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A pearl from the Bobfather
A pearl from the Bobfather
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Old 18th September 2018, 03:51 PM   #1
CharlieM is online now CharlieM  United States
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Default A pearl from the Bobfather

Hi all,
Last week I spent some time with Bob Carver at the annual Carverfest retreat in NC. We discussed the OB hybrid ESLs that I had brought with me, and Bob had an interesting suggestion which I will share below.

I had mentioned to Bob that, with hybrid ESLs, the woofer exciting the diaphragm’s resonance, and phasing errors between the two are major concerns. I had also mentioned that I use a DSP to time-align the woofer and ESL.

Bob replied that time-alignment after the fact can’t make the two drivers work together as one, if they aren’t in proper physical alignment to start with. And he followed up with a suggestion on how to find the correct physical alignment between the two drivers, in the design phase:

With ESL mounted on a baffle of design dimensions, and the woofer hand-held at its mounting location:

1. Paint a white dot in the center of the diaphragm for reference.
2. Using a tone generator; play an 18-20Hz sine wave through the woofer only.
3. Observe the white dot and resulting diaphragm motion induced by the woofer.
4. Find the point of physical alignment by moving the woofer forward or backward until the diaphragm motion stops or is maximally reduced.
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Old 18th September 2018, 08:47 PM   #2
bentoronto is offline bentoronto  Canada
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A pearl from the Bobfather
I think the assertions in the OP post need some examination, starting with why pay attention to the thoughts of Carver who seems addicted to "imaginative" theories and designs.

Next I have to say, I've always just taken for granted that DSP time alignment works roughly as advertised, at least if you do the set-up empirically.

B.
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Old 19th September 2018, 12:04 AM   #3
CharlieM is online now CharlieM  United States
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I figured this thread might spur an interesting discussion. Hopefully, others will jump in with their thoughts as well.

Bob certainly has a flair for unusual designs and catchy labels to market them. That said; over the years I've grown to admire his imagination and intellect-- especially in his Sunfire amp designs. He's definitely old school though--more often than not opting for analog over DSP.

Regarding Bob's assertion that the woofer and ESL diaphragm must be physically aligned before they can act together as a single element, I too was initially skeptical, but after further thought I'm prepared to concede that he may be right-- insofar as time alignment with and without physical alignment are not equivalent, and physical alignment may offer an advantage.

Consider the case where the woofer is rear-mounted on the baffle, as in the speaker shown below which started this whole discussion with Bob.

In this case, the two drivers are not on the same plane, and a digitally applied time delay is required to shift their outputs into phase at the listening position. In this case the woofer must start ahead of the ESL because its output must travel further to reach the listening position. I believe Bob was asserting that the drivers can't work together as one when one must start before the other, for their outputs to reach the listening position at the same time.

Now consider the case where the woofer is moved forward into physical alignment with the diaphragm. In this case the woofer and diaphragm are simultaneously in physical and time-alignment, both moving in sync, and their outputs reaching the listening position at the same time. Or in Bob's words; working together as one.

The questions now are whether the two cases noted above are equivalent, and whether case offers an advantage over the other.

Video

Click the image to open in full size.

Last edited by CharlieM; 19th September 2018 at 12:26 AM.
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Old 19th September 2018, 01:29 AM   #4
bentoronto is offline bentoronto  Canada
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Quote:
Originally Posted by CharlieM View Post
...In this case, the two drivers are not on the same plane, and a digitally applied time delay is required to shift their outputs into phase at the listening position. In this case the woofer must start ahead of the ESL ...[/IMG]
Yes, obviously requires quantum physics analysis. Or what has Carver been drinking?

Granted, alignment can only be perfect at one location; but that location is broad and the alignment only slightly erroneous elsewhere.

B.
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Old 20th September 2018, 03:34 AM   #5
stokessd is offline stokessd  United States
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I'm pretty sure there won't be a null no matter how you translate the woofer as long as the motion of the woofer is perpendicular to the plane of the ESL.

Unless you are crossing over really high, the woofer is going to be pretty close to phase aligned with the panel, due to the shortest wavelengths of the woofer being several meters long or more

sheldon
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Old 20th September 2018, 10:59 AM   #6
wolf_teeth is offline wolf_teeth  United States
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Hey, it's Sheldon! Nice to see you post, bud!

