JBL 4646A LF Cabinet - Port Dimensions?

[robmuso]

Hi, to anyone who may be kind enough to help,

For my keyboards rig, I am building a couple of 12" LF speaker boxes to be as close to a replica of the vintage JBL 4646A which uses 1 x JBL 2206H transducer.
For the top end, I will then biamp them with some 2426 HF drivers and 2370a flare's.
I have downloaded the JBL 4646A specification file from www.jblpro.com/pub/obsolete/4646a.pdf
The diagram of the box and components shows that there are 2 x Ports, JBL Part Number 57932 (these are not for sale).
From my study's of this cabinet so far, I assume that both ports might have an inside diameter of 3.0 inches (76.2mm), and might be 5.75 inches (146mm) in length.
The enclosure volume is known to be 34 litres or (1.2 cubic feet).

My Question Is: :confused:
If anyone out there has access to a JBL 4646a LF cabinet or knows the answer, I would be extreemly grateful for any help. I would like to ask if anyone could put a tape measure inside the port and measure the length as this is what I need to find out before I glue the ports that I have made into the front baffle. I think I'm on the right track but I like to check thoroughly and be correct with these things.

Please help???? :)
Thank you,

robmuso

[4343]

Don't have a 4646A, but I just happened to have Bass Box 6 Pro open running the numbers on my 4648. (Have a pair of 2234H's and an empty box...)

It says that a 2206H in 1.2 cu. ft. would like 2 3" ports of 7.1" for an Fb of 54.6 Hz, and F3 of 79.7 Hz. This with a normal amount of damping (fiberglass) on all sides but the front. That's the optimum flatness, I'd guess.

Changing to flared ports the length goes to 7.6" for one flared end, and 8.1" for both ends. That's probably not needed for 4646 unless you want to play very loudly near the 80Hz end. The flares will lower the distortion a bit, but maybe you want a little to clue you in on the lower limit of the box.


Changing to 5.75" ports, the Fb goes to 58.4 Hz, and F3 goes down to 71.6 Hz. Sounds like a winner to me.

I'm going to use 4" flared at both end ports for the 4648. Just ordered Precision Port from PE.

[robmuso]

Thank you for your reply Mike.

I have been experimenting a bit today using a 5 ohm 20watt resistor in series with the 2206H speaker whilst connected to my Yamaha powered mixer.
On my PC I then use a software program signal generator set to sine wave out whilst connected to the mixer with output set at a low level but with adequate volume.
With a digital AC volt meter connected across the 5 ohm resistor, I start the signal generator at about 45 HZ and press the arrow button on my PC repeatedly to increase the frequency 1 HZ at a time.
Watching the multimeter, I then note where the voltage reads it's highest point. I am told that the frequency of the signal generator at that point is the box tuned frequency.
With my speaker box, I used a vent 84mm long to obtain a 60HZ tuned frequency which is the tuned frequency of the 4646A.
But this was just a first time experiment for me and I understand what is happening with the theory but with so many different calculations, I still feel confused as to whether this method is correct.

I welcome any opinions or advice here from yourself or other members. :confused:

Many Thanks.

Regards, Rob :)

[Baron030]

Hi Rob

A sealed enclosure has only one impedance peak and a bass reflex enclosure actually has two impedance peaks, with one peak above and the other peak below the port tuning frequency. A valid method for testing port tuning is to measure at what frequencies all 3 of these the impedance peaks do occur, meaning that you will need to temporarily seal the ports to get a frequency measurement of the close box. There is a formula for calculating the actual box tuning frequency based on these 3 impedance peak measurements. Please read my postings from #28 to #33 of the following link:

http://www.audioheritage.org/vbulletin/showthread.php?11664-My-Drew-Daniel-s-inspired-DIY-project/page2

Just follow the formula in posting #33 and it should give you an accurate measurement of your enclosures tuning.
From there you can adjust the port tube length to best fit your application.

Baron030:)

[robmuso]

Thank you Baron030

I will check that out and let you know how I go.

