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Old 3rd September 2008, 01:43 PM   #151
gedlee is offline gedlee  United States
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Quote:
Originally posted by Tom Danley
Hi Earl
I am struggling to see why viewing a horn or pipe as its lumped parameters is a better way to see it than the wave view, like as in a quarter wave antenna.Best,
Tom Danley
Tom

Thats not what I said. I said that at LFs the pipe must look like a lumped parameter mass. It has to. I keep talking about what the systems and models have to look like as the frequency falls and you keep talking about what they look like as the frequency increases. They aren't going to be the same things - not even close.

I am not sure the antennae analogy is of much use either as its different enough from acoustics to be misleading. At any rate, I'm pretty sure that this level of detailed discussion cannot be effective in posts like this.
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Old 3rd September 2008, 02:34 PM   #152
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Hi Earl

In that case, I misunderstood what you were getting at, we are talking about the same thing.
The Tapped horns always have a horn path length that is greater than a quarter wl within its operating range.
I agree that when the port is made much shorter than the lowest mode ( its low cutoff in a TH), it does act like a lumped element like the mass in a port etc.
I thought you were saying that in its operating range it was best considered a lumped element.
Best,
Tom
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Old 3rd September 2008, 02:59 PM   #153
jbell is offline jbell  United States
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Quote:
Originally posted by David McBean
You wouldn’t have an AMD Athlon processor in your computer, or be running another 16-bit application at the same time as Hornresp, by any chance?

Kind regards,

David
AMD processor and firefox 3.x running is a 'crash all the time' scenario.
However, I don't care... I can just shut down firefox and work just fine. THANKS for an awesome tool david.
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Old 3rd September 2008, 03:25 PM   #154
iand is offline iand  United Kingdom
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Quote:
Originally posted by Tom Danley
Hi Guys

If you want to try modeling the real thing, I’d be willing to send you a drawing of what’s inside of a TH-115.

Hi Iand,
when I model a pair of the 18ps76’s in a vented box, what I see is a sensitivity between 101 and 102dB 1W1M, about –3dB down from the TH-115.
Also, with that alignment one reaches Xmax in the 65Hz to 35Hz region at about 350 Watts total input and even a 12 inch port has noticeable loss.
So far as Tapped horn excursion, I believe the issue arose from Akabak giving a different predicted value from Horn response at the time, I don’t know if they agree now or not.
I can tell you is that if you want to know what the excursion is, the easiest way to do it is to put a window in the box with a light inside and a white dot on the cone next to a scale.
My point then was that if one sees an increase in sensitivity of say 10dB, one also sees a reduction in cone motion of about 3 for the same SPL.
Also, it has been my observation that often the “Q’s” of the hf resonances are over stated in the computer models, some times a hf feature is predicted that is totally absent in the measurement.
“P.P.S. Also a large array of tapped horns may indeed gain more in efficiency, but this is only relevant in big concert sound systems”
I would agree in part, clearly I have seen that the advantage one can get from a TH is related to being “big enough”.
The TH allows the Horn portion to be somewhat smaller than a normal horn but it still has to be large enough.

Best,
Tom Danley
Hi Tom

When simulated in Hornresp the 15TBX100 tapped horn (TH-115) and dual 18PS76 reflex have almost identical sensitivity and maximum output, as I posted some time ago.

When you say the reflex is about 3dB down from the TH-115 I assume you're comparing your measurements with simulation? This efficiency (and cone travel) discrepancy has still never really been explained, and other people have reported recently that simulated and measured cone travel do agree.

So either what I thought was inside the TH-115 was wrong (do please send me the drawing so I can model it!), or Hornresp is still wrong (which now seems less likely), or your measurements are wrong (which also seems unlikely) -- or maybe a bit of all three :-)

Cheers

Ian
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Old 3rd September 2008, 03:51 PM   #155
gedlee is offline gedlee  United States
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Quote:
Originally posted by Tom Danley

The Tapped horns always have a horn path length that is greater than a quarter wl within its operating range.
I agree that when the port is made much shorter than the lowest mode ( its low cutoff in a TH), it does act like a lumped element like the mass in a port etc.