I'm in agreement with you in the above statement.

Later,
Wolf
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Old 20th September 2018, 01:15 PM   #7
CharlieM is online now CharlieM  United States
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Thanks all,
Good to know it's not really a problem if the drivers aren't in perfect alignment.
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Old 20th September 2018, 06:42 PM   #8
CharlieM is online now CharlieM  United States
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Quote:
Originally Posted by stokessd View Post
I'm pretty sure there won't be a null no matter how you translate the woofer as long as the motion of the woofer is perpendicular to the plane of the ESL.

Unless you are crossing over really high, the woofer is going to be pretty close to phase aligned with the panel, due to the shortest wavelengths of the woofer being several meters long or more

sheldon
The concern really was the woofer driving diaphragm, rather than any audible phasing difference between the two-- and whether having the two drivers in exact physical alignment would be advantageous to that. The speaker sounds really good, so the concern is more theoretical than actual.
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Old 22nd September 2018, 09:40 PM   #9
bolserst is offline bolserst  United States
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Quote:
Originally Posted by CharlieM View Post
…Bob replied that time-alignment after the fact can’t make the two drivers work together as one...
Bob is right that time-alignment by delaying signal to one of the sources will not make them work together as one the way physical alignment will. It can provided identical results for one point in space (like the listening position) which can improve things, but not all points in space. Taking your thought experiment in post#3 a bit further...think about what the signal phases would be like in the reward direction once you had them delayed for in-phase arrival in front of the speakers. Adding delay to correct things in the front will actually make things worse toward the rear! Fortunately, as has already mentioned, the physical misalignment is a small fraction of the wavelength at your low crossover frequency so not a practical concern.

BTW using electrical delay to compensate for physical separation may not be ideal for your situation, but it can be useful for creating directional speakers…in particular subwoofers. Basically, you separate two monopole woofers by a distance and then invert signal to the rear woofer and delay it by the separation distance. The result is a cardioid radiation pattern with a null toward the rear. See attached excerpt from 1972 Olson AES paper. Internet searches for “cardioid bass array” will uncover more details if interested. This is becoming a popular pro-audio solution for steering bass.
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File Type: png cardiod_sub_array_Olson.png (13.8 KB, 217 views)
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Old 22nd September 2018, 09:49 PM   #10
bolserst is offline bolserst  United States
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Quote:
Originally Posted by CharlieM View Post
…Find the point of physical alignment by moving the woofer forward or backward until the diaphragm motion stops or is maximally reduced.
Bob is not correct about the null.
See attached plot of pressure and particle velocity trends around a dipole speaker. It is interesting to see the direction of the particle velocity flip around as you move from in front to along side the woofer. If you happen to have a ribbon microphone, you can actually measure its magnitude and direction.

Notice that although there is a very clear pressure null to the side of the dipole piston, the particle velocity(what is exciting your ESL diaphragm) does not. In fact, if you look just at Vx, the X-axis component of velocity (ie the velocity normal to the plane of the ESL diaphragm) you can see that it would actually be slight larger when your ESL is in perfect physical alignment with the dipole woofer pressure null than if it was a bit forward or aft. It’s a really shallow trend though, not at all like the distinct pressure null. One big thing to note from the equations is that at low frequencies the particle velocity will fall as the cube of the distance from the dipole source. So small additions of separation distance can make a big difference in how much the woofer is exciting the diaphragm.

One further thing that interested me, was that at a 54.7 degree off-axis angle from the source Vx = 0, so that is where you would need to place the ESL to find a null, or minimum in excitation. The reason this interested me actually has nothing to do with ESLs or Acoustics, but antique radios! Back in the 1920s the “magic” 54.7 degree angle was patented by Hazeltine (US 1577421) for use in cascaded RF amplifiers in early unshielded radios.
Attached Images
File Type: png Dipole_pressure-velocity_field.png (49.1 KB, 222 views)
File Type: png neutrodyne_01.png (240.7 KB, 224 views)
File Type: png neutrodyne_02.png (182.4 KB, 224 views)
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