Free air resonance?

What is free air resonance and does it make a big difference if one tweeter's free air resonance is high or low? Would 1500 be better than 600 or vice versa? I am thinking of changing my Vifa's which came with my Hales T5's which have a free air resonance of 1500 or so to the Seas Millenium which has a free air resonance of 600.
Any explanation or advice would be greatly appreciated.
Sorry i didn't get back to you on this via email Mitch. It's been one of those weekends for me....

A tweeter with a measurably lower Fs can typically use a shallower crossover slope and / or be crossed over lower without as much of a problem. In order to achieve a lower Fs, the suspension of the driver would have to be measurably different and would also probably have greater mass. This mass could come from a heavier diaphragm and / or a heavier winding in the voice coil. Either way, increased mass is what you have to move and is directly related to transient response.

Having said that, the added weight due to increased motor structure is what you would want, not a heavier diaphragm. This is especially true of a tweeter since it has to oscillate in fast, short strokes. If the diaphragm / cone was heavier, not only would it be harder to get moving, once it was moving, it would be harder to stop. That's what overshoot and ringing are derived from i.e. lack of motor strength and internal damping.

As such if everything else looks good, take a look at the moving mass of the drivers. If the driver with the higher Fs is a sealed back design and the driver with the lower Fs is vented, that would also explain at least part of the differences in resonant frequencies. Sean
Does that mean that since I'm going from a tweeter with an fs of 1500 to an tweeter with a fs of 600 that it is a good thing? Do I have more versatility with the tweeter with the fs of 600? I ask because this is what I'm going to try.I'm going from the Hales Vifa (fs 1500) to the Seas Millenium(fs 600).
Considering the lower fs ALONE, you're better off: the lower fs tweet will be well within its linear range if the x-over frequency was chosen for a higher fs tweet. Generally speaking, the further away from fs you cross a tweet, the better it is (lots to be gained in avoiding distortion).
Going by Fs alone to predict performance will not do anything for you. Unless you're absolutely sure that the other parameters do not change, a new driver can throw off performance. The biggest change can occur if the impedance curve is different, especially at the 15khz and up range. Frequently, this is where the tweaking is done - parallel resistors, Zobel circuits etc. If such corrective crossover elements exist, a tweeter with different impedances at this range can change things for the worse - or better if the planets align - regardless of a lower Fs.
Ditto on the opinions that going to a tweeter with a lower Fs is at least a wash.

And, I am of the opinion, like Gregm, that getting the crossover point further away from the resonant frequency is a boon.

At the end of the day, unless the speaker was stressing the tweeter, you are not going to realize any benefit now, but if you ever want to reengineer the crossover, you have so much more room to play with now. You are moving to a tweeter which would allow a first order crossover, though that is not the question at hand.
I agree with all of the comments made above : )

Greg and Joe are right on the money regarding avoiding driver resonance. The further away from resonance that you can get, the better off you are.

Gs is also right on the money. He points out factors that i was trying to stress in a previous post that Mitch made regarding this same subject. That is, if the crossover has specific "optimization circuitry" for the existing tweeter and / or the impedances are different at the point of crossover, you could run into problems. Sean

There are many other factors, which need to be considered. How about Seas efficiency? If it's different from Vifa, you may change the balance of your speaker. I wouldn't change the drivers of high performance speaker. The maker definitely was matching the units or spec. ordered the units. The crossover was most likely balanced to match the particular drivers.
In the original design of the speaker, avoiding resonances is
desired for the reasons Sean, Greg, and Joe... have stated.

However, Tkwolek has a point here.

If you are going to replace drivers with drivers of a different
design - then you effectively have to re-engineer the entire
speaker system. If you are up to that task...

However, the original designer certainly took the resonance
characteristics of the original drivers into consideration
when the speaker was originally designed and made provision
for same.

Substituting drivers into a pre-existing speaker system is
probably an "iffy" proposition - unless you know an awful
lot about speaker design. It's not an "amateur's mod".

Dr. Gregory Greenman
Like i said in my second post in this thread...... Sean
While your point is well taken, Dr., with all due respect, I have upgraded tweeters many times, including this spring. Never with less than terrific results.

Replacing my Vifa D27 with the Scan Speak Revelators was a tremendous improvement in air, detail, not having the sound become irritating when the volume goes up, etc. Other than the money, there is not one facet of this changeover that took me a step backwards.

But, again, you raise a caveat.
Trelja wrote:

> But, again, you raise a caveat.

That was my intent.

I'm unfamiliar with your level of expertise - which is why
I stated it was not an "amateur's mod".

If you are a seasoned "speaker-mod" expert - then be my guest.

Dr. Gregory Greenman
Hello Dr,
Would you then reccommend that I leave well enough alone? I should note that an amp designer (the one who designed my amp in fact) replaced his Hales T5's Vifa's with the same Millenium tweeter that I am going to get. He did remove the .33 ohm resistance from the crossovers tweeter section however but he was extremely pleased with the results. What I am saying is that I am not doing this blind but following the experience of a proffesional who had success with exactly the same upgrade. The only difference is that I redid my crossovers(with success) and he did not upgrade the crossovers. I did keep the Hales T5's crossover's design and kept all the origional values.
Mitch, I am not sure what the doctor has to say, but I emphatically feel that with all of the money you have invested in upgrading the crossover and wiring, considering the pretty high level of the Hales woofer and cabinetry, the weak point of your speakers at this point is clearly, Clearly, CLEARLY the tweeter.

Tweeters are the drivers that have possibly come the furthest in the past 10 years. And, they sure had far to come. At that time, the Dynaudio Esotar, Esotec, Morel MDT33, and ScanSpeak Revelator were the best you could get. Outside of the Esotec and Morel, none could really be considered reasonably priced, but all were MUCH (orders of magnitude) more expensive than the everyday Vifa/Seas/whatever metal domes in most speakers.

In your case, unfortunately, you were on the cusp of the arrival of the better affordable tweeters, but fell on the bad side of the fence.

JUST UPGRADE THE DAMN TWEETER!!!!!!!!!!!!!!!!!!!!!!!!!!!!! You'll love it! But, as I also told you before, please give it 50 hours before you really judge it.
I think you should go for it! Those tweets will surely out perform the stock tweets in your speakers. There's just some things charts and graphs cannot explain. If you use those things the good lord gave you on both sides of your head..I think you will be alright!

Or put the other way JUST UPGRADE THE DAMN THINGS!!! LOL

Good Listening!
I put in the Millenium tweeters about 10 hours ago and am extremely impressed and excited. Perhaps my speakers won't read well on paper if tested, I don't know but the sound is more sophisticated sounding. It is not "dark" or have lower levels of high frequencies. If anything what's not there from the old tweeters shouldn't be there. In other words with my crossover mods my Hales tweeters were sounding a little harsh but acceptable. Know the sound is smoother and yet very detailed. A pluck of a bass string sounds ever more like a pluck from a bass string. Symbols and other high frequency sounds sound more natural, less hyped. Piano notes actually sound like a piano.
I am pleased. I hope I like how they burn in.
Thanks to everyone for all your help and advice.