.22 LDC bore clearance study print files

[subscriber]

Bruce,

As you can see from the attached images; I took a wire EDM to your PDF design.  Then, while keeping as much of your original dimensions intact as I could, I recaptured expansion volume at the front and rear of the tube, keeping the long 7.5 mm bore tubes at your design lengths. 

The 25 mm ID section is still 75 mm long, but with a "soft wall" that communicates into the new secondary spaces.

While this design may violate your principles, my approach is that you have to worry less about the frequency of reflected waves, if the reflections are weaker; and more diffuse.  That is what I expect the short support ribs to do - act as diffusers.  And "confuse" the resonant frequency of the 25 mm ID tube.

It would be great, if you could throw this chimera into the mix for your testing purposes.  Ditto, anyone else that is curious about how your single chamber LDC works - please add my interpretation to the mix.

STL attached below

[mrbulk]

I forgot to mention; these mufflers need to be printed standing vertical on the printer platen, with the threads uppermost.

I have the 8.5 version in hand, just printed for me by a friend in his spare time. Am establishing no-pellet, to-the-ear shooting noise baseline for now, will try to get at least the 6.5 printed soon for comparo's sake. If promising will find an appropriate venue and set up the meter and shoot real pellets through...thx!

[subscriber]

Thanks, Charlie

Any chance you could test the 7.5 mm one too, if you are using some sort of dB meter on the 6.5 and 8.5 mm versions?  There appears to be something interesting going on with bore size that was not expected; hence my question.

[subscriber]

So, Chimera 1 is the moderate hack of Bruce's test piece.  Chimera 2 is how I would optimize it for reflected wave damping with some surfaces having the 45 degree aspects flipped; and some flat.  Then further restricting the flow area into the quench zones between the support structures neat the ends of the 7.5 mm bore "tubes".  Then adding fillets in may places to make like easier on the printer; and outside esthetics.

STLs for both Chimera 1 and 2 attached below

Image in next post compares Chimera 1 and 2.  If you are inclined to print only one of these two, go with version 2 please.

[subscriber]

Image below compares Chimera and 2:

C-1 and C-2 designations are embossed in front cap.

[WobblyHand]

If I did this right, and I'm not sure if it is correct, I calculate approximately 4dB attenuation of the lower frequencies per mm of tube length, due to the small tube ID structure.  Which is why I think the ID small sections need to have some length.  But I see some weird stuff that I need to sort through if I have appropriate formulations.  A lot of the references use different ways of talking about the same things, and use different variable names which makes things tough to sort out.  My old book is still missing in action, it might be in storage.  Have to say understanding EM and acoustics after a 25 year pause is tough, especially for an old dog.  It's like getting slapped in the face, repeatedly.  But it's fun trying to dust off the cobwebs.

[WobblyHand]

Went off to where I store junk stuff and rummaged through some old dusty boxes.  I found my ancient copy of Matthaei, Young and Jones, which was originally copyrighted in 1964.  I bought that book in 1985 for the princely sum of $79.  That was big money for a book then.

In it are some really old school methods of design, a lot of which were prevalent in the 50's and 60's, before people had much access to computational methods.  (Before wide spread adoption of computing available to the public.)  There are some interesting filter synthesis methods in there.  I will browse through the book looking for ideas that may be transferable to acoustics.

[WobblyHand]

Image below compares Chimera and 2:

C-1 and C-2 designations are embossed in front cap.

Printing a C-2, just because.  Has some nice design elements in it.  Just wanted to mention that I appreciate your efforts, and your design abilities.  You have an artistic flair, which is nice to see.

I goosed up the hammer on my PP700, so it is a bit noisier.  My LDC is letting through more noise than I'd like.  Looking forward to trying this C-2.  And good idea on printing on the designation!  I need to learn how to do that in my version of CAD.

[subscriber]

Thanks for your eager persistence, Bruce

Without the identifying characters on the front end, confusing too many similar mufflers would be easy. Especially the set of three from the OP study.  One might just assume that the rank order of dB reduction is smallest to largest bore diameter. 

Speaking of which, OldSpook on AGN tested the same exact muffler set as Mike (miksatx), and saw the same rank order anomaly, when the meter was placed 3 m to the side of the muzzle.  Ahead of the muzzle, the results followed the direction of increasing baffle bore diameter.   So, meter placement matters, probably because more high frequency noise is heard ahead of the muzzle, than to the side.

OldSpook's sound recordings showed a significant lowering of the report's tone, with reduced sharpness, between the 6.5 mm muffler bore and the 7.5 mm bore.  With a smaller change between the 7.5 mm bore and the 8.5 mm bore:  https://www.airgunnation.com/members/oldspook.34264/#profile-post-309

[WobblyHand]

Well, drat.  Print failure after 4-7/8".  Somewhere around layer 435 out of 588.  The print fell over.  That's it for this evening.   I'll try again tomorrow morning.  Next time I will reapply the hair spray on the sheet.  Haven't had an LDC or anything else fall over before.  Guess there's always a first time.

Maybe I will saw this C-2 in half.  It's not good for anything else.

[subscriber]

Bruce,

It would be interesting to see what the print looks like inside.

[WobblyHand]

Thanks for your eager persistence, Bruce

Without the identifying characters on the front end, confusing too many similar mufflers would be easy. Especially the set of three from the OP study.  One might just assume that the rank order of dB reduction is smallest to largest bore diameter. 

