Tweeter distortion

[AntM]

Does anyone know of any standard tests that can be applied to determine whether tweeters are damaged ?

The tweeters I'm using (Morel MDT32-s) sound a bit unpleasant (possibly a resonant peak making them very lispy), but I don't know whether they are damaged, or if it is a design problem.

Any help would be greatly appreciated.

[carlosfm]

At what frequency are you crossing them?
At what slope?

[AntM]

Around 3kHz, 2nd order, with a series attenuator.
The problem does not seem to particularly amplitude-dependent, so I don't believe it is due to using a Xover freq too low (also the resonant freq is 700Hz on these tweeters).

The problem is still there if I use a simpler 1st order crossover, which seems to rule out a peak due to an underdamped crossover.

When I disconnect the midbass unit, the problem is still there, which rules out driver interaction.

The drivers are not recessed, so I also wondered if the front-plate edge-effect could be introducing a resonance.

[pajazo]

The non-recessed edge of the tweeter front plate could cause diffraction anomalies in the frequency response. However I don't believe that those would be very easily detected by ear, particularly if not listening exactly on-axis.

http://www.zaphaudio.com/mtg-surface.html

[wintermute]

Have you tried them on a different amp?? Might be worth a shot. I was testing my amp today and it sounded like I had a blown tweeter, but it was actually the amp distorting badly in the high frequencies.

Tony.

[Andy Bartha]

The problem may be the attenuator. If its wire wound its inductance may be interacting with the cross over circuit and tweeter inductance. If the level of the tweeter is what you want, measure the pot and replace it with a resistor. Wintermule's suggestion is worth looking into also.

[AntM]

Thanks for the replies. I'll try changing the source, in case it is an amp problem, I'll also look at changing the louspeaker cables in case this is the problem.

Andy, the attenuator is actually just a resistor anyway, but that reminds me... time for another post on tweeter attenuators

[MBK]

Hi,

I noticed that small differences in the crossover / the frequency response over a certain frequency range can have a large influence on perceived "harshness" , while more gross aberrations go by unnoticed elsewhere.

In the 1-3kHz region, even 0.5 - 1 dB difference in level, or a small peak (1-2 dB) in frequency response, can turn the system from natural to unpleasant sounding. Given that frequency responses of real life speakers are flat at best within a 3 dB band, an unfortunate little peak might cause your problem. You might try to play with level (L-pad) or purposely tweak the crossover (read, slightly misalign it) to fix that.

If you have measurement capability, this will help identify possible targets. As for level, when I set the level in my tweeters MLS/FFT measurements were practically useless. Unless heavily averaged the differences were hardly visible on screen, and looked insignificant at any rate. But 1 dB less on the tweeter made the difference in my system.

[carlosfm]

Excellent post.
Yes, that's it.

[cocolino]

originally posted by carlosfm
Excellent post.
Yes, that's it.

I agree, that`s true and very well said.

But as AntM said "very lispy" I assume the problem being rather elsewhere.

The Morel MDT32 is a textile dome tweeter (I`m otherwise not familiar with this brand/model) and I`m sure it is coated (as almost all textile domes are coated) in one way or another to damp it`s inherent resonancies in the upper frequency range.

I have seen textil/fabric dome-tweeters where this coat failed after a few years just because of expose to sunlight (UV-rays) and thereby producing a sharp 10db (!) peak resonance at 13kHz(backed up by own measurements).

Such a thing could well produce what "AntM" described as "very lispy".

As I mentioned, I don`t know the Morel MDT32, I don`t know if it`s new or used (if it has ben exposed to UV or not) , so take this just as a wild guess.