THAM15 - a compact 15" tapped horn - Page 4 - diyAudio
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Old 21st January 2011, 10:19 PM   #31
epa is offline epa  Netherlands
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Quote:
Originally Posted by Brian Steele View Post
For a useful exercise, try working out what resonances might be set up with the parallel walls and the like in the tapped-horn, or at what frequencies those reflectors are actually going to be useful, then determining whether or not those frequences are actually going to be in or even near the tapped-horn's passband. I'll bet that you'll find that those frequencies are outside, perhaps even way outside, the tapped-horn's passband.

IMO good bracing is going to be a lot more useful in the tapped-horn than inserting reflectors or ensuring that no walls are parallel. With that in mind, don't skimp on bracing in the 1st or 3rd sections of the tapped-horn. If you do, the end result will be panel flex and increased distortion at the upper end of the tapped-horn's passband (my experience).
agreed with the "flappy"ends needs bracing(got them on my flh's, althoug it keeps them clear of beerglasses)
maybe upper harmonics wil come in to play?so testing is the way to go.
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Old 21st January 2011, 10:37 PM   #32
jbell is offline jbell  United States
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Quote:
Originally Posted by epa View Post
intresting,so avoiding simular lengts first,and then if nessecary correct with non 45 degrees deflectors.
i gues the best way to go is bilding a test box with a removable side.
could one make them of some sort of stiff foam?
exactly. Glue with PL everything except for the second side. Lay the 2nd side on and draw as many lines as you can so you can exactly locate where all of the interior panels intersect with the 2nd side. Screw down through all of the interior and exterior panels, at least 2 screws per panel. You can then take 3/8" door/window gasket foam and line all of the panels that intersect the 2nd side.

Install the reflectors you want to test with screws, and then screw down the 2nd side. When you are done testing, you can remove the side and change your reflectors. Go ahead and make the reflectors out of wood, no need to foam.

repeat

repeat

repeat

Quote:
Originally Posted by Brian Steele View Post
For a useful exercise, try working out what resonances might be set up with the parallel walls and the like in the tapped-horn, or at what frequencies those reflectors are actually going to be useful, then determining whether or not those frequences are actually going to be in or even near the tapped-horn's passband. I'll bet that you'll find that those frequencies are outside, perhaps even way outside, the tapped-horn's passband.

IMO good bracing is going to be a lot more useful in the tapped-horn than inserting reflectors or ensuring that no walls are parallel. With that in mind, don't skimp on bracing in the 1st or 3rd sections of the tapped-horn. If you do, the end result will be panel flex and increased distortion at the upper end of the tapped-horn's passband (my experience).
well, I can say a single 12degree reflector COMPLETELY changed the behavior on my 2x2x4 TH. I had a stuffy 70hz that was 8db down. That ONE reflector reduced my severe 50hz bump by 2db, and increased my 70hz by 4db.

The only way you find these kind of things is to build it and measure.

Last edited by jbell; 21st January 2011 at 10:43 PM.
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Old 21st January 2011, 11:01 PM   #33
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Quote:
Originally Posted by epa View Post
agreed with the "flappy"ends needs bracing(got them on my flh's, althoug it keeps them clear of beerglasses)
maybe upper harmonics wil come in to play?so testing is the way to go.
I've got TWO layers of 3/4 ply AND a center brace on the first section of my test TH, and it STILL vibrates too much for my liking, and the panel flex introduces measurable distortion too. I'm thinking of trifurcating the complete length of the horn for the next build with braces - let's see if there will be noticeable panel flex after that .
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Old 21st January 2011, 11:05 PM   #34
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Quote:
Originally Posted by jbell View Post
well, I can say a single 12degree reflector COMPLETELY changed the behavior on my 2x2x4 TH. I had a stuffy 70hz that was 8db down. That ONE reflector reduced my severe 50hz bump by 2db, and increased my 70hz by 4db.
That could be due to the increased bracing the reflector provided rather than anything else, but the only way to really tell would be to look at the impedance response before and after the addition. Panel flex seems to show up as unexpected blips in the impedance curve.
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Old 21st January 2011, 11:20 PM   #35
jbell is offline jbell  United States
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Originally Posted by Brian Steele View Post
That could be due to the increased bracing the reflector provided rather than anything else, but the only way to really tell would be to look at the impedance response before and after the addition. Panel flex seems to show up as unexpected blips in the impedance curve.
uhhh... no.

