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?
 

roddyama

diyAudio Moderator Emeritus
2002-01-19 9:25 am
Michigan
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.:D

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
 
Xyberz said:

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....
 
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.
 
Hi

About vent calculation

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 don´t 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
 
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
 
So how do you tune the ports? I've seen people just use PVC pipe in thier speaker systems. Also is there any easier way or some example on what size and length should a port be for a speaker? Like a 12" Sub should take a 3" dia x 6" long port. Something like that. Also is the formula different for different type of speakers? Or can I just use the same basic formula and just change a few measurements around?
 
The lenght as well as the port Area are tuned to the frequency you need the Enclosure to resonate at. this resonent frequency is determond by the Thiele small parameters of the Woofer you want to use. Computor Programs are available that allow you to play with Box volume and Vent area and lenght and get a Frequency response graph of the complete system a free program is http://www.geocities.com/sdecaudio/boxport/ Also other for sale programs that i have used and work good are LEAP sorry can't remember the url on that..
 

kelticwizard

diyAudio Moderator Emeritus
2001-09-18 2:33 am
Connecticut, The Nutmeg State
Just thought that I would add that flared ports give the port noise of a straight port of twice it's cross-sectional area. The larger the cross-sectional area, the less the noise.

So a 2" flared port has the noise of a 3" straight port, but the shorter length of a 2" straight port.

Other members have said that, when using a flared port, add one inch extra to the straight port length to make up for the flare. So a 10" flared port will tune a box the same as a 9" straight port of equal diameter.

Programs will calculate the length of multiple straight ports of any crossection. WinISD is one of the easiest.
http://www.linearteam.dk/
 
Re: Re: Port Sizes and Effects???

electricashman said:


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....

Sorry guys I left out the last correction conatant term in the LV equation:
-(1.46*R)

Claus: Im trying to convert your multi port equation to inches and cuft,

I got a new constant of 41384.5.

(cm^2)/(in^2) =.062in^2
(cuft)/(leters)= 28.32 cuft
1cm>in= .39in

.062*28.32=1.755

so 1.755* 23562.5=41384.5

and the new constant k should change to inches

.39*.732=.285

so .285=k

Is that correct?

thanks guys...
 
The rule of thumb of making the port an inch longer if it is flared seems a little too easy, but is probably close. I was forced to use metric when doing a drawing for the DIY turntable thread. Boy is that EASY. I couldn't believe how much less hassle it was.
I'm an architect and spend all day worrying about fractions and adding them up!