288-8k extension

[sfogg]

On 288-8k drivers on 805b horns realistically how high up should they go reasonably well before rolling off?

I've seen a few measurement where they look like they can get above 10k or so pretty flatly.

Reason I ask is I am trying the above combo and they are rolling off sharply by 6k. By 10 or 12k they aren't even registering on the RTA so they are down at least 10 or 12dB. This is measured by a calibrated 1/3 octave RTA.

The do have Altec aluminum diaphragms in them. I cleaned the VC gap and made sure the diaphragms were centered properly but it didn't make any difference in the high end extension. Is this a sign the diaphragms need to be replaced or would that be wasted money and this is just what the driver can do.

Thanks,

Shawn

[GM]

Greets!

According to my notes, the pair of 805/288B that I had back in the late '60s began rolling off at ~7 kHz, so a faster roll off with a more powerful Ferrite motor doesn't surprise me, but I don't know if any other design changes were done to offset this, so not much help.

GM

[sfogg]

GM,

OK, thanks for the info.

Shawn

[vuki]

Hi,
here is the measurement of my 288-8k in 450Hz LeCleach horn at 1m.
[IMG][/IMG]

[sfogg]

Thanks, my 288-8ks aren't getting up anywhere near that high. They are dropping like a rock at 6k and don't even register as having any output at 10k so they are down at least 12dB.

I'm still waiting on the new diaphragm from Great Plains for my third 288 to see if that will make any difference in the high end.

Shawn

[vuki]

These are Great Plains alu diaphragms. Yes, they are going high and with some correction in the crossover they even can be used without supertweeter. If you have good diaphragms and clean ferrite 288's there's no reason for drivers not to go that high.

[sfogg]

In case anyone was curious I finally received the diaphragm from Great Plains and tried it in my third 288-8k on an 805B.

My first two were rolling off hard by 6k with Altec diaphragms of unknown age in them.

However with the new Great Plains diaphragm it is much more extended. I'm getting to around 12k before it is rolling off. It are also much flatter over the range and more extended on the bottom end too. The other two were starting to roll off around 800hz or so. This one is now getting down to around 300hz then dropping hard.

The difference in listening to pink noise between the two is dramatic.

I have some quick/dirty 1/3 RTA measurements and will probably do a few 1/24 octave measurements just to have. If anyone is curious in seeing them let me know.

Need to order two more diaphragms from Great Plains now....

Shawn

[vuki]

I would like to see RTA's.

[sfogg]

OK...here are a couple of quick RTAs. Measured with True Audio on 805b horns with the mic right up to the mouth of the horn.

The old diaphragm...



The new diaphragm....



Needless to say the new diaphragm sounds much better. Not sure what the peak is about 20kHz. Could be driver ringing, might be the noise source or it might be the diaphragm rubbing slightly. I'll have to use ETF to look at some time domain measurements to see if it is the diaphragm ringing or not and will also measure my noise source with the FFT.

Shawn

[bfish]

Shawn,
You mentioned using a calibrated RTA, but both screenshots say differently. Until you load the proper mic calibration file to match the mic you're using (assuming you've already created and have loaded a soundcard calibration file), comparative results are the best you can expect, as you'll also show every peak and dip characteristic of your chosen mic. Unless your soundcard requires otherwise, you'll get the best results with in/out set to the same sampling rate, too. TrueRTA is a great tool, but like any test equipment, all components utilized must be calibrated, and that calibration subsequently verified, to produce usable and trustworthy results. If you don't use one of the microphone models that TrueRTA included a cal file for, you'll have to write your own, from the measured response of your mic from a different (preferably hardware) RTA, again with verified calibration. This procedure is likely best, as the included cal files are simply written from the selected mic's published performance, and may vary from actual, even one right out of the box.(Mine did)

I noticed the battery icon in the tray, so I assume you're running from a laptop. If so, the results can be really bad, as few even have a hard sound card. For remote stuff, I use an external (Extigy), and with it, get good enough results with slow sampling.

It's pretty unlikely your 30-pound driver is ringing, but after you get calibrated, if you still think there's something there that shouldn't be, check the driver off the horn (with mic far enough away to ensure you're not rattling the windscreen). If it's still there, verify the protective screen has no loose threads, and that the phase plug isn't loose. Same for loading cap, terminals, etc. Coil rubs from poor centering or over-tightened screws are usually very audible (read; nasty!). I found a "floating" tangerine once, but that too, is rare. If you want to compare your results to published data, you'll have to duplicate the entire test setup, whether plane-wave tube, free space, etc.