Box for JBL 2245s

[David Ketley]

I have a pair of boxes built by cabinet maker to my measurements resulting in 10.06 cubic feet enclosures for a pair of JBL 2245s with new JBL cones. I need to take off the area for the cones .3 cubic Ft and additional bracing say .7 cubic Ft
The ports are two x 6” for each cabinet and the design in conjunction with Wilmslow audio UK gave a port length of 11.5. I’m looking to add fill to about 10-15%
On the subject of fill I understand a good substitute for fiberglass is lamb’s wool insulation?
As I’m using a Marchand 4 way active crossover (2245, 2123, 2446, 2405) I understand I might need a series .5 Ohm resister to bring the QTs up to the required value but I don’t have a programme to model the actual figures.
Can you confirm the figures I have given? Have I got the right idea?

[Ruediger]

<snip>
I understand I might need a series .5 Ohm resister to bring the QTs up to the required value ...
<snip>


If possible leave the QTs where they are. If the cainets are larger than required, make them smaller by making the ports larger and longer.

If the cabinets have been designed with a passive xover in mind, they will have been designed for a larger QT, and thus be larger than normal.

Larger ports take up volume, in particular because they need to be longer.

Ruediger

[ivica]

I have a pair of boxes built by cabinet maker to my measurements resulting in 10.06 cubic feet enclosures for a pair of JBL 2245s with new JBL cones. I need to take off the area for the cones .3 cubic Ft and additional bracing say .7 cubic Ft
The ports are two x 6” for each cabinet and the design in conjunction with Wilmslow audio UK gave a port length of 11.5. I’m looking to add fill to about 10-15%
On the subject of fill I understand a good substitute for fiberglass is lamb’s wool insulation?
As I’m using a Marchand 4 way active crossover (2245, 2123, 2446, 2405) I understand I might need a series .5 Ohm resister to bring the QTs up to the required value but I don’t have a programme to model the actual figures.
Can you confirm the figures I have given? Have I got the right idea?

Hi David,

From my point of view, You have about 255 Lit internal volume ( 9 cb.feet) and using the mentioned ports , box tuned frequency is about Fb=28 Hz . Using mentioned 0.5 Ohm resistor would have almost no influence, as the driver Re=5.8 Ohms.
The placement of the speakers in the room would have much, much more influence to the response, then mentioned resistor.

regards
ivica

[David Ketley]

That's the conclusion I'm coming to just get the system up and running and listen.
Im crossing to JBL 2123 at either 160 or 260hz any issues with either crossing frequencies?

These are the cards I have for my Marchand active crossover.

[Ruediger]

Hi David,
...
Using mentioned 0.5 Ohm resistor would have almost no influence, as the driver Re=5.8 Ohms.
...
ivica

Qt w/o the resistors would be 0.27, w/ the resistors it would be 0.29.

The difference in Qt for Thiele alignments 3 and 4 is 0.04. To me this is significant.

Ruediger

[ivica]

Qt w/o the resistors would be 0.27, w/ the resistors it would be 0.29.

The difference in Qt for Thiele alignments 3 and 4 is 0.04. To me this is significant.

Ruediger

Hi Ruediger,

I have put all the mentioned data in BB6 simulator (before sending the post) and there were almost no differences in the response up to 500Hz (neglecting some small reduction of the sensitivity)

regards
ivica

[Champster]

Are you sure you need two 6" ports? I mirrored my 2245 based sub on the article Greg Timbers wrote in 1983 (http://www.lansingheritage.org/html/jbl/reference/technical/1983-subs.htm) for an 8cf cabinet with a single 6" diameter/11" long port.