You spec the system to go down to 28 Hz, which has a wavelength of about 40 feet. So your horn paths are all greater than 10 feet? It is really difficult to see from the photos how you achieve that. And if they are not at least ten feet then at the lower edge of the passband they are lumped masses.
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Old 3rd September 2008, 04:02 PM   #156
gedlee is offline gedlee  United States
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Quote:
Originally posted by iand

So either what I thought was inside the TH-115 was wrong (do please send me the drawing so I can model it!), or Hornresp is still wrong (which now seems less likely), or your measurements are wrong (which also seems unlikely) -- or maybe a bit of all three :-)
This is the point that I was making (lets hope you don't get chastised too for saying it) that until the theory and measurements agree there is something that we don't understand going on. And quite honestly, from what I have seen they are pretty far off.

Absolute levels are always difficult to correlate and arguing about a few dB in level is not going to be very fruitful, but one should be able to get shapes and passbands to come out right. The models show, and this seems logical to me, a highly resonant response above the first two resonances. Toms results don't show this, and they, in fact, show a very very smooth response that one would not expect from either the theory of a highly resonant system or the models. If the absolute passband level is off a few dB, well thats the nature of the beast, but the disparancy between what the models predict, which is what I would expect, and what is measured is a real mystery to me.
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Old 3rd September 2008, 04:46 PM   #157
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One thing that's ironic about your inventions is that one of them reduces high order modes (the foam that Geddes uses in his waveguides) while the other one depends on it to some extent (the tapped horn.)

Here's what I mean by this:

A Geddes waveguide is meticulously designed so that the wave originating from the diaphragm doesn't suffer from diffraction. To really wallop high order modes, foam is placed in the waveguide to absorb waves which are reflected at the mouth and back into the waveguide.

In a tapped horn, there are four mechanisms meticulously designed to generate high order modes. First, the horn is not driven at the apex, so reflections are created almost immediately, and in large numbers. Second, under sized mouths create more reflections than properly sized mouths IIRC (and a tapped horn has a horn mouth which is dramatically undersized.) Third, a tapped horn does not have any mouth treatment to reduce reflections. For example, a labhorn uses a significant flare at the mouth, which increases output and reduces reflections. Last but not least, a tapped horn has a number of 180 degree bends, which add more reflections than a series of 90degree bends. For example, the Danley DTS-20 has two or three 180 degree bends. In comparison, the labhorn has none IIRC.

In a recent post Dr Geddes wondered how the tapped horn can have such a long path length. Part of the answer is that a great deal of it's output is the result of internal reflections.
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Old 3rd September 2008, 07:09 PM   #158
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Quote:
Originally posted by gedlee
You spec the system to go down to 28 Hz, which has a wavelength of about 40 feet. So your horn paths are all greater than 10 feet? It is really difficult to see from the photos how you achieve that. And if they are not at least ten feet then at the lower edge of the passband they are lumped masses.
Earl,

The 28Hz box is the much larger TH-215, where the TH-115 extends to ~38Hz.

I haven't seen inside of any of Tom's more recent designs, but most all of the tapped horns are folded significantly to allow the path length, and the longer path length past the tap is as long as you describe.

I expect the folding is a big source of the unknowns or approximations in the HF resonant Qs as the programs don't really allow a good approximation of this as frequency rises or the width of the bend becomes significant.
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Old 3rd September 2008, 07:52 PM   #159
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Hi Earl

The quarter wave resonance is what defines the low corner of the response for a TH.
Essentially, one finds that wherever the knee in the response is, is where the quarter wave resonance is.
As with a conventional horn, having a compliance volume at the throat combines with the throat mass and forms a low pass corner. To a degree, this volume also shifts the lowest resonant frequency downward slightly.
Also worth mentioning, at least with low frequency horns, my Mathcad program and Akabak both generally over state the Q’s one see’s in the computer models made with them and underestimate the extra acoustical losses one encounters with very small horns.
Occasionally, one will find a predicted hf feature, which is entirely absent in the measured result.

