THAM15 - a compact 15" tapped horn - Page 12 - diyAudio
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Old 24th January 2011, 03:25 PM   #111
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Quote:
Originally Posted by Brian Steele View Post
At this point you are only assuming for the Spud and the PA Horns, and you're totally wrong on why Helmholtz resonators were used in the DTS-20. They're used primarily to cut the audibility of driver distortion, not affect in-band frequency response - see PSW Sound Reinforcement Forums: LAB Subwoofer => Helmholtz resonators?
Following on on this, have a look at Don's suggested alignment. Notice the peaks @ around 160 Hz and again around 220 Hz? These will tend make any distortion generated by 40 Hz and 80 Hz tones, and 110 and 55 Hz tones, more audible, because this distortion will be amplified by the horn by it looks like around 6dB. How to effectively deal with this? Well, EQ won't work, unless you want to cut the frequencies that are causing the distortion too. So, what's the next best solution? Two Helmholtz resonators, tuned to 160 Hz and 220 Hz might produce good results here. In fact, perhaps even another one could be added - to deal with the other peak at 340 Hz or so.
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Old 24th January 2011, 04:01 PM   #112
soho54 is offline soho54  United States
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Originally Posted by jbell View Post
Hmmm.... seems like danley is saying akabak doesn't show you everything, and hornresp agrees with akabak.... hmmmm.....
Hornresp agrees with AkAbak on a generic straight horn, yes. When you sim an actual folded horn with corners, no. Neither can get path length differences right around the corners exactly right, but the builder should be checking for this.

There is the whole small/large signal issue as well...

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Originally Posted by jbell View Post
Also, wonder what function you think sound absorption accomplishes in this location? hmmm.. maybe eliminating some unwanted 'reflections' or 'standing waves' ????
There is no need to wonder about it, as it has been explained. It is used to help dampen out'o band high frequencies. To help out the LP filter the bends are creating anyway.

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AND if you follow the diyaudio thread from danley's quote above you'll see where adding additonal absorption where it wasn't necessary created a dip that hornresp said shouldn't be there, and when removed the dip was removed..... How do you suppose absorption created an extreme dip? shouldn't it have just lowered the total spl only?
If you put it in AkAbak you could see what caused the dip(s.) The extra foam raises air resistance which adds acoustic path length to the horn. It also takes away horn volume where it is located. This is what reduces amplitude, and the area of amplitude reduction depends on where the volume reduction falls along the horn path.

If you build a horn, and stuff a big rubber ball in it, it won't measure like the HR sim either, but I would hardly blame the sim because you added something that wasn't put in the sim.

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Originally Posted by jbell View Post
You just can't bring yourself to say that Tom uses various techniques to tackle IN BAND reflections and standing waves.
1/2" of light foam in the corners is not doing anything to any wave in a subwoofers passband, other than reducing the air flow volume through the horn.

Last edited by soho54; 24th January 2011 at 04:04 PM.
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Old 24th January 2011, 04:19 PM   #113
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Default Tom Danley's notes on horns and bends...

Copied this from another thread - I'm emphasized a particular statement in bold:

"...in folding your horns, think in terms of how big is the quarter wavelength your dealing with.

What one cannot do is significantly change the total cubic volume of the interior, air is after all this curious combination of spring and mass and a "connectedness" to the outside. In making a bend, a common mistake is to assume the bend is longer than it really is. Going by cubic volume alone is close but also in a bend, the air is moving in an arc and so temporarily has slightly more mass (as the bulk of the volume is on the outer half of the radius of motion).

Also, at the frequency where the difference in path length between the inside of the bend and the outside of the bend is N 1/2 wavelengths, there are deep notches in the response. This is a result of the opposite phases recombining (and canceling out) after the bend. A good rule of thumb is make sharp bends with a short radius -or- bend angle is inversely proportional to radius). The point is again remember the "acoustic size" of what ever your dealing with. As you can see on the LAB sub the only large degree bend has a small radius (the one at the nose).

Does making the fold with a radius make a difference? Yes, it looks cool and I make them that way because something says "it should be". I have tested a couple low frequency horns with and without and to be honest it made little or no difference . Again what are the acoustic dimensions? In the LAB sub, I would suggest making at least the first two bends with the radius because it "feels right" (a non technical term I admit).

