I wonder why the 2245 exhibits more problems with room placement(ground planes and such) vs the 2242 that seems less affected by its surroundings?
Just curious!
Ron
I wonder why the 2245 exhibits more problems with room placement(ground planes and such) vs the 2242 that seems less affected by its surroundings?
Just curious!
Ron
Hey your up early!
I went to bed?
Odd they did not make it 10 cuft 3 and be done with it. The previous iteration I had was around 10.5 cuft tuned to 27.2 hz with Keele's equations
I have tales to tell...
I wonder why the 2245 exhibits more problems with room placement(ground planes and such) vs the 2242 that seems less affected by its surroundings?
The answer is actually in the Product Brochure. The 2242 is designed to be augmented by boundary reinforcement while the 2245 was designed to yield rock solid response within it's bandwidth without boundary reinforcement.
Why is there so much difference in response(ie: 10 cu ft vs 10.5 cu ft)?
I would think there would be more variances with the room loading and speaker position than with a difference of a 1/2 cu ft in cabinet size.
Ron
Well do tell.....Originally Posted by Lancer
Hey your up early!
I went to bed?
Odd they did not make it 10 cuft 3 and be done with it. The previous iteration I had was around 10.5 cuft tuned to 27.2 hz with Keele's equations
I have tales to tell...
.
I went over all this stuff ages ago with Witold Waldron, the inventer of Calsod and owner of Audiosoft..and he laughted and said it all depended on the assumptions made with a particular model........this was well before I found out it was empirically tuned. I mean you can doodle till the cows come home but what the Calsod guy did explain was the ear is very susceptable to variations in LF flatness and rolloff.........and group delay.
By running a series of simulations and by deduction and I therefore reasoned that the 9 cuft 3 tuning at 28 hz was to attain a flat response with maximum extension ie 32 hertz and reasonable group delay.(transient response).
They also had to deal with a 0.5 ohm 5.4 hm choke in series with the woofer.
Plugging that into the simulation is not that pretty...0.7 db ripple in 10cu ft 3 with an fb of 30, the 9 cu ft 3 tuning at 28 herts contols the ripple more, being only .2 db. Perhaps this was borne out in the empirical trials JBL did.......
This is all by the by when one actually builds and starts tuning a real box as there is QL and all the other attendant issues that can make the model less predictive.
There are also alignments by others of the JBL fold incl Gary Margulius that offer tunings on a make to fit box size.
Ian
There are also alignments by others of the JBL fold incl Gary Margulius that offer tunings on a make to fit box size.
Yeah, check out Drew's legacy stuff.
http://www.drewdaniels.com/Audiotech.htm
Joe Etrick (Barath Acoustics years and years ago) assisted on the spreadsheet.
Yes I know,
I am back looking at this again!!
But there is a reason. I wanted to fine tune for my 0.2 ohm 5.4 mh inductor. Up until now I have used 0.33 ohm resister in series to mirror the 0.5 ohm JBL choke in the stock 4345 vol enclosure (9 cu ft3 net)
As I recall Greg suggested that the box Fb might need to be put up a notch.
I tried this and chopped an inch off the ports.
Its highly subjective, initially I thought it was better, more punch but after playing a number of cd's I came to the conclusion it was not as good as my original tuning per the stock 4345 8 1/4 inch ports. I then realised after looking at a BassBox simulation that the ideal box should reduce in volume and the fb should go up accordingly.
Rather than alter the volume at this point I decided to go the other and keep the volume as is and reduce the Fb, so I blocked one of the ports.
Very interesting result, somewhat overdamped.
The thing as mentioned earlier is everyone simulator has adifferent set of assumptions. Drew's calculator gives 11.27 ft cu3 , Fb 26.4 with a QTS of 0.28. I use that value to allow for the added DCR of my choke 0.2 ohms.
Bassbox gives as sub 8 cuft volume without ripple? Drews simulator says there will be ripple at this volume. I just wonder if there are varying interpretations of what maximum flat response means.
My feeling is that I will need to manipulate both the volume and fB to nail this.
Ian
Page 46 is very intuitive
http://www.drewdaniels.com/BINDER.pdf
Ian, have you measured the actual Fb of the box with the different port configurations? Also, are you sure the t/s parameters of your pair of 2245s match the spreadsheets?
This is probably of more interest to would be/are JBL 4345 builders.
Bi amping these monitors is the most popular mode of operation. Its is intuitive however to understand the contraints placed on the system in internal passive mode and how this effects performance.
Those interested in using this monitor in passive mode with a high quality amplifer may find this information useful.
I contest that unless your active crossover is of the highest quality you will loose resolution and transparency and this will mitigate the benefits of an advanced passive crossover. The full range amplifier should be a high current type, preferably with class A operation at least at low power levels.
After carefully review the JBL data and some design details of the 4345 I was able to model the 2245 in the stock 4345 enclosure reasonably accurately in Bassbox 5.1
There are a couple of things that are not entirley obvious. The placement of the ports adjacent to the side wall effects the effective length as does the use of mutiple ports, hence the actual physical length is shorter than the designed Lv for a Fb of 28 hertz. The dcr of the 5.4 choke used in the stock system is about 0.5 ohms. The manner in which the biamp switch is arranged causes additonal Dcr in the order of 0.3-4 ohms. The stock enclosure has QL of 10.
The effect of the total dcr added to the system creates a modified QTs of 0.29- 0.3 (JBL 2245H QT =0.27) . The Ql of 10 and the modified QTs gives rise to ripple in the woofer passband, starting at about 0.1 db 200 hz, 0.2 db at 98 and a maximum of 0.4 db in the 60 hz zone. I would contest this results in a lack of definition and clouds the midrange based on my own evaluation using the stock 3145 networks. While there is a known effect on the midrange due to interaction of the motional impediance of the drivers, the ripple in the woofer passband contributes to a less than ideal performance in internal passive mode.