Speaking of which, OldSpook on AGN tested the same exact muffler set as Mike (miksatx), and saw the same rank order anomaly, when the meter was placed 3 m to the side of the muzzle.  Ahead of the muzzle, the results followed the direction of increasing baffle bore diameter.   So, meter placement matters, probably because more high frequency noise is heard ahead of the muzzle, than to the side.

OldSpook's sound recordings showed a significant lowering of the report's tone, with reduced sharpness, between the 6.5 mm muffler bore and the 7.5 mm bore.  With a smaller change between the 7.5 mm bore and the 8.5 mm bore:  https://www.airgunnation.com/members/oldspook.34264/#profile-post-309

Persistence is one of my character failings...

Interesting stuff.  Did an audio recording of a few shots but not satisfied with what I got.  I need to play with Audacity more.  I was getting a lot of background noise due to the mic gain being turned up, and my 3d printer running in the background.  I generated a spectrum, but didn't think it was good due to the background.  Slow incremental progress.

As for the failed print, maybe I will add a brim next time.

[WobblyHand]

Bruce,

It would be interesting to see what the print looks like inside.

I think so too.  Will use a slitting saw on it tomorrow.  Unfortunately printed it black, which is tough to photograph.  Next one I will print in red PETG.  For testing that's fine.

[subscriber]

So, the idea of piping more air to the front of the muffler, directly from the blast chamber, without "pointing" the duct openings directly at the frontal exit, to avoid transmitting shockwaves directly out, inspired the monstrosity below. 

It looks like Nicola Tesla and Werner Von Braun's love child - if such a thing were possible.  The bypass tubes would act to some degree as a Tesla Valve, although 12 tubes each with a 3 mm bore diameter are not going to flow that much air.  But the idea is to pipe a controlled volume of air into the last expansion chamber, so that it can leave ahead of the projectile (and the air) travelling down the sequential baffle bores; but without its energy, or more specifically, shock being being transmitted directly out the frontal bore.  So the momentum of the piped air will need to stagnate in the last chamber, and turn around before leaking more gently out of the front bore.

Can this happen in the amount of time it takes the projectile to leave the front bore?  This would seem like expecting the air to exit very suddenly, but yet merge with the ambient air more gently, is asking for a lot...

Will this contraption do anything useful or interesting?  I don't know, but there is a way to find out.  The zipped STL is attached for anyone wanting to have a go.

[Mr.P]

I am not going to claim to be up-to-date on all of the details of this thread, but I like peeking in from time to time. Just curious, why did you go for separate pipes instead of a single outer sleeve?

[WobblyHand]

I am not going to claim to be up-to-date on all of the details of this thread, but I like peeking in from time to time. Just curious, why did you go for separate pipes instead of a single outer sleeve?

I'd think one might have a lot more flow with your idea of an outer sleeve.  Might have 4 dividers, maybe six, rather than so many little pipes.

[subscriber]

I am not going to claim to be up-to-date on all of the details of this thread, but I like peeking in from time to time. Just curious, why did you go for separate pipes instead of a single outer sleeve?

Quite honestly, because that was the idea that woke me; begging to be brought to life.  An outer tube with a space between tubes would have been so much easier to model.  If there is a good reason; I wanted the lower duct entrances to point upwards at 30 degrees.  I also wanted to be able to change the duct ID and number of ducts easily in Solid works.  Probably possible the way you suggested.  If there was one "excuse", it might be the amount of added material.

So, no compelling reason, other than Bruce has been telling us about the acoustic damping properties of long skinny ducts.  So, that must be the reason this was the vision that popped into my mind.  Certainly, there will be no looking at this monster and confusing it for something else

[subscriber]

I'd think one might have a lot more flow with your idea of an outer sleeve.  Might have 4 dividers, maybe six, rather than so many little pipes.

Quite right, but the idea was "some flow", rather than as much as possible.   

Modelling that part was intense work.   It was a challenge, and took considerable jiggery-pokery to complete.  So, fun and showing off might have been factors.

But, mostly, Bruce, I am going to credit you with the idea that long skinny tubes have acoustic damping properties.  And here they are....

[WobblyHand]

I'd think one might have a lot more flow with your idea of an outer sleeve.  Might have 4 dividers, maybe six, rather than so many little pipes.

Quite right, but the idea was "some flow", rather than as much as possible.   

Modelling that part was intense work.   It was a challenge, and took considerable jiggery-pokery to complete.  So, fun and showing off might have been factors.

But, mostly, Bruce, I am going to credit you with the idea that long skinny tubes have acoustic damping properties.  And here they are....

I'll bet it was a bit of work.  How about an outer tube with radial sectors to partition them, I'd imagine it would be a lot easier to model, and it would preserve the small tube aspect without being onerous to model.  On the other hand, it seems you rather enjoy it!  The sectors could be fed by holes coming in at similar angles from the main chamber.

At the moment, your design ability far outstrips my ability to analyze or print things.  If I am lucky, the Chimera-2 will print in about 4 hours.  I will try variable layers primarily in the thread region.  I didn't like how 1.27mm pitch threads came out with 0.3mm layer height, they tapped ok, but it wasn't easy starting the tap.

I took the liberty of slicing open the failed print from yesterday.  I used a 1/16" slitting saw and roughly sawed it in half lengthwise.  Had to clean up the edges, but otherwise isn't bad.

[subscriber]

Bruce,

That thing looks almost organic. 

Well, I may or may not produce a cleaner looking bypass design; but I have added a bracing structure to the middle of the tube span.

Another reasons for the ducts that seems obvious:  I want to feed the air through the ducts fast to the front chamber; ahead of the projectile.  If the air first filled an annular plenum, it would lag behind while pressurizing that space.