Measuring speed with acoustics and a tape recorder...
 

Follow the link to the info:

http://groups.yahoo.com/group/halfApylonracing/

Measuring speed with acoustics and a tape recorder...
 

Can't find it. That could just be me of course, but could you be a little more specific?

Probably using some sort of Doppler method...?

Here ya go...
 

http://www.alphalink.net.au/~hughan/frames.html

It could be used, but they only claim a 10% +/- accuracy...quite a bit if you are going very fast. And you need to fly near the mic to get good results. Looks like it could be a good audio tach, but I don't have Excell, and didn't even download the program.

The results are in
 

It Works!

We tested the doppler speed measurement technique at the field yesterday and it worked very well. We had a radar gun to compare the results to and they were all within 2MPH. It's pretty easy to tell if you got a good sample. As long as the plane was travelling straight both approaching and leaving the microphone and the engine was running consistent, the readings were reliable. The best part is this method tells you your RPM with the prop unloaded. It takes some fiddling with the acoustic spectrograph program to get a clear graph, but once you get it right, the frequency shift is very clear. Just plug the frequecy approaching and leaving into the spreadsheet and whala, you get speed and engine RPM.

I originally contacted the guy in Australia, but his spreadsheet calcucated for KPH. A member of our yahoo group, HalfAPylonRacing, posted one converted for MPH. I found the spectrograph program through some Google searches and posted it in the group too. We used a micro cassette recorder to capture the audio and digitized the audio using a simple sound recorder program.

Our test subjects were 1/2A Shrikes with Norvel .061 Big Migs. Some sample results:

Master Airscrew 5.5x4
Pat's:
Static RPM - 18,800
Fastest pass - 74MPH
Flying RPM - 21,500

Jason's
Static RPM 18,300
Fastest pass - 68MPH
Flying RPM - 20,700

APC 4.75x4
Pat's:
Static RPM - 20,100
Fastest pass - 81MPH
Flying RPM - 22,800

Jason's
Static RPM - 19,700
Fastest pass - 74MPH
Flying RPM - 21,200

Pat was using 20% nitro, Jason had 15%.

Jason

Measuring speed with acoustics and a tape recorder...
 

Results with +/- 2mph accuracy?
Maybe not good enough for the FAI, but it sounds good enough for some comparison here at RCU...

Measuring speed with acoustics and a tape recorder...
 

That accuracy can be improved as one gets more comfortable with the technique. Also, the results are almost always a little less than the radar because the farther the plane is from the mic when it passes by, the calculated speed is reduced.

I've noticed that by tweaking the settings on the spectrograph program I can get graphs that are much easier to analyze. Ideally, you want to see a clean consistent approaching freq and a consistent departing freq. Sometimes the graph is hard to read, but by adjusting the record volume and lowering the sampling rate I can get clean graphs.

The biggest increase in accurach will be when I get the recorder mounted on a pole so someone doesn't have to stand in the field ready to dodge out of the path of a speeding plane. Once it's on a pole I'll be more comfortable flying closer to it.

Jason

Measuring speed with acoustics and a tape recorder...
 

I would like to see the spectrum analysis of the recording if you actually hit the pole... :D :D :D

I'm going to give this method a try too, "sounds" fun to experiment with.

I've just been thinking about something: how would the rpm calculations turn out on a 4-stroke? Half the real rpm?

Also, would the program work on a Works Spreadsheet too?

Measuring speed with acoustics and a tape recorder...
 

heres one of my graphs.. clean enough?
this was about 170 mph

Measuring speed with acoustics and a tape recorder...
 

That looks very clean, I haven't managed to get graphs that sharp. On the up side, I don't see so many harmonics on mine, just a single line that at a frequency identical to engine RPM.

Philly, how do you keep the graphs clean at the point of flyby. Do you normalize the volume? When I play a tape into the spectrograph, I get a big blob on the graph at flyby because it's so loud.

Jason

Measuring speed with acoustics and a tape recorder...
 

i dont know why there so many, i just do them till one has the right RPM, so far thats near the bottom...
you can turn the recording volume down right?
i dont really know what im doing with this yet, i just took them from vids on my web site of Magnums doing flypasts for my camera and analized that.
the mess afterwards is the guys laughing that he got that close to us at 170MPH
try to grab WAV from the vids on my site, theres a few that can be used for fly pasts. i'd like to see how fast Jamies Typhoon went past

Measuring speed with acoustics and a tape recorder...
 

Cool, I'll check them out and post a pick of the spectrograph I get.

I wondered what that was at the end if the pass. I don't know if I'd be laughing if I got buzzed at 170. At least in th 70MPH range (with the 1/2a shrikes we are flying) you have a chance to dive out of the way.

Measuring speed with acoustics and a tape recorder...
 

watch the videos, some are longer just to get those reactions :D
heres somes files to play withy, i know what my results where at least....
ok.. cant post wavs... email me via my site or button below and i'll send some over

A clean analyzer pic
 

Attached below is a clean analyzer pic of one of my half-a's going in the high 90's. I used the settings shown on Hughan's page, and optimized them a bit from there.

Measuring speed with acoustics and a tape recorder...
 

