Sometimes, the fluid if not evenly "applied" correctly (I assume)
even with new drivers, will for sure cause response issues, sensitivity and resonances and alter the tweeters Fs and distortion levels.
Read the above for an example if you wish.
I have to add, I am NOT an expert in this either (ferrofluid issues), but found the above quite interesting none the less.
It's hard for me to tell what that person did with those tweeters, they only include the measurements after the ferrofluid was removed. No way to compare to the performance of the tweeter with the ferrofluid in place.
Some comments...
This author's sighted assertions are dramatic, and unhelpful:
I proceeded to play some music. It was apparent immediately that something was not right. The tweeter sounded constipated. It’s like the highs were impeded. I don’t ever recall a fabric dome tweeter sounding this horrible before.
The author's ferrofluid observation is naïve:
I noticed the voice coil was not fully immersed in ferrofluid. Maybe that’s where the problem is.
This is actually how the ferrofluid is supposed to be, never supposed to be fully filling the gap.
And, the distortion graph doesn't look right, and even if it was is
not comparing before and after ferrofluid removal, so no idea what to make of this except the author isn't good at making distortion measurements.
Removing the ferrofluid has a very subtle effect on the measurements, may or may not be audible depending on the usage and tweeter. Old dried up ferrofluid has an effect on the measurements, and is often audible. Filling the gap completely has a large impact, and can be audible.
Most of the internet discussions on ferrofluid are hysterical and non-factual, including massive FUD on DIY-Audio on ferrofluid. The author's assertion that the tweeter sounded "constipated" sounds like they read these incoherent statements, and got a fever in their head that the tweeter was suffering ferrofluid-constipation, and couldn't listen objectively as a result, and didn't or couldn't perform proper measurements to understand if they were hallucinating.
Ferrofluid is another typical audio reproduction tradeoff. It does help with power handling and power compression, it does slightly damp some resonances. It does dry out and is sometimes extremely difficult to remove the metalized varnish. I am just realistic about what it can and can't do to the sound.