NuBee Question: 2 ports = half the length

I downloaded an application which tells me the minimum port diameter and length for a vented box tuned to 22hz should be somewhere close to 22" in length and 2.4" in diameter. What happens if I select to use two ports?
a) do I split both the length and the diameter in half?
b) do I split only the length OR the diameter in half?
c) something else entirely

Pete :)
 

ShinOBIWAN

diyAudio Member
2004-02-25 9:13 pm
UK
Pete H said:
I downloaded an application which tells me the minimum port diameter and length for a vented box tuned to 22hz should be somewhere close to 22" in length and 2.4" in diameter. What happens if I select to use two ports?
a) do I split both the length and the diameter in half?
b) do I split only the length OR the diameter in half?
c) something else entirely

Pete :)

Strange as it may sound but the theory states that you should double the length of each port for a fixed diameter and tuning frequency :bigeyes:

For example:

1 x 50mm port at 40hz would be 25cm
2 x 50mm ports at 40hz would be 50cm each

The advantages of using two ports are two fold, firstly you decrease port noise and by that I mean the turbulance created by the movement of air and secondly you lower the port resonances so they start at a lower frequency and are less severe than a comparable single port.

One tip that I can give you is to use flexible hose and bend it around inside the cabinet, that way you can have a much longer port than a rigid type. I use the tubing thats commonly made for tumble dryer heat extraction believe it or not but there's quite a few others too.
 
It makes sense if you consider that for a given tuning, the driver needs to see the same load from the ports. Doubling the number of ports halves the effort needed to move the air. To preserve the load, you need to double the mass of air - ie double the length of the port.

The good aspect of this is lowering of airspeed in the ports. The WinISD software recommends 17 metres/sec, but I like to aim for 10 m/sec.

The bad aspect is that if your port gets too long, it can cause "pipe mode" resonance. Here as some examples

600 mm 286 hz
800 mm 215 hz
1000 mm 172 hz
1200 mm 143 hz
1400 mm 122 hz

Once you get over 800mm, you need to consider a low pass filter that has a good rolloff, say 24 dB/octave. The filters that come with most DIY plate amps are not this steep.

regards, Collo
 
Hi Pete,

With two ports the vent legnth will actually be longer than with one port. Big bummer when trying to reduce port velocity.

Check this site:
http://www.ajdesigner.com/phpvent/subwoofer_vent_port_equation_length_l.php

Example box/port:

vent dia. =2.5inch
Number of ports=1
box volume=15
fb=40hertz
k=.85 (port flanged on both ends)
vent legnth=13.46inch

vent dia. =2.5inch
Number of ports=2
box volume=15
fb=40hertz
k=.85 (port flanged on both ends)
vent legnth=29.046inch

legnth would be the same if the area of the port(s) are the same.

-Hans