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

**Earl K** · May 28th, 2012, 07:44 PM

1335d1338245049 why do my 288 8ks sound like cr40p 288 1005

Oh yeh, that misfit . I remember now . Obviously, that was as good as it got , right ?? ( Mind-you , looks great up to 8K )

It's seeing curves like that really damp-down my enthusiasm for acquiring new GPA diaphragms ( since the cause of that suckout is still not determined ) .

( Maybe it's a function of the 1005H horn loading on the driver/phragm combo ).

Did you ever try to fitup one of ABs diaphragms into your driver, as a "yardstick" reference ?

**Panomaniac** · May 28th, 2012, 08:05 PM

No, never tried other 'phrams other than the symbiotics that came with them. May have that graph somewhere. Jean-Michel Le Cleac'h and I went back and forth via email quite a got and never could figure it out. I just gave up and roll them off at 7K.

Coen, holy rising response, Batman! If your measurements are correct, that's got to sound awful. As Earl mentions, let's see something else to be sure. Do you have an EQ or something in front of them? A passive EQ with a big HP shelf?

**Altec Best** · May 29th, 2012, 01:19 AM

Originally Posted by Coendert2011

Did you put the mic close to the horn (like 2-6 inches) or at a 1meter (40 inch) ?

Coen

I tried to get the horn equal distance from the ceiling and the floor with Mic dead center in front of the horn.To limit room reflections.I also measured at low volume from my amp to minimize that as well.IMO it is better to keep the Mic close to speaker to minimize adding any noise into the measurements.Three feet one meter is fine,but in a living room with walls,furniture,etc... It's best to keep the Mic close to the speakers IMHO.It helps, otherwise you will need to adjust the gating on the measurements as well... Zilch used to measure just a couple of inches away.

@ Pano, WOW there is something wrong with those ! Are they all aluminum Frams ? If so what model #? It looks like that dip goes off the chart.Literally !

**vuki** · May 29th, 2012, 02:16 AM

Coen, do the IMP measurement from 1m distance without crossover (just dc blocking capacitor).

**Coendert2011** · May 29th, 2012, 04:39 AM

Thanks all for the thinking along.

My setup is quite simple. The motherboard terratec codec to a 75W Hafler mosfet amp (with attennuators and dc servo) and then to the drivers. Behringer mic -pointed straight on-axis to the hornmouth- is connected to behringer 8in-2out mixer with some gain, all eq to 0 and then to the codec again. All wiring is simple low L, low C stuff. The terratec codec can do 24/192 out and 16/192 in, not bad! Levels are quite low though:1V rms out and 1V peak-peak in.

So, as you can see no filter and no eq in the chain. I pondered a little on the results and came to the conclusion that I have been integrating reflections as well. The rta is averaged and the imp has a long sequence. So next measurement will be imp with a 3mSecond window, now I only need a meter of free space. 330Hz as a lower boundary is fine with me. I am still a little sceptical wheter this changes the FR a lot. Probably a notch and peak or two, but we'll see. I'll keep the mic close to the hornmouth.

I wondered though if the on-axis position is the best. The center fin of the horn is in front of it and the two adjacent sectors might have some lobing and peaking at certain frequencies even at a close distance. Otoh off axis measurements can probably not be compared to all other material...

One one account the measurements make sense: they sound awfull allright!
I am struggling a little with the correlation with my first auditory inspection. This seems consistent with the measurements, but should that be the case? I mean hearing sensitivity varies with frequency and loundness, right?

Thanks for the cap suggestion in an moment of unattention this may save the phragms!

Thanks, Coen

**Coendert2011** · May 29th, 2012, 05:51 AM

I found another 288-(16)K Freq graph here: MachMat mainpages.

288-103.jpg

Much smoother on the circular "Oris" tractrix horn. The black line shows the un-eqed measurement and shows the rising response up to 1KHz, The upper part looks a lot flatter though. Save for the reflections, the 511-E throat blobs may have something to do with this too.

Coen

**vuki** · May 29th, 2012, 06:15 AM

IMPulse measurement from 1m distance would tell a lot. 3ms window is just about right. Do it also with the crossover and then do the RTA of the complete loudspeaker at listening position.

Since there is clean 6dB/oct. rise in your measurement I suspect that it is something with the measurement setup (wrong generator signal?) because if corrected (by 6dB/oct.) you would get nice 288k response

**Coendert2011** · May 29th, 2012, 06:35 AM

Originally Posted by vuki

IMPulse measurement from 1m distance would tell a lot. 3ms window is just about right. Do it also with the crossover and then do the RTA of the complete loudspeaker at listening position.

Since there is clean 6dB/oct. rise in your measurement I suspect that it is something with the measurement setup (wrong generator signal?) because if corrected (by 6dB/oct.) you would get nice 288k response

Thanks,

I will fire them towards the ceiling for maximum free space and measure at 1 meter like you suggest. I have planned to recheck the complete setup to eliminate all hidden faults. The strange thing is that it is a high pass characteristing which implies a series capacitance or a shunt inductance. Where to look for those?

Fyi i used the same settings in a loop feedback configuration (as part of the calibration process) and the response was completely flat. Let's first put the amp in that loop...

Coen

**vuki** · May 29th, 2012, 06:45 AM

IMHO that high pass is too linear to be caused by passive component. I still think it's measurement setup. Try the Impulse measurement.

**Coendert2011** · May 29th, 2012, 07:09 AM

Originally Posted by vuki

IMHO that high pass is too linear to be caused by passive component. I still think it's measurement setup. Try the Impulse measurement.

Thanks,

First measurement was done with IMPulse setting. The Impulserespose was measured with a log sweep. In fact to have ARTA determine if there are any excessive distortion components. At the level of my measurements these all appear to fall into the noise floor of the ADC.

The high pass can be acoustical as well. Think of the lower horn cut-off or bass reflex which both have inductive air-impedances in their modelling. Checking all bolts and nuts is also part of the setup inspection!

Coen