Beyond the Ariel

[gedlee]

Originally Posted by augerpro

... you MUST be converging on the same ideal realtionship of some kind. Otherwise you would end up with a prefectly flat on axis response that may suddenly peak or dip just 10 degrees off axis, then suddenly better another 10 degrees off, etc. In other words terrible lobing. A crazy wavy mess off axis.

Such a situation is not physically possible without going to every higher and higher slopes, which can't really be done passievely. The number of possible lobes at the crossover is directly related to the order of the filter (for a fixed driver spacing) so once you pick the order the number of lobes is fixed. And remember that all of this only applies to the vertical polars in my case (NOT the horizontal ones), which I have always said are far less important.

Originally Posted by augerpro

... So...What is your ideal? Perfectly in phase drivers on axis with symmetric lobing, that gradually deteriorates off axis? If so that is pretty much the LR relationship, whether or not the actual slopes of the driver responses are textbook LR. At least that is how I look at it. What confuses me is you once said you use BW3, which at the time made sense as I fiugred you used it's lobing behavior to offset the tilt caused by the waveguide in the first place. Were you talking about using a 3rd order electrical network? As you said I don't really care about that, it's the acoustical response that I'm talking about.

I only ever said that I "typically" use electrical third order filters, but they are seldom symmetrical. "Ideal" is obvious isn't it? Flat listening axis response with as wide a lobe free polar pattern as possible. If the drivers are acoustically in-phase or not at the crossover I really don't know (but I doubt it), I don't look at phase per see. The vertical lobes are generally NOT symmetrical. If this is "pretty much the LR relationship" then its purely coincidental.

[gedlee]

Originally Posted by Lynn Olson

... the horn always adds its own sharp rolloff below the cutoff frequency, and the driver must be electrically protected from out-of-band excursion. Granted, different horn profiles have different behaviors through and below cutoff, but they all have cutoff frequencies, and resulting loss of diaphragm control below those frequencies.

Lynn

I don't think that I'd agree that below cutoff the horn "unloads" the driver - certainly not in the snse of a ported enclosure below cutoff for example. The driver is still "loaded", but it can't radiate sound hence it is ineffective and as such any signals in that frequency region are wasted excursion. That's the reason for the HP filter. But, 6 dB is remarkably effective at excursion control even if it is not so effective at SPL limitations (excursion is linear not log in nature). I have often used 6 dB filters for compression drivers as this is really all that's needed in any home setting. Even in pro the problems are usually high frequency energy above the passband from clipping, which have incredible electrical energy and burn out the voice coil. Its possible to crash a diaphragm in a compression driver, but its a fairly rare cause of failure.

[skorpion]

Sometimes I regret my playing with the great elephants as I have certainly no commerce interest to promote, but beeing that as it is, I will give some clarifying statemants around my design purposes.

First, regarding my 6 db/oct crossover for the MJAO, I tend to agree with Lynn. You can't press the MJAO above much more than 100-105 db in the lower midrange or it will go wild. But my last solution find was in fact associated with the midrange no crossover fall. I definitely heard noices that I couldn't really associate but to this effect. So I introduced a 6 dB crossover also for the HP of the midrange and this set the solution. It was measured to go with the tweeter.

As you all know OB tend to put out a, with the help of room reflections, relatively homogeneous soundfield. I usually listen to classical music and jazz. That's what I use to estimate my speaker constructions.
Most listening is on axis or at a very small departure.

Regarding Bratislav's question,

The MJOA came in as a selfcontaind project before a planned EP clone construction. During this period I was challanged to timealign the MJAO (still has to be done) and there was also presented the Vifa tweak: http://www.tweakaudio.com/EVS-2/Modd...ysics_spe.html . All in all i don't really think about these speakers as competing design. They do things different, but well, that's all.

With the regard to the explicit mentioning of Emerald Physics. They never tried the Vifas, I think that's probably why their speakers look the way they do now.

Ok guys, that was another overstatement. But my find was actually that, when properly EQed the Alphas are very solid performers up to 1 kHz and over there is not much energy actually transmitted. So the Vifas can absolutely take care of the rest.

/Erling

[Pano]

Just on a practical note: I've used the Le Cléac'h 3rd order filter for several years and it works well for me. Never tried it with a passive x-over, but that should be easy, too.

[Pano]

Originally Posted by gedlee

The driver is still "loaded", but it can't radiate sound hence it is ineffective and as such any signals in that frequency region are wasted excursion.

I know you've spoken of this quite a bit thru the threads. I would be very interested to know more. The conventional view seems to be that the diaphragm is not well controlled below the horn cut-off, so it risks damage and distortion. But is it really any less well controlled? How could this be measured?

Of course large excursion that results in no usable sound is not going to be a good thing, controlled or not.

[mige0]

Originally Posted by Lynn Olson

If I'm counting up things correctly, the electroacoustic summed response results in a 6th to 8th order highpass filter two octaves or more below the desired crossover, and a more moderate slope closer to the actual crossover.

Please allow me to re-introduce my handy tool in that context :

http://members.aon.at/kinotechnik/di...distortion.xls

If you have your horn simu already done - by Hornresp of David J. McBean for example - its a quick and powerful way to calculate XO slope needed in order to not exceed excursion limits.

All explanations on the "how to" are given in a few words on the second sheet.
The "IM barrier" is what you have to check and to make up your mind what your minimum XO requirements in a specific case may be.


Michael

[gedlee]

Originally Posted by panomaniac

Of course large excursion that results in no usable sound is not going to be a good thing, controlled or not.

This is completely the point. The load on a diaphragm for any transducer is fairly small so changes in it are not that big a factor. The problem is that any excursion below cutoff is simply a waste, and total excursion is a factor. But -6 db is an extremely effective lowering of excursion since excursion is linear. But the idea that cutoff has to be well below the crossover is a waste of space in my mind. This requirement makes the horn much larger with a much slower flare - more pressure in the horn and more distortion (although thats not a significant factor). My waveguides have the "horn cutoff" - such as it is - at just about the crossover point. So the acoustical response drops like a stone below the crossover with only a single pole electrical HP filter. The combined acoustical response does not have an electrical analogy in this situation since the HP function of a waveguide is not a simple HP filter.

[thend]

Originally Posted by panomaniac

The conventional view seems to be that the diaphragm is not well controlled below the horn cut-off, so it risks damage and distortion. But is it really any less well controlled? How could this be measured?

Measure the excursion not realy possible, but measure the distortion that can generate it's more easy.

Not with the acoustic response but with the electrical impedance measure.
See the pre-impulses of distortions.

[augerpro]

Originally Posted by john k...

I guess I don't know what you mean by driver over lap.

Just referring to the width of the band where each driver contributes significantly to the response. LR2 being much wider than LR4 for example. Thanks for the idea, I have a project right now that this filter may help deal with some issues.

[Pano]

Originally Posted by gedlee

T But the idea that cutoff has to be well below the crossover is a waste of space in my mind.

My only worry here is that phase and distortion may get "squirrely" ( to use a technical term) near the cutoff. Thus the recommendation to cross an octave above the cut off. It should not be too hard to see this in measurements, right? I'm sure you and a few other folks here have examples.

Originally Posted by thend

Not with the acoustic response but with the electrical impedance measure. See the pre-impulses of distortions.

This is the sort of thing I'm looking for. Would be interested to see the cut-off behavior of different horns. I need to do some measuring myself.