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Old 21st November 2007, 06:01 PM   #51
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I appreciate all these measurements very much... and may be you could help me, since I am not an expert on designing drivers.

Comparing the on and off axis of a number of "recent tweeters" (like 6600 and HDS) I somehow get the suspicion, that the "better 3/4''-like dispersion" (which is basically non-sense to me) is achieved by designing a resonance somewhere in the 15kHz region (you can suspect that also in zaph's CSD for both 6600 and HDS I think).

Would you agree? Wouldn't that be counter-productive in the end for designing a neutral system?

I know that the relevance of the top octave is debatable. However, from my own experience in recording orchestras and from an extensive small diaphragm cardioid microphone test, I came to the conclusion, that resonances or unlinearities in the top octave can be quite crucial for the last bit of "realism" in orchestra recordings, I would say more crucial than harmonic distortion.

Thanks for answers.

Regards,
Leif
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Old 21st November 2007, 09:28 PM   #52
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Quote:
Originally posted by leifislive

Comparing the on and off axis of a number of "recent tweeters" (like 6600 and HDS) I somehow get the suspicion, that the "better 3/4''-like dispersion" (which is basically non-sense to me) is achieved by designing a resonance somewhere in the 15kHz region (you can suspect that also in zaph's CSD for both 6600 and HDS I think).

Would you agree? Wouldn't that be counter-productive in the end for designing a neutral system?

Leif
I can say with some conviction that the better Tweeters around right now, and the HDS is that at a lower price than the others, they DO sound like they have lower distortion AND they measure that way.

I am inclined to believe that it is the lower end (bandwidth wise) of these Tweeters, that is the key to lower distortion overall, but not exclusively. The better Tweeters have good low end bandwidth with lower distortion.

I like the idea of the Vifa (now V-Line) XT25 being a ring radiator. The problem lies that in the top octave even a dome tweeter has to behave like a ring radiator, the centre of the dome should be stationary and the area nearest the voice coil should radiate. It's not hard to visualise that this is then a 'ring' radiator. But will the centre of the dome now behave properly? Especially as it does not have a natural point of termination? I remember the early 19mm STC Coles hard dome tweeters used in BBC monitors, some would cut out the centre of the dome.

But this 'stationary' trick that allows a dome to behave like a ring radiator, the better materials used in the best Tweeters, well, the audible results speak for themselves.

But coming back to the XT25, being a true ring radiator it ought to have an advantage over the HDS in the top octave. But it does NOT sound wise. But XT25 has rising distortion below2-3KHz where the HDS is much better behaved. Don't forget that 2KHz 5th order harmonic distortion turns up at 10KHz, 3rd order 5KHz is 15KHz, etc. I think it can be linked to the fact that Tweeters do not like amplitude (the XT25 is worse than HDS), they don't like to behave like pistons a la bass drivers. Use the crossover to limit amplitude at its Fs using LCR Traps or Null Filters. The distortion throughout the Tweeter's range is then audibly reduced right up to the top octave.

Joe R.

PS: Re Condenser mics, I am no expert but my understanding is that some resonance is built in to get a slice of the top octave and then they fall off. This explains why ribbon mics sounds less toppy and yet have far more extended response. They are just cleaner. Same goes for the better Tweeter's. They sound kinda less 'trebly' - if you get my drift.
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Old 21st November 2007, 09:39 PM   #53
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Thanks for your reply.

Do I understand your answer correctly, that the unlinearities off axis result from interferences due to the "pseudo-ring-radiator" like behavior of the diaphragm rather then resonances

Have you listened to the different tweeters within the direct field (is this term existing in English language?)?

Have you also done pink noise listening tests?
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Old 21st November 2007, 10:19 PM   #54
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I am not aware of any ribbon that can beat a DPA 4006, but that is comparing figure 8 to omni...

To make it short, I have not completely understood how a mic is build (I am not an electro engineer and I do not have the time to read a lot in this field presently...).
I think the problems vary among the different types of capsules (pressure vs. gradient transducers, small to large diaphragm) and are specific for each. Actually, resonances are a part of building a condenser or a dynamic mic, as you stated, I remember that from an article from one of the Schoeps engineers. But it depends on the polar pattern for each capsule, if I remember correctly, where the resonances in the spectrum are placed.

I have only evaluated cardioids so far in detail (searching for perfect cheap spot mics - something that does not exist... ) and developed the hypothesis, that resonances may show up in cardioid mics also as an unlinearity in the polar pattern (especially at 180°). But I have to deal with that in detail again. Maybe someone more experienced can comment on this.
I do not exactly know about omni or figre 8 mics; those mics might have other problems based only on the specific polar pattern. So, ribbons could potentially work around these problems a little, since they are only working in figure 8, according to my knowledge.

I do not know, whether comparing a SD condenser to a tweeter is valid at all (I always thought mics may "shut-off" the outer membran part rather than the inner as you stated for tweeters).

I welcome any comment to clarify things
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Old 21st November 2007, 11:53 PM   #55
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Quote:
Originally posted by leifislive
Thanks for your reply.

Do I understand your answer correctly, that the unlinearities off axis result from interferences due to the "pseudo-ring-radiator" like behavior of the diaphragm rather then resonances

Have you listened to the different tweeters within the direct field (is this term existing in English language?)?

Have you also done pink noise listening tests?
I think I understand what you mean by 'direct field' is ON axis?

