Cab port question??

[basss]

What would be the best port size to tune a 2 cf cab to 50 hz? According to Win ISD online I can do 2 1.476" ports without using port tubes assuming that the baffle is 3/4" ply. That seems small to me - am I going to run into port whistling problems? Win ISD says no...

[lbwdog]

I'm glad you asked this. I have a sealed, 2 cu ft. 1x15 cab that I want to rear port, and need the same info. Mine is almost identical in size to a BagEnd s-15. I'm also curious if my port should be directly behind the magnet, or offset towards a corner.

In the past, when I asked, everybody said "it's too small","make it a twelve", etc., but I want to load it with a Neo eminence. Either a 3015 or 2515, and use it for light duty work. I know this cab won't ever sound like a monster stack, but I know it will do what I want if I can just get the correct port size and placement.

Sorry to hijack, but it seems we're going after about the same thing.

[will33]

According to WinISD, a 2cu.ft. cab using a pair of 3" ports, each port needs to be 5.39" long. Using a single 3" port it needs to be 1.6" long. That's not all there is to consider in a port though. The biggest concern is having enough port area to keep the wind speed down. If the air coming through is moving too fast through too small an opening it will make weird noises and even affect the tuning. Ports are cheap and can be made from most anything round. Cardboard tubes be they from a roll of TP or a shipping tube for blueprints, whatever size you need. PVC pipe comes in about every size that's common to port tubes and some that aren't.

To the second poster, a sealed box will almost always be larger than a ported box for the same speaker. Physics, it's just the way it is. A box that is suitable for a sealed 15 will likely be a good size for a ported 12. A sealed 12 may be a good size for a ported 10 and so on. The 2 are different animals. The saying is loud, low and small...pick 2, you can't have all 3. Always tradeoffs to be made. Ported boxes play louder, lower but the small is out. Sealed boxes can play loud and be small but they don't go as deep so the low is out, etc.

Many commercial offerings split hairs somewhere where you get 80% of all 3. Aren't great at any of them but still sound good.

[billfitzmaurice]

Originally Posted by basss

am I going to run into port whistling problems? Win ISD says no...

It says no at the default power input, which is 1 watt. Change that to the actual power you will be using and the port velocity will go off the chart.

[lbwdog]

Originally Posted by will33

To the second poster, a sealed box will almost always be larger than a ported box for the same speaker. Physics, it's just the way it is. A box that is suitable for a sealed 15 will likely be a good size for a ported 12. A sealed 12 may be a good size for a ported 10 and so on. The 2 are different animals. The saying is loud, low and small...pick 2, you can't have all 3. Always tradeoffs to be made. Ported boxes play louder, lower but the small is out. Sealed boxes can play loud and be small but they don't go as deep so the low is out, etc.

Thanks Will, and this is all stuff I know. I'm just trying to find the best port size and placement given the compromised size of the box. It was actually the cabinet of an inexpensive Acoustic combo from 1986, which was damaged in transit to the music store I managed. Acoustic sent me a new cabinet so I took this one home, cut off the amp section and loaded it with a JBL K-140 I had laying around. I've used it for Jazz gigs and it sounds pretty good, but it's heavy. A good neo will make me happy with the weight and fine with the sound if I can ever find a good size for the port.

Many commercial offerings split hairs somewhere where you get 80% of all 3. Aren't great at any of them but still sound good.

That's what I'm looking for....I mean, BagEnd is identical in size.

to the O.P. I surely didn't mean to hijack your thread, so I'll bow out after this. I was hoping some one would come along and tell us both something like "Hi guys, a 4" x 6" port will give you the best sound for your small box"

[basss]

Originally Posted by billfitzmaurice

It says no at the default power input, which is 1 watt. Change that to the actual power you will be using and the port velocity will go off the chart.

I had the acoustic power field set at 10... I'm using the online version of Win ISD - is that my problem?

Originally Posted by lbwdog

to the O.P. I surely didn't mean to hijack your thread, so I'll bow out after this. I was hoping some one would come along and tell us both something like "Hi guys, a 4" x 6" port will give you the best sound for your small box"

No problemo - we're asking similar questions. I've had a 3015 loaded in 2 different very small boxes and it sounded great in both. No massive lows of course but it sounded like a bass.

[billfitzmaurice]

Originally Posted by basss

I had the acoustic power field set at 10... .

I don't know what that is. I use the Alpha pro version.

[uglyrumor]

Can't give you any help on size, but placement is immaterial. You can put the port anywhere you like, so long as the dimension works. Front, side, back, top, bottom (but you have to leave room if it is at the bottom!). One half of your question answered!!

[T-Bird]

Hi.

There's an on-line WinISD now? Great news.

"Baffle ports" do work, but are seldom good. Those are usually used only when replacing the OEM driver if the existing port is just a hole in the baffle and the shape of the port opening isn't a round hole. Who can spell Peavey .

I would port the cab with round tubes, using any program, or formulae.

Regards
Sam

[NoGraveConcern]

With such a tiny box you are limited on port size by the resulting length requirement. I wouldn't go any smaller than a single 4" diameter or dual 3" diameter, though even those will have their limits. Decide what material you are going to use for the port tube, measure it's inside diameter, and then calculate the appropriate length based off an exact measurement. However, just drilling a hole in the baffle is not acceptable practice.

[Sundogue]

Originally Posted by NoGraveConcern

I wouldn't go any smaller than a single 4" diameter or dual 3" diameter. Decide what material you are going to use for the port tube, measure it's inside diameter, and then calculate the appropriate length based off an exact measurement. However, just drilling a hole in the baffle is not acceptable practice.

Just drilling a hole in the baffle is acceptable if the diameter of the hole and the thickness of the baffle is what is required for the port. It depends on the cab design and port tuning.

I've experimented with higher tunings and the length of the port "tube" ended up being no more than the thickness of the baffle (3/4"). No point in inserting a tube the same length as the thickness of the baffle, when the baffle itself just needed the correct diameter hole.

Typically though a port "tube" is required as most tunings wouldn't allow such a short port.

[NoGraveConcern]

You are confusing the issue though. There may be odd cases where it is mathematically acceptable, but the OP has neither the enclosure volume nor the tuning goals to allow simple baffle hole drilling to make a useable port. The ideal port for this box & driver would be 7" dia x ~16" depth. Compare that to a 2 3/8" hole in the baffle. Both tune the same, but the result of one is acceptable (though impractical) and the other is unacceptable and generally frowned upon.

[basss]

If you had to use a 2 cf cab and wanted to tune it to 50 hz what size port would you use?

[billfitzmaurice]

Originally Posted by basss

If you had to use a 2 cf cab and wanted to tune it to 50 hz what size port would you use?

That depends on the driver used and the power level. There is no one answer. But for an average 15 inch driver the minimum port diameter is 6 inches.

[basss]

Originally Posted by billfitzmaurice

That depends on the driver used and the power level. There is no one answer. But for an average 15 inch driver the minimum port diameter is 6 inches.

I'll be using a 12. Generally speaking would a 4" port be OK? When reading the graphs in Win ISD what should I be looking for as it relates to ports?

[NoGraveConcern]

Rear port air velocity is the graph you want. You have to set the input power to the level you expect to be your max at the frequency where port air velocity is the worst. Keep vel below 17 m/sec and you are good.

For a lower excursion 12 I have had good luck with a 4" port. You can cheat that 4" for a higher excursion 12 by using a flared port like a precision port.


Alternately, you can use this handy link to figure minimum port diameter based on driver size and excursion.

http://www.carstereo.com/help/Articles.cfm?id=31