Still more complication, at the point where the wall area is a significant acoustic size, the sound pressure couples to the width mode resonance (caused by the parallel walls) which puts the first in a series of notches in the response coming out of the mouth. In this situation, there is a 1/2 wavelength standing wave with the pressure maxima at the walls and velocity max in the center. Coupling to this mode saps off energy at frequencies related to the N 1/2 wavelengths. Here, your horn mouth width (where it has parallel walls) also kind of sets your upper frequency limit
For a 21 inch wide horn like the LAB sub, the first width mode notch should be in the mid 300's which is a non issue. "



Based on these notes, I suspect the last bend in the THAM15 is what's going to create the most deviation between sim'd response and actual response, seeing it's a 180 at the wide end of the path. This afternoon I'm going to do a few calculations to see where that response difference is predicted to lie for my POC#2 (which shares the same folding), to see if it actually shows up where it should in my measurements.
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Old 24th January 2011, 05:03 PM   #114
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Bad Brian ... Tom is talking about using an arc instead of a flat reflector. This doesn't make your point at all!
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Old 24th January 2011, 05:07 PM   #115
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Originally Posted by Don Snyder View Post
Bad Brian ... Tom is talking about using an arc instead of a flat reflector. This doesn't make your point at all!
You forgot to add your smiley .

Perhaps I should have bolded Tom's subsequent rhetorical question concerning acoustic dimensions as well .
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Old 24th January 2011, 05:23 PM   #116
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Sorry, I did need a smiley.

Tom keeps giving us rules, all of them based on good physics, and then violating them. I sometimes feel i'm being taunted.

My only complaint was that you bolded the wrong sentence.
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Old 24th January 2011, 05:59 PM   #117
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Brian,

Simming the TH as shown in the schematic, peaks are evident at 160 and 230Hz. The "Q" of these peaks is very narrow. When the TH is folded, you will
no longer have a single, unique path thru the horn, and the "Q" of these peaks will be wider and the peaks will be much lower.

Or am I wrong ...
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Last edited by Don Snyder; 24th January 2011 at 06:11 PM.
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Old 24th January 2011, 06:20 PM   #118
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Quote:
Originally Posted by Don Snyder View Post
Brian,

Simming the TH as shown in the schematic, peaks are evident at 160 and 230Hz. The "Q" of these peaks is very narrow. When the TH is folded, you will
no longer have a single, unique path thru the horn, and the "Q" of these peaks will be wider and the peaks will be much lower.

Or am I wrong ...
It's possible, but I suspect the answer is... I think it depends. My POC #2 doesn't show significant changes in the out-of-band peaks until +350 Hz, for example. The first "dip" seems to be deeper than expected, but that could be due to the fact that I went with a stepped config for part of the horn. I'm not sure.
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Old 24th January 2011, 06:39 PM   #119
jbell is offline jbell  United States
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Quote:
Originally Posted by Brian Steele View Post
At this point you are only assuming for the Spud and the PA Horns, and you're totally wrong on why Helmholtz resonators were used in the DTS-20. They're used primarily to cut the audibility of driver distortion, not affect in-band frequency response - see PSW Sound Reinforcement Forums: LAB Subwoofer => Helmholtz resonators?
still not buying....

Click the image to open in full size.

now, if this pipe is 31 3/4" long, making the total length of pipe around 40" or so, and closed end pipes are 1/4 wave resonators -- what is the fundamental of that 1/4 wave resonator? possibly around 80hz?

I know ivan is quoted as saying it was for 'distortion' reasons, and that closed pipes only resonate on the fundamental and odd order harmonics...

still I personally believe that the 2 pipes are set for fundamentals of the 2 normal peaks of a TH to tame them down, and the inductor is there to fill in the gap. As a side bonus, you also get reduction in 3rd order harmonics of the 2 'peaks' of a typical TH.

meaning, the helmholtz is there to tame a resonance that is IN BAND.... which was my point.

Last edited by jbell; 24th January 2011 at 06:41 PM.
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Old 24th January 2011, 07:09 PM   #120
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Originally Posted by jbell View Post
still not buying....
...even though a representative from the company specifically indicated the purpose of those pipes??

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Originally Posted by jbell View Post
now, if this pipe is 31 3/4" long
That's a very big assumption. What if they're only 20" long?

We need to know their true lengths before making further assumptions about their purpose, particularly if those assumptions are at odds with those of the inventor .

In any case, I think you missed the major point. Theory supports the use of Helmholtz resonators to modify frequency response in a bass horn. There's no accepted theory that provides the same support for the use of "reflectors" to do the same thing.
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