# Larger port = length less critical?

Discussion in 'Amps and Cabs [BG]' started by Remus_Redbone, Mar 2, 2014.

1. ### Remus_Redbone

Dec 27, 2010
Western AR
Building a single 15" cab (actually modifying an existing cab) for an Eminence CB 158 http://www.eminence.com/pdf/Legend_CB15_cab.pdf

My cab is 17" x 21.75" x 23" for an internal volume of 4.92 cubic feet. I'm going to tune the cab to about 48hz.

Question for those who know; is a larger port diameter (4") more forgiving of the port length than a smaller port of 3"?

The port length calculator I'm using shows two 4" ports each 5.99" long. If I go with two 3" ports, it shows the length of the ports at 3.37".

So, if I get the length of the 4" ports somewhere between 5.9" & 6.1" am I closer to my freq than if I get 3" ports 3.3" to 3.5" ?

I've read a single 15" cab will like the 4" ports better for air flow & chuffing elimination, so I'm going with 4". It was just my curiosity on the how critical the length is to achieve the desired freq of the box.

Feb 21, 2012
Kent NY
The port length changing by even an 1/8 of an inch will move the tuning frequency, not by a lot but it will move. I just did a plot in winisd, 15 n eminence kappa 15lf. 2 4in ports tuned for 48hz is a length of 3.32 each. 47hz gives a length of 3.59, 49hz gives 3.07. so roughly a 1/4in will move your freq 1hz in this example.

3. ### fdeckSupporting MemberCommercial User

Mar 20, 2004
HPF Technology LLC
Moving the port tuning frequency by 1 Hz won't make an audible difference. There will be larger variation between calculated and measured frequency due to the shape of the port ends, and how close the throat of the port is to the back of the cab.

It never Hertz to cut the port a bit too long, measure the tuning frequency, and trim to suit.

4. ### Mr. FoxenCommercial User

Jul 24, 2009
Bristol, UK
Amp tinkerer at Ampstack
With a fixed port area and box, length is the determining factor of the tuning, so its all of the important. Going on the long side, and thus lower tuning is the safety, its a sound compromise, going the other way is a safety of your speaker compromise.

5. ### Remus_Redbone

Dec 27, 2010
Western AR
I can check it with a tone gen app and some ground up rubber pieces on the cone to verify the freq of the box. I've used that process on a cabinet with 3" ports and found them to be very sensitive to length. I hoped the 4" ports would be a bit more forgiving.

6. ### Arjank

Oct 9, 2007
Above Amsterdam
Use Boxsim to calculate the port tuning and dimensions. This tool will take the ports distance to the back,- upper,- and sidepanels into it's calculations