Audio Chronographing

[MrCrowley]

Tried some audio chronographing today, more difficult than I thought.

Parameters:
Cannon: V.A.L
Projectile: Golfball
Distance to target: 5m
Mic Distance: 2.5m from target and cannon
Target: MDF

Two of the audio files used an external mic, one used the laptop mic.
On shot 1 and shot 3 I got velocities of about 125m and 135m but that was sort of guesswork really. Wondering if anyone can help identify the impact points.

I also tried using SoftChrono but that gave me 220m/s, which is a bit out of GGDT's calculation. Think GGDT said about 160m/s for this cannon at 120PSI.

Attached in the .rar are the wav files and jpgs.

[Technician1002]

For the files to be of any use, the mic distance has to be known from the targets as it takes time for sound to travel from the sound source to the mic. If the distance is uneven (near muzzle and far from impact) then the sound travel time from the target back to the mike will need to be subtracted. Ideally a mixer board (stereo) and 2 microphones are used with each microphone set near each sound source at the same distance. That way the 2 mic to source distances cancel and the time difference is the flight time. A future plan of mine is to use a home built contact mic for the target mic. The plywood target would be the mic diaphragm.

When I'm not at work, I can do some evaluation of the sound files. I like to both listen to it, and look for the high frequency components of impact if they are not lost due to clipping or a speech input filter (common for voice use on laptops as a noise cancelling feature).

Shot 2 shows severe over compensation by the automatic volume control (sound mutes). If you have a way to attenuate the mic signal so the auto volume control is disabled, the sound would be much easier to evaluate. The flat portion in shot 3 shows this too. Do you have any recording equipment where the recording levels can be set manually with no auto gain control? The response from the sudden loud sound is causing problems with the recording.

[MrCrowley]

Just exactly was the mic positioned in relation to the muzzle and target?

Distance to target: 5m
Mic Distance: 2.5m from target and cannon

It was half way between the two as I said in my original post.

I'm thinking I may have to put the mic on the target though, to help reduce the sound of the cannon and enhance the sound of the projectile hitting the target.

I can do more tests no problem, I was looking for some ideas about getting better results as well. I think it was you who recommended SoftChrono awhile ago, did you ever use it? I'm sure I put in all the correct parameters with just Muzzle Volume and Impact Volume being the unsure ones.

I guess this means trying to Audio Chrono my hybrid is no longer an option. Not to mention how loud the damn thing is, it can shoot well over Mach 1 according to GGDT.

[Technician1002]

I may PM you later on how to build a contact mic into your launcher and target. They pretty much ignore ambient noise and work on just the vibration in the material. Do you have any way to attenuate your mic? Some condensor mics will saturate the built in pre-amp in loud noise. Using dynamic mics instead is highly recommended.

Attached is an audio file of the Mouse Musket fireing at 80 PSI. It is an indoor dry fire. The levels were set with an audio mixer so the sound capture device did not operate auto level controls. You can hear the pilot hiss, the main valve pop and room reverberations following the shot. Are you able to get this type of recording? The mic was near the breech to make the pilot venting louder in relation to the main discharge.

[MrCrowley]

Do you have any way to attenuate your mic?

Not sure. This is the external mic I used.

I'm open to any relatively cheap alternatives. Don't mind making a project out of this.

[Technician1002]

That is a condensor mic. They are powered by 5 volts from the PC. Note, this is a "Noise Cancelling mic" Some cancel noise by filtering out the high frequency stuff. Not good.

You are at the mercy of the mic and the audio card it is plugged into. I use the same type of mic a band would use with a sound board. I have 2 mixers, a small DJ one and a larger one. The DJ one is shown in my test setup photo. It will input 1 mic and either 2 turnables, tape decks, or mp3 players for DJ use. I use it to adjust the mic level before the sound card goes nuts with high volumes. The mixer is on the table to the right of the printer. In this case I used the pickup coils on the barrel and a magnet in the projectile as the microphone for in barrel accelleration measurements. Details of the proceedure are on this page;
https://inteltrailblazerschallenge.wiki ... rim+method

Directly taping a flat coil of 3 or 4 turns directly onto a pice of wood, and mounting a magnet on a piece of foam over the coil, makes an excellent in wood vibration detector using the wood as the mic diaphraghm.

Edit; In chat we discused using another sound adaptor and dynamic mic. I expect better recordings soon.



Edit, I was able to pick some stuff out of one of the shots, but due to the clipping, the impact info is lost. The hiss, the pressure wave of the valve opening and the pow of the projectile (crack) leaving the muzzle are identified. After that the sound sytem is hopelessly swamped.

[MrCrowley]

Wow, amazing how you can pick out stuff like that. Just saw your edit now. Tomorrow I'm busy all day, but the day after I should be free all day for testing.

As Tech said, in chat we discussed using a dynamic mic I found (good quality Akai microphone) and using a tone port to adjust the microphone settings.

[mark.f]

Well...

Not to get into you guy's business or anything, but I think this setup would actually be easier to implement than microphones and "vibration sensors".

[MrCrowley]

I'm looking in to that as well now. Tomorrow I'll do some mic testing and I might decide to go the phototransistor route if it is easier or I have the time.

[Technician1002]

A good recording will look like this. The boom is still clipped, and could have been recorded at much lower volume, but the end of the hiss and the pressure wave of the boom is shown as a very sharp shift off the centerline. This is the Mouse Musket discharge indoors with no projectile. With a loud target impact after the boom, it would be easy to see in the trace.

[MrCrowley]

Think I might need to put the target back at 10m to help with the recording, 5m seems a bit close. Wonder how much the golfball slows down in that time, probably takes 20m/s or so off the muzzle velocity.
Guess that is an advantage of the phototransistor setup.

[Technician1002]

5 Meters would be OK if the noise is loud enough to be seen in the tail of the launch noise and the recording is clean. I marked where it would show up with my cannon at 100 M/S. 10 meters will give better separation.

[MrCrowley]

Thanks that should help. By the way, would this phototransistor be suitable?

I'll continue with the audio recording and see how well it performs but i'm also in the mood for a little project and this seems like a fun one.

[mark.f]

Looks good, but what kind of electronics stores do you have around where you live?

[MrCrowley]

I have that store, Jaycar, (which is the best place in terms of range) or Dick Smith Electronics which have a more limited range but will probably have some phototransistors.
Jaycar is probably more like a RadioShack, perhaps better stocked. The Radioshack's I went to were fairly small (although, CBD stores).