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Old 14th May 2013, 12:19 AM   #891
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Quote:
Originally Posted by JLH View Post
Take this picture and show me exactly what you mean. What port are you talking about?
Two of the four ports are visible in the shadows to the right and left of the upper coil.

The proximity of the wood blocks right and left of the coil to the port probably would lower Fb somewhat compared to the 3/4" depth round cutouts without them, though that sort of "ducting" would be hard to simulate.

At any rate, the the SH-50 Fb looks to be around 80 Hz, resulting in a frequency response "bump" centering at 70 Hz.
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File Type: png Two Ports.png (53.8 KB, 352 views)
File Type: png SH-50 FB 80Hz.png (41.0 KB, 353 views)
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Old 14th May 2013, 02:58 AM   #892
hulkss is offline hulkss  United States
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Quote:
Originally Posted by JLH View Post
The distance from the woofer to the port is irrelevant because as soon as the woofer moves, the entire enclosure is pressurized and the sound emits out of the port. This is very much like a syringe. It doesn’t matter how far away the plunger is from the syringe exit, as soon as the plunger moves forward the fluid begins to exit the syringe. Air is compressible, but for all practical purposes the effect is instantaneous.
Time out......
Are you stating that when the woofer cone moves sound will almost "instantly" emit from the port. That the acoustic pressure wave will travel at supersonic speed reaching the port quicker than traveling at the speed of sound??

The length of an organ pipe sure makes a difference.
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Old 14th May 2013, 03:24 AM   #893
hulkss is offline hulkss  United States
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A short update on the Synergy mids I'm working with (AuraSound 2").

I've done some more modeling in Akabak and read a bit more about port noise and harmonic distortion. The port edges need a slight edge radius, about 15-20% of port diameter. The end conditions should be symmetrical to avoid harmonic distortion as the direction of airflow reverses.

Here is the frequency response unfiltered, and then filtered with a bandpass filter to achieve a -6 dB crossover point at 300 and 1200 Hz:

Click the image to open in full size.

Click the image to open in full size.

Here are the speaker diaphragm displacement and port velocity curves (with the bandpass filter):

Click the image to open in full size.

Click the image to open in full size.

Here is a cross-section through two of the drivers showing the design as currently planned:

Click the image to open in full size.

I guess the next step will be to build a simple tuning test rig with the driver, rear chamber, front chamber, and port to verify the tuning resonant frequency.
Attached Images
File Type: png SPL.PNG (40.1 KB, 413 views)
File Type: png SPLfiltered.PNG (39.2 KB, 412 views)
File Type: png Displacement.PNG (39.5 KB, 413 views)
File Type: png Velocity.PNG (40.0 KB, 413 views)
File Type: png DriverPorting.PNG (95.2 KB, 412 views)
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File Type: txt synhorn.txt (3.4 KB, 7 views)

Last edited by hulkss; 14th May 2013 at 03:33 AM.
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Old 14th May 2013, 03:56 PM   #894
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Originally Posted by JLH
The distance from the woofer to the port is irrelevant because as soon as the woofer moves, the entire enclosure is pressurized and the sound emits out of the port. This is very much like a syringe. It doesn’t matter how far away the plunger is from the syringe exit, as soon as the plunger moves forward the fluid begins to exit the syringe. Air is compressible, but for all practical purposes the effect is instantaneous.

Quote:
Originally Posted by hulkss View Post
Time out......
Are you stating that when the woofer cone moves sound will almost "instantly" emit from the port. That the acoustic pressure wave will travel at supersonic speed reaching the port quicker than traveling at the speed of sound??

The length of an organ pipe sure makes a difference.
JLH's syringe analogy "kind of" works when thinking of a bass reflex enclosure small relative to Fb, but the hydraulic analogy obviously falls apart for systems like a pipe organ (or tapped horns ), sound waves travel at a fixed speed in air, with slight variations due to temperature etc.

The frequency of the box/port system resonance, Fb, is determined by the length and diameter of the duct, the internal volume of the enclosure, and the speed of sound in air.
The port output is phase inverted from the back wave of the speaker, the inversion brings it in phase with the front wave (lagging by one wave period) reinforcing the front wave.

The SH-50 Fb is around 80 Hz, speed of sound is 1130 feet per second, 1130/80=14.125 foot wavelength.

Within 1/4 wavelength, sound waves reinforce, 14.125/4=3.53 feet.

The SH-50 horn path is less than 2 feet, regardless of the port placement in the horn, the port output is within 1/4 wavelength (one wave period behind) of the speaker's direct output.

Looking forward to seeing how your real measurements compare to the Akabak sims.

Art

Last edited by weltersys; 14th May 2013 at 03:59 PM.
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Old 14th May 2013, 05:29 PM   #895
JLH is offline JLH  United States
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Thanks Art. That is more or less what I was trying to say, you just said it better.
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Old 14th May 2013, 10:53 PM   #896
kessito is offline kessito  Netherlands
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Quote:
Originally Posted by JLH View Post
Take this picture and show me exactly what you mean. What port are you talking about?

Hi,

please see the attached picture,
What I mean is the path from the driver to the low(rear) port has to follow the red lines.
I believe the blue marked square area, or maybe a larger area between the dividers form part of the the actual bassport, not only the small holes which enter the hornwall.
Hopes this makes sense.

greetings,
Kees
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File Type: jpg horn.jpg (123.6 KB, 116 views)

Last edited by kessito; 14th May 2013 at 10:57 PM.
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Old 14th May 2013, 11:23 PM   #897
JLH is offline JLH  United States
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Quote:
Originally Posted by kessito View Post
Hi,

please see the attached picture,
What I mean is the path from the driver to the low(rear) port has to follow the red lines.
I believe the blue marked square area, or maybe a larger area between the dividers form part of the the actual bassport, not only the small holes which enter the hornwall.
Hopes this makes sense.

greetings,
Kees
The pieces between the crossover board and the horn wall do not look like they are tall enough to contact the rear cover to form a duct/port tunnel. Those pieces look they are there for bracing the horn.
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Old 15th May 2013, 06:39 AM   #898
kessito is offline kessito  Netherlands
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...I mean the green cirkeled ones, they definately touch the wall, the outer cabinet is attachted to it with some screws.
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Old 15th May 2013, 10:02 PM   #899
Phil B is offline Phil B  Australia
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Originally Posted by kessito View Post
...I mean the green cirkeled ones, they definately touch the wall, the outer cabinet is attachted to it with some screws.
This is what I was originally refering to ......

.p.
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Old 15th May 2013, 10:14 PM   #900
jeno is offline jeno  Norway
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I think Tom Danley is refering to the bass reflex section as a tapped horn, and I think it was the work with the Synergy horn that originally gave him the tapped horn idea.
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