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I'm having trouble setting up a file for a Dayton Audio UM18 on WinISD. Can someone tell me what I'm doing wrong?
You entered too much information. WinISD calculates to a higher degree of accuracy than manufacturer data sheets, so if you enter all the data sheet information WinISD will find the data sheet inaccuracies as errors. For instance, don't enter Qes, Qms and Qts. Just enter Qes and Qms, let the program calculate Qts. You only need to enter about 1/3 of the data, the program will handle the rest.
 

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Discussion Starter · #6 ·
Thank you for the replies. I followed the instructions and came up with a different outcome.


I made up a spreadsheet for calculating the dimensions of the box based on the "Golden Ratio".


WinISD is showing a single round 4 inch vent 4.16 inches long, This does not seem right. Is it? The box has a volume of 19.825 with a tuning frequency of 15.66. Does the bracing for a vented sub have to be extreme? I seems like the vent pressure is never going to build up very high.


I'm going to be using this sub (s) for below 40 Hz in a 5,000 cu. ft. room. Will 2 of these be enough to play flat down to 15-20 Hz at 110 dB? I have a lot of bass traps in the room.
 

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You might need to tweak the venting some. Seems a little small and short for a UM18 tuned under 20 Hz.

The process I'd use is this:

1. Set the box size and tuning, which you've done.
2. Set the power you'll be using on the Signal tab.
3. Look at the cone excursion graph. Go to the Filter tab and configure a high pass filter so the driver excursion doesn't go through the roof below the tuning frequency.
4. Look at the Rear Port Air Velocity graph. Go to the Vents tab and tune the vent size/count so that air velocity is in a reasonable level. Generally, the ballpark is between 15 and 25 m/s. Lower is better to prevent chuffing, but may lead to a vent that takes up too much space in the box, or is so long that the port resonances can be audible. Another loose rule of thumb is to keep the first port resonance at least double the frequency you expect to cross the subs over to the mains, so commonly 160 Hz. This step can require a bit of compromise.
5. Be sure to account for the volume the port takes up in the box volume. The vent in a HT sub could easily take up to 1 cubic foot.
 

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WinISD is showing a single round 4 inch vent 4.16 inches long, This does not seem right. Is it?
You have to model the box at the maximum power level you intend to run at, then look at the rear port velocity. Use as much vent area as required to keep the velocity less than 20 within the pass band. You can specify one large port, or any number of smaller ones, of any shape. What matters is the total area.
Will 2 of these be enough to play flat down to 15-20 Hz at 110 dB? I have a lot of bass traps in the room.
Look at the Maximum SPL chart. In an average room you can add 8dB/octave of sensitivity below 30Hz to account for cabin gain. Also add 6dB for two subs versus one. BTW, bass traps have no effect below 80Hz or so. They really should be called midbass traps.
 
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