Hi all, i would like to know how bass ports are tuned. If I have a box with a port 2"x13.5"x21.5" how must I tune it to lower or higher frequency? Just curious....
Make the box larger for lower and smaller for higher. Adjust port tuning to effect the relative gain. Those specs are worthless unless I know how much volume is in the box. If the box has a volume of 2.5 cu ft then the tuning will be around 34 Hz, if the box is around 4 cu ft then the tuning will be around 27 Hz.
The volume is approx 1.4 cu.ft. I got this box along with my previous sub (an Audiobahn 12" SoundQ Series) and it was built by a friend of mine. Now after I've been "studying" how to build my own box I see that it's not really how it should have been done. For example the port has a dimension of 15.5x3.25" inside and around 2x10.5 to the outside flange of the port. In other words the port has two kind of measurements, it decreases in volume. If I'm correct it should have one size all the way. The box looks like the drawing below adn it's been made from 3/4" plywood (not MDF) I'll be using this box for now until I have the chance to build my own.
t/s data of my sub: 12" Soundstream tarantula Driver displacement: .12cuft RMS power: 900Watt Res Freq: 26.58 Hz Qts: .383 Qms: 4.412 Qes: .422 Vas: 43.686 ltr XMAX: 14mm According to the spec sheet my Optimum Enclosure should have a net volume of 1.75 cuft, vent design 3"x8.75" (is this a round port or what because it doesn't say the length of the port), tuning freq 31 Hz. I've also read a few articles on DD (Digital Designs) website and they say I need a port area equal to 16x net volume so in my case 16x1.75= 28 sqin. and it should be 18-22.5inch long. I've done some calculations and came up with a box with the external dim 15"x15"x22.7". I chose 15" to maximize trunk hight and not to long enclosure so I still have some bagage space if needed. My port is 2x13.5x22.5. Do you think these calculations are correct? Am I on the right track?
No reason to be rude, many of us have jobs and other obligations. We don't sit on the computer all the time. Your port is too short if you want to tune to 31 Hz. But seriously, there is no reason to tune that low. Lets redesign this thing a little. Outside dimensions should be around 25-wide, 14-tall and 16-deep. Now for a more reasonable tuning, lets shoot for 35-36 Hz. This would mean the port will need a dimension of 12.5- tall and 1.75- widw. The port length will be around 18-inches long.
Sorry, didn't mean to sound rude. Was just dissapointed that with the info I provided I couldn't get a responce and I'm someone who doesn't just want to follow what others say but want to know a lot in detail. But thank you for your reply. Did you get these numbers from some kind of software? Because that's what I'm aiming at. I want to be able to calculate these my self. I now have a new spec sheet from the same sub but got this one from their website (the previous one I downloaded from a website - don't remember which one) and I assume the one I have now is an updated version, maybe the sub has been re-designed. If I put these values in WinISD I come up with other dimensions as yours. If I use the specs given in the attachment below then I would go with optimum enclosure. These are the specs:
Win ISD is a good program, but there is nothing like experience. According to your specs the woofer wants a box around 2 to 3 cu ft. I'm in that range. The box has a volume of around 2.2 cu ft (the port volume does not count). I use a calculator (ti-89 or my Voyage 200) to figure what I need, but here are my rules, manufacture recommendations are OK but always use the largest recommended enclosure possible (ported boxes), if you have the room a 2.5 to 3.0 box would perform better. When I use WinISD, I look at the graph. First I adjust box size (increasing size) and look for the lowest F3 point. Don't exceed the manufacturers max recommended size by too much. Then I adjust the port tuning and try to get a relative gain around +3 to +6. Typically this will put the tuning around 36 to 40 Hz. Now if we want to compete, then I tune even higher and make the box even larger.
I designed above box with flairings, is the total length of the port INCLUDING the flairing? Or should the flairng be extracted. The flairings are just rounded of corners of the sheet so .75 inch thick thnx alot
Lets include the flare as part of our length. However, the 90-degree turn in the box, lets not include it as part of our length. By the way, I always add an inch to my calculations to be on the safe side. But stay with the 18-inches anyway.
I found some formula's on the web and also have "The Car Stereo Cookbook 2nd edition" and compiled them in an excel spreadsheet. Can you take a look at those? They pretty much give the same outcome but I get a longer port calculated with these formula's (22" compared to your 18").
Not sure why the difference. The equation I use is from the Loud Speaker Design Cookbook by Vance Dickeson. It is similar to yours except it uses inches Lv is vent length in inches r is radius of the port (If its a square port or rectangular port, I calculate the cross sectional area, divide it by 0.7854, then square root the result. This give me the equivalent round port size.) Fb is the desired tuning frequency Vb is the volume of the box in cu in
I'll look at your equation more later, I have to finish making up my finals for my students. I have a lot of work to do this week (last week of school). But I will get back with you.
No prob, I'm using the exact same equation and the last one is from JL Audio website. I'll build 3 kinds of boxes, one sealed for when I really need the space in my cargo area, 1 nice ported box but not too big and one massive head banger. With recommended specs I can get 1 ported and 1 sealed box out of 1 sheet of MDF so those will be built first. Off to work now. thanx
