.22 LDC bore clearance study print files

[WobblyHand]

@subscriber it's hard to address your posts!  Lots of good stuff there.  I can't effectively answer much on the phone, but may comment later.  I hope some of the stimulus I provided gets your creative juices going.  Hadn't thought about jet engine noise management, but this stuff is all similar.  Good to see some effort into trying this out.  When I get a chance I will take a look at the stl files.

Based on what I've seen, a multiplicity of short chambers of the same diameter and differing lengths correspond to multiple low pass filters of different cut off frequencies.  I'd wager that they interact with each other rather than be totally independent of each other.  Until one understands the interaction, I'd stay with a single longer chamber, which does actually have a lower frequency cut off.  But this is conjecture at the moment.  It should be possible to design multi-pole filters, for more attenuation, but at least for me, I need to walk before I run.  And finally, one can perform complex filter synthesis which gives even more options.  But as you well know, sometimes the options are not physically realizable within the packaging constraints.  But in general, simple structures are better and potentially easier to construct.

[Back_Roads]

 I did a simple man test of basic aerosol can mufflers, found they did the trick on low power, most sound was resonating from cylinder ping.

[WobblyHand]

I did a simple man test of basic aerosol can mufflers, found they did the trick on low power, most sound was resonating from cylinder ping.

Makes sense.  Yeah I'd expect that baby to ring!  Tried a dense rubber foam sleeve to damp the ping?  Might make it a little more tolerable.  Or even wrap it with a wet towel...

[WhatUPSbox?]

Nice!
I think that's one of the basic questions for LDC designs. How much noise is coming from the front opening and how much is coming through the walls. If you were to overlay your aerosol can over the video I posted in #39, I would guess all of the muzzle turbulance is gone by the time it hit the end of your can.

[WobblyHand]

@Back_Roads moderator is a giant low pass filter, with ringing walls.  It would be very effective with some damping, I'd bet.  For my simplified calculations, the walls are assumed to be infinitely stiff, but unfortunately for us, they aren't.

[WobblyHand]

Another thought, the cavity is so large it is possible to excite other modes.  These modes could resonate at undesired frequencies.  It is possible to design some internal structures that favor a single mode.

[Back_Roads]

Another thought, the cavity is so large it is possible to excite other modes.  These modes could resonate at undesired frequencies.  It is possible to design some internal structures that favor a single mode.

Something as simple as a depinger and inner wrap sound dampener IMO.
 I do have a sorta name brand LDC  2nd so no name attached, that rang, I gave it an outer wrap of heavy self adessive vinyl  to deaden the ping.

[WobblyHand]

Another thought, the cavity is so large it is possible to excite other modes.  These modes could resonate at undesired frequencies.  It is possible to design some internal structures that favor a single mode.

Something as simple as a depinger and inner wrap sound dampener IMO.
 I do have a sorta name brand LDC  2nd so no name attached, that rang, I gave it an outer wrap of heavy self adessive vinyl  to deaden the ping.

Can't argue with what works! 

If something like this were printed, I think it's possible to do something to the diameter that would reduce pinging of the structure.  Or, at least it is possible to do in microwaves, hoping there's an acoustic analogue. 

[WobblyHand]

Printing something.  We'll see if it works at all.  There's nothing like the acid test of reality to shatter all your dreams.  I'm not sure that I printed the best of my options.  It might be it is better to locate the expansion chamber sooner.  I'll test it on a low power 2240 first, no need to blow it up first try!

Anyone got a recommendation for a not too expensive microphone I can plug into my PC? 

Here's an off the wall question, anyone got a recommendation for a mini speaker, oh maybe only 12mm wide?  I was thinking of mounting at the input of the LDC, and sweeping a tone, and measuring the output at the exit.  I could measure with and without the LDC.  Poor man's filter response.  A while back I programmed an Arduino to make a stepped frequency test tone.  If I could use it to power a speaker, then I could measure the relative output of the moderator.  Shooting is fun and all, but I think the measurement would have less noise in it using a swept source.

Edit: the ultimate test is always shooting through an LDC!

[TorqueMaster]

Ok, you've piqued my curiosity.  What you've described is braindead easy to implement.
OD 30mm
Chamber ID 25mm, Chamber length 65mm
Bore ID 12mm
Are the lengths of the bore ahead and behind the chamber important or will any length work?   I just used 50 and 50 with overall length 165.

Do the chamber ends need to be exactly as shown, flat, perpendicular to the bore?  Does the exterior need to be cylindical, or would it work just the same if these cavities were inside a long square block, for instance?  My reason for asking is to simplify or overcome 3D printing limitations.

I neglected to answer one of your questions.  The derivation only applies to circular cylinders, not square or rectangular. 

Something else that I ignored for this is the possibility of multiple propagating modes in circular guides.  Might need to add some features for mode control, but hey it's rather early in the design cycle for me.  I thought it was exciting to start to see a design moving in the right direction.  It's expected that some iteration may be required.

