Damping 4345 enclosure

[richluvsound]

Hi Folks ,
can anyone help with a damping solution for my 4345 project. Fibreglass or wool ? And what thickness ? Do I understand correctly ?.........too much insulation will drop the freqency of the box.
I'm nervous as hell about this as the next step is to seal them up . I dont fancy spending time with my head stuck in an 18" hole trying to correct a mistake.


Rich

[John W]

I used Wrap-On 1" fiberglass insulation, glued with spray on contact adhesive.

Here is a link to more information on a source, though I'm not sure if they ship to the UK.
http://audioheritage.org/vbulletin/s...hlight=wrap-on

Also for reference, I found this picture looking up the woofer hole of an original 4345.

[Ian Mackenzie]

Rich,

I would try and source the wrap fibre glass insulation. The particular grade appears easier to handle and has the same properties as that used in the stock design.

The purpose of the fibreglass is to absorb a certain amount of internal energy (which is converted to heat). This helps control internal resonance and sound waves at higher frequencies coming out the front of the woofer cone. The fibreglass also adds a certain amount of damping to the driver at systen resonance.

The effect of ithe fibrelgass will also add some virutal volume to the enclosure.

Before installing the fibreglass (known as fill) the enclosure has a gross internal volume (of approx 10uft 3 less 1.0cuft 3 = 9.00 cuft gross volume) taking into account the space occupied by the dog box, the ports, the compression driver & horn and crossover.

Subtracting the volume the rear of the 2245H (0.3 cuft 3) woofer and the bracing (0.35 ft 3 approx) the fibreglass adds back the volume so the net volume is 9 cu ft 3.

Sometime ago I asked the designer (Greg Timbers) about the JBL 4345 box volume and tuning and the above was basically the explanation.

Because the amount of fibreglass is going to effect the net volume this will have some effect on the final system tuning. If you end up using a different grade of fibrelgass or another type of material some adjustment of the tuning or the thickness of the fibreglass maybe required.

Avoid using too much fibrelgass and make sure it is well clear of the port openings.

System tuning +- 2 hertz is quite acceptable depending on your room acoustics and preferences. The tuning of the system is about 28 hertz according to Greg Timbers (based on real motion of the 2245H ). The fibreglass should be attached to all internal surfaces except the front baffle.


Ian

[boputnam]

Also for reference, I found this picture looking up the woofer hole of an original 4345.

Hey, that's mine!!!

[Ian Mackenzie]

I have edited my post to provide more accurate details of the enclosure gross and net volume.

In practise the stiffness of the enclosure and losses caused through any air leaks will have a more pronounced effect on the response than subtle variations in the enclosure volume.

The QL (system losses) of the stock cabinets was in the order of 10 apparently indicating a fairly stiff enclosure . The norm for large enclosures is 5.

Cabinet making skills are therefore relatively important.

Below is the ground plane measurement which is fairly meaningful indication of the low frequency response.

The in-room response will show significant room gain below 50 hertz in a dometic listening room.

The impedance curve shows the system box tuning (Fb) which in reality (looking at real motion of the woofer) is a bit higher than shown by the graph.

I trust these details will provide a more well rounded understanding of LF aspects of your JBL 4345 diy project.

Ian

JBL 4345 Ground plane measurement courtesy of JBL Professional

[X_X]

It (4345) also had a reputation for less than detailed bass, which I have always thought was due to the enclosure not being quite large enough.

Net volume of the 4345 is 9 cubic ft.

Recommended enclosure guide for the 2245 is 10 cubic feet.

Ahh, if only they made them bigger...

[richluvsound]

Thanks guy's , i found 25mm insulation in black.
So, 10 cu ft is better . BOLLOCKS ! OK , i can sort that out . Bearing in mind the internal depth is 16" as per Rick's drawings ,what would that depth need to be to acheive a volume of 10 cu ft.
As the baffles are not fixed yet i can add to the front and skin them in 12mm walnut veneered mdf. 32mm thick , top, bottom and sides.25mm baffle and back panel. Would I be achieving enough to warrant such changes , has anyone done an a,b comparison ? You guy's know your bloody apples from your pears so I'm willing to listern and learn on this one.
Would such a change affect xover design ? I need to consider Guido in this .
He has given me so much time already , I don't feel it would be fair. Besides this is my first ,not my last project.
Rich

[John W]

My advice would be to stick with the original plan. It is a proven and well engineered speaker.

[John W]

Hey, that's mine!!!

And thank you very much for posting such a fine set of pictures.

[pmakres1]

I've never had the pleasure of auditioning a set of 4345's but I have little
doubt it is a very fine speaker. And Rich your photos of your project show that you are doing a superb job! I continue to watch with interest.

Peter

[X_X]

Richluv,

Yes, do stay with the original design. You would have to retune the cabinet, etc...

Get aquianted with the original, intended design. My test run was thumbs up.

[Ian Mackenzie]

The stock system was tuned empirically.

It was (I think) the volume that worked best in the full passive mode with the 0.5 ohm dcr inductor on the woofer. I also think the final cabinet volume probably had some influence from marketing. As it was the stock 4345 was just too large for the end market.

I personally think the big woofer frightened them off!

If you are biamping there is lot of scope for aternative alignments.

I have used the D. B Keele alignment of 10.3 cu ft 3 with Fb at 27.2 hz quite successfully. Remember this is the net volume.

[Guido]

Leave it original!

I did also choose the inductor for that original design.

[John]

Why don't you add another sq.foot with extra fibreglass. ???

If I was building one and after reading what Greg T. mentioned I would go bigger, and retune the ports.

[richluvsound]

Thanks guy's .
i'll leave it be and stop fretting . The router is back on the shelf .
Whats an inducter ? (stop ) I know i'm out my depth.
In an ideal world I would apprentice with one of you guy's . Austalia sounds good, but as a cricket fan I dont think i could take the ribbing. Hayden !!!!!!!
Thanks again all, Rich