port modification

[ryanc]

I am unhappy with the way my ONS mtm's came out due to the paint. To repaint would cost about $100+. I want to go a more traditional route and use a Oak/Walnut combination. I would like the ports to be front firing but I know I do NOT want to use the cheap sliding/small ones I got with my "kit". TO calculate the two into one would I just double the diameter and keep the length? I know to keep the baffle the same size but the overall dimensions will change, but keeping the internal vol the same. They will be converted into towers.


Thanks again for the help everyone!! One day I will be on the giving side of answers.

[isaeagle4031]

Re: port modification

You need to match the port surface area, not diameters. For example, two 1.5" ports do not equal a single 3" port. Keep the surface area the sane and port length stays the same. increase area and length goes up as well.

[Paul K.]

Re: port modification

There's also the port's end correction factor to consider. If you use one larger port, whose area equals the sum of the areas of two smaller ports, with the lengths of all three ports identical, the larger port would be a bit too long, thus lowering the tuning frequency a bit. The effective length of a port is its actual length plus 0.6 times its radius. This is likely a small correction and may not be necessary, however, as far as the overall sound is concerned.
Paul

[ryanc]

Re: port modification

Thanks for the input. However I'll have to try and figure this out in layman's terms. That totally blew my mind!! But I do think I understand what is being said. Thanks again!!

[ryanc]

Re: port modification

So if I am thinking correctly here. I find the total volume of the 2 ports and then calculate the volume of the single port then they should, in theory, have roughly the same tuning?

[dustinbagby]

Re: port modification

So if I am thinking correctly here. I find the total volume of the 2 ports and then calculate the volume of the single port then they should, in theory, have roughly the same tuning?

yes, essentially you will have the same volume, but what you are aiming for is first the area of the port (I.e. area of a 3" dia circle).

You then just make your new port the same length. So for an example, if you are starting with (2)1.5" diameter ports that is a total area of roughly 14in^2. So you now want a port that's opening is 14in^2. (7in^2 each).

So with the OS MTMs, you start with (2) 1.375" dia ports, 6" long. This gives you a total port area of 3in^2, so your new port needs to have an opening that is 3in^2, and 6" long. So you could use a single 2" diameter port, 6" long.

[ryanc]

Re: port modification

Thanks a lot!!