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Old 9th February 2008, 03:51 PM   #1121
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GM, what you said about using Leach' math for Tapped Horn design got me thinking about how applicable his whitepaper is to Tapped Horns, so I made a few AkAbak scripts to test some things. Since I have no idea how you use his Math for your designs (the only thing I could think about was for sizing the back/front chamber and choosing the driver Fs), maybe you want to play around with them?
I have tried to comment them as much as possible, so maybe you can use it as a jumpstart to learning new tri ... err ... AkAbak

Can you PM me? I have tried to send an email to you but you have it disabled.
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Old 9th February 2008, 04:18 PM   #1122
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Since i dont have any data on the drivers, i use a Peerless caraudiodriver in hornresp. It work in the previous TH i built. I know its not realy a very good idea, but i have the drivers so why not. The wood cost me about 3 dollar.

I calcualated the driver as one with the same VAS and Fs but twice the BL and SD, and four times the Mms.
The two ohms "rg" is to simulate a higher Qes and Qts.

Click the image to open in full size.
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Old 9th February 2008, 04:29 PM   #1123
MaVo is offline MaVo  Germany
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Quote:
Originally posted by MaVo
Will the resonance occur at pathlength = wavelength/2 ?
Quote:
Originally posted by jnb
Is it worth the effort of rounding the bend and flaring the mouth (so it looks like a sax?)
I found a post from Tom Danley about this subject. Its about the Labhorn, but aplies here as well, as it seems. The part in " " is from Tom Danley.

"Picture a long wavelength, lets say it is of the highest frequency of interest, say 120 Hz.
120 Hz has a wavelength of 9.43 feet, its quarter wavelength is 28.3 inches.
When the dimensions of a chamber or passageway are less than 1/4 wave in dimension, one can treat that
space/volume/compliance as a lumped element or elements.

What that means may not be clear but here is an example of it.

Imagine a horn, that was 6 meters long, with a 28 Hz flare and a 7 sq. meter mouth area.
Instead of making a traditional horn, one made it out of pipes of stepped diameters, a "digital" horn.
How many steps do you have to have before it works like a horn, any guesses?.

Well if one made each pipe 1 meter long and did the expansion in 6 steps what would you get?
You get the exact same thing as a true exponential horn up to 150 Hz.
What you see in the radiation resistance is a small wiggling around compared to the expo BUT with the right driver, that does not show up in the acoustic output.
The effect of the steps does put a pretty deep notch in the response but the 1 meter step's first notch is at 200 Hz and the next at 400 Hz and so on. Using this box below 150 Hz, there is essentially no difference between the two.

What all this means is that when you are dealing with acoustically small dimensions, one has an extra degree of freedom
in where one puts a given air volumes etc. (like where I "hid" the front volume)
Where in a 1 inch compression driver and horn , having the rubber gasket obstructing even a tiny part of the horn throat
can kill the hf response, one can get away with a significant deviation from "the book" if its acoustic dimension is small.

Sven 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."
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Old 9th February 2008, 05:11 PM   #1124
AndrewT is offline AndrewT  Scotland
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Quote:
Originally posted by MaVo
"Picture a long wavelength, lets say it is of the highest frequency of interest, say 120 Hz.
120 Hz has a wavelength of 9.43 feet, its quarter wavelength is 28.3 inches.
When the dimensions of a chamber or passageway are less than 1/4 wave in dimension, one can treat that
space/volume/compliance as a lumped element or elements.

What that means may not be clear but here is an example of it.

Imagine a horn, that was 6 meters long, with a 28 Hz flare and a 7 sq. meter mouth area.
Instead of making a traditional horn, one made it out of pipes of stepped diameters, a "digital" horn.
How many steps do you have to have before it works like a horn, any guesses?.

Well if one made each pipe 1 meter long and did the expansion in 6 steps what would you get?
85Hz has a wavelength of 4m, leaving the quarter wavelength at 1m.

Where does the 150Hz come from?
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Old 9th February 2008, 06:17 PM   #1125
GM is offline GM  United States
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Quote:
Originally posted by Cordraconis

GM.............
Can you PM me? I have tried to send an email to you but you have it disabled.
What's this? The 'pot calling the kettle black?' Your PM is disabled also.

Anyway, I use it for Vas, Qes and 'M' factor.

GM
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Old 9th February 2008, 06:25 PM   #1126
MaVo is offline MaVo  Germany
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a segment length of 1 meter equals a resonant frequency of: 344m/s / 2*1m = 172hz. 150 is just a bit below that, to allow the crossover to filter all resonance exciting frequencies.
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Old 9th February 2008, 07:22 PM   #1127
GM is offline GM  United States
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Quote:
Originally posted by AndrewT

85Hz has a wavelength of 4m, leaving the quarter wavelength at 1m.

Where does the 150Hz come from?
Where did you get 85 Hz/4 m?

The ~150 Hz comes from his 6 m long, 28 Hz flare frequency expo terminating into a 7^2 m mouth (At) example, though AFAIK you would have to reverse calc it's throat area (St) to find its theoretical Fh3, or St = ~105.9 cm^2 = ~944 Hz/2/pi = ~150 Hz.

GM
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Old 9th February 2008, 08:23 PM   #1128
jnb is offline jnb  Australia
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Quote:
Originally posted by MaVo
The effect of the steps does put a pretty deep notch in the response but the 1 meter step's first notch is at 200 Hz
Quote:
Originally posted by GM
The ~150 Hz comes from his 6 m long, 28 Hz flare frequency expo terminating into a 7^2 m mouth
Intentional coincidence? He may have designed the step related notch and the throat cutoff to be equally out of band.

200Hz does sound more like a practical 1/2 wavelength yet he did say a quarter wavelength. The segment steps must be OK below some frequency?
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Old 10th February 2008, 12:59 AM   #1129
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Quote:
Originally posted by GM


What's this? The 'pot calling the kettle black?' Your PM is disabled also.

Anyway, I use it for Vas, Qes and 'M' factor.

GM


I checked it right before I posted that ... My PM was enabled.

Anyway, I checked it again and this time I also enabled my email adress. It should work now.

Goodnight! *yawn*
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Old 10th February 2008, 02:04 AM   #1130
GM is offline GM  United States
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Don't know, I clicked on it in your profile screen a couple of times and got the 'does not accept............' response, but it works now, so YGM.

GM
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