My A7-500s. A journey

Thanks Marvin, good to know.


I saw a posting on a French Hi-Fi site about an A7 that had been pushed to an F3 of ~28Hz. No other details. I'll have to pursue that.

FWIW, both tunings are so far above the driver's Fs and the cab is large enough that there's very little difference in Fb between the two (~55 Vs 53 Hz) and considering that in an acoustically small room it dominates down low, i.e. any typical HIFI/HT app, I'm betting that the difference is inaudible. A few of us prefer tunings from around Fs up to 32 Hz (Per Hasling has championed it on the joelist), though at one point we found 16 Hz to be a good tuning for corner loaded in a basement dedicated HIFI/HT before converting them to a two way with a 411 sub in the bottom.

GM

Hey Gee.
Could you expand a little on that, please?
Are you speaking in general, or of the 828/825 cabinet specifically?

The pair of 416-8As I'm using now have an Fs of 28 & 33 hz. Qt is 0.42 & 0.38 respectively. Running sims and assuming a 12ft box, the best compromise seemed to be in the low 30s. That puts the 2 drivers closest to each other with decent low end extension.

But I'm just not sure how these light cone woofers with a horn flare in front match the software predictions. So far, they are not matching well.

Greets!

It was aimed at the A7, but it applies to typical large, low tuned vented alignments too.

Hmm, those must be re-cones.

The horn has virtually no affect on box tuning down low with low Fs drivers, though it will affect it with higher tunings, adding acoustic mass that lowers its in-box Fs same as a BLH. What is affecting sims Vs reality is the driver/vent offset that's long enough to have 1/4 WL action mass loading the vent and the fact that the cab has a stepped flare that creates a crude BLH if no baffle boards are used, ergo a ML-horn with a vent. Typical box calculators assume a Helmholtz resonator's uniform particle density acting on the air mass in the vent.

Anyway, a ~32 Hz tuning with these specs seems a good one to try based on your room and will handle all but some pipe organ symphonies unless there's enough room gain to fill in below tuning. The big problem with these cabs is getting a well sealed cab for the vent system to work properly.

GM

Thanks GM, I'll work on it.

No idea if they are recones. Look Altec. Much lighter cones than my GPA recones. These have an Mms of about 54 grams. Vas somewhere in the 500 L range, I think.

Greets!

You're welcome!

'Sounds' like you can't catch a break with Altec drivers, these specs makes the cabs too small for getting the widest/flattest response from these too, i.e. at around a 0.4 Qts Vb = Vas, Fb = Fs. Doesn't mean they won't perform well of course, but for best transient response you'll actually have to either tune them below Fs and/or heavily damp the vent system to completely quell their 'ringing'. FWIW, using an averaged 30 Hz Fs/0.399 Qts and 9.4 ft^3 net Vb, ~25 Hz gets you close enough.

Regardless, either a single 8" i.d. dia. or (4) somewhat longer 4" vents is required for decent cab efficiency. If 3", then at least (7) and preferably (8). Too small a ducted port vent system is acoustically no different than a simple baffle thickness reflex vent, so might as well just use a slit vent reflex port like on an A8 if using just (3) 3" dia. tubes and save yourself the added expense/hassle.

GM

Yeah GM, my quest for the 0.25 Qt Altec woofer seems destined to fail.....
Oh well, that's life! But it's not too bad really, driving them with a high DF amp like the ICEpower I'm using seems to suit them well.

8" diameter port? That's a big 'un. Sonotube? Was just thinkin' 3" tube cause that's what I have on hand. Agreed that getting these sealed up tight enough for good tuning is going to be a bit of work.

Yeah, it takes a huge air mass plug to efficiently quell such a large resonant cab tuned low. Cardboard tubes, metal vent ducting (if damped), plastic drain pipe, etc..

That said, with a low tuning you can get by with as small as a single single 6" or (4) 3" dia. tubes with the understanding that there could be audible vent compression at higher power with some pipe organ symphonies, special effects CDs and of course many ~current movie DVDs if no suitable sub system or hi-pass is used.

GM

Thanks GM, I'll see what I can dig up (Not literally, you understand....)

I'm guessing that the 6" ID pipe will need to be about 2.5" long and an 8" pipe about 6-7" long, yeah? Should get me near Fb of 30Hz. Different calcs say different things.

Got back to work a little on these. Tried some more port tuning.

Since my port is now a simple slot about 26"Wx3"H I wanted to stay with that and just make it a tunnel instead of a hole. The hope was to lower the tuning frequency to near 30Hz and get a better balance a a little more low end.

So I made a tunnel out of cardboard that would fit into the slot. It's is sealed and held in place with gaffer tape. The first tunnel port I made was 6" deep. (26Wx3Hx6D). That certainly changed things, but maybe not enough. Next I went to 9" deep and that made an even bigger change, tho did not get me down to 30Hz Fb. Maybe ~37Hz.

I will post some port frequency plots and impedance plots later today.

The sound is certainly different. Much more "matte" or "dry" sounding. The flat port had a lot of energy near 60Hz, and that was audible in room. The 9" deep port sounds like it has less bass, but is mostly just missing the 60Hz bump. Better balanced, but less lively.
Going to take some time to decide if I like it.