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Multi-Way Conventional loudspeakers with crossovers

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Old 5th June 2013, 02:14 PM   #11
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Quote:
but the distance between the mid and the compression driver in the Synergy is far more than 1/4 wave at the xover point of the compression driver
You are mixing things up. The 1/4 wavelength rule accounts for drivers of the same type/frequency-range. The distance between drivers of different working range is determined by the group delay of drivers and crossover.

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Charles
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Old 5th June 2013, 03:24 PM   #12
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Except if both drivers are radiating at the same frequency, then the distance between them, including group delay and xover phase flips and time delays, if not properly set will certainly cause lobing, which means variations in the polar response in the axis that is lengthwise between the two drivers... like at the xover region.

As far as WTW systems, they suffer from this very problem, thus the interest in full range and coaxial drivers - an attempt to obviate this issue.

But regardless, take the relationship between the mid and woofer, the question is by exactly what means do the mid and woofer's output sum coherently given their physical offset along the mouth of the conical horn? I'd find it hard to believe that group delay or the delay/phase flips of the xover can account for the differential. The reason I am skeptical is that one does not need or see such an offset when (for example) one stacks a three-way system with individual boxes (permitting physical positioning). So why the difference?

Or stating it the other way, why don't all the drivers enter at or near the throat, as per earlier art?

Is an all-pass delay used in the "passive" Synergy xover??

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Old 5th June 2013, 04:04 PM   #13
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Hi All
Bear, that is correct, it is the mutual radiation which the drivers share when closely coupled which raises the electroacoustic efficiency of the direct radiator and it is the increase in radiation resistance which causes a horn to have a higher efficiency. For multiple drivers to coherently drive a horn as one source, that spacing must also be recognized.

Dumptruck poses questions;

#1, yes the physical / acoustic spacing MUST be held (that ¼ wl spacing or less) in order for the sources to radiate spatially as one acoustic source. With a larger spacing than about 1/3 wl, then the sources radiate independently, the mutual coupling is gone and what you get, depends on the vector sum of the two sources.
As you move around the speaker, whenever the difference in path length is N X 1/2wl, one has a cancellation notch and null in the polar pattern. In commercial sound while unavoidable using multiple sources, an interference pattern is very bad because the lobes that point out in the wrong directions (not the listening area) excite room sound and the directivity (energy in the right direction compared to energy going everywhere else) directly relates to being able to understand random words etc (and preserving information related to the stereo image in the recording).

#2, in all “normal crossovers” like those with names, once past first order, the summed output has an “all pass” phase response. That is a flat amplitude but phase rotation equal to the number of orders times 90 degrees. This places the upper and lower crossover outputs at two different times (hf first, lf later) and this can be seen looking at the Group delay of any simple crossover. Most crossovers do this and is normal behavior.
For the Synergy horns, part of the design uses the inverse spacing of the low, mid and high sources, conceptually like an FIR filter where passively, the hf is delayed the most, the mids less and lf the least to offset the electrical part.
This will not work with any “named” crossovers even without the magnitude and phase of the individual frequency bands added in. In this case, with all the drivers directly coupled acoustically, like signals through a resistor network, one cannot get away with an error or on gets a whopping big cancellation notch everywhere not just one spot..

This then requires a crossover who’s magnitude and phase is what is required to mate the magnitude and phases of each range and this is never a normal test book shape but one that is adapted to the conditions. How i do that has evolved but is certainly based on careful measurements of the real thing.

The SH-50 referenced that I designed in 2005 will reproduce a square wave from near perfect to fair on an O-scope, from about 250Hz to about 2900 Hz, spanning both crossovers and like most all of them, even the largest 10 feet tall with over 100 drivers, sounds like, acts like, measures like a single crossover-less driver.

A technical idol of mine was Dick Heyser, a person I only met once and didn’t really have the nerve at the time to talk to. I have had his TDS measurement system for over 30 years and the last push to get rid of the crossover phase shift was an effort to reach a goal he had set out in one of his very last papers before his death about 25 years ago.
A friend and co-worker Doug Jones knew Dick and is caretaker of the Heyser library and archives at Columbia College in Chicago where Doug (now professor emeritus) taught acoustics and was department chair.
If I had a magic wand, I would want to show Dick what we have done and took 15 years of my life to finally reach the target he drew.
Then, I would have a list of questions on things he wrote about I am still trying to understand.
The man was just plain brilliant, he saw things a different way than anyone before, maybe since.
Best,
Tom Danley
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Old 5th June 2013, 04:12 PM   #14
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I don't understand why you guys think the path lengths look so different. Like I posted earlier, I don't actually know how the relative phase ends up, but the alignment looks pretty close already to me.

