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Old 27th April 2009, 10:12 AM   #101
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Quote:
Originally posted by Lynn Olson
Thanks, Rudolf, for posting the Linkwitz Orion curves. They look a little different than the ones I saw at the RMAF, since the curves I saw at the show had a sharp downturn below 20~22 Hz, obviously to protect the bass drivers from destruction. The mid and HF curves look very similar, though.

I'm still amazed just how much bass boost there is - at 1% or so, it's not like the bass drivers are all that efficient to begin with, and the 20 dB of EQ effectively makes them 100X less efficient. No free lunch; more EQ means more excursion, more IM distortion, and less headroom, even if amplifier power is unlimited.

The mid curve isn't anything I'd like to do with a passive crossover - the driver would only be at full efficiency (which less than 1% for the Scan-Speak midrange) from 120~240 Hz, and attenuated everywhere else.

The curves demonstrate why Linkwitz takes the approach he does; multiple op-amps and multiple amplifiers with substantial power reserves. Combine inefficient drivers with 6~20 dB of boost, and I can see why big amps are needed. Still wonder about the overall system headroom; Scan-Speaks never struck me as having all that much headroom compared to professional drivers, and EQ reduces the headroom in direct proportion to the boost.
The heavy use of op amps is why I designed the NaO II as a hybrid system with passive crossover between mid and tweeter. I was able to optimize the circuit to use only 5 op amps.

I know you don't particularly like lower efficiency drivers, but the issues with head room aren't so much efficiency but excursion limitations. As I'm sure you know the max SPL limitations are a function of volume displacement, Sd x Xmax. The driver efficiency only indicates how much power is required to push the driver to Xmax. The potential benefit is high efficiency drivers is the lower thermal compression, all other things being equal. The question is, are all other things equal?
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Old 27th April 2009, 11:41 AM   #102
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A few days ago I said I would discuss "basket" resonances and the effect on dipole/cardioid designs. First, let me say that these resonances can be grouped in to the broader category of spurious resonances in general, be they from the basket, enclosure, baffle cut out, etc.

I set up the same circular baffle dipole SL used, a SS 8554 mounted in a 16" diameter circular baffle and took some data. This first figure shows the front and rear near field response: Red is front, green is rear. I have flipped the rear phase so that it overlays the front phase. Note that the departure of symmetry in the amplitude response also results in a departure in the phase.

Click the image to open in full size.

This next picture shows the response measured at 19" in from of the dipole in read and an ideal dipole is blue. The delay for the ideal dipole was set to place the null at the same frequency.

Click the image to open in full size.

The third figure shows the near field front and rear phase overlaid again, but with the delay determined from the ideal dipole included in the rear phase. The rear phase remains flipped. Note that the front and delayed rear phase cross (are in phase) at 1500 Hz, the notch frequency. Since the rear phase is flipped this actually means the front and rear at 180 degrees out of phase at 1500 Hz as would be expected.

Click the image to open in full size.

In this last figure the front and rear phase are again overlaid, but the rear phase is flipped back to as measured. What it shows is that the front and rear are in phase at 625 Hz. Based on the null at 1500 Hz, the dipole peak would be expected to be at 750 Hz. But the departure from symmetry shifts the dipole peak (the "in phase" frequency) lower. The phase asymmetry effectively increases the delay in that frequency range. At lower frequency symmetry is observed and the delay is reduced to the propagation delay. What this indicates is that, due to the departure from symmetry the response deviates form the ideal dipole response from differences in both amplitude and phase.

Click the image to open in full size.
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Old 27th April 2009, 12:08 PM   #103
MJK is offline MJK  United States
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Quote:
Originally posted by john k...
First, let me say that these resonances can be grouped in to the broader category of spurious resonances in general, be they from the basket, enclosure, baffle cut out, etc.
John,

Why do you thing that these departures from symmetric response are resonances?

Could they just be geometry or reflection generated?

I would have expected to see a plus and minus swing in the phase about the average if it were a resonance. Also the humps and dips look very broad which would seem to imply a lot of damping.
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Old 27th April 2009, 12:56 PM   #104
dlr is offline dlr  United States
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Quote:
Originally posted by MJK


John,

Why do you thing that these departures from symmetric response are resonances?

Could they just be geometry or reflection generated?

I would have expected to see a plus and minus swing in the phase about the average if it were a resonance. Also the humps and dips look very broad which would seem to imply a lot of damping.
Good questions. Since the driver will have an AC offset behind the baffle relative to the front of the driver, there ought to be a small time delay added to the front wave reaching the baffle edge with a lessened delay of the back wave reaching the edge prior to any effective acoustic filter delay of the back wave. But considering the wavelengths in question I would guess that these time differences are relatively insignificant.

