why can I hear a 1uF cap (XO=19K) ???

[bbaker6212]

When you use a cap for a 1st order high pass XO of say 19K hz - about 1uF - why is my tweeter so audible? Is it because the slow 6db drop off below 19K of the 92-94db tweeter means that it is still contributing a significant amount of sound be low 19K?
So when you choose a 1st order XO of 19K by using a 1uF cap is the XO point of 19K the -3db point or is 19K the point where the output *starts* to drop (ie the high point) ???

thanks, -Brad-

[GRollins]

19kHz?
Is this a tweeter or a supertweeter?
When you plug in the numbers and crank the handle, the frequency that comes out is the 3dB down point.
6dB/oct is a pretty gentle slope. It's only going to be down 10dB or so at 5kHz. That's not all that much. Think in terms of the speakers you see these days with 'usable response' spec'ed at +-10dB. Or, put another way, if you've got a volume knob with dB gradations on it, listen at one volume, then reduce it 10dB. The volume is clearly lower, but it's still audible.
1st order crossovers are your best bet for sound quality, but it takes a good driver to make it work...simply because the driver is left covering so wide a frequency band.

Grey

[kelticwizard]

Brad:

One thought is to ask how sensitive is the woofer or mid the tweeter is crossing into.

You said your tweeter was 92-94 dB. If your woofer or mid is 87 dB@1W/1M, then a 6 dB/octave slope would bring your tweeter to the same level that a tweeter rated 87 dB@1W/1M would be at 8500 Hz.

[bbaker6212]

Sorry, I should have gave more info. The mid-woof is a Fostex FE103Z which is rated at 92db and out to 18K but drops pretty fast after 15K - moreso off axis. I'm trying to use an Audax tweeter to pick up the slack.

[bbaker6212]

Here's the driver's graph.

[bbaker6212]

Here's my question in a nutshell...

If you series a 1.33uF capacitor with a tweeter, which means a 1st order 15K crossover point, does that mean it begins to roll off at -6db starting at 15K and going down, or that it will be at -6db (of the drivers normal response) at the 15K XO point?

thanks, -Brad-

[schizeckinosy]

It will be -3db at that point. Grey said it in his post earlier.

Cheers, Dan

Originally posted by bbaker6212
Here's my question in a nutshell...

If you series a 1.33uF capacitor with a tweeter, which means a 1st order 15K crossover point, does that mean it begins to roll off at -6db starting at 15K and going down, or that it will be at -6db (of the drivers normal response) at the 15K XO point?

thanks, -Brad-

[navin]

lets see now
-3db at 15k
-6db + (-3db) at 7.5k Hence -9db at 7.5k
-15db at 3750hz
92db-15db = 77db

however your woofer is 92db so the tweeter is contributing very little at this point. the tweeter might be audible but the woofer will definetley be dominant

[GRollins]

About the only thing you gain by having the crossover point that high is having a "seamless" midrange, i.e. one not marred by crossover points and phase related problems.
On the other hand, there are tradeoffs. I'd cross the critters over lower:
--Dispersion will be much better from a tweeter than from a mid-woofer.
--Transient response will be better if you let the tweeter handle more. The tweeter's smaller, lighter, and faster.
--Reduce the load on the mid-woof by a smidgen.
--Almost certainly lower distortion. Most cones achieve their high frequency response by utilizing cone breakup. The less you have to rely on a distortion mechanism for your high frequencies, the better things are going to sound.
--At that high a crossover frequency, you're going to get some pretty fierce interference from the two drivers, leading to very uneven polar response out in the room.
--and, uh, whatever else I meant to say but forgot...
Hmmm, going to have to see about getting more sleep.

Grey

[kelticwizard]

Another consideration is the tweeter resonance.

I cannot seem to locate an impedance chart for Audax tweeters, but other manufacturers have models without ferrofluid that have an impedance at the resonance frequency of over 30 ohms.

Some Audax models have a resonance as high as 3,000 Hz. Ferrofluid damps the resonance-I don't know if your Audax model has ferrofluid or not.

A 1 uF capacitor has a reactance of 53 ohms at 3,000 Hz.

If the resonance point is at 93 dB at 3,000 Hz-I don't know if it is-then the tweeter will be playing just a few dB's under 93 at the 3,000 Hz.

That would certainly be audible. I believe that 3,000 Hz is one of the most audible frequencies on the sound spectrum.