Three actually. 
Is there a preferred length for the 12mm (or 8mm) bore ahead of the chamber?
Is there a preferred length for the 12mm (or 8mm) bore after of the chamber?
And I asked, for these cavities, does the exterior shape matter?  Does the math require that moderator outside shape is circular, and the cross-sections look like pipes, or thick pipes?  My understanding was that only the cavity shape and dimensions mattered, but I may have just assumed that.

I would usually print such a thing vertically, but the "upper" inner flat chamber wall will not be very clean (droopy) if I did that.  Next choice is print horizontally, which would be pretty clean with the exterior as a cylinder, but better if the printbed side was flattened.

Curious what your design looks like and what orientation you're printing it.  And how it performs.

I use this mic on my phone.  Nothing fancy, but it had pretty good reviews from folks who sounded like they knew what they were talking about.  It's disco now, but maybe it's still available under a different name.

[WobblyHand]

Ok, you've piqued my curiosity.  What you've described is braindead easy to implement.
OD 30mm
Chamber ID 25mm, Chamber length 65mm
Bore ID 12mm
Are the lengths of the bore ahead and behind the chamber important or will any length work?   I just used 50 and 50 with overall length 165.

Do the chamber ends need to be exactly as shown, flat, perpendicular to the bore?  Does the exterior need to be cylindical, or would it work just the same if these cavities were inside a long square block, for instance?  My reason for asking is to simplify or overcome 3D printing limitations.

I neglected to answer one of your questions.  The derivation only applies to circular cylinders, not square or rectangular. 

Something else that I ignored for this is the possibility of multiple propagating modes in circular guides.  Might need to add some features for mode control, but hey it's rather early in the design cycle for me.  I thought it was exciting to start to see a design moving in the right direction.  It's expected that some iteration may be required.

Three actually. 
Is there a preferred length for the 12mm (or 8mm) bore ahead of the chamber?
Is there a preferred length for the 12mm (or 8mm) bore after of the chamber?
And I asked, for these cavities, does the exterior shape matter?  Does the math require that moderator outside shape is circular, and the cross-sections look like pipes, or thick pipes?  My understanding was that only the cavity shape and dimensions mattered, but I may have just assumed that.

I would usually print such a thing vertically, but the "upper" inner flat chamber wall will not be very clean (droopy) if I did that.  Next choice is print horizontally, which would be pretty clean with the exterior as a cylinder, but better if the printbed side was flattened.

Curious what your design looks like and what orientation you're printing it.  And how it performs.

I use this mic on my phone.  Nothing fancy, but it had pretty good reviews from folks who sounded like they knew what they were talking about.  It's disco now, but maybe it's still available under a different name.

Sorry about not answering all your questions.
1.  I don't know actually.  I just finished printing one with the big cavity in the middle.  But it may be better to have the cavity shifted elsewhere. 
2. Same.
3. The exterior shape (first order) shouldn't matter.  The primary acoustic properties should be dominated by the interior dimensions, if the material is stiff enough.  But, I'm not an ME.

I ended up with a design like attached.  I printed it vertically, threads up so I had to accommodate for the overhang.  I'll find out within the day if this is just a stupid idea or not...  Believe me, I will let everyone know, good or bad.  I printed in draft mode and used 50% infill, simply to reduce print time.  It isn't beautiful, but hopefully it sort of works, at least for low power.

[TorqueMaster]

Thanks for the answers!  Your design is what I had in mind for the 2nd generation if it didn't mess up the calculations -- 45 degree angled end walls on the chambers.  Which, if you used two or more chambers, with a 2+ mm space between them -- it starts looking like...conical moderator baffles inside a cylindrical shell.   I have a hunch this single chamber will work to some degree.

[WobblyHand]

I just chased out the threads with a tap and dry fired it on my 2240. 

It is noticeably quieter than my slim LDC.  It was lower in pitch and sounded more like a snort than firing.  The action of the hammer and valve are a lot more pronounced.  It doesn't sound like a gun, more like something pneumatic.

Think I made a mistake on it for the seat of the threads, but this unit impresses me.  No, a 2240 is not a high power PCP.  I'll fire it with a pellet to see if there is any significant difference in tone.  Then I will try it with my PP700SA.

[WhatUPSbox?]

Anyone got a recommendation for a not too expensive microphone I can plug into my PC? 

I think you said you had a 96Khz sound card. This is the mic I got for these type of measurements.
www.amazon.com/gp/product/B00ADR2B84

[WobblyHand]

I can report that firing a 15gr Daisy HP pellet sounded identically the same, save for the pellet slap when it hit my rubber mulch target.  It's a lot more pleasant to shoot, especially in my office.  I used a 7.5mm bore for the narrow section.  I'd bet it printed slightly small, which actually makes it a more effective moderator.  Sort of a snort, burp like sound then whap.  I would need to set up something a bit less casual to check for clipping or sliding down the moderator.  The pdf file is what I printed. 

All in all, I'm astonished really.  Definitely going to have a celebratory beer tonight.

