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Old 22nd April 2009, 01:52 AM   #11
MBK is offline MBK  Singapore
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Now that we saw the polars of the muffled driver compared to a large open baffle a number of next questions arise:

1- let's see the rear radiation (full polars) - it could look like anything from omni transitioning to directional regime, to open baffle, to cardioid-like

2- how do baffle size and shape change the picture assuming we leave the muffler as-is?

3- if this was just an open baffle, say a U-baffle, with similar separation distance front to rear, how would this be different?

4- if this was acting more like a "transmission line" (a damped but open box, also see current "Beyond the Ariel" discussion, say this post by Lynn Olson .

To address 1- see pic attached: sonogram shows rear radiation is indeed mostly absent, not perfectly so, but largely 6+ dB down in the range that interests me, i.e., 300-2000 Hz at most.

To address 2- I used bafle templates attached to the front of the muffler.

To address 3- I put the driver on top of a naked PVC pipe of driver diameter, 6.5", and 3" depth (as deep as the driver itself).

To address 4- I closed off the sides o the muffler, leaving only rear ventilation holes (stuffing othewise identical).
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File Type: jpg muffler no baffle sonogram gated 7 ms.jpg (62.5 KB, 1375 views)
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Old 22nd April 2009, 01:58 AM   #12
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Thank you for the clarification. Very interesting experiment indeed. Just when I thought I knew a good amount, you go and throw me a curve ball

Keep us posted.
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Old 22nd April 2009, 02:02 AM   #13
MBK is offline MBK  Singapore
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Selected full polars of naked muffler. One could call it a strongly damped asymmetrical double-8 (dipole) pattern ith the rear lobe 12 dB down over most of the range, or an imperfect cardioid. Interestingly, the rear "blade" of radiation at -180 degrees is strongest at higher (1600 Hz here), not at medium frequencies.
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Old 22nd April 2009, 02:09 AM   #14
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Hi MBK,

I'm following up your experiments with keen interest. Gotta love audiophile experiments with measurements which sometimes is a rarity.

Some question:
- How did you do the polar plots? Do you use rotating base?
- What is the gated time? ms ? Do you have a big room?
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Old 22nd April 2009, 02:18 AM   #15
MBK is offline MBK  Singapore
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Thanks for the interest! - Gating is 5 to 7 ms for the polars, that is the best I can get even with x=2m and h=2m measurement height and distance.

I measure outdoors and for the FR graphs I realized that it was better w/o gating because gating decreases the frequency resolution so much. Outdoors the first reflection is the floor bounce on grass, it's visible around 300 Hz as a dip and bump. There will be wiggles caused by plants as well but nothing serious.

Below a pic of rotating table, graduated in 5 and 7.5 degrees, base is plywood and rotating mechanism is a lazy Susan assembly that I found for USD10
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Old 22nd April 2009, 02:41 AM   #16
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The pic in post 15 shows the PVC pipe assembly BTW, a damped version which I also tried. But first things first, placing increasing baffle sizes (thin circular cutouts taped to front of muffler), what FR do we get?

Here is a combo of naked muffler - 12" baffle - 16" baffle FRs. The polars look less clean and at HF curves start to invert (off axis higher than on axis). The low end is lifted due to increased dipole separation distance, but at the price of a bump, described by Linkwitz here .

But still: the muffler improves a lot on the Linkwitz picture above (he uses a 16 in diameter baffle, so compare with the 16 in baffle FR graph attached, bottom). The resonant bump around 400 Hz is less pronounced, and e get no nulls higher up, just smooth on and off axis FR. The graphs of course are completely unequalized.
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File Type: jpg nobaf-12-16in fr combo lores.jpg (52.2 KB, 1342 views)
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Old 22nd April 2009, 02:51 AM   #17
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And how does the full +-180 degree sonogram look like? It looks as though a larger baffle makes the rear lobe increasingly stronger, i.e., the baffle dipole separation mechanics take over the muffler flow resistance effects. Picture, sonograms of naked muffler vs muffler with 12" baffle vs muffler with 16" baffle.
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Old 22nd April 2009, 02:57 AM   #18
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And how does the recommended rectangular baffle fare (to smooth out the separation distance)? I say, not very good. The FR is actually more uneven than that of a circular baffle, we see dips as well now, and rear lobe is strong.

Pic is FR and sonogram of an assembly with muffler, with frontal baffle of 12x21.5", a typical bookshelf speaker size.
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File Type: jpg muffler and 12x21.5 baffle fr sonogram combo lores.jpg (96.5 KB, 776 views)
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Old 22nd April 2009, 03:07 AM   #19
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Wow, progress. The 16 inch baffle looks great, pretty darn smooth.

Excuse my ignorance, but how is this trend affected with a smaller driver? Say, a 3-5 inch midrange...
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Old 22nd April 2009, 03:54 AM   #20
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Yeah, apologies btw for dumping all this at once, but I was trying to present a useful, i.e., coherent picture. So after many months of thinking about it and weeks of measurements finally I am getting towards some understanding, but to explain it it takes a lot of graphs

A smaller driver would become directional later. Say you see the lines diverge after 1600 Hz or so in the FR graph. That is the driver becoming more directional by itself. This starts at about 800 Hz already, pics to this effect to follow, in the graphs of the muffled driver the cardioid pattern actually masks this and makes the driver more even in its radiation pattern/power response, which is the point. A smaller driver would have FR lines widening (i.e., directivity narrowing) higher up in frequency by a ratio of (my driver = 6.5" diameter) / (your driver diameter).

Let me first address question 3- above. Just now we addressed Q 2- , what if we keep the muffler and increase separation distance. Now in Q 3- we want to know, what happens if we keep the separation distance and add a muffler. I did this using a 6.5" dia tube, 3" deep, as an "U" baffle, and compared it to the muffler with a 12" circular baffle in front. This is about equivalent to a "flattened" 3" deep tube in separation distance: 6.5+2x3" =12.5". I did that instead of just a flat circular baffle because Linkwitz showed that version already and it's not all that great, uneven and with sharp dips.

The graphs attached show muffler with 12" baffle (top) vs. no muffler, driver on undamped 3" long PVC tube of driver diameter (similar separation distance). The tube looks much more "dipole-like" in that the directivity is not constant, widens first then narrow, and is generally uneven. No obvious 1/4 effects observed btw, the tube is too shallow w.r.t. frequency band and its diameter as well is large w.r.t. its depth. I should note I tried to add damping to the tube by adding a length of cotton filled tube, 1" and 3", but this made things worse, I started seeing 1/4 resonances (not shown).
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File Type: jpg 12 in baffle muffle vs 3 in tube undamped.jpg (67.8 KB, 758 views)
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