A single 5" long reflector.... and this was 9 ply 3/4".... The reflector had NO effect on bracing, or reducing cabinet flex. All it did, was prevent a null where 2 waves of opposite phase were interacting with each other.
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Old 21st January 2011, 11:36 PM   #36
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Originally Posted by jbell View Post
uhhh... no.

A single 5" long reflector.... and this was 9 ply 3/4".... The reflector had NO effect on bracing, or reducing cabinet flex. All it did, was prevent a null where 2 waves of opposite phase were interacting with each other.
A 70 Hz wave is about 16.1 ft. long
A 50 Hz wave is about 22.6 ft. long

I'm not sure how a 5" reflector can prevent a frequency null when we're talking about wavelengths that long, but as always I'm open to correction - horns design is fairly new to me.

Was this reflector in the first section of the horn? Did you include in your model the change in the horn's response that may be caused by reducing the cross-section at that point with the addition of the reflector?
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Old 21st January 2011, 11:45 PM   #37
Djim is offline Djim  Netherlands
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Brain, its all about a quater of wavelength not full wavelengths...



Jbell, but why did you leave in your suggestion for the Tham still so much space open for the standing waves of about 50cm (170/180Hz)??? See green area in the picture with Jbells suggestion for reflectors…

Click the image to open in full size.
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Old 21st January 2011, 11:53 PM   #38
jbell is offline jbell  United States
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djim:

ok, so you got me on the 45 degree reflector, it should be a bit bigger... I was giving an off the cuff answer.

In terms of the lower section, I don't know why, but the shallow reflector works there.

In the last vertical section, I never tried to fix that. My 2x2x4 cabinet has a HUGE hole, just like martisson has, and I gave up trying to fix it. Since I crossed it low, I ignored it.

My ss15 only has a 2hz wide notch in the 170hz range, so narrow that you'll never know... that's how I really fixed it.

Last edited by jbell; 21st January 2011 at 11:56 PM.
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Old 21st January 2011, 11:59 PM   #39
Djim is offline Djim  Netherlands
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Sorry Jbell, it was not meant to ‘get’ you but I was curious if the lower section would give enough reflection to kill the problem this design seems to have. Btw Shallow reflectors seem to work better simple because they are not as obstructive and still are steep enough to do the job. In my opinion 45 degree reflectors seem to bounce the original signal backwards (partly)…

Last edited by Djim; 22nd January 2011 at 12:05 AM.
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Old 22nd January 2011, 12:05 AM   #40
jbell is offline jbell  United States
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Quote:
Originally Posted by Djim View Post
Sorry Jbell, it was not meant to ‘get’ you but I was curious if the lower section would give enough reflection to kill the problem this design seems to have. Btw Shallow reflectors seem to work better simple because they are not as obstructive and still are steep enough to do the job. In my opinion 45 degree reflectors seem to bounce the original signal backwards…
or... 45's make 2 adjoining segments work kinda as a single larger segment -- I never know which it's going to do. I do know that multiple 45's and multiple paths don't work this way, but a single has somewhat of a different effect. (sorry, trying to give a generalization on something that you can't give a generalization on.)

There's a place for 45's... but it should be a 'careful' placing of 45's in my opinion.

Also, Tom has commented that sharp bends should be closer to the throat, not the mouth... that's also why I have my 180 bends up front, and my 90's at the end. The martisson style cabinet as well as my 2x2x4 cabinet are the opposite with the 180's at the end.

regardless... I know I've mentioned before... TH's are like a soup, and it's all in the seasoning at the end...
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