THAM15 - a compact 15" tapped horn - Page 13 - diyAudio
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Old 24th January 2011, 07:16 PM   #121
jbell is offline jbell  United States
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you missed seeing the measurement on the side of the pipe.

And no, I did not miss the point. I do large room acoustics. You can deal acoustically with a wave with:

1. absorption. (like the 1/2" foam used in the spud knock off that DOES NOT have an acoustic absorption peak where the dip showed up in that testing, meaning that extra foam did not cause an non uniform reduction in spl.)
2. tuned cavity. (like the helmholtz in the dts-20, that tom obviously designed with tall peaks and a deep valley between them, and then used a combination of the helmholtz/inductor to flatten that out.)
3. oblique reflection/diffusion. (like the reflectors that I have installed and measured in the MANY tapped horns that I have built.)

All three work in room, and all three work in cabinet.
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Old 24th January 2011, 08:36 PM   #122
jbell is offline jbell  United States
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Here's a thought for you... (hey actually putting my music degree to work here....)

A clarinet is a quasi closed ended pipe 1/4 wave resonator. Think about a clarinet here.... then think about the shape of a typical tapped horn.

I know a clarinet is a quasi closed ended pipe, as it has no even order harmonics in the lower register. It resonates at 1/4 wave. (yes, you guessed it, I'm a wind player)

I'll submit a guess that the sections of a TH behave like a clarinet and are actually 1/4wave resonators due to the pressure coming from the 'throat side' of the driver, and that the reflectors actually disrupt that segment from acting like a 1/4 wave device.
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Old 24th January 2011, 08:43 PM   #123
Djim is offline Djim  Netherlands
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So, see if I got your theory right Brian…

The reflectors seem to work fine as an optimiser for the expansion problems in the bends (and the name should be changed in volume optimizers).
As a soundwave reflector (for preventing ¼ wave cancellations) they have almost no use cause then they need to have the surface for ½ wavelengths (following your rules of room acoustics).

This is what you try to say, right?
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Old 24th January 2011, 09:00 PM   #124
tb46 is offline tb46  United States
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Hi Y'all,

1/4 tube are not Helmholz resonators, and they work differently:

From diyaudio Tom Danley's TOWER OF POWER, Post #128: "...Helmholtz resonators are phase inverters, produce a 180 degree phase shift between the driving pressure and output pressure. A quarter wave resonator, the lowest mode of such a thing, only has a 90 degree phase shift I / O. ..."

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Old 24th January 2011, 09:04 PM   #125
jbell is offline jbell  United States
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Quote:
Originally Posted by tb46 View Post
Hi Y'all,

1/4 tube are not Helmholz resonators, and they work differently:

From diyaudio Tom Danley's TOWER OF POWER, Post #128: "...Helmholtz resonators are phase inverters, produce a 180 degree phase shift between the driving pressure and output pressure. A quarter wave resonator, the lowest mode of such a thing, only has a 90 degree phase shift I / O. ..."

Regards,
yep, you are right oliver -- my bad for calling them helmholtz... I should say 1/4wave resonator. (you always seem to catch my goof ups, thank you.)

And that also goes with Tom's statement about the 'limited' amout of phase shift within a TH cabinet vs a ported cabinet, that I quoted earlier.

(back to that clarinet example)

Last edited by jbell; 24th January 2011 at 09:08 PM.
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Old 24th January 2011, 10:33 PM   #126
soho54 is offline soho54  United States
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Quote:
Originally Posted by Don Snyder View Post
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 ...
Speaking generically, it isn't the bends, it is the material the horn is made out of. Everything in the HR simulation assumes absolute values, or think of it as infinity hard surfaces. Anything striking the surface is reflected at the same amplitude.

This is what produces the high sharp peaks and dips in all the HR graphs, FR, displacement, impedance... When you build the horn in real like actual materials do not work this way, and small amounts of energy are transferred in several different ways. When you go to measure the real horn (let's keep it a straight perfect copy for now,) the peaks and dips will be there, but they will not have the same absolute amplitude in gain or loss.

Bends alter the horns expansion rate, volume, and air resistance in the immediate area. Depending on the horn and it's properties slight variations can cause drastic changes to the peak/null, and random overal amplitudes in certain locations along the horn.

