Here is a Freq. Chart for MY 288-16K.......

**Cut-Throat** · December 31st, 2009, 01:39 PM

I am just starting to use REW and took a measurement with the microphone 2 inches out from the Center Bottom Cell of the 1005B horn. This is with the amp connected directly to the driver. No crossover used. This does not look good to me and suggests that I could use new Diaphragms. What Say you????

**bfish** · December 31st, 2009, 03:05 PM

Looks pretty normal for a 288/expo horn to me. 500-5kHz is relatively smooth, and it's diving by 7k.

**Panomaniac** · December 31st, 2009, 03:29 PM

That looks about like what I get with the 291 driver on the 803B horn. (Symbiotic)
I was hoping that the aluminum 288 'fram would be flatter.

You might try pulling back the mic a bit. And if you want to do good measurements, I can highly recommend HOLMImpulse software. Very good and it's free.
HOLM Acoustics

**Cut-Throat** · December 31st, 2009, 04:23 PM

Originally Posted by Panomaniac

You might try pulling back the mic a bit. And if you want to do good measurements, I can highly recommend HOLMImpulse software. Very good and it's free.
HOLM Acoustics

I loaded HOLMImpulse, but could not figure out how to use it. The documentation with REW was much better, so I thought I's start there. Maybe I'll try the Holm again when I get comfortable with measuring.

**Cut-Throat** · December 31st, 2009, 04:54 PM

Originally Posted by Panomaniac

You might try pulling back the mic a bit.
HOLM Acoustics

Well, I pulled the Mic out 2 feet and here is what I got. Strange to me!

**Cut-Throat** · December 31st, 2009, 04:29 PM

Originally Posted by bfish

Looks pretty normal for a 288/expo horn to me. 500-5kHz is relatively smooth, and it's diving by 7k.

Here is what someone else posted in another 288 Thread. - Looks much better to me.

forums

**bfish** · December 31st, 2009, 05:19 PM

Originally Posted by Cut-Throat

Here is what someone else posted in another 288 Thread. - Looks much better to me...

IIRC, (and as verified in the picture), that measurement was made with no cal files loaded, so IMO is of little use. It's also 1/3 octave resolution, which smooths out the peaks/dips. Still, it and both of your plots reveal the basic characteristic of a large format Altec, with the "best" BW from 500-5k, and a rolloff above that, which is the signature trend I was alluding to.

As you've found for indoor DIY measurement, little things can mean a lot.

**whitebroncoii** · December 31st, 2009, 06:50 PM

Altec always made measurements in the chamber and as far away as was possible from the mouth of the horn. This minimizes junk reflections. Due to the size of our chamber, the mic was usually about 12 feet away.