Once you have the data into the spectrogram, how do you use the cross hairs? I have 10 or 12 parallel, horizontal lines that have a step down. Kinda hard to describe. How do you measure the data?

phillybaby,
I got the wav from John's fast run off of your site.

Measuring speed with acoustics and a tape recorder...
 

I'm getting ballpark speed figures, that seem to be correct, but the rpm is like 171906.96, 288332.51.

I still don't think I'm putting the cross hairs in the right spot.

Measuring speed with acoustics and a tape recorder...
 

i assume your using the Excel program.
Right heres how i do it.
So far the correct line seams to be the second one up. move the cross hair to the top of the curve where its still flat and put the frequency into the first box ( coming ) then put it down the bottom of the curve where its flattens out and put that in the ( going ) box type in a factor of 10 and you get an answer, the RPM should be roughly what your ground based was, tho so far i've never seen two lines close to the same RPM so its obvious which is correct.
your RPM should be in the 20-22k range.
I'm not sure why i get lots of lines, maybe its because my camera just records that way, I'm not sure about all the settings yet for analyze file

Measuring speed with acoustics and a tape recorder...
 

I got it! I also get multiple lines. The second one up will give you the accurate engine rpm, but they all give accurate speed. By tweaking the window gain, and analisys windows, you can get clean lines.

Here are the settings for one line
 

and correct RPM measurement.

These are what I used to create the pic above, and to create all my findings.

I have found it usefull to make many runs both upwind and downwind as well. An average of them all is the best. I also threw out 'odd' runs that just don't sound right.

Measuring speed with acoustics and a tape recorder...
 

matchlessaero:
what settings would you use to scan a file rather than an input? i cant input sound and record at the same time :(
Still waiting for a nice day so we can do this. i'll put my cam face up on the patch and record just sound, then shout at it whos comming from where and get lots of runs, out on the patch they can fly right over it safely.

I was surprised i was able to test the 200mph car from the video in the RC cars threads, very short unclear lines and about 2 per second:D

Measuring speed with acoustics and a tape recorder...
 

Philly, are you referring to those cars that run in circles on a control line. Those won't work for doppler because they don't go in a straight line. I suppose there is a highly complex formula that could be used to compensate for the path, but you'll need a degree in mathematics to sort it out. hehe.

I've improved the quality of my graphs by not pumping the recording directly into the PC using Line-in. Rather, I'm using "Scan Input" and playing the tape recording into the mic. I don't get the signal over strength (blob on graph) problem and it's much easier to queue up and analyze a pass by manipulating the microcassette tape, rather than a wav file.

Jason

Measuring speed with acoustics and a tape recorder...
 

thats actually how i had to do the control line car because my video editing software cant edit that type of file. i got exactly the same kind of signal. I didn't however change the settings to match the screen shot above.
However for how i get the sound in the first place this is a very round about way of doing things when i can just scan the file. More tinkering with settings required me thinks.
The car in the video i sampled came right at the camera which was set on the side of the track, not much different from having the plane not fly right over your head. the graph did differ from plane recordings as theres was a flat line which first picked up sharply as the car came around into shot and closer range then very quickly fell as the car went past the camera and out of range.
Not ideal but interesting that i got a figure within 10MPH of the advertised top speed of these things.

Measuring speed with acoustics and a tape recorder...
 

Hi.
I got the software last night. I can't really get the nice graph,or lines. I have experimented with the file from phillybaby (nice homepage by the way),but the spectrum is really just a mess. I have had a look on the colorsettings,but can't figure out what to do. Can anyone specify the changes that has to be done in the settings,on top of the ones mr Hughan describe in his setup

Measuring speed with acoustics and a tape recorder...
 

Look for the posts from Jason Devine on the 1/2A pylon racing group below. Jason and matchless do a great job walking you though the steps. Also, all the files and programs are available for downloading.

http://groups.yahoo.com/group/halfApylonracing/messages

Measuring speed with acoustics and a tape recorder...
 

down-shift

I couldn't really find any info on setting up the graphics display on the 1/2A pylon racing group. My signal is ok and recording seems to be ok,but the graphic result isn't possible to analyze. I would like the nice curve...
Anyone tried the trigger option?I will try to use my laptop on the field,and the trigger option will start the recording automatically if I set it right.

Measuring speed with acoustics and a tape recorder...
 

On the color palette/scroll display, play with the db, range and gain. It helps if no-one else is flying. Play the tape into the mic, then hit scan input, then in the lower right hit record enable. Then when the spectrum starts to come in hit save, then stop when you are done. Then hit analize file, and mess with the color palette/scroll display until you get something you can use. I use 16384 for the FFT, and bump the band up to 150. Put the time scale on the lowest number your pc will handle, I think 0 might be automatic.

I got 108mph yesterday as the fastest run, after tweaking half the night, I got 121 today. The difference in reaction time is exponential. Great prog!

Hmmmm, question: I have a Mag 25 spinning a MA 9x4 at 17.5K on the ground, you could really hear it unload in the air, I just had no idea how much. The results say consistantly 75 to 85 mph and 18.5 to 19.2K. How is this possible? 4 pitch at 19.2 should be only about 73mph?