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Old 16th February 2004, 07:51 PM   #11
Zaph is offline Zaph  United States
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Default Re: I don't mean to threadjack...

Quote:
Originally posted by coolkhoa
If I were to design a 2-way based on the Seas L12 4.5" aluminum woofer, would you guys recommend the 25TFFC over the 27TFFC, 27TDC, and 27TDFC? Seeing that it's a 4.5", I don't need a low-Fs tweeter but rather want good dispersion, so I think the 25mm is better. Is this good logic?

Oh...I know this is a "fabric dome tweeter" thread but is an all-aluminum Seas system wise?
I have an aluminum cone design rule that I adhere to whenever possible. I try to place the crossover point at the frequency of the breakup node divided by 3. With the L12, that's roughly around 2300hz, a fairly low crossover point for a 4" driver. The reason I do this is to limit the audibility of harmonic distortion.

Basically, you may be able to filter out the breakup node but you can't filter out the harmonic distortion peaks because they are mechanically induced by the driver *after* the filter. Lets say a speaker has a large breakup node at 8Khz. That means a 2Khz tone is going to have a high 3rd order harmonic distortion component. Likewise, a 4Khz tone is going to have high 2nd order harmonic distortion. The peak at 8khz may be completely notched out by the filter, but harmonic distortion components at that frequency are going to come through loud and clear. Now if I cross over 2nd order LR at 2Khz, the main signal is already 6db down, taking a good chunk of the distortion harmonics with it. And then of course the 2nd harmonic at 4Khz is going to be 18db down, reducing it's audibility.

How closely I adhere to this metal cone crossover rule is going to depend how harsh the breakup node for the driver is. If it's a mild breakup node, I don't mind pushing the crossover point up a bit.

A lot of people pay no attention to a driver's distortion, and focus only on a response curve. My problem is that I've learned what to listen for, and these harmonic distortion issues annoy me when the right frequency pops up in the music. (same with energy storage issues) On the other hand, I'm also easily annoyed by harsh sounding tweeters that are crossed over too close to their Fs. I typically like the tweeter to be at least 25db down by the time it gets to the Fs. Edgy sounding brass instruments are often blamed on a harsh tweeter, (and often rightly so) but if it's a metal cone driver, don't rule out that it might be crossed over too high.

All that said, any of those tweeters would work fine with the Seas L12, but I'd cross them over differently. I'd give the 25TFFC a 3rd or 4th order rolloff, where as the others could likely get by with a 2nd order rolloff. IMHO the L12 needs a notch filter for a 2nd order slope, but 3rd order or steeper can get by without a notch. (all IMHO)
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Old 16th February 2004, 08:50 PM   #12
Zaph is offline Zaph  United States
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Default Oops

Quote:
Originally posted by kan3
there IS an edit button
I looked all over for an edit button. I see now that the edit button only appears for a few minutes after a post and then it disappears. That's not too usefull but I suppose the admin doesn't want people making outrageous statements and then going back to change them after there have been responses.

The ability to delete a message would be helpfull too. Well, I don't come around here much. I'm not familiar with this forum software and I appologize for my typing errors.

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Old 16th February 2004, 10:25 PM   #13
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Quote:
Originally posted by Zaph
....You are correct about the difficulty of designing for tweeters without ferrofluid. They can't be crossed over nearly as low and they don't handle as much power. Occasionally they will need a conjugate notch to flatten the impedence curve. On the plus side, they will have more detail in the range that they operate as a direct result of the lower energy storage.

However, don't discount tweeters with ferrofluid as being low on detail and clarity, particularly Seas tweeters. The designer's selected viscosity is often a choice made to be the best tradeoff between damping and detail. Seas tweeters have a lower viscosity ferrofluid and sound quite a bit more detailed than some Asian made tweeters that use thick viscosity ferrofluid to hide serious design flaws.
Hi Zaph, what's "detail"? If it doesn't refer to frequency response, or distortion, or frequency response + taking into account dispersion, then it must be something else. Delays and ringing caused by resonances perhaps?

Your claim that there is lower energy storage without ferro-fluid doesn't make sense. The whole point of ferro-fluid is to absorb resonant energy, and I could intuitively guess that soft domes are inherently much more resonant than hard aluminium domes. Of course, at ultrasonic and possibly at near-ultrasonic frequencies aluminium domes will have strong "break-up modes" AKA: resonances, but this is nothing compared to fabric domes which tend to have resonances across a much wider range of the audible spectrum.

Electrical compensation can be used to eliminate impedance peaks, but I don't think this will eliminate the associated mechanical ringing at the resonant frequency. I might be wrong though, can someone enlighten me?

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Old 17th February 2004, 01:01 AM   #14
Zaph is offline Zaph  United States
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Quote:
Originally posted by CeramicMan


Hi Zaph, what's "detail"? If it doesn't refer to frequency response, or distortion, or frequency response + taking into account dispersion, then it must be something else. Delays and ringing caused by resonances perhaps?

