My A7 Mods for Cinema

[martyh45]

In the theatre world mains are run full range and the subs are only an effect channel. 100 Hz is the general LP point on the subs but taking that down to 80 cleans up the sound of the woofers. HP filters are used on the mains just to keep the woofers from unloading. 40 Hz makes sense since most of the newer mains are tuned somewhere about in that area.

[Horn Savant]

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

[Horn Savant]

In the theatre world mains are run full range and the subs are only an effect channel. 100 Hz is the general LP point on the subs but taking that down to 80 cleans up the sound of the woofers. HP filters are used on the mains just to keep the woofers from unloading. 40 Hz makes sense since most of the newer mains are tuned somewhere about in that area.

Interesting info

[GM]

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).

LOL! That's only because in my 'stream of consciousness' composing I tend to forget to put in periods!

True, limiting the LF BW below 125 Hz as well as rolling off the highs above 2 kHz maximizes speech intelligibility and I've suggested it to a number of older HT forum posters that have a hard time following dialog even with a phantom CC, but apparently they'd rather continue to either miss dialog or do as a buddy of mine does and just bitch about how poorly mixed movies have become.

Simple compression horn theory, couple an under-damped sealed chamber with an (over-damped) horn 'chamber' and when the system is balanced it extends/flattens the LF gain BW down to the flare frequency, so in the 825/828's case this is around 110 Hz, ergo when we find a net Vb that creates a 110 Hz resonant peak we should be close enough and since driver specs matter and accurately calculating net Vb is a tedious affair it's easiest to make it a little large and reduce it until the impedance trace hits the mark.

515B specs varied quite a bit over the decades, so worst case I came up with just doing T/S sealed cab calcs starts at ~29.5 L net and if HornResp is accurate then you may have to shrink it up to 10 L based on the various specs I have.

Note that with such a truncated horn it won't be flat to a 110 Hz F3 unless a much higher Qts driver and/or EQ is used, but it should have enough extra bottom end to justify the effort.

GM

[GM]

Yes, right behind screen........

As in not spaced back from the screen?

GM

[bfish]

...since driver specs matter and accurately calculating net Vb is a tedious affair it's easiest to make it a little large and reduce it until the impedance trace hits the mark...

Yep, for mine w/515-8GHPs it started with a full-width baffle to seal off the flare cavities, followed by a new bottom panel at the bottom of the baffle. Further volume reduction is easy by adding any solid material to take up space, until you get the desired result, then make it permanent.

[Horn Savant]

As in not spaced back from the screen?

GM

I put them (A7 & JBL L + R) right up to the screen . Disadvantages ? How far back would you say ? - there's plenty of room.

[Vathek]

Simple compression horn theory, couple an under-damped sealed chamber with an (over-damped) horn 'chamber' and when the system is balanced it extends/flattens the LF gain BW down to the flare frequency, so in the 825/828's case this is around 110 Hz, ergo when we find a net Vb that creates a 110 Hz resonant peak we should be close enough and since driver specs matter and accurately calculating net Vb is a tedious affair it's easiest to make it a little large and reduce it until the impedance trace hits the mark.

I knew it

This is very interesting, as I have gotten pretty tired of the standard 828 cab because of the 200 Hz bump it creates. When you look at a 515B response curve, it has a rising response to about 200 Hz from where on it's pretty flat. In a 828 cab this results in a massively rising response with actually a peak at about 200 Hz when the horn kicks in where the driver doesn't really 'need' it anymore to flatten it out. Now my plan was to get rid of the cab and probably go dual 515 non horn loaded. But if it's possible to load the driver almost an entire octave lower by adding a properly tuned compression chamber to the 828 and therefore flatten the response from about 110 Hz up, I might reconsider it.

IIRC you have a dual non horn loaded 515 setup. How would you compare it sound quality wise with such a properly tuned midbass horn? The standard 825/828 cab has just become too compromised to my ears with its roller coaster response.

Thanks!

PS: sorry if I'm hijacking this thread, but after all this is about A7 mods?

[GM]

I put them (A7 & JBL L + R) right up to the screen . Disadvantages ? How far back would you say ? - there's plenty of room.

Well, when I wrote that I was thinking about WE's 2" minimum rule and a later paper that in essence considered horn mouth end correction distances, but haven't been able to find it yet, though I noticed that Cinerama spec'd them to as close as practical without actually touching it and a later RCA doc that also specs it, so now I'm wondering when/why the switch?

As for the 'why', the only thing that comes to mind ATM is that unless the speaker cavity is quite large, such as a live stage doing double duty as a cinema, it's necessary to either fill all the screen space not occupied by the speakers with rigid/damped baffles and/or suspend a thick acoustic blanket behind the speakers/screen for best performance, so I guess to save $$$, using the relatively narrow baffles designed for stage or acoustic blanket apps plus minimal air gap was an acceptable trade-off to a more theoretically correct one. Not having ever experimented with speaker/screen spacings though, I don't know if there's any audible difference.

GM