Choosing between 2 types of Jensen Ultra-flex (onken) cabinet designs

[Salectric]

I am making plans to build a cabinet based on the Jensen Ultra-flex design. A Jensen DIY manual from the 1950's shows two versions---one that looks like a modern Onken with vertical vents on the front of the cabinet, and a second that has the vents on the back corners at a 45 degree angle. Both types have about the same footprint so size isn't a factor. The two versions are shown on the sites below:

http://www.dcaudiodiy.com/jensen_imperial/page_13.jpg

http://www.dcaudiodiy.com/jensen_imperial/page_14.jpg

Does anyone have experience with both types of designs to help me choose one over the other?

I will be using Jensen P15LL woofers which are the intended drivers for each of these Ultra-flex designs. The woofer will hand off around 1K to an Altec 802-8G in an Altec 32A horn.

[kevinkr]

I built the front firing design and it is certainly easier to build, and perhaps is a little less critical as to placement, but that is a swag. The rear port version might avoid the comb filtering issue that exists with the front firing design, but I suspect might be happier in a corner.. All a guess on my part, all I can say is I am pretty happy with the ones I built 5yrs ago, and they are the "built in" forward firing style. With some sadness I might note that I am absolutely the only one with a pair within the group I run with - plenty of interest, just too big I guess for most hobbyists. So far I have not heard anything else that makes me think I made a mistake..

I am crossing at 800Hz and 8kHz to a JBL 2440 on 2311 short conical horn/2308 vane diffuser and 2402 ring radiator.. Directivity is deliberately narrow in both the horizontal plane and vertical plane, and imaging is very good at my listening spot - the room is fairly modest in size with a 7' ceiling.. Cross-overs are 12dB/octave butterworth passive types of my own design.. Power is 7-8Wrms per channel of broadband 300B power..

[GM]

For me, the corner cab is the only Ultra-flex, but then I prefer corner loading regardless of cab design. It works best against a really rigid corner though, so ideal for the below grade concrete constructed basement I had mine in.

GM

[Salectric]

Thanks for the replies. I agree with Kevin that the possible comb filtering with the front-facing ducts may be a concern, especially since I will be using a slightly higher crossover (1K vs. 800). Right now, however, I am leaning towards the front-facing approach mainly due to the simplified woodworking; only right angle cuts are needed. Plus my cabinets will be a few feet out into the room, not in corners or near the back wall.

Dave

[kevinkr]

Hi Dave,
Although I can measure some bleed through the ports at higher frequencies and there is probably some comb filtering going on as well due to the configuration I don't hear anything untoward. It may also be that my cabinets are a bit under damped due to my light use of acousti-stuff on the back, bottom and side panels - my ports aren't stuffed either. I'm quite happy with the results and have heard VOTs, RCA Ubangis, JBL C37, C40 and a number of bass reflex cabs and remain satisfied. Efficiency is high, extension is into the low 30s and they are very tight & clean sounding..

[GM]

Originally Posted by kevinkr

Hi Dave,
Although I can measure some bleed through the ports at higher frequencies and there is probably some comb filtering going on as well due to the configuration I don't hear anything untoward. It may also be that my cabinets are a bit under damped due to my light use of acousti-stuff on the back, bottom and side panels - my ports aren't stuffed either.

Greets!

Vents 'close up' with increasing frequency, so what you're hearing are the strong half WL pipe harmonics of all those crude musical instruments and with minimal internal damping you may also be getting some bleed through due to reflections modulating the back of the driver diaphragm, both of which are comb filtering with its output and at higher SPL may be loud enough for the higher orders to comb filter with the mids/HF horn.

This was a major problem with the large Altec 210 combination horns and why I finally whacked off all the excess to make them compression loaded and added a sub system.

Well, there's clean and then there's 'clean' which is more accurate, but folks who prefer the 'richer' harmonic distortion of these types of cabs, especially when tube driven, don't much care for the price of increased accuracy, i.e. it sounds too 'dry'/'sterile' in comparison.

GM