Lynn Olson said:
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?
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.
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.
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.
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.
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.
An externally hosted image should be here but it was not working when we last tested it.
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.
An externally hosted image should be here but it was not working when we last tested it.
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.
An externally hosted image should be here but it was not working when we last tested it.
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.
An externally hosted image should be here but it was not working when we last tested it.
john k... said:
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.
MJK said:
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.
Dave
Lynn Olson said:
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.
MJK said:
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 k... said:
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.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
I´m 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.
john k... said:
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.
john k... said:
This is all that Linkwitz is also saying. On the page I quoted he does talk about basket structure (he does peak about low pass filter, not resonance in general), in another location that I can't find now, but I remember it well, he says it almost literally what you say here, along the lines of "the rear due to its low pass behavior exhibits a phase shift, which in turn produces the aberrant first dipole peak".
The crux of the matter is that this aberrant dipole peak is now shifted a bit toward lower frequencies while the frequency where the peak should have been according to theory, is now a bit flatter.
If you visualize this now, as a net result: as you go down in frequency, instead of dipole peak and dipole rolloff you have a shelf, then a stronger initial rolloff than it should have been (over 6 dB/oct), which later settles to around 6 dB / oct unless the driver is low Q at least. Linkwitz's curves show this kind of pattern at varying degrees, so do my dipoles, and so do my muffler's in this thread. Each looks a bit different of course.
In conclusion I think this peak / shelf / knee thing is just not that much of a surprise to me, as much as I'd like to get by without it. I don't think we have discovered anything novel so far.
MBK said:
To simple say the rear response is LP filtered by the basket structure is a gross oversimplification. I am not saying that at all and I don't see SL saying that with regards to the basket effect. For example, consider this snip form Backman:
An externally hosted image should be here but it was not working when we last tested it.
That there are such resonances is clear from just looking an my front and rear near field FR data. between 400 and 1k Hz the front response zigs when the rear zags. This is a result of some type of resonance loading the back side of the cone and impeding the cone motion, just as the cone motion is damped in a venter box, of in an undamped, open ended tube do to the 1/4 wave resonance. Additionally, the front impulse shows substantially less ringing than the rear.
Note that in my comparison the phase of the front and rear match up at low frequency, and again between 1200 and 1600 Hz. And looking at the amplitude, is this really LP filtered or just highly irregular? Than amplitudes match again at 10 K Hz. The phase really doesn't show a complete departure until above 3.5K.
Looking at my plot of the front SPL at 19" (or SL's data which might be cleaner than mine),
the ideal dipole using the geometrical delay for the 16" dia. baffle
seems to have to correct roll off. It is only the peak that is off. That is, it is only in the region of the peak where there seems to be excessive delay, at least for the case of the SS8554, as is indicated by the "burst" of phase difference between 500 and 1200 Hz. Also, where SL talks about the acoustic filter what he says is "The peak is caused by an acoustic filter formed by the basket openings and trapped air between cone and basket. This filter is the reason for the differences in high frequency response between front and rear." Note he does not call it anything else until then next paragraph, "The combined response of the two 8" drivers mounted on the PHOENIX baffle also exhibits a peak due to the basket resonators."
Anyway, it's way beyond the point to argue this further. As I said, it is what it is and the only way you know exactly what you will have in any case is to measure.
By the way, looking at my phase plots also notice how "fuzzy" the phase is in that burst. It is not apparent in the amplitude because the amplitude is 1/6 octave smoothed. But the phase is raw, unsmoothed. It is again apparent that there is a complex resonant structure here and that the phase variation is greater in the rear (green) response than in the front (red) is further evidence that this is a resonance emanating from something on the back side.
bzfcocon said:
I can only speak of the Orion Builder's Manual I saw at the last RMAF. In that manual, there was a large difference - several dB - between the lower peak of the "M" and the upper peak. The LF curve looked quite different than the curve published here, though; just as much boost - I counted more than 20 dB - but cut sharply below 20~25 Hz.
I want to offer thanks to MBK and John K for terrific, very valuable measurements published so far. Much, much appreciated.
Lynn Olson said:
Thanks for the flowers Lynn. Just two remarks re: EQ, efficiency and all.
In my post 28 the FR of the 12" muffler is pretty close to a L-R 2nd order highpass (red overlay). Save for a few wiggles this configuration could practically stand without EQ and without electrical HP section should one want to choose an L-R 2nd order crossover at 320 Hz.
The efficiency of my ScanSpeak 8543 6.5" is a bit low at 88.5 dB/2.83V/1m. I used the Scan Speak because I had it around and in other respects than efficiency I believe it is a great driver. Anyway I looked at higher efficiency alternatives. But, not few of those pro mids rated at 100 dB/2.83V, say the available 18Sound 6.5" models, do actually fall off drastically towards the mid bass and by the time you hit 300 Hz you are barely over 90 dB/2.83/1m as well.
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