Subwoofer calculation questions

Discussion in 'Archived Threads 2001-2004' started by Robin Smith, May 4, 2002.

  1. Robin Smith

    Robin Smith Stunt Coordinator

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    So, at long last I am finally embarking on my sonotube subwoofer project and am having difficulty with some (most)of the calculations.

    Problem #1:

    What is the volume displacement of the Dayton DVC 15" (295-190) driver?

    Problem #2:

    How do I account for the volume of the portion of the driver that will actually be "in" the tube? I am flush mounting it to 1.5" of MDF, only 0.85-0.90" of which is "inside" the tube. Surely this means that not all of the volume of the driver is considered to be inside the tube and thus I should only account for a portion of the driver's volume.

    Problem #3:

    I am having difficulty understanding which numbers to enter first to determine the length of my sonotube cut.

    I am using 20" sonotube, which has an actual inside diameter of 20 1/16" and I plan on using a 4" flared port (unless someone recommends otherwise).

    How do you decide the tuning for your sub? Why tune to 18Hz instead of 20Hz, etc? I will be running the sub with the MCM 250W plate amp. How do you then use this tuning frequency to get the volume?

    Problem #4:

    I played with LspCAD using the Tempest (whose numbers seem close to the DVC 15") and got mighty lost. It seems to want me to enter the desired volume first, but isn't this what I am trying to determine? Plus, I didn't have all the numbers it needs to give me the answers.

    Ultimately I would like to know:

    What is the "ideal" tuning (and why)?

    What is the desired internal volume and port size/length I need for achieving this?

    How to use all these numbers (with the volumes of each item) to determine the cut length of my 20 1/16" sonotube.

    In other words I need to pretty much be told everything.

    I did a search through some previous posts and checked out Patrick Sun's web site (among others) but didn't really find the more basic info I am seeking.

    Thanks in advance

    Robin Smith
     
  2. Jeff Rosz

    Jeff Rosz Second Unit

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    hello

    1) 5.4 liters

    2) dont sweat it, its negligible either way.

    3) tuning frequencies are set by alot of different factors, box volume, driver specs, box dimensions. there is no optimum tuning frequency, its all relevant to the particular application. mostly you use a tuning frequency to extend the response. there are trade-offs...generally, tune lower to gain output in the lower freqs but you give up output in the higher freqs, and vice-versa. follow?

    4) goto the adire website and look up the application papers for a good starting point. then adjust the numbers in the program as you see fit. how tall can you stand for the tube to be in your room? 4' 5' 6'? once you know that then calculate the gross volume of the tube, then start subtracting volumes of whats inside the tube, thats the Vb to enter into the prog. have fun. hope this helps.

    p.s. LSPcad has a learning curve for most. it had all but the really adept(not me) folks scratching their heads for a while. there used to be a LSPcad question(me) every other post a while back. play with it and then one day, bam, its easy.
     
  3. Robin Smith

    Robin Smith Stunt Coordinator

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    Thanks for the reply Jeff.
    My sub will be used for 90% home theatre , so I want to optimize for that application, but I have exactly 50" of the sonotube, so it can't be any taller than that. Plus, the SAF has determined that the less tall, the better.
    5.4 liters sounds like a lot!
    What is this spec: P-Vd (cm3): 1218
    I found this listed on the PE site in the specs for the DVC 15". Is Vd = Volume Displacement (or venereal disease) and that is the "P" part for? If not volume, what is that number?
    I am still lost in LspCAD [​IMG]
    Thanks
    Robin
     
  4. Dustin B

    Dustin B Producer

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    Just use 5L for the DVC15" a liter either way isn't going to affect anything for the size of enclosure you will be dealing with.

    Vd is not the actual amount of air physically displaced by the driver. Vd is the volume of air the driver is capable of displacing with its motion. To determine Vd you take Sd (the effective radiating surface area of the driver) and multiply it by the linear travel the diver is capable of (ie peak to peak excursion or the one way Xmax multiplied by two).

    I don't know what the P is, but that number is the cubic centimeter number that would result if you multiplied the Sd of the DVC 15" by one way Xmax.
     
  5. Jeff Rosz

    Jeff Rosz Second Unit

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    just guessing but i think the P is one of these two: piston or peak. it really doesnt matter...
     

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