Help needed ! U-frame dimensions ?

[Empee]

Hi all,


I got a pair of very cheap "woofers".

I put "woofer" between quotation marks because they have
a Qts of 1.009......... not much of a woofer, èh ?

Anyway, here are the rest of the T/S parameters:

Diameter 8"
Revc=7.9 Ohm
Fo=47.634Hz
Sd=15.416msqM
BL=3.851TM
Qms=4.621
Qes1.291
Qts=1.009
No=0.376%
SPLo=87.8dB
Vas=46.507mcuM
Cms=1.378mM/N
krm=170.017uOhm
Erm=1.003
Mms=8.101g
Mmd=7.000mkg
Kxm=2.701mH
Exm=0.792

Now, there are not much options with a driver like this,
except open baffle / dipole frames. Now I got a nice 8" FR speaker
(vintage Onkyo) to put above it, so I'd like to make a U-frame with
the "woofer" near the floor, and the Onkyo at approx. 90cm. from the ground
(ear-level-when-seated).

I was thinking something like the sketch below.

Who van help me on the dimensions ?

Cheers,

Empee

[Kensai]

MJK used 7.5" deep frames in his article about them. I used folded wings of the same depth for awhile with my RS100s (~3" aluminum Dayton drivers) and they helped the little driver hit 70Hz with reasonable slam. I figure if you're using real wood and just want stock pieces to use, 8" lumber should be just fine (really 7.75" since everything is planed down at the mill). And I like your design. You'll get good reinforcement from the floor, and as long as you're sitting back from the speakers a bit, integration should be fine. Also, it kinda looks like a cheapo Orion

Kensai

[holdent]

I modeled the driver using MJK's MathCad worksheet for U-frames. I would suggest that the U-frame be 10" x 10", 6" deep (inside dimensions) using a 2nd-order Linkwitz-Riley xover at 150 Hz. The actual rolloff will occur at 300 Hz at roughly 7 dB per octave. At the low end the rolloff will occur at around 50 Hz. Bad news is that the sensitivty of the combination driver/U-frame will low at roughly 80 dB/watt.

You'd probably be better off going with a larger OB, and as Kensai notes, with "wings" by the woofer. It won't go quite as deep but will be more sensitive.

[Empee]

Hi all !

thanks for the replies !
In the mean time I read a lot of sheets by MJK and others,
and found that the so-called "woofer" isn't the best option
'cause it's sensitivity is far less that the fullrange 8"...

Yes I could temper the fullranger but since I play with SE pentodes
with about 4watts I do not see the point in doing that.

Now I got a pair of 15", unbranded, but measurement (by myself)
say a Qts of 0,35, Fs of 25... Maybe I could use that on bottom,
although I understand a high Qts is better suited for OB.
Maybe I could get the Qts up with a series resistor on that 15" woof ?


Cheers !

Empee

[holdent]

I understand a high Qts is better suited for OB. Maybe I could get the Qts up with a series resistor on that 15" woof ?

If you stick with Martin's approach you need a woofer with a much higher Qts than 0.35. You can use your woofer in an OB but it will require significant amount of EQ to produce usuable low bass. MJK has a great paper on OB design here http://www.quarter-wave.com/OBs/OB_Design.pdf that discusses this.

Adding a series resistor can bring up the Qts but will also lower the sensitivity of the driver. I don't have access to the MJKs worksheets right now but it wouldn't surprise me if the resister had to be greater than 10 ohms.