The original drawing posted ignores the actual entrances to the horn. The HF is at the back and the VC is right there at the entrance. The MF is a bit forward in the horn, but its VC is also farther out from its entrance. The LF is farther forward in the horn still, but its VC is even farther from its entrances. Makes sense, no?
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Old 5th June 2013, 04:22 PM   #15
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Whoops, I've been cross-posted by Tom himself . So you can space them to inverse the phase shift of the crossover and it keeps working off axis because of the horn? That's pretty cool.
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Old 5th June 2013, 04:58 PM   #16
Zero D is offline Zero D  United Kingdom
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@ Tom Danley

Hi, Thanks for the info & links in Post # 4

I saw the square wave pics in one of your PDF's, very impressive.

Quote:
Large and not pretty.
They are as big as they need to be, so no worries there Looks aren't everything, it's how they sound/perform that's the main criteria !

Quote:
my general objective is very large scale hifi sound
From what i've read/seen you've achieved it.

So using different order Xover slopes for each set of fo's realigns the TD ?

Re sensitivity limited to about 100dB

I see. Obviously with higher dB drivers that would improve. But i imagine you've tested quite a number from various manufactures already Plus i know that there's more to good sound than just dB @ 1 W !

I can envisage the SH-50 producing Extremely convincing stereo images, & all without DSP etc =

Next time i visit the USA, don't know when, i'd Love to actually hear the SH-50's + your TH's together.

Quote:
Originally Posted by bear

Also the OP was asking about the physical distance/offset between the drivers, high, mid, low and how they are maintained in phase, since the woofer (for example) is a good distance in front of the HF driver
.

Indeed i am, Thanks

Quote:
Originally Posted dumptruck

The OP's drawing is based on centers of voice coils,
It was

Quote:
which sometimes works as a crude approximation of difference between acoustic centers,
I accept my drawn lines were just an estimate.

Thanks for your input
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Old 5th June 2013, 05:08 PM   #17
ra7 is offline ra7  United States
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One question I have, and it might be a silly one, is how does drilling tiny holes along the length of the horn allow the drivers to radiate properly? I mean, doesn't the hole, which can be thought of as a small pipe, have an impact on the frequency response? Doesn't it act as a bandpass and limit the max and min frequencies that will be passed... kind of a port in a vented enclosure?
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Old 5th June 2013, 09:00 PM   #18
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Hi
Zero, this might be hard to believe, but that hf driver is already one of the top couple most efficient drivers available at any price. It is the wide dispersion, maintained up high, which forces it down. On a horn which narrowed up high, that driver can deliver over 110dB on axis sensitivity.
Keep in mind, on a CD horn, it is usually the hf drivers sensitivity in the top octave that limits the overall sensitivity.
The holes are another part of the design. You may have noticed that as you drive loudspeakers harder and harder, they get “bright” and eventually harsh sounding. Harmonic distortion starts an octave above the real signal and extends upwards by 2,3,4,5 etc times the input frequency.
While that brightness may not be objectionable, the object here is to be a faithful reproducer and one thing that means the spectral balance should not change with level if possible.
The trapped air volume under the cone and the small holes, form an acoustical 2nd order low pass filter, like an electrical filter but in air. The object being that the harmonic distortion the drivers invariably produce, will be attenuated and not enter the horn. The effect does limit the bandwidth and that is the object of it. The idea is you DO NOT want sound the driver produces on it’s own (not part of the input signal) which is always above the electrical crossover and the short obstruction does not affect the horns radiation resistance.
Best,
Tom
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Old 5th June 2013, 09:28 PM   #19
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Tom,

why don't you do a DIY version for home use? I can't see why this would be a bad idea.
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Old 5th June 2013, 09:53 PM   #20
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^^^the million dollar question!

I've been wanting to build one for some time now... but I'm not sure my woodworking skills are up to the mark. Bill will be updating his build with available drivers. I plan to copy it.
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