First venture into ported box

[Moble Enclosurs 10+ year member]

Well, your'e right about the 3.5" port cubic volume. BUT.....and this is very important for everyone to understand. You cannot retain a ports physical dimensions based on volume and retain tuning!!! Here is why:

Example: Take a 8-9" round port, or a 10x6 square port...same volume. Now, if you give is a 23.25" length, you get a certain tuning, say with a chamber volume of about 2 cubic ft for a specific driver ( I am using one that I am working on right now as an example). You get about a 44Hz tuning with a 4th order Im working on for this driver. Now, if we get volume of the port and keep the chamber volume, we get 1464.75in^3. now, if we want to use a smaller port for example, say 10" and retain volume of the port...naturally it should be shorter right? Well, we know making it shorter, we are decreasing the area to maintain tuning. This is well known. But if we keep volume, making it shorter and also making it smaller, well, physics will say, you will have a pretty large area compared to the longer port if you are to keep volume. So, lets say the ideal of 44Hz for a 10" port in this example is about 5.88" round, but with retaining volume, you get about 146.48in^2 area, which is a 13.65" round port.

So, that is not going to give you the same tuning AT ALL. In fact it will be around (for my example box/driver combo) 80hz tuning! Almost a full octave above the original tuning! not going to work that way.

best to figure for minimum port area needed, then tune it based on length required afterwards. Not by retaining port volume. Not a good idea.

Note: My math is a little off only because I averaged and rounded fast to get this typed. But the idea is solid that it increased tuning by retaining port volume if making the port shorter, and vice versa due to physics.

 

[Moble Enclosurs 10+ year member]

That is why the calculators are giving you different numbers. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif

 

[chawski 10+ year member]

could you point me in the right direction as for calculating how long my 4" port would need to be, assuming a rectangle box. ideal box size is 1.6 cu, 35hz

 

[Moble Enclosurs 10+ year member]

Yes. To start, you know your volume of the chamber, and your desired tuning so far. the complex way is to get everything together, but for simplicity and lack of accuracy, i can give you an idea of what to do. First, you know to subtract displacement form the chamber volume of the driver and port, or simply the driver, and add the port to the exterior of the volume. I am going to keep it very simple and basic for now. Try using a port(round) of half the area of the driver or around that. For an 8, 3", for a 10 4", for a 12 6", for a 15 8" or two 4", etc. This is very very basic and is not a way to be accurate 9and some may chime in and say other things as well, but this is what I can do without getting too involved, unless you want me to do a design for you.

Now, you have tuning, chamber volume, and port area. You need length. So, say you are using the 4" port and you want a 35Hz tuning. Well, if you cannot change chamber volume for fine tuning, you have to get the length right on. So, the best way to do this is with this:

Take the radius of the port, say 2". Square it to get 4. Then take 10^7 and multiply it by 4. Then take 1.463 times that. Write that number down. You should get 58520000. Then take the 1.463 and multiply by the radius of the port (which is 2). Write that down, which should give you 2.926. Then take the box volume, 1.6 cubic ft. Multiply by 1728 to get cubic in. You get 2764.8. Then take the tuning of 35Hz^2, which is 1225. Multiply 1225 by 2764.8 to get the third needed number to write down which is 3386880. Then divide the first number, 58520000 by the third number 3386880 to get 17.28. Then subtract the second number of 2.926 to get 14.35" port. This is the common form to get the length and is based on a restricted chamber volume. hope that helps. //content.invisioncic.com/y282845/emoticons/smile.gif.1ebc41e1811405b213edfc4622c41e27.gif

 

[chawski 10+ year member]

If I got you the order emailed to you tonight, how long would it take for a basic design? btw, I was off on the speaker model. Ported box actually comes to 1.5cu, not 1.6. I think ill just do a slot port.