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Old 27th April 2009, 01:04 PM   #105
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Quote:
Originally posted by Lynn Olson

The mid curve isn't anything I'd like to do with a passive crossover - the driver would only be at full efficiency (which less than 1% for the Scan-Speak midrange) from 120~240 Hz, and attenuated everywhere else.
.

Does Orion still use a 100-200Hz shelving high pass filter for the half space/fullspace transition as in Phoenix ? My guess would be yes (this part is applied to both bass and midrange):

http://www.linkwitzlab.com/images/graphics/x1.gif
http://www.linkwitzlab.com/images/graphics/x2.gif

This would make the first peak of the "M" lower, actually much to the level of the second one - thus the midrange would be significantly attenuated only in the notch portion.

Of course, still not something easy to do with passive.
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Old 27th April 2009, 01:11 PM   #106
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Quote:
Originally posted by MJK


John,

Why do you thing that these departures from symmetric response are resonances?

Could they just be geometry or reflection generated?

I would have expected to see a plus and minus swing in the phase about the average if it were a resonance. Also the humps and dips look very broad which would seem to imply a lot of damping.

What I am trying to demonstarte is that the asymmetries effect both amplitude and phase and that the effect of phase asymmetry impacts the delay with result that the dipole peak isn't neceaasrily where it might be expected form observation of the null when a circular baffle is used. FWIW, both front and rear near field are minimum phase, at lest to 5K Hz. I know what you are saying about plus and minus swings, but that isn't necessarily so obvious with a system with mutiple humps and dips. The characteristics of the humps and dips are those of the 8554. Other drivers will vary.
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Old 27th April 2009, 01:12 PM   #107
MJK is offline MJK  United States
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Quote:
Originally posted by dlr
Good questions. Since the driver will have an AC offset behind the baffle relative to the front of the driver, there ought to be a small time delay added to the front wave reaching the baffle edge with a lessened delay of the back wave reaching the edge prior to any effective acoustic filter delay of the back wave. But considering the wavelengths in question I would guess that these time differences are relatively insignificant.
An interesting plot to go along with the frequency plots would be the time trace. Then looking closely at the time trace you could try and distinquish between a resonance (ringing response) and a reflection (secondary positive or negative spike).
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Old 27th April 2009, 01:19 PM   #108
Rudolf is offline Rudolf  Germany
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Quote:
Originally posted by john k...
First, let me say that these resonances can be grouped in to the broader category of spurious resonances in general, be they from the basket, enclosure, baffle cut out, etc.
I have "researched" this for a much smaller driver, the 3" Visaton FRS 8. All measurements were done at 30 cm distance from the dust cap. Black is 0, red is 180. All SPL values are relative only, and all values are faulty above 7 kHz because of limitations of the mic.

First measurement was for the driver in free air, second mounted from the back to a 3 mm thick baffle of 13 cm width. Third mounted to a 15 cm wide ply baffle with tapered back.

Click the image to open in full size.

Click the image to open in full size.

Click the image to open in full size.

Im not sure what happens in detail, but to me it looks like the mounting situation is of much more influence than any "built-in" basket-cone relationship/resonance.
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Old 27th April 2009, 01:22 PM   #109
MJK is offline MJK  United States
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Quote:
Originally posted by john k...



What I am trying to demonstarte is that the asymmetries effect both amplitude and phase and that the effect of phase asymmetry impacts the delay with result that the dipole peak isn't neceaasrily where it might be expected form observation of the null when a circular baffle is used. FWIW, both front and rear near field are minimum phase, at lest to 5K Hz. I know what you are saying about plus and minus swings, but that isn't necessarily so obvious with a system with mutiple humps and dips. The characteristics of the humps and dips are those of the 8554. Other drivers will vary.
John,

I don't disagree. What bothers me is the instant assumption that many DIYers make that they are battling a resonance. Then comes all of the snake-oil and pseudo-science required to get rid of the nonexistant resonance. The real cause is never found. Some people love complex solutions to "difficult" problems.

The latest versions of my MathCad worksheets take into account more of the enclosure and room. As a result I see many more dips and humps, similar to what you are showing, that are all reflection generated and not extra resonances. My own in room measurements also show many of the same dips and humps and seem to correlate reasonably well. That is all that I was trying to introduce into the discussion.
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Old 27th April 2009, 01:46 PM   #110
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Reflections are the root cause. Resonances are the magnification of self-reinforcing reflections.

Anyway, that the behavior of the driver I measured is resonance is clear in the time response which shows ringing as opposed to a single secondary reflection.
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