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Larger port = length less critical?

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


  1. Remus_Redbone

    Remus_Redbone

    Joined:
    Dec 27, 2010
    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.
     
  2. gearhead1972

    gearhead1972

    Joined:
    Feb 21, 2012
    Location:
    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. fdeck

    fdeck Supporting Member

    Joined:
    Mar 20, 2004
    Location:
    Madison WI
    Disclosures:
    HPF Technology: Protecting the Pocket since 2007
    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. Foxen

    Mr. Foxen

    Joined:
    Jul 24, 2009
    Location:
    Bristol, UK
    Disclosures:
    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.
     
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  6. Remus_Redbone

    Remus_Redbone

    Joined:
    Dec 27, 2010
    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.
     
  7. Arjank

    Arjank

    Joined:
    Oct 9, 2007
    Location:
    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 ;)
     

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