Porous fill moderators, any success yet?

[subscriber]

Old Crow, that looks like an OldSpook design.

[OldCrow]

Yeah, it is funny how that works.   

[WhatUPSbox?]

Bruce, to answer your instrumentation question from a few days ago, here is what I'm working with and some output from a checkout shot. It provides some additional timing and spectral information. I'm not sure it is much more useful than the basic soundmeter outputs.

[OldCrow]

That's pretty clean clean.
Nice setup.

[OldCrow]

Bruce, to answer your instrumentation question from a few days ago, here is what I'm working with and some output from a checkout shot. It provides some additional timing and spectral information. I'm not sure it is much more useful than the basic soundmeter outputs.

Muzzle velocity approximate?  15" equates to about 1.5 ms between the moderator and the backstop.  I don't see any sign of a return from the backstop?  What is going on with that LASER gate?  It looks like it triggers goes ~24ms and triggers again and then goes ~21ms and triggers again?  Am I reading it wrong?

[WhatUPSbox?]

This was set up for check-out in a small room. I'm sure there are many other bounces spread out in that trace. The intent is to use it outdoors and minimize those.  The trap is close but I used the quietest trap I've found (open cell foam over old clothes in a trap.) 500 ft/sec .177. The laser trigger looks like the image below when it triggers. It is very consistent and triggers on the tail of the pellet exiting the muzzle (plus about 2 mm). You could almost use the width as a chrony except that the beam has some finite width. It does tell you if you are out of CO2 on that gun.

[WobblyHand]

Bruce, to answer your instrumentation question from a few days ago, here is what I'm working with and some output from a checkout shot. It provides some additional timing and spectral information. I'm not sure it is much more useful than the basic soundmeter outputs.

Thanks for the pictures and explanation.  If in a small area, echo sources can be tracked down using the time delay and the speed of sound to get the range.  One needs to figure out the corresponding round trip path, but it can at least reduce the number of candidates.  Ruling those out, there's hammer bounce which I would imagine is in the 10's of milliseconds since things with mass take a while to move.  Probably a lot easier to optimize an LDC if one reduces bounce.  My 2240 has an SSG, but I've only done a sloppy adjustment.  Probably needs some more attention.  I also see you are digitizing at 96KHz, which I ought to do as a minimum.  The MIL handbook for impulse measurements actually recommends 200KHz minimum.  I don't know if my Mac can do that, but I'll check.

[WobblyHand]

Yeah, it is funny how that works.   

Thanks for your explanation Mitch.  Appreciate it.

[WhatUPSbox?]

I'm using a 2240 with a 1377 barrel. It has a power adjuster but not an SSG (I really need to make one). I was mostly interested in the response in the first couple of millisec so reflections from more than a foot away were less of a concern. In the initial shots, the mic (2) aimed at the base of the LDC started responding consistent with the sound coming from the wall of the first segment. The soundmeter mic (3) aimed at the LDC exit started responding before the pellet exited the LDC. I did not figure out when and where the bulk of the sound energy came from. I don't think I can do that in the small room. I did measure the lags of the three DSO processed mics and, accounting for the air gaps, they ranged from 22 to 40 micro-sec. I used my Copper Contact Clapper Calibrator (apologies to Johnny Carson and Jack Webb).

[WobblyHand]

My mac can support 96 KHz sampling.  Also seems to record at 192 KHz, but I'm not so sure about the microphone response.  I sent an email to Dayton Audio requesting the calibration file for the IMM-6, but haven't heard anything from them.  I'll check the generic response.  It is 18 Hz to 20 KHz +/- 0.5 dB.  Have no idea what the response may be outside of that region.  May not be worth digitizing at high rates if the response falls off a cliff at above audio frequencies. 

After messing with this, I think my Mac only supports 96 KHz audio.  I attempted to digitize at 192KHz, but Audacity only offered to save at 96.  I did change it to 192K when I saved.  An FFT of captured noise shows no energy past 48 KHz.  It also shows the microphone has practically no response after 20 KHz.  Not surprising, since it is commodity sold as an audio mic.

[WhatUPSbox?]

I think you can just download the file https://support.daytonaudio.com/MicrophoneCalibrationTool

[OldCrow]

This was set up for check-out in a small room. I'm sure there are many other bounces spread out in that trace. The intent is to use it outdoors and minimize those.  The trap is close but I used the quietest trap I've found (open cell foam over old clothes in a trap.) 500 ft/sec .177. The laser trigger looks like the image below when it triggers. It is very consistent and triggers on the tail of the pellet exiting the muzzle (plus about 2 mm). You could almost use the width as a chrony except that the beam has some finite width. It does tell you if you are out of CO2 on that gun.

Sweet.  At 500 fps (5.45 in/ms) you can also calculate pretty accurately where the pellet exits the moderator.

