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In Reply to: Re: horn stuff and final conclusions posted by hancock on July 13, 2002 at 15:20:31:
Hi John!You wrote:
> > It's not the distance between diaphragms you need to worry about.
> > It's the total acoustic delay of each driver--geometric,
> > electrical, mechanical, and acoustical factors all affect this.That's right. But since some delays are fixed and others are moving, no collection of diaphragms can ever be a point source radiator. In fact, a single diaphragm cannot even be a point source beyond a certain frequency. You can "pinpoint a target," and optimize performance for phase alignment in one position. But you cannot be accurate in all positions and at all frequencies simultaneously.
> > Do you have an email address, I'll forward some articles on
> > designing crossovers for offset drivers. This really is not a
> > problem if you take the physical separation into account when
> > designing the crossover.My E-Mail address is Wayne@PiSpeakers.com . And I'd also like to invite you to read my crossover document, at www.PiSpeakers.com/Speaker_Crossover.doc .
There is no way that multiple point sources can be in phase at all locations and at all frequencies, so any solution at one frequency or position ensures that it is not correct at another. This discussion is illustrated best here , and in the "children" posts under it.
> > Since the horn provides a constant resistive impedance in this
> > range and because the horn is a constant dispersion horn, the
> > drivers will show a minimum phase response, meaning that if their
> > magnitude response is flat they will show no phase shift. Anything
> > you do to flatten the magnitude response will also reduce phase
> > shift.This is the oversimplification that makes the statement actually misleading. The Unity horn we've discussed on this thread does not provide effective loading below the midrange band because it is too small, and above the midrange band because of motor mass. It is not providing a "constant resistive impedance" except in a relatively narrow range, or a subset of the entire response curve. That's why electrical compensation filters are employed in the crossover circuit. Tom describes compression driver performance in the Unity device here .
> > The compensation network for the compression driver does exactly
> > that, it compensates for both the magnitude rolloff and the phase
> > shift induced by the mass of the diaphragm and the inductance of
> > the voice coil.You can't have it both ways. Either the filter is accurate in the time domain or it is in the frequency domain. We discussed this here . Since the filter is designed to correct the frequency domain, it makes a shift in the time domain. The apparent source is shifting away from the listener as frequency rises.
All speakers - truly, all filters - act this same way.
> > The resonance of the drivers works the same way--the resonance of
> > the midranges actually flattens response in the operating range
> > and so improves the phase response.The phase of a system at resonance moves rapidly. The Unity system is designed to crossover generally where phase is "intercepted" between two subsystems. This is not an attempt to make a correction in the time domain for all frequencies, and is rather a way to "splice" together two adjacent subsystems where they meet. This is discussed here .
> > At and below the crossover frequency, the mechanical resonance
> > induces exactly the same phase shift that a passive crossover of
> > the same slope would add.Mechanical resonance acts like a tank circuit, not a low-pass or high-pass filter.
> > I'm not sure which Unity you are talking about, the two way version
> > maintains horn loading down to 300Hz. The three ways go lower.
> > The Unities horn load compression drivers as well.We've discussed this and found that the horn unloads due to compression motor mass and it begins to act as a direct radiator just above the vocal range. The horn unloads due to size just below the vocal range. These are things that Tom has said about his device.
> > The compression drivers see a pure resistive acoustic impedance
> > over their entire operating range. The mass inductance of the
> > diaphragms as well as the the voice coil inductance that cause the
> > high frequency rolloff have nothing to do with horn loading and
> > have everything to do with the driver itself.That's right, but output of the horn system is limited to direct radiation of the compression device above 2500Hz. The horn is no longer effective, and the mass (reactance) of the diaphragm becomes significant. The horn has unloaded above the vocal overtone range.
Thanks for your civil tone in this correspondance and I look forward to your E-Mail.
But please understand that I maintain the device does not "correct phase" and Tom's comments appear to concede this with statements like "I can't do anything about the phase response of the woofer, one is stuck with that and in that case the closest one can come is to match the time /phase at and around crossover". He also has said, "as the mids are not 50% efficient and the compression driver is even less (around 20-25%), there is mass reactance showing in the acoustic phase ."
These are important technical matters, because fundamental distinctions have been claimed between the Unity design and other comparible loudspeakers, including my own. But it has been my position all along that these distinctions are somewhat exaggerated. From my discussions with Tom Danley, I can see that he has paid great attention to detail, and has no doubt made the Unity perform as best as it can. But the technical details are important to clarify, nonetheless.
Take care!
Wayne Parham
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Follow Ups
- Re: horn stuff and final conclusions - Wayne Parham 07/13/0223:23:04 07/13/02 (0)
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