Port Calc Sites (inconsistencies)

[robert J]

I am confused by some port calc sites I have been researching.

in this

from CarAudioFabrication, he builds a box with gross volume of 3.5cf, net Vb 2.75cf tuned to 32Hz. The port size is 3.75x29.xxx inches.

When I do the same 2.75cf net volume port calc on the12volt.com their calc says the box needs a 42.5" port to tune to 32Hz. That is a 12" difference.

Can someone give me a link to a reliable calc site please or tell me where I am going wrong?

Cheers.

EDIT: I think I may see my problem. I should have used the gross volume of 3.5cf in the port calculation and not the net volume? If I do that, the port size is within a couple inches.

Should not the tune calc be done with the net volume after port volume and driver displacement?

Click to read more...

 

[BOOMINGRANDPA 5,000+ posts]

wow i have seen like a 1.5-3" diff in couple of them cause some subtract like 1/2 width of port per common box/ port wall but not that big of a diff.. this one is fine as long as u do it right and when in doubt add 1.5" to port if u want a low tune.. Port Length Calculator

 

[Jeffdachef 5,000+ posts]

I am confused by some port calc sites I have been researching.
in this

yeah dont use the 12 volt calculator, its pretty cr@p.

Download xplicit audio ultimate car app, its the closest one to google sketchup. It does all the calculations for you.

You most likely have the port width completely wrong, there is no way you can get a 32hz tuning with those box dimensions and port dimensions.

 

[robert J]

Thanks very much guys,

this site seems pretty detailed Enclosure Volume Calculator - Subwoofer Box Calculator

 

[Jeffdachef 5,000+ posts]

only way to make it work is with a 2.75 inch wide port.

 

[Jeffdachef 5,000+ posts]

Thanks very much guys,
this site seems pretty detailed Enclosure Volume Calculator - Subwoofer Box Calculator

Just a load of stuff you can calculate out yourself.... Whats important is the calculation of the port length as the box shrinks from the port being longer. Less airspace needs a longer port to achieve the same tuning, nothing online can accomadate these changes so everything you found so far is pretty useless. Again torres box calculator or xplicit audio ultimate car app is the most accurate in terms of knowing actual proper dimensions and port length.

 

[THATpurpleKUSH 5,000+ posts]

only way to make it work is with a 2.75 inch wide port.

OP had 3.75 port width, not 2.75

There is nothing wrong with the port length calc on the12v, it's always within a few tenths of an inch compared to Winisd and Bass Box Pro 6.

Winisd runs shorter by a .1 - .2 inches and BBP6 runs longer than the others by .1 - .2 inches. A couple tenths of an inch tolerance is good enough for me.

 

[robert J]

Thanks again guys, Just downloaded the calc.

Can you tell me how batting fill effects the volume calc if at all?

 

[THATpurpleKUSH 5,000+ posts]

Thanks again guys, Just downloaded the calc.
Can you tell me how batting fill effects the volume calc if at all?

You don't need fill in a subwoofer box because there are no standing waves to absorb.

 

[robert J]

in the video the port width is 3.75". total box dimension is 25w"x15"h and the speaker face panel is 20.5" less the .75" of the side panel, leaving a net width of 3.75". Port is 13.5" x 28.75" in the video.

12v.com says the port should be 32.19" using the gross Vb of 3.5cf. That is almost 3.5" difference.

 

[Jeffdachef 5,000+ posts]

OP had 3.75 port width, not 2.75

There is nothing wrong with the port length calc on the12v, it's always within a few tenths of an inch compared to Winisd and Bass Box Pro 6.

Winisd runs shorter by a .1 - .2 inches and BBP6 runs longer than the others by .1 - .2 inches. A couple tenths of an inch tolerance is good enough for me.

3.75 will leave him with very little cubes of airspace and way too much port area, thats why i said he might have gotten the port area wrong from the start.

All of those calculators assume you know how to subtract port and overall displacement from the net airspace which most people have no clue on doing.

 

[Jeffdachef 5,000+ posts]

Here are both your calculations and now you know why those calcs are garbage unless you know how to build around them. You simply cannot achieve that port tuning without making the box a lot bigger to accomadate. Hence why I used 2.75 which is a lot more reasonable and doable. Port area is also based on your amplifier power as well so unless he has a giant amp, He's not gonna get maximum performance unless he does get one if he uses 3.75 width for port area.

 

[robert J]

Can someone explain

"Port Area Range" and "Port Area"

I have not been able to find "help" on those figures. I have read that the the port area should be around 12cu/in per cub foot of volume if using tube porting? is that right.

The speaker I am referencing needs 34Hz port tune and optimum 1.43cf of volume. Oops, I see I left out the woof displacement.

 

[Jeffdachef 5,000+ posts]

Can someone explain
"Port Area Range" and "Port Area"

I have not been able to find "help" on those figures. I have read that the the port area should be around 12cu/in per cub foot of volume if using tube porting? is that right.

The speaker I am referencing needs 34Hz port tune and optimum 1.43cf of volume. Oops, I see I left out the woof displacement.

You can disregard port area range.

port area is the cross sectional area aka how big your port is so for round ports its pi x radius ^2 so for a 3 inch diameter port, 1.5 x 1.5 x 3.14 = 7.07

Port area per foot: = cross sectional area divided by net cubic feet of the enclosure.

This next value is your relative amount of cross sectional area compared to how big your enclosure is. You usually want the numbers to stay within 8 to 13 port area per foot for round ports and 12 to 16 for slot ports. This is not absolute, its just a general guideline for newbie builders. Usually when you have more power going to the sub, you need more port area to reduce compression and port noise. With less power, you need less port area. Also if the sub is a very powerful high dispacement sub aka very high Xmax on 2x rated rms, you need a lot more port area than the basic beginner recommendations i listed. On the otherside, if you have a very weak sub with low motor strength and xmax or even midrange/midbass speakers on low power, you'll need FAR less port area per foot.

As of now, your port is way too small for most subs out there. Minus some home theater subs.

 

[robert J]

You can disregard port area range.As of now, your port is way too small for most subs out there. Minus some home theater subs.

Thanks for your reply.

tricky balancing the port area which needs to go up and Vb that needs increasing while the tuning freq needs to go down. I can manage to get acceptable net Vb 1.43 and tune 34Hz but can't seem to get the port volume number above 9 and keep those numbers. I see that port length drops Hz while hardly changing port volume and net Vb, while port diameter greatly increases port volume but kills my net Vb and Hz tune. I will figure it out, just need to play with it for a while.

Also,

the 'DESIRED NET VOLUME" is confusing in the help section. What exactly does entering a number there accomplish? The two pics below are the same except for the second one has "desired net volume" plugged in? I don't get it.

I notice the port Hz, net volume, port volume numbers change but not the dimensions.

Cheers.

EDIT:

 

DUH!!! add another port. Seems like a lot of port though for the box.