To compensate for the modified Qts in the system design would call for a larger enclosure and a different tuning frequency.
Over the weekend I explored some interesting approaches to improving the bass and hence the midrange performance in passive mode.
The choke in my diy 3145 crossover has a DCR of 0.2 ohms. My choke is hard wired without any biamp switch to ensure minimal additional losses.
The lower DCR choke results in a modified QTs of 0.28 on the 2245H. This is a more ideal value to minimise ripple in the woofer passband. The Bassbox simulations showed no ripple for this alignment.
Using Table X of Small's T/L data (from Bullock on Boxes 4th order QB3 alignments) for a value of Qts for 0.28. The Fb given by H =1.3747 = 27.5 hertz, Alpha =3.4971= Vb 8.29 cu ft3 net and F3/fs =1.6826 = 33.6 hertz.
I implemented these updates into my cloned 4345 system for assessment. This required a reduction of the net enclosure volume and an adjustment to the port length.
The bass balance with the midrange which is a fairly subjective aspect of loudspeaker performance was similar to the stock alignment. The clouded almost woofey character was far less pronouned, there being less overhang and better definition. I tried a slightly lower Fb of 26 hertz as a comparison and found this made the low bass more pronounced but the mid and upper bass was less aggessive.
I will continue to assess these variations on the tuning and update this thread in due course.
Ian
Hi Ian,
You've probably noted this elsewhere, but it might be useful, toward understanding your
tuning/development process in this thread, to briefly describe your listening environment
(room, placement including any stands, and listening position), as this would be integral
to what you are hearing/reporting. Cheers, -grumpy
I would be interested to hear what actual Fb and box volume you think sounds best.
I guess my point is that even if you follow the nice set of 4345 plans exactly, there will still be slight variations in the final volume of the cabinet. You may have a thicker front baffle, a little more stuffing, slightly wider bracing, etc. Also, I guess I am a little suspicious about the 9.0 cubic feet in the published specs.
Also, how much of a constant is the QTs of a 2445? Once the driver is initially broke in, does this value change much throughout the life of the driver, or with environmental factors like temperature, altitude and humidity. Maybe the actual QTs of your drivers started off at .29 and now they have been changed to .32.
Grumpy,
I have them in a relatively small at the moment( by Yank standards..its not a Red Neck barn!) that measures about 4 x 5 metres. (apartment).
The loudspeakers are 2.2 metres apart (from the horns) , about 0.25 metes from the rear wall and 0.45 metres from the side walls. They were elevated 0.2 metres off the floor on wooden stands but for safety reasons I have them on 50 mm alumuminum cone tip toes while I work on them. This provides a degree of de-coupling from the floor. The listening position 2.8 metres from each speaker and is 1 metre from the rear wall and within arms reach of the Bar tap! They are toed in at a slight angle, not much. I find this arrangement works well in terms of imaging.
Excellent. Thank you kindly! -grumpy
John,
I am coming around to the opinion that there is no best Fb and Vb and this is explained in Drew's binder pdf ..the server is down at the moment. There are two extremes on paper...maximumly flat ....overdamped and extended bass...underdamped. Somewhere in between will be happy medium to suit the individual application.
The 4345 box tuning was somewhat empirically, based on listening tests.
If you follow the plan (in the tech reference area) you can't go wrong, just do everything they say. If you stray from the plan..look out. The vol is respect to the net volume after accounting for the woofer and cleats from the gross volume in the woofer chamber and the effect of the fibreglass fill which adds real volume. I spent hours labouring on this and this was Greg's explanation. I have checked it and its correct.
The design was tuned on the real world, things were done a certain way (like the port length) with the actual box to get a desired result.
'Both of my 2245H are relatively new (less than 5 years old). To the best of my knowledge the parameters (electrical/mechanical) don't change, the 2245 is such a robust assembly its unlikely to vary much.
The point of what I have been doing here is highlighting and addressing the woofer performance in internal crossover mode. If you are going to bi amp don't loose sleep over it. But the coil (the choke ) the capacitors shunting the LP filter on the woofer effect the bass, noteably at FB but as I have indicated they can influence the whole bass range. Basically the way its set up for internal passive mode is a best fit without having to re tune the box but its obviously not as good as biamping where that coil anc capacitor are out of the picture and there is no motional impediance interaction of the drivers.
What I am attempting to do is tailor the tuning for internal passive only mode. Yes the room has an influence, but that is aside from the woofer sounding correct with the mid cone and as a system as a whole.
If you put them right in a corner it will obviously have an effect.
I am talking about 4th order QB3 alignments here, other alignments may suit certain rooms and situations better, ie 4th order SBB4 and BB4 aligments yield H=1=20 hertz, Alpha=3.0127=Vb 9.62 cuft3 net, F3/Fs=1.9805=39.6 hertz.
This alignment would be an interesting exercise as the Vb is no excessive in terms of real estate and as it has the best transient response and the gradual rolloff may suit the room gain of a domestic living environment. There being less output in the 40-60 hertz rang than the QB3 alignment
The problem with this alignment is the vent constraint as long ports are required, limiting 2 4 inch ports to about 15 inches..too long for the stock 4345 , perhap an elbow port, or one 4 inch port only 6 inches long. Side mounting the ports might be an option for a diy project.
I hope all this makes some sense.
Ian
I attempted the above BB4 alignment.
Very interesting! Almost a dry studio sound.
It certainly has the transient response, but lacks some of the punch of the typical JBL sound.
Ian
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