Port lengths: which measurement?

[rajivs]

Hi after reading a lot of material here and elsewhere, I am presently building my first pair of enclosures: an MLTL design with MarkAudio Alpair 10 drivers.

A question I have is about port lengths. Suppose a port is specified as 4 inches long. I am cutting PVC tubes and rounding off the MDF opening in the baffle. Do I measure the 4 inches from one end of the tube to the other, to the outside front surface of the baffle, or only to the inside surface of the baffle? The baffle is 3/4" MDF. The tube will be 3/8" into the MDF (seated in a rabbet).

Thanks

 

[DaveThreshold]

Hi.

You need to measure, or add the total length of the port to = your goal. Let's say the port is supposed to be 4" long. If you are gluing a 4" port on to a radiusd 3/4" baffle the total effective length would be 4-3/4". So you would have to reduce the tube length by 3/4" to = a 4" vent. Stuffing, and other factors can throw off your desired tuning freq.

I would STRONGLY recommend something like, the "Loudspeaker design Cookbook" by Vance Dickason- Latest edition. It will cover all of your questions, including about 500 more that you haven't thought of yet. To out do the many great mid-priced speakers that are on the market, is not an easy task.

http://www.madisound.com/catalog/product_info.php?products_id=142

 

[rajivs]

Dave, thanks for the info and the link!

 

[Stu]

A general rule of thumb when rounding over the mdf at the port, or using flared ports is to subtract half the flare/rounding radius.

So if you need a 4 inch port and have a 1/2 inch rounding radius at the port entrance, the back of the port should be 4.25 inches from the front of the baffle... in other words, air near the front of the roundover belongs to the room, not the port.

 

[gedlee]

Honestly, I have found it nearly impossible to get accuarte tunings from measured port lengths. This is two things: inaccuracy of the port length and inaccuracy of the enclosure volume. Between these two things its all a guess.

So I suggest you simply "guess at the longest possible port, measure the system and then cut to the final tuning. This will always work. But if you cut the port too short its a real problem to get the tuning right after that.

 

[col]

Iv'e extended ports using 3mm thick cardboard postal tubes and duck tape with success. It's not perfect but it does work. The tubes come in a whole bunch of sizes and you can all ways reduce enclosure volume with more stuffing.

col.

 

[gedlee]

Stuffing an enclosure tends to increase its volume, not decrease it. Only when it is stuffed to the point where the material is becoming compressed does the enclosure volume begin to decrease. This is not a good way to "tume" a cabinet, because too much is changing at the same time (volume and resistance). Port length is the correct way to tune.

 

[DaveThreshold]

A general rule of thumb when rounding over the mdf at the port, or using flared ports is to subtract half the flare/rounding radius.

So if you need a 4 inch port and have a 1/2 inch rounding radius at the port entrance, the back of the port should be 4.25 inches from the front of the baffle... in other words, air near the front of the roundover belongs to the room, not the port.

Yes, as soon as I answered it, I thought part of the depth of the radius should be written off.

Honestly, I have found it nearly impossible to get accuarte tunings from measured port lengths. This is two things: inaccuracy of the port length and inaccuracy of the enclosure volume. Between these two things its all a guess.

So I suggest you simply "guess at the longest possible port, measure the system and then cut to the final tuning. This will always work. But if you cut the port too short its a real problem to get the tuning right after that.

Hi Earl. I met you at a P.S.A.C.S. meeting, at your place before you moved. The subject was your room acoustics design.

When I build or design a box for bass, I always measure every little thing, (obviously real accurate measurements on the bracing, and driver to subtract them) etc. That helps the accuracy of the port estimate a lot. When I built my 6.5 Cu Ft. sono-subs, I also weighed the amount of fiberglass stuffing for each. I cut the vents which were supposed to be tuned to 17.5 HZ, and the Z plots were dead ringers for each other. They looked like one Z plot. I now listen to them sealed anyway. LOL

 

[rajivs]

First enclosures

Thanks all. My enclosures should be ready for testing this weekend. Now I just need to figure out the best way to break in the drivers and measure the SPL.

 

[doug20]

[

When I build or design a box for bass, I always measure every little thing, (obviously real accurate measurements on the bracing, and driver to subtract them) etc. That helps the accuracy of the port estimate a lot. When I built my 6.5 Cu Ft. sono-subs, I also weighed the amount of fiberglass stuffing for each. I cut the vents which were supposed to be tuned to 17.5 HZ, and the Z plots were dead ringers for each other. They looked like one Z plot. I now listen to them sealed anyway. LOL [/B]

Its great to be nitpicky but if you are off by 1/2 inch any port its not going to change the tuning frequency that much. (assuming subwoofer designs and tuning below 20Hz, loudspeakers 1/2 inch might be meaningful)

The Audible difference between 17.5 and 18 Hz is meaningless so most people need not worry about the exactness of a port measurement unless they are the pedantic type

A person just needs to use WinISD and change the port lengths to see the changes in the F3 value. You can get a rough estimate from it then you build the box, try to get close to that value but do not sweat it when you are off by 1/8" inch

 

[gedlee]

Its great to be nitpicky but if you are off by 1/2 inch any port its not going to change the tuning frequency that much.

That completely depends on the port length. You should state it in terms of % change and the statement would be more accurate. I've use ports as short as 1-1.5", a 1/2" is going to make a big difference (but I did note your caveat). % changes of as much as 20% aren't going to matter much.

 

[doug20]

That completely depends on the port length. You should state it in terms of % change and the statement would be more accurate. I've use ports as short as 1-1.5", a 1/2" is going to make a big difference (but I did note your caveat). % changes of as much as 20% aren't going to matter much.

Good point, my original intent was to just say the port change in a subwoofer being 1/2" then I realized main speakers sometimes have extremely short ports.

Percentage would have made it more accurate.