A7 vs Model 19 for home HF use

[VolvoHeretic]

Quote" These drivers were originally designed to be driven with a ~matching impedance, ~doubling its Qts which if done with the proper coupling results in a ~current driven system where the speaker’s frequency response ~ mirrors its varying impedance, ergo can have considerable LF efficiency if tuned to Fs in a sufficiently large cab.

If some form of EQ is used with a very low output impedance, then more power is required, though with an acoustically large cab’s extra tuning capability it can be better blended to the room’s gain curve, reducing power/excursion consumption.

With all that in mind, using GPA’s 604-8G II specs, a T/S max flat alignment = ~33.9 ft^3 net Vb/24.8 Hz Fb, but for most efficient electrical coupling it would have to be slightly under-damped with an actual Fs tuning. These tunings though would typically be too high and may even need to be sealed for best room blend.

The pioneers decreed that the ‘best’ vented alignment was a net Vb that increases a driver’s Fs 1.56x in a sealed cab, then vent it to an Fb = Fs alignment or ~ 11.35 ft^3 net Vb.

Of the two, the larger cab has a bit more output around/at Fs which may/may not be worth the all the extra size/cost to the builder depending on room conditions and whether or not it’s misaligned to a lower Fb.

FWIW, in my own builds/recommendations, I generally ‘draw the line’ at Vb = Vas in these situations as an acceptable trade-off or ~16.35 ft^3 in this case.

GM"

There might be a lot of typos (in my understanding of) in that quote, but I love my A7-500's. But let's suppose I wanted to modify them. If I screwed on a sheet of plywood onto the fronts and increased the Vb to the exterior dimensions of the boxes to something like 12.5 CF (or what ever, since i can't remember), would that be perfect with the right sized port for inside listening? How wound something like that affect the horn bell box loading? Would the horn bell matter for interior dimensions?

[GM]

Typos? Please elaborate.

Understood, this was my first pair of Altec loaded DIY speakers.

Not sure I follow??. Cover up the horn mouth, vent area? What horn loading in this case? Best to make a drawing???.

Regardless, if the horn cavities aren?t blocked off [825 cab], then it already has ~12.66 ft^3 of gross rear chamber volume [Vb]. The later 828 is reduced to ~11.4 ft^3 IIRC.

Ideally you need to measure both your driver?s specs and the room?s gain curve to find the speaker alignment?s closest match, so normally you make it probably too big and fiddle with EQ and/or vent tuning till it sounds good to your ears to at least driver Fs or down to ~16 Hz if you want to get the most out of the few pipe organ symphony CDs available.

AFAIK, only the 411 can do this without corner loading, so you would either need a couple of good, solid corners [outside walls] and even then you ideally need to double up the woofers as me, AB and a few others have done to go ~?live?. Otherwise, best to use a separate sub system and optimize the mains for max mid-bass efficiency. The downside is getting an efficient and/or powerful enough one to keep up.

GM

[VolvoHeretic]

Sorry GM, what I ment was that I didn't understand a word of you post. Does the outside (the room) of the speaker affect it's inside tuning, box size and port tuning? I was thinking that for an experiment, I could clamp on a new woofer baffle onto the outside front of the A7's, but the sound output coming out of the back of the woofer would have to pass through the horn bell and existing baffle to reach the interior of the box. I suppose I could just turn the speakers around and mount the woofer and round tube port in the (a new) rear access panel along with covering the front of the speaker, but then the horn would act like some kind of Helmholtz resonator. I calculated the overall inside volume of the A7's once, but can't find my figures, but if its 30" x 42" x 24", it's something like 17.5 c.f. minus the thickness of all of the panels, speaker, and port. Good idea to try? Bad, stupid?

[GM]

Bummer, oh well????

Yes, it does in that down low, the room dominates due to its complex structure of
eigenmodes which in turn affects the speaker?s response at the listening position [LP] or as you walk around the room.

OK, so you do want to mount the driver in the mouth of the horn. This is not a good plan nor mounting it on the back. I don?t have a speaker program loaded that will sim it, but in either case the horn will act as a resonant chamber that will add a deep notch somewhere in the speaker?s upper bass/mid-bass/lower mids response depending on where you place the woofer.

You need to also subtract the horn?s net volume [Vb] to find its reflex Vb.

GM

[cradeldorf]

I guess my question would be what's wrong with them that you would want to make this change? what aren't they doing that you would like them to do?

[cradeldorf]

Should be over 100 dB and some women can get into the 120s in a scream.

GM

Some little girls could probably reach 200 DB with their screams....LOL not that I would know anything about that...Eeesh now I sound creepy.

[VolvoHeretic]

I guess my question would be what's wrong with them that you would want to make this change? what aren't they doing that you would like them to do?

Nothings wrong with them, I do notice the low bass is lacking although the EQ takes care of that nicely and I also plan on reducing the reflex port volume. I was just wondering if they could be improved with an increase of Vb, since I don't really need the horn loading mid range boost and the 511's are already attenuated a lot. I don't see a sub in their future. They are positioned about 4 feet each way out from the corners measured at the outside corner of the speakers (closest I can get them).

" Some little girls could probably reach 200 DB with their screams....LOL not that I would know anything about that...Eeesh now I sound creepy."

I was at a Rush concert and Cory Heart was the warmup band, when He started singing "I wear my sun glasses" the sound coming out of 3000 14 year old teenage girls screaming almost split my head.

[cradeldorf]

I was at a Rush concert and Cory Heart was the warmup band, when He started singing "I wear my sun glasses" the sound coming out of 3000 14 year old teenage girls screaming almost split my head.

All I can remember is seeing those old Beatles clips of them showing up in the Limo and 800 crazy teenage girls screaming at the top of their lungs...I'm surprised the sound didn't shorten out the electrical system of the Limo.

[GM]

Some little girls could probably reach 200 DB with their screams....

They can't scream nearly as loud as an adult, but they do it at a higher frequency and pitch that we adults are particularly sensitive to.

GM

[GM]

I was just wondering if they could be improved with an increase of Vb.........

They could, reducing the need for digital EQ, ergo less distortion, but we're talking a cinema sound reflex's ~37 ft^3 cab here [assuming originals or at least properly re-coned] tuned to at or below Fs. This big typically are built into room corners. An A7's 30" W x 24" D would need to be 100+" tall, so normally would be folded in half and still ~ as tall as an A7-8 with the horn on top except wider and/or deeper. Even if re-coned with a newer kit, it would still be up around 27 ft^3, but without measured specs of your drivers, no way to know where the point of diminishing returns is WRT cab net Vb.

GM