Kind regards

Rob

[robmuso]

Hi Baron030

I have read on another website since, that the 4646A has 2 x 3.0" diameter ports at 6" long. The person posting that info also shows a photo of the cabinet.
So I hope he's measured it from that box.

I have been experimenting with the formula that you had posted in 33.

First, I designed an excel spreadsheet to enter the 3 frequency values.
I calculated them using my digital multimetre across the 5 ohm 20w resistor whist reading the AC voltages at different frequencies.
I then calculated the tuned frequency and all is looking good.

I am also experimenting with 2 programs that I purchased online:
Winspeakerz (Box Design Program) (Pretty Good and close impedance readings to your test formula)
TrueRTA (Signal Generator with Analyzer) (I had previously tried the Analyzer using my DBX RTA microphone, and was getting all sorts of confusing readings)
Maybe I'm not doing something right. For now, I just use the Signal Generator with your formula with the port tuning experiments)

There are some JBL4726ap cabinets at a club I work at, and I will test the port tuning with the formula next week.
That should then give me all the options to weigh up before I decide what length port to use.

Another thing that seems to be possibly affecting my tests with my prototype box is that I have lined the inside with dampening wool made for speakers, so if you have any advice on that, I would be grateful. From what I have read, the dampening wool simulates a larger box size. As a basic rule, I've always understood that it also improves the bass response.
I'm not an expert with this stuff, but very passionate about learning.

I want to thank you heaps Baron030, for your advice about the formula in 33.
You were very kind to give your time in responding to my post.
This is an experiment that will definitely help me in my future studies with speakers.

Take care
Kind regards,
Rob (robmuso)



[QUOTE=Baron030;380416]Hi Rob

A sealed enclosure has only one impedance peak and a bass reflex enclosure actually has two impedance peaks, with one peak above and the other peak below the port tuning frequency. A valid method for testing port tuning is to measure at what frequencies all 3 of these the impedance peaks do occur, meaning that you will need to temporarily seal the ports to get a frequency measurement of the close box. There is a formula for calculating the actual box tuning frequency based on these 3 impedance peak measurements. Please read my postings from #28 to #33 of the following link:

http://www.audioheritage.org/vbulletin/showthread.php?11664-My-Drew-Daniel-s-inspired-DIY-project/page2

Just follow the formula in posting #33 and it should give you an accurate measurement of your enclosures tuning.
From there you can adjust the port tube length to best fit your application.

Baron030

[Baron030]

Hi Rob

Wow, you are hitting, your project with a lot more design tools than I used for port tuning a project. Of the two most popular tools, of course I am referring to WinISD and Bass Box 6. I think Bass Box 6 is a better tool. And the reason why I say that, Bass Box 6 takes into account the amount of “Fill” in its calculations. And I don’t think that WinISD takes "Fill" into account. One of things to keep in mind with all of these design tools is that they are just computer simulations of reality.

Joseph D’Appolito is a pretty smart guy. And his formula for calculating port tuning seems to be pretty accurate. So, I would trust the measured results of the impedance peaks from your project more than just a computer simulation.

Of course, Can we really trust computer simulations? You have a choice, take the blue pill and simply go back to life as before or I can take the red pill, and see just "how deep the rabbit hole goes." Sounds like you have taken the red pill...

Good luck with your project.
Baron030:)

[4343]

...
I have read on another website since, that the 4646A has 2 x 3.0" diameter ports at 6" long. The person posting that info also shows a photo of the cabinet....

Falls right in line with the 2 x 3" ports 5.75" long that Bass Box 6 gives.

Do consider the flared ports.

[lgvenable]

Rob,
Just replying back, as I'll get you the details on measurements to my 4646A speakers I have in my theater.
I'll post them to this thread for everyone, as well as emailing the measurements to you so you can build four new 4512LF cabinets, that you can fill with 2206H 12" bass drivers.

That'll remove all the guesswork on exact port measurements right??

Cheers Mate!