Your claim that there is lower energy storage without ferro-fluid doesn't make sense. The whole point of ferro-fluid is to absorb resonant energy, and I could intuitively guess that soft domes are inherently much more resonant than hard aluminium domes. Of course, at ultrasonic and possibly at near-ultrasonic frequencies aluminium domes will have strong "break-up modes" AKA: resonances, but this is nothing compared to fabric domes which tend to have resonances across a much wider range of the audible spectrum.

Electrical compensation can be used to eliminate impedance peaks, but I don't think this will eliminate the associated mechanical ringing at the resonant frequency. I might be wrong though, can someone enlighten me?

CM
Admittedly, detail is kind of a vague term, but to me the item most affecting detail is energy storage. Energy storage is just one form of distortion, as are frequency response aberations or harmonics.

It's pretty much a known fact that tweeters without ferrofluid have lower energy storage. Just compare the waterfall plots for these drivers which are the same except for ferrofluid:
http://www.seas.no/seas_line/tweeters/H1189.pdf
http://www.seas.no/seas_line/tweeters/H1149.pdf

The one without ferrofluid stops on a dime after the signal ends. The one with ferrofluid comes to a stop slower. The ferrofluid is like a shock absorber in a car. It stores the energy by resisting the compression stroke (think impulse response) and then releases the energy by slowly returning to zero. When the tweeter with ferrofluid is trying to reproduce a complex signal, waveforms are "rounded out" and information is lost/distorted. What you hear is a lack of detail.

To restate things, even these Seas tweeters with ferrofluid have impressive detail. Just compare the waterfall plot of a 27TDFC to one of those tiny Audax polymer tweeters. Tweeters with no rear chamber are high Qts drivers that require thick ferrofluid to even make them usable at all.
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Old 17th February 2004, 09:27 AM   #15
kamppi is offline kamppi  Finland
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After reading your comments and I checking of the 27TDFC data sheet more carefully, I decided that 27TDFC could be the one which I will use in my project. Waterfall figure is very nice though this tweeter has ferrofluid. Seems to be very low viscosity fluid.

Other driver will be Vifa PL18WO 09-08.

This is my first speaker project which will be fully designed by me and I think that these drivers are easy to start with.

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/jarno
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Old 17th February 2004, 10:14 AM   #16
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I have to disagree with Zaph's comments on ferrofluid storing energy. By definition something that has resistive mechanical properties such as ferrofluid will dissipate energy by producing heat, it doesn't store the energy. What ferrofluid can do is increase the damping factor. It decreases both the Qms and the Qes of the dome, so without it the damping factor would be lower and there could be more overshoot and ringing.

The 2 waterfall plots are hard to compare because on one of them you can't see the time or amplitude scale. The 2 plots could easily have a different window size which will give different results.

However, I think you are partly right; I'll explain. If the ferrofluid is too thick, the system could be overdamped, which could make mechanical ringing worse - the very problem it is trying to reduce. You have to remember that unless the membrane is made of a hard material such as aluminium alloy, in a highly optimized shape such as a dome, then it can't move as a piston except at very low frequencies. The softness of soft-dome tweeters is their inherent weakness, because is allows mechanical flexing and break-up ripples to occur even at low frequencies such as 4kHz. To absorb these resonances it might help a little bit if the soft material has a gooey coating. However, the resonances are still mostly absorbed at the perimeter of the dome where there is both electrical and mechanical damping. If there is either too much or too little damping, it is equivalent to an electrical impedance discontinuity, and there will be significant reflections. (The electrical version is an antenna where energy is lost by EM radiation, just as the tweeter's mechanical resonances are gradually absorbed when they're radiated as sound into the air.)

With soft-dome tweeters there's more to it, because at high frequencies like 10kHz, the effective mass that's moved directly by the voice-coil is a lot smaller than at 1kHz, due to the ripple-like motion of the membrane. The amount of damping therefore may be inadequate at low frequencies, and too high at higher frequencies. As seen on many waterfall plots there are a lot of resonances at very high frequencies, and at a low frequency the "fundamental" tweeter resonance is also inadequately absorbed. The best performance might be at 6 or 8kHz for example.

I'm not a fan of soft-dome tweeters for these reasons. There may be an optimum level of damping, but it is always a case of finding the best remedy to a problem that can't be avoided. You simply can't win with fabric or polymer tweeters. IMO the best option is to find a tweeter where the lowest break-up frequency is outside (or at least almost outside) of the range of human hearing. Accuton tweeters do this, so do some Seas aluminium tweeters, and probably others too.

CM
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Old 17th February 2004, 01:15 PM   #17
kamppi is offline kamppi  Finland
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After reading your comments and I checking of the 27TDFC data sheet more carefully, I decided that 27TDFC could be the one which I will use in my project. Waterfall figure is very nice though this tweeter has ferrofluid. Seems to be very low viscosity fluid.

Other driver will be Vifa PL18WO 09-08.

This is my first speaker project which will be fully designed by me and I think that these drivers are easy to start with.

Regards,
/jarno
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