TalkBass Forums Round port to shelf port conversion?

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#1
09-02-2013, 06:20 PM
 Registered User Join Date: Sep 2010
Round port to shelf port conversion?

I'm hoping to get an answer to a question without coming across totally confusing.
If I need 4 round ports at a depth of 5.5", could I just calculate and combine the cubic feet of all 4 round ports and convert that to a shelf/slot style port with the same cubic feet at the same 5.5" depth?
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#2
09-02-2013, 06:37 PM
 only immortal for a limited time Owner & speaker designer, AudioKinesis Join Date: Nov 2008 Location: Preston, Idaho
Quote:
 Originally Posted by Phendyr_Loon If I need 4 round ports at a depth of 5.5", could I just calculate and combine the cubic feet of all 4 round ports and convert that to a shelf/slot style port with the same cubic feet at the same 5.5" depth?
The short answer is, no. A shelf port will have a lower tuning frequency than an equivalent-length round port.

The geek answer goes on to explain that there are two mechanisms which come into play that both tend to lower the tuning frequency:

First, as the shelf port's area becomes more slot-like than box-like (that is, as the aspect ratio increases), the amount of surface area relative to the internal cross-sectional area of the port goes up, which increases drag due to boundary effects, lowering the tuning frequency.

Second, assuming the shelf port is on the bottom of the box and is the full width of the box, the bottom of the shelf port as well as its sides run the full depth of the box, while only the top part of the shelf port is of reduced depth. So the effective length of the shelf port is considerably longer than the length of the shelf itself. In cases where the shelf extends fairly close to the rear wall of the cab, the effective length makes the turn and extends upwards a bit, sort of like a short-footed "L" lying on its back.

I don't know of a handy conversion formula, as the specifics vary from case to case.
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Last edited by DukeLeJeune : 09-02-2013 at 06:43 PM.
#3
09-02-2013, 06:47 PM
 Registered User Join Date: Sep 2010
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#4
09-02-2013, 07:25 PM
 Registered User Join Date: Aug 2009 Location: New Zealand
Even then, the area and length are variables important in their own right. Together they make a volume. One long skinny port is a completely different tuning to two short thick ports.
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#5
09-02-2013, 08:34 PM
 Registered User Join Date: May 2001 Location: Orangevale, CA 95662
Link to Vent Calculator with End Correction

Link to End Correction Factors by Vent Type

These two will get you started.
#6
09-02-2013, 08:48 PM
 Registered User HPF Technology: Protecting the Pocket since 2007 Join Date: Mar 2004 Location: Madison WI
Quote:
 Originally Posted by bgavin Link to Vent Calculator with End Correction Link to End Correction Factors by Vent Type These two will get you started.
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#7
09-03-2013, 08:30 AM
 Registered User Join Date: May 2001 Location: Orangevale, CA 95662
Francis,
They are fairly close, at least the ones I have tested.

My small test box is 1.0 cuft net internal volume, with interchangeable top baffles.
I use a 4.0" ID PVC cut into different lengths for the external vent.
The vent is located away from the walls, and falls close to the 0.732 correction factor.
The external vent allows tuning changes without changing the net box volume.

I don't manufacture, so I have not (yet) built a test box for Slot ports.
However, I have need to build a slot port type for a JBL M151-8, and will probably have to fiddle with that one a bit.
I need 52 sqin of vent area to keep the velocity within bounds.

FWIW, I would trust the correction factors to be a very good starting point.
#8
09-30-2013, 07:35 AM
 Registered User Join Date: Oct 2012
boundry effects?

Quote:
 Originally Posted by DukeLeJeune The short answer is, no. A shelf port will have a lower tuning frequency than an equivalent-length round port. The geek answer goes on to explain that there are two mechanisms which come into play that both tend to lower the tuning frequency: First, as the shelf port's area becomes more slot-like than box-like (that is, as the aspect ratio increases), the amount of surface area relative to the internal cross-sectional area of the port goes up, which increases drag due to boundary effects, lowering the tuning frequency. Second, assuming the shelf port is on the bottom of the box and is the full width of the box, the bottom of the shelf port as well as its sides run the full depth of the box, while only the top part of the shelf port is of reduced depth. So the effective length of the shelf port is considerably longer than the length of the shelf itself. In cases where the shelf extends fairly close to the rear wall of the cab, the effective length makes the turn and extends upwards a bit, sort of like a short-footed "L" lying on its back. I don't know of a handy conversion formula, as the specifics vary from case to case.
Wondering what the term boundry effects means in laymans terms? thanx, PP
#9
09-30-2013, 08:12 AM
 Registered User Join Date: Oct 2007 Location: Above Amsterdam
Quote:
 Originally Posted by DukeLeJeune I don't know of a handy conversion formula, as the specifics vary from case to case.
Well explained Duke!
All though there isn't a conversion formula/tool there is Boxsim
In Boxsim you have a tool to calculate the port lenght, this tool is pretty extensive and you have to fill in some extra parameters to get the exact results.
Boxsim takes into account the port-boundary distance
E.g. with a shelved slot port on the bottom the boundary distance to left, right and bottom will be zero.
Boxsim works very accurate and it shows clearly that when you have a regular tube port with the same lenght the tuning frequency goes up.

