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Old 22nd January 2011, 01:17 AM   #41
Djim is offline Djim  Netherlands
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And, if you make the 45’s much bigger they will narrow your horn path. If you make them smaller they wont work over the whole reflecting surface. To me that is the main reason why I don't want to use 45's.
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Old 22nd January 2011, 02:14 AM   #42
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Quote:
Tom has commented that sharp bends should be closer to the throat, not the mouth
No disrespect to Tom but could that be wrong? Andy
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Old 22nd January 2011, 02:33 AM   #43
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Brain, its all about a quater of wavelength not full wavelengths...
Ok...

1/4 wavelength of 70 Hz is about 4 feet. How is a 5" reflector going to "reflect" this in any appreciable way? That's a bit like trying to stop and change the direction of an ocean wave with a spoon (ok, that may be a bit of an exaggeration, but still)...

Based by your diagram, *maybe* replacing part of one parallel surface with a surface with a shallow slope might change the 'Q' of the resonance between the two walls a bit. This might explain why a shallow slope works better than the steeper 45 degree reflector. But then again we're talking about a 5" difference, max.. The mathematicians here can work out how much the Q will change with that difference (it's Friday night and my mind is more on beer instead, LOL), but I suspect that it's not going to be much. However, it should be possible to get better results by just lining one side (e.g. the end closest to the driver) with enough fill to cut that resonance significantly.

Further down the horn the reflectors will get larger, and therefore more effective at lower frequencies. But still, none of them approach 4 feet in width...
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Old 22nd January 2011, 02:50 AM   #44
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Hi there Anders: With regard to adding 45 degree (or similar size) deflectors, MK II & III versions may be signifiquently reducing the total cabinet volume, possibly as much as 15-20L ( estimated: 4@ [13x13x50] + 1@ [15x15x50]/2=[5625 +16900]=20+L. This much reduction may effect the response of the TH, just as lowering the box volume for other types of enclosures. ...regards, Michael
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Old 22nd January 2011, 03:51 AM   #45
jbell is offline jbell  United States
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Brian:

I am obviously not going to change your mind, or convince you of anything. I will not try. I will respectfully disagree with you, and state that a single reflector can in fact make a huge difference in a TH, that I actually measured.

I have built and rebuilt dozens of TH's. Each time I measured. Each time I learned. There have been many bonfires of the carcasses of the under performing.

It's kinda like the old saying about two farmers arguing about how many teeth a horse has. The argument stops when one of them opens the horses mouth and looks.
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Old 22nd January 2011, 04:08 AM   #46
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Brian:

I am obviously not going to change your mind, or convince you of anything. I will not try. I will respectfully disagree with you, and state that a single reflector can in fact make a huge difference in a TH, that I actually measured.
I'm not questioning your observations. Just trying to determine the actual cause. Once the actual cause is known, the next step is determining how best to optimize it. Could lead to less bonfires

Last edited by Brian Steele; 22nd January 2011 at 04:13 AM.
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Old 22nd January 2011, 04:38 AM   #47
Djim is offline Djim  Netherlands
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Well Brain, the problem(s) with acoustics is often more complex then our first thoughts, especially on Friday nights! BTW in an open-field environment you would be right. With a spoon you can’t stop an ocean wave, but sure a spoon (with enough backup) can bend a beer ‘jet stream’ that is leaving a barrel under pressure. This is a matter of controlling environmental circumstances. A horn is all about controlling the environment for the sound waves.

Now back to your question; sound waves with bigger wavelengths then the surface they reflect from, will simply bend. They can’t leave the box nor causes the corner enough friction to change the energy of the sound waves into heat. So for the sound wave there is only one solution and that is bending and following the path we carefully set out for them.

Standing waves however are the result from sound waves bouncing (reflecting is a better word actually) between two parallel surfaces. The more bends we have in a horn the more parallel walls are introduced and the more ‘spiky’ the outcome will be.

Every bend in horn design means a loss in energy by the introduction of unwanted physical side effects. One of problems is a different expansion factor within the bend. The other factor is that the waves runs into a straight wall and can possibly change into a standing wave. In that case the energy will partly bounce forwards and backwards between the walls until its energy is gone (actually changed in another form of energy).. This we will hear (and see in our measurements) as a dip.

With reflectors you can strive to a more optimum wave bend with less differences in the expansion path and more importantly to prevent standing waves. If you make the reflector to big it will influence the horn path negative by obstructing the sound wave instead of bending it. If you make it to small it wont have the effect (or minimal) what you are after.

Hopefully this makes sence at half past seven (my time) cause it aint no longer to me...
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Old 22nd January 2011, 04:45 AM   #48
Djim is offline Djim  Netherlands
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actualy I could use a beer! (and a bed)

Last edited by Djim; 22nd January 2011 at 04:48 AM.
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Old 22nd January 2011, 05:28 AM   #49
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Originally Posted by Djim View Post
Standing waves however are the result from sound waves bouncing (reflecting is a better word actually) between two parallel surfaces. The more bends we have in a horn the more parallel walls are introduced and the more ‘spiky’ the outcome will be.
I agree with all you're saying there. However, the standing wave problem occurs between parallel walls when the wavelength of the sound being produced is twice the distance between the walls. If you look at the dimension of the box being discussed here, that's puts the standing waves over 280 Hz (guesstimate). Yet JBell says he's observed significant (4dB) improvement at less than quarter of that frequency by using in a shallow reflector.
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Old 22nd January 2011, 12:11 PM   #50
Djim is offline Djim  Netherlands
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Nope Brian, standing waves start to occur between walls at a quarter of the wave length (and half wavelength and full wavelength).
If you look at Sanders Tham, you see 3 places where standing waves can develop (and they all have similar length). My point in the picture of post # 37 was that Jbells suggestion for reflectors will not deal completely with the standing waves because of two reasons:

1.) There are still enough parallel surfaces where standing waves will develop (I marked that with the green area). Jbells example of his significant improvement of 4dB was to solve his problem at 70Hz. This means he had two parallel walls around 122cm (48inch) causing a standing wave.

2.) The second reason needs more explanation. How a relative small and shallow reflector can give such results is a matter of careful placement and understanding of standing waves. In literature (published in AES papers for example) you will find experiments that show that walls start to suppress standing waves as soon they have a > 10 degree difference. However this is frequency related. The higher the frequency the bigger the angle you need. If you want to suppress standing waves over the whole spectrum you need an angle of about 30 to 40 degrees between the two walls. That’s why most walls in recording studio’s have each a traditional angle of about 15 to 20 degrees.

Now, Jbells 4 dB example was at 70Hz and Anders Tham problem is around 175Hz. It means that a similar angled reflector of 12 degrees for the Tham is not going to be as effective. Therefore I suggested more steep angles. The problem with steep angled reflectors is they start to narrow the horn path. Instead of reflecting they start to obstruct the sound waves. That’s why I think you cannot get away with just 2 reflectors as Jbell suggested. In my opinion the Tham would benefit from multiple reflectors that ‘see’ each other. But the use of multiple reflectors will give unwanted side effects and Jbell was trying to warn us for that...

Last edited by Djim; 22nd January 2011 at 12:33 PM.
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