Why do my 288-8Ks sound like cr@p?

**vuki** · May 28th, 2012, 02:57 AM

Coen: this could help you.

**Coendert2011** · May 28th, 2012, 09:57 AM

Originally Posted by vuki

Coen: this could help you.

Thanks, i ran into that site yesterday. I've made some uncalibrated measurements with ARTA and the behringer ECM8000 to get the big picture. So far pretty consitent with AB and Earls measurements. A bump aroud 650Hz, a deep notch at 8kHz and a strong resonance around 12kHz. I will post some pics after calibration.

Coen

**Altec Best** · May 28th, 2012, 10:48 AM

These are the frequency response graphs of the 288's I measured on a 511E horn for my DIY project that I'm building. These graphs are both the 0 degrees on axis measurements.They both have the 23763 diaphragms.While driver #2 has the 8 Khz notch, driver #1 does NOT... It also has a pretty flat response.

I was told when I bought them from an attorney handling the estate of an Altec engineer both drivers had a pretty close matched set of diaphragms.One pair received the 23763 and the second pair received the 25884 and that they were rebuilt and never put into use after rebuild.And I have to agree with that statement.

**Panomaniac** · May 28th, 2012, 01:02 PM

Both mine have the sharp 8Khz notch. What you see above is smoothed, so the notch appears wider and shallower than in a raw plot.
Sure wish I could figure out why one of your drivers dos NOT have the notch!

**Earl K** · May 28th, 2012, 02:00 PM

Pano,

Just how deep is the 8K notch on your drivers ?

Here's a close look of mine . The 8K notch is only +/- 2.5db ,, which is nothing, really .

Another ( higher rez ) shot ( raw file//no smoothing ) ;

**Coendert2011** · May 28th, 2012, 02:52 PM

Originally Posted by Altec Best

These are the frequency response graphs of the 288's I measured on a 511E horn for my DIY project that I'm building. These graphs are both the 0 degrees on axis measurements.They both have the 23763 diaphragms.While driver #2 has the 8 Khz notch, driver #1 does NOT... It also has a pretty flat response.

Interesting. So what are you saying? The notch is related to the diaphragm? That would be nice

. For now my units seem a lot rougher. They're the radial tangerine ones though....

Those are some nice 288-Cs you're having there. Maybe I should get some of those, only if they wern't so expensive.

I was told when I bought them from an attorney handling the estate of an Altec engineer both drivers had a pretty close matched set of diaphragms.One pair received the 23763 and the second pair received the 25884 and that they were rebuilt and never put into use after rebuild.And I have to agree with that statement.

Any preference toward one of the diagrams? Those graphs are shure good reference material. Did you put the mic close to the horn (like 2-6 inches) or at a 1meter (40 inch) ?

Coen

**vuki** · May 28th, 2012, 06:05 PM

Make a driver+ horn measurement with IMP function at 1m. Make SPL measurement of the whole loudspeaker at listening position with pink noise as source signal and with averaging.

**Panomaniac** · May 28th, 2012, 06:44 PM

Pretty damn deep on mine. Here is the raw response on a 1005 horn
288-1005.png

**Coendert2011** · May 28th, 2012, 06:52 PM

Here are some graphs with calibrated in and output. Never mind the details, for now this is about the big picture. I couldn't reproduce the graphs of yesterday night, that might be the result of the calibration. Oh and no mic FR compensation.

I have measured (coarsly) with two different methods and the results are very similar. Even when the difference in mic distance is taken into consideration.

288-8k-1-Imp-25cm-SC-003.jpg
25cm from the horn, Log sweep, Impulse setting in ARTA. 100dB is -below- the noise floor of the 16 bit dac. For starters I used the mobo codec. Very little distortion (0,03%), limited DR (16 bits).

and the second one with noise and RTA (averaged).
288-8k-1-Fr1-15cm-SC-002.jpg
15cm from the horn, RT Freq analyser (noise cut off at 300 Hz)

By no means does this resemble any of the above measurements. Different horn and different driver though. Probably better matched.
Looking at my graphs I'd say this response could have been designed as a compensation for the natural HF fall of CD style horns... Just wyld guessin' here, OTOH they have been reported excellent on tractrics style horns.

So what should I think of this? Any quick tips to get a better measurement or does this indicate something?

Thanks,

Coen

**Earl K** · May 28th, 2012, 07:30 PM

Originally Posted by Coen

>snip< So what should I think of this? Any quick tips to get a better measurement or does this indicate something? >snip<

If what you just posted can be believed / then yeh / that's going to be pretty unlistenable ( IME ) .

The problem I have now, before I can comment on the 2 response curves that you've shown, I need to be convinced that the data you just got is worthwhile .
I ( we ) don't want to be chasing phantoms ( that are actually caused by some fault in the setup process / such as, inline EQ in the test chain ) .

How about measuring something else & then posting those results ( like some 2-way HiFi speaker that you figure is smooth & is enjoyable to listen to ) .

How are you driving signal into these devices ? ( it should be with a simple basic amp, plus a 40 to 50uF in line cap just before the driver offering DC protection ) .