Nelson Pass: The Slot Loaded Open Baffle Project

[pengesluk]

Originally Posted by Octavia

Damn you got a extra Accuphase A50V in your surplus box??!!!! What the hell else do you have in the surplus box?!

Actually, I'm trying to look more "outside of the box" these days...

[Octavia]

Touche

[mortron]

In regards to the slot loaded baffles at Burning Amp this year, did anyone get the 411 on the design of his speakers with the slot on the bottom. Going by his article, the slot is 1/3 surface area of the woofer. My question is how important are the size of the side wings? Or how much do they contribute? Reason is, we are just working with materials we have so there are two 18x42" panels, two 10x36"panels, and two 48x7.5" pieces. The rough sketch is here, a smaller version of those burning amp speakers Papa had, but I am not sure what the wing size should be, or how important it is. Also, is tilting the baffle back simply a matter of changing the angle of the wings where they fasten to the baffle?

[CLS]

I've tried a similar one, with 2 problems:

1. the deep slot forms a strong 1/4 W.L. resonance, deep notch filter is needed if you play it up to midbass.

2. the cone (and air load) moving up and down, causing strong vibration to the whole base, efforts on isolating that from floor and other part of the speaker would be needed.

I dropped that eventually.

[StigErik]

Let me say that I have not read the whole tread, but would like to share a few observations.

A friend of mine build a prototype SLOB with two 18", compression ratio approx 1:10.

We did notice that the quarter wave resonance in the slot was a big problem, showing a large response peak and also a impedance peak around 200 Hz.

There were two surprises: Fs of the driver was pressed down from 37 Hz (free-air) to 24 Hz in the SLOB. The other was that there was no gain in efficiency at distances above 1 meter. In the near-field we measured 9 dB difference between from and back, but at 1 meter there was no difference at all.

All in all we concluded that the SLOB's best feature is smaller size compared to a H- or U-baffle.

[markus76]

Correct. The main feature of a SLOB or Ripol is the decrease of Fs (about 10Hz) and the small size. The efficiency actually decreases.
Axel Ridtahler holds a patent on the design.

I'm currently building two of these from 4 SLS-12 (the SLS-10 would work too).

[StigErik]

We did not compare with a H-baffle of the same size, but its interesting to hear that the efficiency of the SLOB is lower. Anyway, we concluded that - if size does not matter - a regular U-baffle would be better efficiency-wise, and the H-baffle possibly the best overall solution, because of the symmetric front/rear polar pattern.

We also saw that above the first dipole peak (around 150 Hz), the FR of the SLOB was total rubbish, as with all dipoles.

[CLS]

Wow, 10% Sd slot is a severe compression in my experience. After some trial & error, eventually I took 40% of Sd for the slot area, bigger than 30% some which Nelson Pass suggested. 10% is a big surprise to me. (BTW, my 40% Sd slot reduces the fs from 27Hz to 16)

Smaller slot tends to suppress the upper half of frequency response, so, relatively the low end would be more pronounced. But there's also a sense of heavy-footed which seems independent to the EQ'ed repsonse. For this reason I didn't make them too small.

I can't put this in numbers, but it's there. Probably related to the driver and the room, I guess.

[StigErik]

We did not see any sign of suppressed upper end, assuming "upper end" to be the octave below the dipole peak (150 Hz in this case). The response looked very much like expected from a regular dipole, and at least 30 dB EQ would be required to hit 20 Hz flat.

[markus76]

The driver parameters probably didn't match the requirements for such a design.