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Last edited by Arjank : 09-30-2013 at 08:39 AM.
#10
09-30-2013, 11:34 AM
 only immortal for a limited time Owner & speaker designer, AudioKinesis Join Date: Nov 2008 Location: Preston, Idaho
Quote:
 Originally Posted by PawleeP Wondering what the term boundry effects means in laymans terms? thanx, PP
Air is a fluid, and right next to a boundary (like the walls of a port) a fluid doesn't move a fast as it does farther away, out in the main stream of the fluid flow. The reason is viscosity - the fluid molecules tend to stick to the surface of the boudary, and to adjacent molecules, just a little bit.

Let me try to give an example, slowing things down a bit:

Imagine that you have a 4" diameter pipe, and a three foot wide smooth board, both angled downwards at 45 degrees. Imagine that you pour molasses both into the pipe, and onto the board. The molasses will move more quickly through the pipe than down the board because more of the molasses is in contact with the surface of the board, and tending to stick to it.

What's happening is, the molasses molecules adjacent to the surface are sticking to it. And the molasses molecules adjacent to those molecules are sticking to them. And the molasses molecules adjacent to those... well, you get the picture. So the speed of the molasses close to whatever surface ends up being a bit slower than the speed of molasses farther away, and in the pipe, we have at least some of the molasses fairly far away from the surfaces (boundaries).

Obviously air is not as sticky as molasses, but the same thing is happening, just not as severely. The effect of the air's friction against the surface of the port or vent is to slow it down a bit, which makes the port or vent act like it's a little bit longer than it really is, which in turn lowers the tuning frequency.

Hope that helps.
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#11
09-30-2013, 11:41 AM
 Registered User Join Date: Oct 2012
killer

Quote:
 Originally Posted by DukeLeJeune Air is a fluid, and right next to a boundary (like the walls of a port) a fluid doesn't move a fast as it does farther away, out in the main stream of the fluid flow. The reason is viscosity - the fluid molecules tend to stick to the surface of the boudary, and to adjacent molecules, just a little bit. Let me try to give an example, slowing things down a bit: Imagine that you have a 4" diameter pipe, and a three foot wide smooth board, both angled downwards at 45 degrees. Imagine that you pour molasses both into the pipe, and onto the board. The molasses will move more quickly through the pipe than down the board because more of the molasses is in contact with the surface of the board, and tending to stick to it. What's happening is, the molasses molecules adjacent to the surface are sticking to it. And the molasses molecules adjacent to those molecules are sticking to them. And the molasses molecules adjacent to those... well, you get the picture. So the speed of the molasses close to whatever surface ends up being a bit slower than the speed of molasses farther away, and in the pipe, we have at least some of the molasses fairly far away from the surfaces (boundaries). Obviously air is not as sticky as molasses, but the same thing is happening, just not as severely. The effect of the air's friction against the surface of the port or vent is to slow it down a bit, which makes the port or vent act like it's a little bit longer than it really is, which in turn lowers the tuning frequency. Hope that helps.
I actually kinda understand Duke, Since I am a cylinder head porter.. Awesome info....
#12
09-30-2013, 12:02 PM
 Registered User Join Date: Oct 2012
Speaker cut out location?

Quote:
 Originally Posted by DukeLeJeune Air is a fluid, and right next to a boundary (like the walls of a port) a fluid doesn't move a fast as it does farther away, out in the main stream of the fluid flow. The reason is viscosity - the fluid molecules tend to stick to the surface of the boudary, and to adjacent molecules, just a little bit. Let me try to give an example, slowing things down a bit: Imagine that you have a 4" diameter pipe, and a three foot wide smooth board, both angled downwards at 45 degrees. Imagine that you pour molasses both into the pipe, and onto the board. The molasses will move more quickly through the pipe than down the board because more of the molasses is in contact with the surface of the board, and tending to stick to it. What's happening is, the molasses molecules adjacent to the surface are sticking to it. And the molasses molecules adjacent to those molecules are sticking to them. And the molasses molecules adjacent to those... well, you get the picture. So the speed of the molasses close to whatever surface ends up being a bit slower than the speed of molasses farther away, and in the pipe, we have at least some of the molasses fairly far away from the surfaces (boundaries). Obviously air is not as sticky as molasses, but the same thing is happening, just not as severely. The effect of the air's friction against the surface of the port or vent is to slow it down a bit, which makes the port or vent act like it's a little bit longer than it really is, which in turn lowers the tuning frequency. Hope that helps.
Does the Mounting Location Have Much Bearing on Tuning and or port or vent location.. I remember reading that off Center (non-symmetrical) Mounting is better, wondering if this aplpies to Ported/vented designs?

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