TH-18 Flat to 35hz! (Xoc1's design) - Page 2 - diyAudio
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Old 11th June 2011, 07:15 PM   #11
Xoc1 is offline Xoc1  United Kingdom
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It depends on how much bracing you want to do, as there is enough excess material in 2 & 1/2 sheets to use some of the birch ply. I,m not 100 pecent decided yet how much to brace the cab.
I may know more next week. it will take some time to get everything tested and optimised.
For example this is a resonance in the back of the cab at 274hz:
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Old 11th June 2011, 07:19 PM   #12
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Well lets assume this will be low passed at 125hz right?
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Old 11th June 2011, 07:37 PM   #13
Djim is offline Djim  Netherlands
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Martin, I’m glad to see your PA sub concept finally got its own thread.

Although we all realise your concept is a continuation of earlier ideas, theories and concepts it is amazing to see them come together with your ideas, skills and true professionalism in modelling. Therefore I really hope the Xoc1-TH18 will get as much attention and appreciation as JBell’s SS15 and other great designs here on this forum.
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Old 11th June 2011, 07:48 PM   #14
Djim is offline Djim  Netherlands
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Quote:
Originally Posted by Xoc1 View Post
It depends on how much bracing you want to do, as there is enough excess material in 2 & 1/2 sheets to use some of the birch ply. I,m not 100 pecent decided yet how much to brace the cab.
I may know more next week. it will take some time to get everything tested and optimised.
For example this is a resonance in the back of the cab at 274hz:
Question; the upper range harmonics as shown in your model do they also take account of the resonances of the materials selves? I mean baltic multiplx has different material resonances as Philippine hardwood ect. The reason I mention is because it also works differently in damping. I'm wondering if your modelling is able to use all these parameters that are involved.
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Old 11th June 2011, 10:32 PM   #15
Xoc1 is offline Xoc1  United Kingdom
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Hi Djim
The results I have were an initial test of the unbraced cab - The material was a plywood but not Birch Ply. The results were neverless very interesting - The software is very capable. The results targeted resonances at increasing frequencies. The lowest resonance (weakest point) was the horn mouth wobble at 60Hz, Most of the other resonances involved the first vertical internal panel (part 9 Ref latest drawing), and the rear panel. Your bracing scheme would sort these out!
Some of the frequencies were higher than the passband, but would probably be present as harmonic resonances.
The 2 other internal vertical panels are quite strong. I think this is because the reflectors brace them.
The two side panels also flex around the exit of the horn. No doubt this is why even a mesh grille makes a difference to similar TH cabs.
I'm away from my CAD PC at the moment but I have added some bracing between the front and first internal (part 9), and the rear panel to the rear vertical internal panel (part 7).
I have also added 2 experimental braces to the horn mouth. These are positioned in a V shape from the centre of the base. The driver can still be removed! The bracing I will test wil be quite minimal but hopefully will be engineered to do enough to kill the major resonances in the cab without over engineering.
I know this will reduce S5 area at the horn mouth but hopefully it will cure more than it kills.
If I get the chance I will get this tested next week.
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Old 11th June 2011, 10:52 PM   #16
jbell is offline jbell  United States
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I can say from experience that a metal grill like this reduces the 'walking' the sub does... (meaning much less flex)

Notice how I bent the ears inside and then bolted the top/bottom. I made sure to have enough pressure on the 'ears' on the sides that no wood/metal vibrations happen. (also dipped them in plastidip just for good measure)
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Old 11th June 2011, 11:21 PM   #17
Djim is offline Djim  Netherlands
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Hi Martin,

The corner bracing indeed makes the panel’s tuff and very roadproof (some people forget they are building PA cabs instead of Ikea kitchens ).

I see you left out the vertical bracing in the last corner that goes on over the bottom panel (my second suggestion for bracing). In our tests that extra bracing (+ ‘broomsticks’ solutions) did help allot in suppressing mouth problems without damaging the LF content to much. Also in case of Vast installation, this bracing suggestion helps very much when the cabs needs to be flying.

To be honest my eyebrows raised a little with your V-type mouth solution. Just based on my own experience it will influence the output more than you like while there are other solutions that already proved their effectiveness. Like Jbell suggested I also favour metal grills (especially with bar/solid surroundings). It suppresses the mouth problem at all places and it acts as a safety tool to keep 'unidentified objects' out.

About the high resonances, you can’t get rid of them as long you don’t tread panels over the full surface with absorbent materials such like bitumen/rubber (high elastic) based sticky layers. I doubt if (most) users have a big problem with these harmonics in the first place and those who have I suppose they already know the solutions

One more question, what does the model say about the 'hollow space'? The reason I ask is that we noticed difference in measurements when we braced the inside of a similar hollow cavity.

Last edited by Djim; 11th June 2011 at 11:25 PM.
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Old 12th June 2011, 11:47 AM   #18
hisense is offline hisense  United Kingdom
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Great work!

Which driver was the design based on the 18 sound driver? I need to build a couple of cabs for two BMS 18N850V2 and would like to give this design a go if the driver is suitable.

Unfortunately i dont have much experience with horn design and driver matching - should i model the BMS driver in this design with hornsresp?

Any input appreciated,
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Old 12th June 2011, 06:08 PM   #19
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Quote:
I have also added 2 experimental braces to the horn mouth. These are positioned in a V shape from the centre of the base. The driver can still be removed! The bracing I will test wil be quite minimal but hopefully will be engineered to do enough to kill the major resonances in the cab without over engineering.
I've made two models with similar bracing in the past. The difference to the picture is that the braces do no touch each other but some spacing inbetween was left, in order to get the driver out.
My initial impression is that it doesn't work as well as doubling the bottom panel. The panels are over the entire depth of the cabinet.

Best regards Johan
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Old 12th June 2011, 08:11 PM   #20
Djim is offline Djim  Netherlands
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Quote:
Originally Posted by hisense View Post
Unfortunately i dont have much experience with horn design and driver matching - should i model the BMS driver in this design with hornsresp?
The BMS 18N850-V2 works in Xoc1's TH18.

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