Altec 511B horn compared to the GPA MR931

**Alien_Shore** · August 14th, 2012, 09:44 PM

I did some measurements tonight, since my test rig happens to be set up right now.

I took a 902 and measured from 1 meter, on-axis. Comparisons of MR931 (Blue), 811 (Red), and 511 (Grn).

54816258254 891bdba280 b

The reason for the lower response from 0 to 120Hz on the 511 is the attic fan turned off (which was running during the other two tests and contributes some LF rumble).
If I am feeling motivated this evening, I'll take some off-axis measurements...

**Alien_Shore** · August 15th, 2012, 12:29 AM

Here are the off-axis (45 degree) comparisons MR934 (Blue), 811 (Grn), 511 (Red):

GM · August 15th, 2012, 12:45 AM

Originally Posted by Alien_Shore

I took a 902 and measured from 1 meter, on-axis. Comparisons of MR931, 811, and 511.

Thanks, once again proof of what me and others say about the 902, 908 needing an 800 Hz or higher XO point for best overall performance.

GM

**aditya** · August 15th, 2012, 01:20 AM

Many thanks Allen_Shore for the graphs. These will be very helpful as references.

**theophile** · August 17th, 2012, 10:00 PM

I found Allen's graphs interesting mainly because of the contrast between them and the ones done by Zilch and posted at AudioKarma. Zilch's graphs showed the Altec horns to have a frequency response like a rollercoaster and rolling off at around 7KHz, being around 10dB down at 10KHz.

Allen's graphs are quite different.

GM · August 17th, 2012, 10:09 PM

Weren't they of the 806, which indeed is a much poorer performing driver compared to the 802, 902 series?

GM

**theophile** · August 17th, 2012, 10:36 PM

Originally Posted by GM

Weren't they of the 806, which indeed is a much poorer performing driver compared to the 802, 902 series?

GM

GM, you are indeed correct about the ones which were rolling off at 7KHz.

Zilch did some graphs of the 811/902 which indeed showed more extended response than the 806, but they were much more uneven than the ones measured by Allen.

GM · August 17th, 2012, 10:43 PM

Don't recall those nor the resolution they were measured at, which can make them look somewhat different, not to mention driver condition, especially the AlNiCo ones.

GM

**Old Guy** · August 18th, 2012, 12:33 PM

Originally Posted by theophile

I found Allen's graphs interesting mainly because of the contrast between them and the ones done by Zilch and posted at AudioKarma. Zilch's graphs showed the Altec horns to have a frequency response like a rollercoaster and rolling off at around 7KHz, being around 10dB down at 10KHz.

Allen's graphs are quite different.

Bear in mind lots of different Altec frams, even in aluminum, and as GM said, Zilch did 806's. A big thanks to zilch BTW for all he's done.

**Panomaniac** · August 18th, 2012, 02:40 PM

Thanks for the plots, very nice reference.

As a long time user of HOLMImpulse here are some tips to give you cleaner results:

Keep the volume up. It looks like you are not running these very loud, at least not compared to the noise floor. You want a good S/N ratio.
Use the high pass filter. If you look down just to the left of the impulse window, you'll see High Pass and Low Pass filters. For horns and drivers, a 200Hz HP is a good idea. No low frequencies going into the driver and HOLM won't pick up junk down there
You can start your sweep at 100Hz or higher. That's in the first tab. It's handy, but the high pass filter is the real tool you need.

I'm a little surprised to see how similar at least two of plots are. That's encouraging.