Later on, I will try out the same moderator on my PP700SA.  Beats me what velocity it shoots, I need to chrony it.  The PP700 is a whole lot louder and more powerful than a 2240.

[WobblyHand]

Anyone got a recommendation for a not too expensive microphone I can plug into my PC? 

I think you said you had a 96Khz sound card. This is the mic I got for these type of measurements.
www.amazon.com/gp/product/B00ADR2B84

Thanks Stan.  I will check it out.

Right now, my ears are not deceiving me, this unit I just printed is loads quieter than the slim LDC I posted in one of my threads.  It's lower pitched as well.  No sharp sounds at all.

[WhatUPSbox?]

I've been using Audacity (free) for the capture and analysis.

[subscriber]

Good job, Bruce

What you have there may be the longer last stage of a multi chamber LDC, where the latter is taking the muzzle blast of a higher power PCP.  In other words, the previous chambers drop the pressure significantly, to feed the last long chamber a more gentle pop (or series of pops), that is closer to a stream.  This theory will collapse, if your single chamber LDC is as quiet at 30 FPE as a multi chamber LDC.

How important is the significant length of the 7.5 mm ID sections?   My latter designs make those short, because the solid material around them seems like wasted expansion volume, in any given length casing.  Also, I have concerns about the pellet being steered aerodynamically by the air stream down such a narrow tube; but one thing at a time.  Certainly, the long narrow sections represent resistors and the open chamber a capacitor, to act as an AC filter.

[WobblyHand]

Good job, Bruce

What you have there may be the longer last stage of a multi chamber LDC, where the latter is taking the muzzle blast of a higher power PCP.  In other words, the previous chambers drop the pressure significantly, to feed the last long chamber a more gentle pop (or series of pops), that is closer to a stream.  This theory will collapse, if your single chamber LDC is as quiet at 30 FPE as a multi chamber LDC.

How important is the significant length of the 7.5 mm ID sections?   My latter designs make those short, because the solid material around them seems like wasted expansion volume, in any given length casing.  Also, I have concerns about the pellet being steered aerodynamically by the air stream down such a narrow tube; but one thing at a time.  Certainly, the long narrow sections represent resistors and the open chamber a capacitor, to act as an AC filter.

The initial reason for the lengths at 7.5mm is to ensure the section of pipe is truly acting as a high pass filter.  If it is short, like a mm or so, it really doesn't act as one.  To be effective as a HPF, we would need the low frequencies to die out.  Technically we need to determine say 5 to 10 length constants, which I have yet to calculate.  The field (acoustic or EM) will decay exponentially with a length constant l₁.  l₁ is dependent on the guide dimensions and the frequency.  In 5-6 length constants the energy has basically vanished.  Since I haven't determined the value yet, I conservatively chose rather long values.  After a determination is made, I can see if it makes sense to shorten up those skinny sections.

I'd think the long narrow section would be predominantly inductive rather than resistive, and the open areas could be thought of as capacitive.  At least that's the microwave analogue.  In the microwave world, we make can make filters from distributed inductors and capacitors.  Resistive elements are infrequently used. 

I'm leery of the narrow tubes myself.  Seems like it's just waiting for something bad to happen there.  Steering, crashing, you name it.  Honestly, I gritted my teeth when I pulled the trigger with the pellet, I was expecting something not good to happen, but things were fine (as far as I can tell).

But the thing that really makes this particular design work is the ratio of the squares of the two diameters, the higher the ratio, the lower the low frequency cut off.  And the narrower the tube, the higher the HPF cut off is.  That's how the math works out.  But there's still a lot of topological tricks that still could be played, so not to worried about that. 

I wish there was an more effective way to push out the HPF cut off than bore diameter.  Going to have to think about that.  I really need an acoustic simulator, but I don't have one.  At the moment, I'm a long way off from running CFD.  I can't even get the examples to run!  I want to be able to see the effects of multiple sections, and their interactions.

That all being said, I think there's still some gain to be made with this simplistic approach.  But to make more real progress, will require better tools.  Then I could visualize turbulent flow and could mess with some more of the more interesting structures and get some quantifiable results.  On the other hand, simulations can take a long time.  Sometimes it's faster to try it...

[WobblyHand]

I've been using Audacity (free) for the capture and analysis.

I have used it before.  Reinstalled it on my laptop a couple of days ago.  You wouldn't happen to know what the allowed windowing functions are?  Never mind, I found them.  It seems you can't put in custom filters (that easily). 

I would strongly recommend NOT using boxcar, or rectangular windows (another name for the same thing).  Spent my whole life telling colleagues never to use it.  You get high spectral splatter, especially for signals with high dynamic range.  This splatter masks other smaller signals.  As a radar engineer my job was to find picowatt signatures in milliwatt noise.  You just can't do it with rectangular windows.  Other window functions are far, far better.  Hamming are not bad, but out of what's available, I'd use the Blackman-Harris.  I used to use modified Kaiser windows.

Hmm, I just read, may be a use for rectangular windows is for impulse analysis...  I'll keep that in mind, although my experience is warning me otherwise...