What are the chances that the signal you labeled "hammer bounce" is really a barrel harmonic acting as a carrier for higher frequencies?

It really doesn't matter but I am curious, why does the LASER trigger seem to fire three times on that dark blue trace?  Sustained barrel  oscillations?

It seems to me that the Audacity trace at the top is also picking up barrel oscillations before the pellet exits the barrel.  That is a good thing.  I've been looking at that particular section (the signal to the left of the red line) as barrel transit time.  Do we agree that is what that signature represents? If so it implies (to me) that the simple setup with a phone/camera to capture and Audacity to analyze is  (if not optimal) certainly sufficient for the study of this problem.  Yes it is better to have more data but at some point enough data is enough data, unless management does not care about return on investment.   

I like it.  Well done. Thanks for the information.

[WhatUPSbox?]

All of the mics are picking up the noise prior to pellet exiting the muzzle. The 2.8 msec is close to the pellet travel down the barrel.

I think the hammer bounce is more likely. It shows up as sound on the other channels, and is distinct with high frequency content.

I'll check on the noise on the trigger channel. It is a simple circuit powered by 3 AAA batteries in a holder. For convenience it was taped to the side of the barrel. In the past, I would see some noise on that channel when the battery holder got bumped (I had it on a springer) and the voltage would respond. When it triggers it goes fully to ground.

Yeah, this is way more data than is needed to look at LDC's, especially if the timing is not of interest. From other testing I've done, the shot cycle noise timing is very repeatable and a gun can be characterized with a well placed mic. The laser trap is cheap (<$5) to implement if one has a channel for it.

[WobblyHand]

I think you can just download the file https://support.daytonaudio.com/MicrophoneCalibrationTool

Thanks for that!  I was searching for that link and couldn't find it.  The datasheet simply said there was a download link on their product page for the IMM-6, but there's no link on the page.

[OldCrow]

All of the mics are picking up the noise prior to pellet exiting the muzzle. The 2.8 msec is close to the pellet travel down the barrel.

I think the hammer bounce is more likely. It shows up as sound on the other channels, and is distinct with high frequency content.

I'll check on the noise on the trigger channel. It is a simple circuit powered by 3 AAA batteries in a holder. For convenience it was taped to the side of the barrel. In the past, I would see some noise on that channel when the battery holder got bumped (I had it on a springer) and the voltage would respond. When it triggers it goes fully to ground.

Yeah, this is way more data than is needed to look at LDC's, especially if the timing is not of interest. From other testing I've done, the shot cycle noise timing is very repeatable and a gun can be characterized with a well placed mic. The laser trap is cheap (<$5) to implement if one has a channel for it.

Thank you, sir.  Again, well done!  We are also discussing the gyroid (porous) infill vs solid infill problem over at AGN.  I'd link but apparently do not have privileges for that.  Bruce most likely does.  I don't have many posts in this incarnation.

[WobblyHand]

Hi Stan, if you want to see what Mitch is referring to, it's at: https://www.airgunnation.com/threads/porous-moderator-design-tests.1302564/page-6

[WhatUPSbox?]

Thanks for the link. Looks like you are moving forward with some interesting concepts. I have more work to do on the basic test methods.

[WobblyHand]

Thanks for the link. Looks like you are moving forward with some interesting concepts. I have more work to do on the basic test methods.

Stan, what did you use for the laser trigger?  I've seen some stuff on Amazon that could be cobbled together, wondering if you had a simple solution.  What I saw was discreet parts, a laser and separate sensor.  Is that what you used?

[WhatUPSbox?]

If you search Amazon for laser receiver module you will see a bunch of these. Some come with the laser. A set-up should be less than $5
-https://www.amazon.com/Ximimark-Non-Modulator-Receiver-Detection-Arduino/dp/B07MYY1ZZH

By the way if you come up with a way to feed it into Audacity while the iMM-6 is attached (second channel), I would be interested.

[WobblyHand]

If you search Amazon for laser receiver module you will see a bunch of these. Some come with the laser. A set-up should be less than $5
-https://www.amazon.com/Ximimark-Non-Modulator-Receiver-Detection-Arduino/dp/B07MYY1ZZH

By the way if you come up with a way to feed it into Audacity while the iMM-6 is attached (second channel), I would be interested.

All I can think of is to scale the detector output perhaps using a resistive divider, or op amp.  Worst case, use capacitive coupling and scaling.  Should be easy to pick up the short rectangular pulse.  My inclination would be to try simple scaling first.

My take away is that you made a custom mount for the sensor.  Not an issue for me, I just was curious what your solution was.  It's often worth a question or two to see if there's a better solution out there, that one hadn't known.  Think I will try to make something.  It might be obvious in some AG's the moment of barrel exit, but there's nothing like a sensor to remove all doubt.