No, I don't think there is anything deliberate going on. The off axis is going to suffer from beaming effects unless we reduce the radiating area. The same applies to cone drivers. The radiating area reduces (at least in theory) above the 'knee' which is determined by the diameter. For example, a 100mm diameter the knee will be around 2KHz. If the cone is infinitely stiff the response would fall 6dB/Octave. But because the cone flexes, the response can be maintained above the knee, but the off axis will not be as successfully extended as the on axis. Dome Tweeters also reduce radiating area towards the voice coil to achieve something similar. Except the radiating area now become 'ring' like and the centre of the dome is now not terminated and thus its behaviour becomes less predictable.

The knee of a dome Tweeter is around 8KHz. The material which the dome is made of, needs inherent damping (some add doping) to control the centre of the dome. I don't think that any manufacturer enhances off axis response by using above 8KHz resonances. I can't see it work like that.

I do believe that Tweeters should have some reasonable off axis response but it is expected that it will not match the on axis. The off axis does help give a better sense of 'air.'

But I reiterate, I think distortion well below 10KHz can show up in the top octave as harmonic distortion - creating sizzle and excess energy. The best Tweeters don't sound like Tweeters.

Joe R.
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Old 22nd November 2007, 08:15 AM   #56
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Thanks for your answer. Direct-field was meant to be within the reverb radius (I googled the term and think is the translation for hall-radius). It is, where the reverb of the room has less energy than the direct beam of the speaker, I hope this is more understandable...

May be lower non-linear distortion numbers point towards a more accurate motor design of the driver; therefore when playing multiple frequencies, IMD redue dramatically. Maybe one could see clearer differences doing extensive (>5 spread over an octave?) multitone testing? I don't know ...

If I sum up would you have said, the Scanspeak ringradiator should theoretically fit your needs best, but in reality, you don't like it, true?

Best,
Leif
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Old 22nd November 2007, 02:58 PM   #57
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Default One other consideration

The off-axis for a dome tweeter is better than that of the ring because of two aspects. The dome occludes increasing large areas the more the off-axis angle increases. Prior to the reduction in radiation due to damping at higher frequencies the dome is radiating in the off-axis with reduced phase deltas due to the blocking of the occluded areas. The non-occluded areas of a dome have smaller phase deltas as well than a ring because the ring of the XT25 has very little occlusion at all.

Add to this the damping factor that a good dome includes and you have additional reduction of phase deltas in the off-axis, not just that of the on-axis. The ring is nearly ideal on-axis, but will always suffer at upper frequencies in the off-axis vs. a good dome.

Dave
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Old 22nd November 2007, 03:56 PM   #58
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Thanks for sharing your view. Taken together, it makes me wonder, whether there are other - due to the design - "negative" point for the ringradiators besides these off-axis responses?

If sitting close enough, off-axis should not matter too much...

Best,
Leif
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Old 2nd December 2007, 12:47 PM   #59
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Default Re: One other consideration

Quote:
Originally posted by dlr
The non-occluded areas of a dome have smaller phase deltas as well than a ring because the ring of the XT25 has very little occlusion at all.
Dave
Here is a curious fix of sorts:

Due to the poor of axis response of XT25, you'd think that adding a waveguide (shallow horn) would make it, at least on paper, more directional - that is what horns usually do, right? Yet the exact opposite is the case. A waveguide actually builds up the off axis response, very noticeably. If you look back with an eagle eye, I have actually given measured examples of this on this thread.

Post #50 http://www.diyaudio.com/forums/showt...78#post1172978

The difference is huge. Kinda fascinating!

It all comes under that curious broad heading of 'diffraction effects.' Right?

Joe R.
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Old 2nd December 2007, 01:51 PM   #60
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Default Re: Re: One other consideration

Quote:
Originally posted by Joe Rasmussen


Here is a curious fix of sorts:

Due to the poor of axis response of XT25, you'd think that adding a waveguide (shallow horn) would make it, at least on paper, more directional - that is what horns usually do, right? Yet the exact opposite is the case. A waveguide actually builds up the off axis response, very noticeably. If you look back with an eagle eye, I have actually given measured examples of this on this thread.

Post #50 http://www.diyaudio.com/forums/showt...78#post1172978

The difference is huge. Kinda fascinating!

It all comes under that curious broad heading of 'diffraction effects.' Right?

Joe R.
I saw that post, but didn't reply as I have only nominal experience with waveguides. The results are interesting, but I have little direct experience on which I can make direct comments.

A waveguide is supposed to increase sensitivity in the front hemi-sphere at the expense of the rear hemi-sphere (below step) and increase it in a cone (if a circular opening) defined by the waveguide geometry. As Geddes does it he's trying, as I understand it, to create constant directivity down to the point where diffraction diminishes the directivity and the dispersion starts to increase.

The source for that waveguide has an impact on the effectiveness (flat wavefront compression tweeter vs. non-flat of a dome or an XT25 for example). The XT looks to be a closer match than some domes from some of the posts I've followed. 25 degrees off-axis as in your measurement is likely, I would think, to be within the defined cone of most waveguides, so an increase in the off-axis at this angle seems quite reasonable. The dip around 750 must be some sort of diffraction issue internal to the horn (I''m guessing here), but the high end looks a lot better than I would have expected. Certainly, as you point out, most of the peaks/dips are diffraction from the internal regions of the waveguide.

The more interesting comparison to me would be 90 degrees. How much signal "illuminates" the edges of the baffle with and without the waveguide? IME the XT25 has far less energy reaching the baffle edge than most domes. This will be in the region of distances that yield diffraction signatures from about 4K down, the region where most tweeters show wide dispersion. IME the XT tweeters are more directional to lower frequencies on flat baffles.

Any baffle (or waveguide) that presents a non-flat barrier is mostly about diffraction, I suppose, though some of it must be reflections inside a waveguide, not diffraction. Interesting topic in any case.
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