you can run a shell on your phones it may still work.

 

[RodneyPierce]

Yes, Phils was for an android app. Wont catch this guy owning an iPhone...... Droid for me!!

 

[hugh hargett]

Yep i-stuff is a slippery slope never ending year after year more $$$$ for basically the same stuff available on all the other top phones with a better GUI. As far as processors go the I touches arent even all that great. killer marketing though. I love my droid x great video camera great phone . The easiest in my opinion for people is the windows based phones coz more people are familiar with how to negotiate within a windows OS and they are much easier to intergrate with a windows pc. Macs cost alot and not as many people can negotiate as well on MAC os though the are some really great pc's.

 

[Tim_Duggan]

RP,

So you now know the rpm, and can measure the leading edge pitch as you have a pitch guage, you should be able to calculate the theoretical speed (if the boat and prop is working efficiently, and make changes to improve the efficiency if it isn't through set-up / prop tweaks etc.).

MPH = L/E Pitch x rpm/1057

 

[RodneyPierce]

RP,

So you now know the rpm, and can measure the leading edge pitch as you have a pitch guage, you should be able to calculate the theoretical speed (if the boat and prop is working efficiently, and make changes to improve the efficiency if it isn't through set-up / prop tweaks etc.).

MPH = L/E Pitch x rpm/1057

yes sir. Been working on it

The prop isnt working efficiently enough I dont think. Im just starting on the prop work now, so its bound to get better

 

[hugh hargett]

Yes Tim but the problem that Jeff is asking about is valid as far as doppler shift is concerned when you are analyzing audio in a video that the boat get closer to the mike then farther away. This shift is even more prominent as both the source and listner are both moving in respect to one another. You know how as things get farther away the pitch seems to change. Think about the sound of and indy car as it passes you and moves father away. This shift doesnt really come into play with an onboard audio tach coz the distance between the soure and the listner stay the same.

 

[hugh hargett]

Rodney you need a way to measure your speed accurately as that equation gives you the theory you dont really know your slip till you know what speed youre actually at, then you can make cup/prop adjustment to bring it into its theory. Rememeber Andys post.Too much cup will plow and make it slower.

 

[RodneyPierce]

Hugh, yes, working on that as well. Stalker is in the future.

 

[RodneyPierce]

And, weird you talk about the dopler shift. The point at which the boat passed directly in front of the camera/mic was when it read 32XXX rpm. It jumped around a bit until right when the boat passes the camera/mic. Then it was solid in the 32XXX range for a second, then went back to jumping around.

 

[hugh hargett]

Based on how dopler shift works is why I said the recording would be likely most accurate when the mic is closest to the source. Dopler can be seen sometimes in the spectral frequencies to in passing cars when the headlights shift from a redish to blue color . I cant remember if i need to reference the fiber optics text or communincations text probably the communications text ill have to look when I get home. Looks like you will soon have all the tools necessary to properly fine tune that rigger. Its running good though.What im interested in is the mods done on that rossi thats putting it a 50K unloaded. Wonder if shes got any torque.

Later

Hugh

 

[Greg Moss]

To track rpm via sound/video recordings this is what I do;

I get a camera video taken; the format doesn't really matter. Strip the soundtrack off into a wav file using whatever software you can find. Load the wav into a spectrogram display program set to have the narrowest frequency separation possible for clarity of display (and accuracy). This gives you a time-based image display, left to right usually, with individual "horizontal" but wavy lines tracking each harmonic component of the engine's exhaust sound. The fundamental frequency of my CMB RS21 on a 3.5cc tunnel is usually between 440 and 500 Hz. IMO; a traditional RTA may not give you a "tracking" display that is useful over an long time period.

The spectrogram program I use has a cursor that you can hover over the detected harmonics to read them off, and some programs will export the data to csv or xls. I have also managed to create a composite video file where the image and the frequency display timelines are merged together so that you can see the rpm variation together with the boat on the water. this is not an easy process though.

Converting Hz to rpm is quite straightforward but I'm not sure how the reality of rpm to speed can be captured without a radar gun to indicate the reality gap. Also the doppler considerations, etc.

If anyone is interested, I could dig up some screen shots and matching video...

 

[Tim_Duggan]

Doppler has been used for years to establish rpm by Pylon racers going WAY faster than what we do. A GPS logger with a tach is better for sure, but not always available or practical.

Greg - perhaps you could tell the masses what you do for a living!

 

[hugh hargett]

Greg I use a rigol scope so I have tracking and filter features. I just saw the pc app and added it to phils post but it not what I use. Its a brand new scope with all the digital crap on it so im still learning it. It not like the old style scopes we used in class. Rod gps is an option. People talk trash on them but they havent been far off a few mph anytime someone has went to the stalker from there gps numbers.An infared speed trap would be most accurate IMO.

THE STALKER IS BETTER THAN GPS BUT GPS IS BETTER THAN GUESSING OR NOTHING AT ALL! before the swarm hits.

Greg, I wasnt suggesting a RTA as a tool for this i was just saying if a phone can have that app it sould be fine to measure the freqs we expect to see. What program are you using to strip the audio into a wav file? That sounds alot better than fooling with the refresh and you tube over again buffering and s%*T.

Rod there are definately gps speed aps for android Im just to scared to fasten a 500 dollar smart phone to a 80 mph boat with duct tape like i see people do the garmins . Screw that. The other problem with gps is they need a line of site to the satellite so that sometime may mean the have to be on the outside of the boat instead of inside where theyd be protected from the elements. If you have the guts,rich or didnt have to pay for you phone retail coz of your plan go for it. But I never want to be stuck 2 years with a plan so I paid head up 571 bucks for my droid when they first hit.

Hugh

 

[Greg Moss]

Well FWIW;

I'm an electronics engineer formerly specialised in sound electronics, television and transmission engineering. I work at our National Film and Sound Archive where over 20 years I've done a lot of development work with analysis of sound for preservation, restoration and forensics. Now I'm just an IT systems manager... looking after the digital media aspects of our National Collection..

I also like boats

 

[hugh hargett]

Would love to see some of your tricks to extract just what you want from the audio.

 

[Greg Moss]

I'm at work now, but I'll see what I can do this evening... Don't want to bugger up the topic too much

 

[hugh hargett]

pm me definately and ill pass u my email.

thanks

Hugh