Not understanding slot port vs. round port - Home Theater Forum and Systems - HomeTheaterShack.com

Old 03-14-11, 12:02 AM Thread Starter
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steve1616

Join Date: Apr 2009
Posts: 315
Not understanding slot port vs. round port

I have a question concerning slot port. I was under the impression that slot port is used to save internal cabinet volume, but when I put the numbers into winisd, the numbers seem to indicate the opposite. For my enclosure winisd says that I need a 4" round port with a length of 14.71" long. That is a volume of 3.14 x 4 x 14.71 = 184.76 cubic inches. The slot port show a 1.5" tall x 16.5" wide x 30.27" long. That is a volume of 1.5 x 16.5 x 30.27 = 749.18 cubic inches.

Am I doing something wrong? This isn't even taking into account the internal mdf to make the slot port. The internal mdf would take more area also, then the thin round port.

I am sure I am missing something, I just don't know what. I just know I am doing something that I will feel stupid about after someone points out what I am doing wrong.
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Old 03-14-11, 07:42 AM
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dboff01

Join Date: Feb 2010
Location: Long Island, NY
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Re: Not understanding slot port vs. round port

Quote:
steve1616 wrote: View Post
I am sure I am missing something, I just don't know what. I just know I am doing something that I will feel stupid about after someone points out what I am doing wrong.
A slot port is not used to save internal cabinet volume. A slot port is used because it is generally easier to construct and it allows for multiple 90deg bends if necessary to achieve the desired port length. I also think they just look cooler.

When sizing a port you must consider the air velocity in the port (which WinISD Pro can show you) when driven by the proper power the sub will see. A conservative target is 24 m/s or less.

In your previous example, if you compare the air velocity of the tube port vs the slot port, you will see the lesser volume tube port will have significantly higher air velocity, which can lead to compression and port chuffing.
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Old 03-14-11, 12:12 PM Thread Starter
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steve1616

Join Date: Apr 2009
Posts: 315
Re: Not understanding slot port vs. round port

Thank you for your input. On WinISD, when it says vent length, does that mean the length per vent or is that the combined length of both vents. Cabinet volume is a premium for my design so I will probably just build what is the most effecient for cabinet volume.
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Old 03-14-11, 12:19 PM
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Location: St. Louis, MO
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Re: Not understanding slot port vs. round port

It means per vent.
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Old 03-14-11, 03:02 PM
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dboff01

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Re: Not understanding slot port vs. round port

Quote:
steve1616 wrote: View Post
Cabinet volume is a premium for my design so I will probably just build what is the most effecient for cabinet volume.
If keeping cabinet size small is a priority, you might want to consider a passive radiator design, which offers the benefits of a ported design (increased output down low) without the volume required by the port. The trade-off, of course, is \$\$\$ as passive radiators cost more than a hole in your box.
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Old 03-14-11, 05:13 PM Thread Starter
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steve1616

Join Date: Apr 2009
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Re: Not understanding slot port vs. round port

I am trying to meet a size and cost budget. I have gotten the maximum size narrowed down. The cabinet will be made of 3/4 MDF and will be 22" tall x 24" deep x 20" wide. That will give me an internal volume of 4.938 cubic feet. I don't know how many watts I should figure to find a good velocity for the ports. I am going to use the trio12 for the subwoofer. It shows 350 watts rms. I will use a 500 watt bash amp from o-audio. Should I figure the port velocity for 300 watts input? Obviously the wattage changes dramatically what port I should use.
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Old 03-14-11, 05:48 PM
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Re: Not understanding slot port vs. round port

Why dont you just buy this kit for trio 12, has everything you need and within the size you want.
http://www.creativesound.ca/details....el=Trio12APR15

read the pdf on the page, shows how to build the enclosure too.
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Old 03-14-11, 07:10 PM Thread Starter
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steve1616

Join Date: Apr 2009
Posts: 315
Re: Not understanding slot port vs. round port

Maybe I should think about the kit that you mentioned. I have seen this kit and thought about it. The only reason that I was going to go the ported route with a slightly bigger enclosure was because it showed better extension into the lower frequencies, and the amp from O-audio offers a PEQ. It would be the same price either way. The ported route seems to offer a little better performance while the kit would be a little easier, and I guess a little cheaper since I don't have to buy flares for the ports.
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Old 03-14-11, 09:20 PM
Senior Shackster

Join Date: Jan 2011
Location: St. Louis, MO
Posts: 131
Re: Not understanding slot port vs. round port

The standard bash 500 has a hpf and peq, instead of a knob you can change the resistors on the board to your prefrence. If you look at parts express on the amp page there is a pdf with the resistor charts. yes those ports come in a gold plated box, thats why most use square ports or home pvc.
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Old 03-14-11, 09:39 PM Thread Starter
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steve1616

Join Date: Apr 2009
Posts: 315
Re: Not understanding slot port vs. round port

Thank you for the information on the Bash amp. I am also wondering how much difference the little bit of extra extension the ported box is going to make. I have heard that sometimes the passive radiators are just as good but they don't measure quite as well because the woofers are on different sides. I am really leaning towards just doing the kit. It would be so simple, and the plans are already laid out. It just doesn't get any easier. I wonder if the ported boxes measurements would even show up in the form of any audible differences.

On a side note, would it be a good idea to make a base plate for the bottom firing woofer to protect it?
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