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Old 24th January 2013, 09:16 PM   #71
xrk971 is offline xrk971  United States
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Dr Boar,
the "Handhole" idea is good, I make little holes (or "windows") big enough to put a hook tool in to add stuffing or remove it. It can be sealed with tape until ready for gluing.
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Old 25th January 2013, 08:56 AM   #72
DrBoar is offline DrBoar  Sweden
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I am getting to the finish line with this horn soon. But lets not forget that it is it a compact horn
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Old 25th January 2013, 03:12 PM   #73
DrBoar is offline DrBoar  Sweden
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Drilled holes for the IHR
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Extended with a tube containing vitamin C previously.
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Horn mouth measurements
The one long vent with the lowest tuning just lowers the 40-60 Hz, so what ever I throw at that 90 Hz peak it is really stubborn.
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Corresponding impedance measurements
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I think I will go with the two short vents, fiddle with the damping of the two chambers and then glue on the side and take it to some serious listening and some final set of measurements.
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Old 25th January 2013, 08:12 PM   #74
xrk971 is offline xrk971  United States
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Dr Boar,
The 90 Hz peak corresponds to a quarter wave cavity length of about 1.05 meters, looking at the MJK plans for this horn, there appears to be wall forming the bottom of the Helmholz resonator and the wall forming the top wall of final passage before it turns 90 deg and exits at the mouth, that has an approximate length matching this distance. Try putting some highly acoustic absorbent material like thick felt on these walls to destroy the resonator cavity. It might be worth noting that avoiding parallel faced walls in the internal passages may reduce these peaks in general. Just 2 or 3 degrees offset from perfect 90 deg normal will do wonders. I run across this all the time in the optical design world where we trace back inadvertent Fabry-Perot etalon peaks by backing out the length and look for corresponding offending parallel walls or surfaces.
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Old 26th January 2013, 07:42 AM   #75
DrBoar is offline DrBoar  Sweden
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Velocity of sound is about 344 m/s a quarter of that is 86 and 90 Hz then would be 0.95 m. Standing waves are at 1/2 length if it is between two surfaces. The bounce at an opening due to the impedance change is at 1/4 wave.

But I think you are right about that it is the pipe that is causing that peak. I will try putting rolls of damping material in the middle of each section (velocity maxima) to see what I can find. I also have some scrap pieces that I can use to make corner reflectors, so I will do that as well. This I can do today but actual measurements have to wait until the wife is out of the house, those test signals fray the nerves...
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Old 26th January 2013, 08:59 AM   #76
xrk971 is offline xrk971  United States
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My mistake, I was doing it in my head and got the 0.95 flipped with 1.05. Yes, standing waves is half wave length but I saw how mouth is at one of the walls to make it quarter wave.
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Old 27th January 2013, 09:30 PM   #77
DrBoar is offline DrBoar  Sweden
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I have glued some corner reflectors and some bitumen loaded pads as damping. I bought the PU glue, so I am getting to the begining of the end. Tomorrow I will try some tweaks of the two chamber and then glue on the side and then it is listening time.
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Old 29th January 2013, 05:17 PM   #78
DrBoar is offline DrBoar  Sweden
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Here you have the corner reflectors 50x50x175mm apart from the corner thing in the IHR that is only about 80 mm long. Then there are some bitumen loaded fibre board added as well
Click the image to open in full size.

Measurment setup, minus my body gracefully draped on top of the box...
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The topmost jagged curve is the empty pipe, then follows with damping in chamber, chamber &IHR and then complete damping the last one remove the dip at 40 Hz and also the one at 150 Hz.
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Driver response 5 cm on axis.
Red undamped, big dips at 40 85 and 150 Hz
Brown with chamber and IHR damped 40Hz dip lower in frequency and less deep, 85 Hz dip unaffected 150 Hz less deep.
Black full damping, 40 and 150 Hz much reduced, 90 Hz slightly lower in frequency and slightly less deep
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Horn opening:
Black undamped humps at 40 90 and 150 Hz, corresponding to the dips in driver response
Red damping in chamber and IHR, as the one above but 150 Hz reduced 6 dB or so.
Brown full damping. 40 Hz reduced by 6 dB, the 90 Hz lowered some few Hz and dB and the 150 Hz is gone by 10 dB also this in good agreement with driver response and impedance plots.

The foam seal was hard to get rid of very sticky but now the side is glued on with 23 g BAF in the IHR and 25 g in speaker chamber. I used PU glue as it expands and fills any minor cracks.

Next up is some listening tests.
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Old 29th January 2013, 08:24 PM   #79
xrk971 is offline xrk971  United States
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Very nice work Dr Boar! Are you going with the "red" line configuration as your final glued shut version? This is a nice roadmap for the very tricky and critical damping strategy for anyone wishing to make these speakers in the future - thank you for this "Lessons Learned" study.
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Old 30th January 2013, 06:18 AM   #80
DrBoar is offline DrBoar  Sweden
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Yes I have left out the damping of the pipe. With a 50 cm2 cone area I try to keep all the radiation resistance I can get. There must be a way to reallocate colors in Omnimic so I get consistent coloring schemes across different graphs as well as better contrast in than red and brown.

I will try out different levels of damping in the speaker chamber, and perhaps try sticking a piece of felt in the first section of the horn.

I also have discovered that the side walls of the horn mouth vibrates, tapping it gives a sustained high pitch tone putting a crossbrace of only 5mm diameter hardwood reduces it to a dead "thunk". I could record it and put in on youtube, but then I have to get an account on youtube and so on so that may or may not happen.
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