My A7 Mods for Cinema

**Horn Savant** · May 12th, 2009, 02:55 AM

Here's some impromptu Mods performed on an innocent A7 that happened to be in a theater I was upgrading:

Since this was the center channel, I wanted clean articulation for speech, and since there was a Sub running at the normal Dolby crossover freq, I first stuffed box with loose fiber fill then sealed off large port to eliminate any out of phase content. This had the added advantage of making it a pure bass horn from approx 80 hz up x'over and adding added mass/cabinet damping to the front. Then I added diagonal 2x4 to lower rear panel to further reduce any cabinet sound.

**Panomaniac** · May 12th, 2009, 10:38 AM

Did it help dialog?

**Horn Savant** · May 13th, 2009, 01:59 PM

Originally Posted by Panomaniac

Did it help dialog?

Yes, no out-of-phase junk in the LF from that gaping reflex port, also uses old N500 crossover for 500hz XO for more clarity from Lo Mass HF transducer with pressure cap removed and felt layers installed in rear cavity - system also EQ'd by system channel equalizers - all these things help

**Steve Mac** · May 12th, 2009, 11:22 AM

Originally Posted by Horn Savant

...This had the added advantage of making it a pure bass horn from approx 80 hz up x'over and adding added mass/cabinet damping to the front.

Nice setup!
Hey, I'm not understanding why you think you have an 80Hz horn
if you defeat the bass reflex portion of the cab. I think the horn unloads
no lower than 125Hz....but I'm thinking it's higher than that. Perhaps others can
shed some more light on this. Apologies if I'm mistaken about this particular setup.

**robertbartsch** · May 12th, 2009, 12:23 PM

I've not seen many cinema set ups before.

I have always wondered about the driver locations; are these typically placed directly behind the viewer screen? Since the screen is not accustically transparent, I would assume you would need a bunch of equalization; right?

GM · May 12th, 2009, 05:01 PM

Originally Posted by robertbartsch

I have always wondered about the driver locations..........I would assume you would need a bunch of equalization.....

No, they need some space for the horns to 'gell', with the largest horn mouth dictating the minimum air gap AFAIK. There may be other factors, but I'm not aware of any.

Yes, they need 'screen EQ': http://www.jblpro.com/catalog/suppor...=864&doctype=3

GM

**Vathek** · May 12th, 2009, 06:19 PM

As usual GM has said it all, but I have some measurements to show the 828/825 stepped response. The horn loads only to about 200Hz, the black line is with relatively smallish ports tuned to about 50Hz, the green line is open back. So yes, with a 80Hz crossover point there will be quite a dip in the response, which may actually be beneficial for a voice/center channel. Voices often benefit from cutting in that region, but for general use, the frequency response would be quite far from linear and hardly ideal.

GM, can you tell me more about the effect of a smaller compression chamber on loading the horn to its actual flare rate? Where would that be with the horn of a 825/828 and how large would the cc have to be? I know this is basic horn theory, but I'm sure you can squeeze it all into one sentence (you usually quite amazingly can).

http://img24.imageshack.us/img24/8076/8281md.gif

(measurement made at 1 meter, driver is a 515B in a 828 cab)

**Horn Savant** · May 13th, 2009, 01:34 AM

Originally Posted by robertbartsch

I've not seen many cinema set ups before.

I have always wondered about the driver locations; are these typically placed directly behind the viewer screen? Since the screen is not accustically transparent, I would assume you would need a bunch of equalization; right?

Yes, right behind screen which has fine perforations to transmit sound - albeit with HF loss - but holes too small to affect picture quality. Processing amplifier in booth has special boost of 8k - 12khz to compensate for screen HF loss.