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Discussion Starter #1
Guys,

How do you calculate a full length dual-flared precision port in Boxnotes? Do you simply select the option for "dual flare" and then punch in 17" for the length?

Also, what would I punch in for a 15" Tempest-X for volume?

And lastly, what is a good way to calculate interior bracing? The bracing that boxnotes gives you by default is less than I would think of building for the box.

 

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Re: How to calcualate Precision Port in boxnotes?

Boxnotes doesn't make it easy to do flared ports, but it is on the list!

There are actually three lengths involved. This drawing should help...


The Effective length is what your design program, such as WinISD gives you.

The Length as seen by boxnotes is the effective length plus half the flare radius for the outside flare. In the case of the PSP ports, this means adding a total of 1/2 inch.

The Physical length is the effective length plus half the flare radius for each flare. In the case of the PSP ports, this means adding a total of 1 inch.

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The screenshot you supplied shows the setting for flanged ends, not flared ends.
An end is flanged when it is terminated in a baffle rather than free air. Most ports are flanged on the outside and free on the inside. This changes the tuning slightly. Both WinISD and boxnotes make the same adjustment. This is a completely separate issue to flares.

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You don't need to enter any volume for the driver. Selecting a driver size of 15 inch from the drop-down list will use an estimated volume. There is currently no facility for you to specify the driver volume directly. If you know the volume displaced by the driver and wish to correct for this, you can change the target working volume.

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Bracing used by boxnotes is square-section edge bracing cut from the same material used for the box itself, plus a single shelf brace. There is no way to directly change this, (That wish-list just keeps on growin).
As for the driver volume, you can fudge it by adjusting the target working volume.

regards,
Collo
Boxnotes author
 

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Re: How to calcualate Precision Port in boxnotes?

Thanks, guys! That all helps a lot! :)

It looks like I should use "15" as the port length then (for my 6" ports). It's 12" for the center tube and each flared end is 3", so if I divide the ends by 1/2 and multiply by 2... :)
 

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Re: How to calcualate Precision Port in boxnotes?

Hmm, the precision calculator doesn't seem to agree with boxnotes though. Boxnotes comes up with about 14.2 cu.ft. or so.

What I want is an enclosure tuned to 16hz using a full length 6" dual flared precision port, for my Tempest-X. Without using polyfill.

 

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Re: How to calcualate Precision Port in boxnotes?

Exodus audio has a page for the Tempest-X that includes an application guide.

The guide suggests the following ported boxes:
8.5 cu.ft / 19hz
10.6 cu.ft / 17hz
12 cu.ft / 16hz

Assuming you wish to build the 12 cu.ft / 16hz design......

Modelling this in WinISD shows that the Effective length for a 6inch port is 20.31 inches (one end flanged)
The PSP calculator will tell you 20.41 inches for the "Straight Length Port required", which is the effective length.

Going over to boxnotes, enter a port length of 20.31 plus 0.5 ie 20.81, calculated as shown in the earlier posting.

Play around with the internal volume until you get a Working Volume of 12 cu.ft
You should come up with around 12.66 cu.ft for the internal volume.

Notice that the tuning frequency is now displayed as 15.8hz, not 16hz
This is because boxnotes is not taking into account the flare when determining the tuning. This method is giving an accurate volume for the port though.

You can now mess around with the external dimensions (one at a time) to get the shape for the box.

Your Physical length for the port will be
20.31 + 1 = 21.31 inches according to winisd, or
21.41 inches according to the PSP calculator. A variation of 0.1 inches isn't going to make any discernable difference.
 

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Discussion Starter #7
Thanks, Collo.

Unfortunately, that application guide uses 100% polyfill in all it's suggestions. Which is a terrible idea IMO. I mean 100% polyfill in a 12 cu.ft. box is going to cost ~$88 at parts express. Assuming we're using 1lb per 1cu.ft.

My 6" Precision port is only the 2 flared ends + a single 12" center tube. So I can't go 20" or anything longer than what I already have.

I pulled out one of my 6" ports and measured it better this time. Each flare is about 1" long. The center tube is 12" long. But assembled, the total length is 18". So if you divide each 1" flare by 1/2 and then multiply by two, then you get 1". So I can subtract that 1" from the 18" to get 17" total tube length seen by WinISD or Boxnotes, per your picture above.

So WinISD & Boxnotes suggest a 13.85 cu.ft. enclosure if I want a 17" port & 16hz tune.

And now that I better understand how the port itself should be measured, I am seeing that Precision's calculator does work properly.


Diycable.com doesn't recommend this box, btw. They recommend the 12 cu.ft. 100% polyfill over this box. But I'm not rich. $95 (shipped) per enclosure for polyfill will get real expensive when multipled by four. I already spent $35 for each port (shipped).


For some reason, I thought these Precision flares were a lot longer than 1". In hindsight, I should have just used sonutube and created a custom flare using my router and a 1" roundover bit for way less $. Oh well.
 

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Now you've got the hang of the port thing......

It's not normally recommended to fill a ported box, as you don't want to interfere with the air coupling between the driver and the port.

Most people just line the walls with some foam or batts to help with any stray resonances.
 

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I mean 100% polyfill in a 12 cu.ft. box is going to cost ~$88 at parts express. Assuming we're using 1lb per 1cu.ft.

Diycable.com doesn't recommend this box, btw. They recommend the 12 cu.ft. 100% polyfill over this box. But I'm not rich. $95 (shipped) per enclosure for polyfill will get real expensive when multipled by four. I already spent $35 for each port (shipped).
The recommendation isn't for Acousta-Stuff. It's for plain old polyfil which is about $0.99/lb at Walmart.
 
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