Quote:
Originally Posted by jbell View Post
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?
85Hz

Quote:
Originally Posted by jbell View Post
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.
The 1/4W resonators are not for the FR peaks. The are to dampen the harmonic content from other frequencies that get boosted by the horn. Let's say the horn has a peak at 80Hz. You can add EQ to lower this peak, but that only effects that one frequency. Now you have a real driver playing a 20Hz tone loud enough to distort. The 2nd harmonic distortion is now being boosted to higher levels by the horns natural peak at 80Hz, and there is nothing you can normally do about it. Enter the 1/4W resonators.


Quote:
Originally Posted by jbell View Post
A clarinet is a quasi closed ended pipe 1/4 wave resonator. Think about a clarinet here.... then think about the shape of a typical tapped horn.

I know a clarinet is a quasi closed ended pipe, as it has no even order harmonics in the lower register. It resonates at 1/4 wave. (yes, you guessed it, I'm a wind player)

I'll submit a guess that the sections of a TH behave like a clarinet and are actually 1/4wave resonators due to the pressure coming from the 'throat side' of the driver, and that the reflectors actually disrupt that segment from acting like a 1/4 wave device.
TH's are 1/4WL resonators, closed end air columns. Most all bass horns are at the cutoff. That is how they work, it's no secret. Have you done any kind of research into this subject? The section folds in a TH are not closed end air columns. They would be open end air columns, and work in 1/2WL modes if anything.

Last edited by soho54; 24th January 2011 at 10:37 PM.
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Old 24th January 2011, 10:54 PM   #127
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Quote:
Originally Posted by Djim View Post
Jbell, thanks for the sharing, oh and Brian you can use your tube you already build, seems a perfect candidate…
I've done some tests tonight, using matt-board taped into the corners to simulate reflectors. I took great pains (ok, maybe not so great) to ensure everything else remained constant, so the measured FR changes will in the main be caused by the insertion of the reflectors. Are you interested in seeing and discussing the results?
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Old 24th January 2011, 11:00 PM   #128
Djim is offline Djim  Netherlands
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Actualy Brian, I asked you some questions which Soho54 (thanks) already seemed to have answered.

But yes, share it with us please cause you seem to have build something that looks more like a panel resonator instead of "reflectors" which I prefer to call volume optimisers from now on. But I really appreciate what you are trying to do here!

Btw and thank you Xoc1 cause he showed the working of those "volume optimizers" already in his post...

Last edited by Djim; 24th January 2011 at 11:21 PM.
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Old 25th January 2011, 12:06 AM   #129
soho54 is offline soho54  United States
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Sorry, about my tone here guys. It's a little more aggressive than I would have liked. I seem to be having some bleed through from real world issues.

I'll try and curb it. Maybe I should start drinking?

Brian Steele, I would be interested in anything you felt like putting up. The more the better. There is a new Subwoofer measurement site in the works, and it will hopefully put some good stuff out over the coming year. It's going to be DIY to commercial stuff, but I know a few well known THs have been through the ringer already, and there are several more in the build stages right now to be ready for when it stops snowing.
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Old 25th January 2011, 12:10 AM   #130
tb46 is offline tb46  United States
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Hi Brian,

Yes, I think everyone is looking forward to see your data (Post #127). Please, include impedance measurements. That matt-board looks a little flimsy though (?), and the size should be about double (going to about the middle of that turn).

Just thinking out loud:

What seems to make the whole thing so untracktable is that you have the major modes resulting from the mouth-to-driver, driver-to-driver, driver to throat distances, individually as well as in combination. Then you also have the pressure waves travelling down the horn path and interacting with every discontinuity, corner, bend and reflector. When a pressure wave interacts with a bend it should affect the propagation in both directions from the discontinuity, wonder how Tom Danley puts that into his custom AkAbak models?

That's why to me the information that screamerusa and jbell have provided - pertaining to the effect of shallow (12 to 15 degree) reflectors placed at the first bend(s) after the driver entry near the throat - is so valueable. It is a not at all obvious, measured result, and because of the magnitude of the effect, Brian is right, it should show in the impedance measurements too. I guess that's why Tom Danley says that "measurements trump models". I guess to get any clarity here will require a series of models and measurements under controlled circumstances.

In his SPUD thread (Dual 8" tapped horn = TH-SPUD - Post #61) geitmans modeled the individual sections and bends as ducts, and added different viscosity values until he got something, that reflects the measured SPL response. That approach may be an entry point. Maybe we can talk soho54 into making an AkAbak model for the THAM?

Regards,
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