How much damping to make U frame cardioid?

[avincenty]

I built a u frame woofer sample to test with open baffle configurations. I choose to get the Eminence 15A and the plan is to EQ and biamp. I have some options for the mids and highs that I want to test and see which one makes it to my permanent setup (if I like OB). For testing I have some Greencone Sabas, AN 12 cast frame, FE126E, RS 40-1354, oval coral alnico fullrange and others. The picture shows a RS 40-1384E 5 inch fullrange.

I have already played with the woofer non eq'd just biamped and there is definitely much better bass that any other speaker I have. I have been using the recommended crossover from the Endorphin P17 OB speaker.

I have also been reading the NAO site that talks about the damped U frame cardioid setup. I want to test this as well. Since I currently do not have any damping material available, I would like to get an idea of how much I need to make it a cardioid. Do I need to fill he thing up with pollyfill? Will a breatheable sheet (or layers) of wool felt do the trick?

The interior dimensions are: 14.4Dx16Wx24H

Any assistance is appreciated.

Alfredo

[a_tewinkel]

A damped U-frame may not be the appropriate way to approximate a cardioid. At low frequencies it tends to behave as a dipole, while it's upper frequency is limited by the U-frame resonance.

Theoretically, a cardioid could be constructed with a resistance box with a specific volume which should behave as an acoustical compliance, and an acoustical flow resistor. The output of the rear wave would then be delayed by T=RC. See Frontiers for theoretical considerations.

See also Kimmo Saunisto's archive for cardioid bass experiments. I suggest you simply experiment with different amounts of damping material (preferably mineral wool).

[avincenty]

a_tewinkel;
I searched for mineral wool. I like the look of it. I may be able to just add tight fit squares of it until I get the desired effect. Got to see if the major hardware store carry it.

[avincenty]

The mineral wool is not so easy to find, it is not a hardware store item. Home Depot carries something called denim insulation that may work. Anyone use it before?

I made test baffles with driver inserts so i can test various drivers. I can accommodate up to a 12 inch driver.

Tomorrow i will listen to the Sabas.

[john k...]

FIY NaO II U-frame woofer design

The damped U is an acoustic resistance enclosure. One thing to realize that it is much beeter to use a U-frame with cross sectional area on the order of the driver Sd, or at least restrict the exit area at the rear to approximately Sd.

[avincenty]

Thanks John, i have been reading the excellent information on your site. Base on your Sd reccomentation it will be a lot harder to dampen my box. If i put a back panel with a circle the same dismeter as the driver cutout, and dampen that, it would no longer behave as a u frame, correct? Or should i just install two shelves, on top and below the driver, then dampen that?

[avincenty]

Currently listening to the Sabas... I like what I am hearing . I am currently using the Endorphin P17 crossover. But... I am just listening at low/mid level, 8 feet from speakers and in my garage that is full stuff everywhere. The next step is to move them temporarily to where I have my main setup. In order to do that I need to minimize the crossover spaghetti as seen in picture. The main setup also has my MiniDSP that I can use to biamp them.

So far what I can tell is that the speakers sound effortless. The standup bass stands out, never heard it as good.

I will get some of the Denim Insulation from HD, it is inexpensive enough to give it a try.

[avincenty]

Some more progress.... I buiit the temp crossover on a proto board and placed the speakers on my main listening room. These things are too tall. 50 inches. I may need to chop the top baffle down a bit.

I am now listening to them with a pp 7591a amp. From the listening position the sound appears to come from behind the speakers with excellent imaging.

I need to play with the placement from the back wall, they are now around 4 feet. This is where playing with the damping may allow me to place them closer to the back wall.

More to come.

[avincenty]

Prepared a few more removable baffle inserts. Next up RS 40-1354A with 1.5" ribbon tweeter and a pair of TB W4-930SF.

[avincenty]

I have been playing some more. Now I am using a minidsp crossover and biamping. For level control I am using a Sansui QS-1. It is a Quadraphonic unit but allows the control of 4 external channels. The minidsp is fed from a rec out output from my preamp, the 4 channels out from the minidsp go the the 4 channel tape PB IN on the Sansui. The lows go to a Hifimediy T2 and the highs to the 7591 tube amp.

So far I have tested (with no measurements) the Sabas, the TB, some cheapo Kenwood I "tried" ENABL'ing and the Radio Shack's. The Radio Shacks I added from behind the baffle and added a second tweeter.

The TB sound good but are missing sparkle on top. The Kenwoods surprisingly sound very good but undoubtedly the larger speakers sound more open. The Sabas sound really good but they cant really be cranked up high. The RS 40-1354A sound very good with the ribbon tweeters. The Sansui has a convenient Front/Rear balance pot that makes balancing the bottom and top quite easily.

The Minidsp is a breeze to use and sounds better than the generic passive crossover I was using.

I have done no stuffing test yet as I need to setup a measuring rig to try and follow JohnK technique. The last driver I will test is the AN 12 Cast Frame. From what I have heard now, the larger the driver the better it appears to sound. Lets see.