Seas 27tdf vs 27tdfc

[stephenmarklay]

Why do most designs seem to use the "c" version of this tweeter. Looking at Zaph Audio Freq and Dist plots don't seem to justify it.

Having said that I don't know the first thing about this (really). But for a few bucks savings the TDF seems really nice?

[breez]

The non-C version doesn't have rear chamber, you need a separate chamber for it inside the speaker enclosure.

[Michael Chua]

Both are equally good. The main difference is, with the rear chamber, you can cross lower with the "C" (Fs=550Hz; FR=1,500-25000HZ).

The "Non-C" version has a higher Fs (Fs=900Hz; FR=2,500-25,000Hz). It'll struggle at 2K5.

I cross the "C" at 2K5 (18dB) without problems. No LCR required to damp resonance peak. No strain on tweeter.

For the "Non-C", I'll have to cross at 3K5 or higher.

[stephenmarklay]

Thanks. I have a lot to learn. I see these numbers now. I understand that and it makes sense. What I don't understand (my ignorance here) is why the "Non C" version has a flat or slightly elevated region from 1k to 2k while the "C" version seems to fall off from 1K to 2k?

[jmsent]

Originally Posted by stephenmarklay

Thanks. I have a lot to learn. I see these numbers now. I understand that and it makes sense. What I don't understand (my ignorance here) is why the "Non C" version has a flat or slightly elevated region from 1k to 2k while the "C" version seems to fall off from 1K to 2k?

This is due to the differing air volume behind the dome. The larger air volume of the chambered version results in both lower resonance and lower Q for the moving system. This creates the greater rolloff in the 1-2K region.

[jmsent]

Originally Posted by breez

The non-C version doesn't have rear chamber, you need a separate chamber for it inside the speaker enclosure.

This is not correct. The non chambered version is completely sealed from the outside, so no separate chamber is required.

[Michael Chua]

is why the "Non C" version has a flat or slightly elevated region from 1k to 2k while the "C" version seems to fall off from 1K to 2k?

It is easy to be misled by the Non-C plot. The trick is to look at the impedance curve. The Non-C has a "High Q" whereas the C version is "Damped".

When its High Q, the tweeter will vibrate quite vigorously at its resonance frq. Its strong enough to push up the curve, giving the false impression of a well behaved, extended range, response. However, that region contains all the nasties, hence the listed FR is 2,500Hz onwards. For such tweeters, a safe frq to cross is 3K5 (12dB). In fact, this is the frq they used in their power test.

The C version, on the other hand, is well damped at resonance. In other words, it is not so excitable. The main advantage is you can cross lower. 2K5 is not unreasonable.

[stephenmarklay]

Thank a lot everyone this has been VERY informative. Man you have to look at EVERYTHING!

[Smoke Screen]

There is also a 27TDC version with rear chamber but no ferrofluid.
Same fs as 27TDFC around 550Hz. Easily crossed between 2k5 and 3k.

[tvrgeek]

You say 2500 and up only second order? Been playing with a 27TBFC/G. Just seems too shrill. I have been trying to match it up to the Zaph mids. I am not quite ready to give up on metal domes as I have heard some ( Totem comes to mind) that were fine. I just have never made it there. Measure great, sound bad. Leaning back on good old HDS's. Have to admit, I have not sprung for a Millennium yet.