Someone found the patent application on line for the dts-20, a package shape Mark Seaton suggested and this shows the enclosure layout.
Here is what it does;
http://www.danleysoundlabs.com/pdf/D...ec%20Sheet.PDF

Here is what is inside.
This cabinet has a low cutoff and as you can see, a very long path length.
http://www.diyaudio.com/forums/attac...amp=1202828040

Best,
Tom


Hi Ian

I am not sure how you modeled the TH-115, but what I got was about 3dB down in sensitivity.
Also, as I said earlier, I am happy when a computer model for a horn is within a dB or two of what you measure when you build it.
In my case, we sell the actual thing and not the computer model so how it measures trumps the model (normally the model is a bit better than the real thing).
We measure at 10 meters most of the time because one would literally be in the mouth bubble at one meter with large woofers. This also compensates for the fact that the speakers aren’t buried in the ground firing up into half space.
We have a mic calibrator, Earthworks mics, use an HP3456a Voltmeter and send some stuff to an independent company for measurement also.

I have made a fair number of boxes, which didn’t work out for one reason or another so this appears to be “the way it is” so far as some computer ambiguity or at least indicates that something’s are still left out..
Your reference to excursion discrepancy is puzzling with David’s reply , yes the cones do move and the models I use tend to overstate the Q’s on its predictions like magnitude and excursion.
The best bet is to observe the motion while in operation.
Best,
Tom

Hi Patrick

I am not sure the concept of HOM’s applies when the horn dimensions are such a small fraction of the wavelength.
For instance, think about how big a wavelength is at 20KHz and then think about how large the horn that wavelength is bouncing around inside of is (many many wl’s).
Then think of a TH with an upper operational frequency of say 100Hz and how big that WL is compared to the inside of the TH, only in its length is does it even approach 1wl.
In other words, it is too small at low frequencies to support non-axial or not length wise modes.
Now, if you drive it higher up at say 2KHz or above, then sure.
To be sure there are several axial resonances, but that is how low frequency horns work and it is by manipulating the series that one can greatly reduce or eliminate the dip between the first and second peak. that “small bass horns” like the Lab sub exhibit when used alone.
With a bass horn it is very hard / impossible to make one that has a flat response curve to its low corner. They essentially always have a significant droop off and then at some point have a knee. With the Tapped horn, one can often make the system Flatter for a given size and cutoff. One drives the shape and proportions according to the predictions.

It has been my experience that bends are often necessary in horns.
A bend can be anywhere from completely harmless and only adds a tiny lump of extra mass at the bend, or can cause a large notch, or even produce scattering, HOM’s,.
Observationally I would say it depends how large the difference between the inner path and outer path length is compared to the wavelength your talking about.
A real example of bending, If you had a 10KHz source driving a 3 foot copper tube that has a bore of say .062”, you would find that you could literally wind the tubing around a coffee cup a number of times and not significantly effect the sound coming out the end.

Best,
Tom
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Old 3rd September 2008, 08:30 PM   #160
gedlee is offline gedlee  United States
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Thanks for the drawing. I guess calling those passages "horns" is really stretching the issue in my mind. But I can see whare you might be getting some decent path lengths with all the folds.

The driver radiates front and back correct? And the front is delay when it reaches the back by the path length so periodically it will be in phase and out of phase.

I still have trouble seeing how such a smooth system results from what should be a chaotic mess of resonances and interferences. Perhaps, like room acoustics and Distributed Mode panels, you are relying on a large density of resonances to yield a smooth sum. But in both of my examples, there is a region where there aren't enough modes for good summation followed by a region of reasonably smooth response with a sort of random variation. I don't see that in any of your examples.
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