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Old 28th August 2002, 03:20 PM   #1
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Question Port Sizes and Effects???

When building speaker boxes, how much does port size matter? If you build a box and didn't want to put a really long port, is it okay to maybe put 2 ports of about half the length? Example:

Instead of a 8" long port use (2) 4" ports

I'm not sure how much difference it would make or if I have to stick with just the longest port possible for best sound quality.

Also when building an compact enclosure that requires a port that would extend out of the box, would there be any way of maybe making a curved port and still retain the same sound?
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Old 28th August 2002, 04:00 PM   #2
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Hi Xyberz,

The size of the port in a ported box is "tuned" to the size of the box and the driver specifications. When sized correctly, the air in the port will resonate with the air in the box to limit the driver excursion at the drivers fs and augment the bass output of the driver extending the f3 of the system.

The dimensions of the port are a function of its length and the volume (mass) of air in the port. A larger diameter (if round) port has to be longer to resonate at the same frequency. That's why two ports aren't always better then one.

You can bend the port if it is too long. This is often done, although it may not be the best thing to do as it will effect the operation of the port.

Your going to want to find some reading material on designing ported or closed box speakers. Try the web. There is information out there on the basics of box design.

If you already have the driver, you need to find out the driver T/S (Thiele/Small) parameters. The manufacturer can provide this information.

That's a start.
Rodd Yamas***a
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Old 28th August 2002, 06:42 PM   #3
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Default Re: Port Sizes and Effects???

Quote:
Originally posted by Xyberz

I'm not sure how much difference it would make or if I have to stick with just the longest port possible for best sound quality.
So if you go with a "large port diamater" around 1/2 the diamater of your speaker, you will need it longer as mentioned. The increased port width will reduce port noise, or reduce the sound of air passing through the port.

LV=(1.463*10^7*R^2)/(Fb^2*Vb)

Edit: Lv is the port length needed in inches
R= port radius in Inches
Fb = port reequency like 30hzish
Fb= tuned resonance of box.

Fb=0.42*Fs/(Qts^(.9))

Fs and Qts are the Thiele/Small paramaters you will need....
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Old 28th August 2002, 06:53 PM   #4
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u can replace one large port with 2 small ports (same depth) as long as teh area is the same for example 4" dia port has area of approx 12 sq in. hence 2 2" dia ports with same lenght of 4" dia port will do same job.

each 2" dia port has are 6 sq. in

area = 3.142* dia.

hope this helps

regards
navin
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Old 28th August 2002, 06:56 PM   #5
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Default sorry aboutthat

area = 3.142*D*D/4

hence 4" dia port has area = 12 sq. in
this can be replaced with 2 3" dia ports (approx)

area = 2 * 3.142*9/4 = 54/4 = 13 sq in.

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Old 28th August 2002, 06:57 PM   #6
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above numbers are approx. 54/4 might be more than 13 but fine tuing will take care of the smaller anomalies.
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Old 28th August 2002, 09:36 PM   #7
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area of a circle = Pi x radius x radius

volume = area of circle x length of port


just simplifying a bit


rob
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Old 28th August 2002, 10:02 PM   #8
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Wow this thread when all over the place.

Ok if you have a 2in Diamater port lets just say long, like10in long,

you can used two ports 2in diamater but 5in long, combining the two smaller ports is the same volume as on elarger port so they have the same Frequency as the singel longer one.

Now you can also change diamater and recalculate the length, that is why I gave those equations.
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Old 28th August 2002, 11:36 PM   #9
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Hi

About vent calculation

Quote:
Ok if you have a 2in Diamater port lets just say long, like10in long,

you can used two ports 2in diamater but 5in long, combining the two smaller ports is the same volume as on elarger port so they have the same Frequency as the singel longer one.
This is not true.
if you increase the area of the ports, you have to increase the lenght too. This has nothing to do with the volume of the port.

If you donīt do that the tuning frequency will increase,

Lv = (23562.5*Dv^2*Np/(Fb^2*Vb))-(k*Dv)



where,

Dv = port diameter (cm)
Fb = tuning frequency (Hz)
Vb = net volume (litres)
Lv = length of port (cm)
Np = number of ports
k = end correction (normally 0.732)

Best regards

Claus
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Old 29th August 2002, 05:23 AM   #10
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me thinks you have to tune the port. each port will have a tuned frequency (one may call it fundamental frequncy).

from what I know is that people usually want to replace a large port with 2 smaller ports if the depth of teh cabinet will not allow a large port.

smaller ports are more prone to air noise. that is why when I hd this problem I installed the largest dia port I could get i the bottom of a floor standing cabinet. since the cabinet was on 2" tip toes the bass coupled to the floo but that is a different topic.

large dia ports are usually longer for teh same tuned frequncy.

am i in the ballpark?
cheers
navin
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