# Port questions.

Discussion in 'Archived Threads 2001-2004' started by Justin Ward, Nov 2, 2002.

1. ### Justin Ward Supporting Actor

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I was calculating the size of ports nescessary for my SBB4 tempest subwoofer I am building. Anyway, I plugged the width of the PVC(3.75 inches according to my measurements)and the desired tuning frequency(18.8hz)into LspCAD Pro. It told me that the required length was 49.4 cm(19.45 inches). Does this sound right? Also, I thought PVC was supposed to be in whole numbers(3 or 4 inch) is it normal for it to measure 3.75 inches?

I plan on crossing over to the sub at 100hz. In the frequency response graph it shows a response of 20hz-100hz(+/-3dB) this seems very good to me. Is this an acceptable result? And does LspCAD take into consider the room dimensions I entered in?

2. ### Dustin B Producer

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Are you measuring inner or outer diameter? Are you sure you are going through the center of the circle?

I prefer to use circumfrence equations to determine the diameter of circles when I don't know the center point.

Use a string and find the circumfrence. Then use C/pi = d to find the outer diameter. Measure the thickness of the pipe's wall, multiply by 2 and subtract from d. That will be the inner diameter.

3. ### Justin Ward Supporting Actor

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I used a measuring tape placed on one side of the circle and moved it until I got the largest distance. I measured on the inside. Later I will try the circumferance to diameter conversion idea.

4. ### Brandon_M_S Agent

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i *believe* they say 3.75 inches cause thats the actual size of the inner circle. the pvc might be 4"s, but the material on both sides will add up to .25"s. 3.75+.25=4".

brandon

5. ### ScottZol Auditioning

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Plugging in the #'s to LspCAD, it looks like the longer ports add to the group delay some (unless I have some parameters wrong in LspCad, which is possible). Just curious as to what "sound" trade offs are. Longer ports = lower tuning frequency and larger group delay; shorter ports = higher tuning frequency and smaller group delay. Having just completed my SBB4 cabinet, and getting ready to put in the ports any info would be appreciated.

6. ### Jeremy Stockwell Supporting Actor

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Are you planning to use two ports or flared ends? One straight 4" port might result in port noise and/or limit a Tempest.

Double check your "Airspeed in port" graph.

JKS

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