My A7-500s. A journey

[Panomaniac]

When doing box tuning and designing crossover, driver impedance curves are a great help!
One of the best audio gadgets I ever bought is the WT3 woofer tester from Parts Express.
Just a little USB gadget, it makes measuring drivers very fast. Only a couple of seconds to measure everything but Vas. And that only takes a few seconds more.

Plots show here are from the WT3.

Below we see 2 graphs, 3 plots. One is the woofer in the A7 box. This is a 416-8A. You can see the single peak at resonance (~23Hz) and the rising impedance typical of a woofer. Also obvious are some small bumps that indicate box resonances. I'll work to get rid of those.

Do notice the single peak. Does not look like the classic twin peak bass reflex curve, does it? More like infinite baffle. Maybe that's because of the big port. Currently 206 sq. inches or about 160% of cone area. We can see why Marvin brought his port size down!
I'll do the same and run a sweep.

The other graph is the impedance of the 802 driver on the 511 horn. One is with loading cap, the other without. As the peaks come near the crossover point, this makes things a little tricky. An LCR trap could be used to flatten this peak, might be worth trying.

Just for fun, I tried running the drivers with an active crossover. 3rd order at 610Hz. It works well and certainly sounds better than the stock crossover. Worth investigating further.
FWIW, the way my 511s are mounted, there is only 4 cm of depth offset between the acoustic centers of the woofer and horn. Very close to time aligned. Just a small fraction of a wavelength. Nice!

[Old Guy]

Altering the xover point will alter the sound a lot.

500, what you have, results in a more mid heavy, very "up front' aggressive sound.

This can be very desirable with a straight bass reflex, but the 828's short mid bass horn tends to boost that area too.

Going up to 800 will mellow them a lot. To 1200 even more...but how much is good is personal preference.

An electronic xover can help , play around with crossover points till you get the sound you like, then design your passives for that point...or just stay electronic.

Personally, I will never go back to passives. Never.

And this from someone who insists on never saying never...

Really glad the horn worked out for you, in the old days would have had a spare fram to send, but those days are gone...

[GM]

Do notice the single peak. Does not look like the classic twin peak bass reflex curve, does it? More like infinite baffle.

The little bump is the reflex's upper impedance peak which is highly damped due to the cab being tuned so much higher than the driver's Fs, ergo if tuned a similar amount below Fs it would become the ~single peak with the lower one just a small bump and of course equal in amplitude if tuned to the driver's Fs.

Anyway, if you recall my remarks about driving vintage speakers with the type of high output or matching impedance sources they were voiced with, this high tuning relative to Fs is desirable since it yields much better woofer control/higher low distortion SPL while still having extension to slightly below Fs same as if tuned to Fs with the trade-off being it sounding a bit 'loose' due to the amp 'stalling out' against the high impedance.

This same high output impedance source would also flatten the HF horn's 'hot' mids somewhat while adding scharron's 'gentle climb' EQ to flatten it out with increasing frequency due to increasing driver inductance.

GM

[Panomaniac]

Hi GM, I was hoping you would "tune in."

Yep, I will be working on the port tuning, for sure. Will aim for equal peaks to test how that sounds. And for sure, a high output impedance or resistive network on the horns sounds better. Not quite sure about it for the bass, yet.

[Panomaniac]

Sorry to report no real progress over the weekend, too busy with yard work and other chores. Mostly just listening. =)

But I did try to figure out the internal volume of the box. I have read and heard various numbers, but nothing definitive. So I started measuring. I figure this:

Gross internal volume = 15 cubic feet
Horn section + woofer = 3 cubic feet.
Net volume = 12 cubic feet = 340 liters.
(for box with horn flares open in back)

Actually did all my calcs in cubic centimeters then converted. Years ago I had figured 11.5 feet going from published plans. So pretty close to my measured results. I sure looks like the horn and woofer should take up more than 1/5th the volume, but I measured and calculated 3 times. Anyone have an opinion?

Given an Fb of 340L, and the port size I have, my cabs must be "tuned" to about 65Hz. Or about an octave too high. A starting point!

[Panomaniac]

Well did a little tuning and testing today. I pulled off the top board from the port and made 3 others boards of different sizes for various port areas. Easy to swap out the boards and change the port size. Or course all were "flat ports" - just 3/4" deep. And all where full width - about 28.5".

Closing down the port definitely extends the bass, no doubt about that. Easy to hear as the port plates are switched in and out. I was trying to get the box tuned to about 35Hz for max flat response. Never sure what tuning I got. Could not tell be the impedance curves.

But about 57 sq in. seemed to work best to my ear. Nice solid bass, less midbass. Listening with a signal generator says that the boxes now play strongly into the mid 30s. Drop fast under that. Tried both more area and less (down to 17 sq inches) but liked the mid 50s best. Will need more time to fine tune.

One thing I can say for sure. Bracing and damping the woofer horn flare is going to be VERY important! They buzz like crazy. I put one hand on the flare while running the sine waves up and down. Real easy to feel the resonances. And they don't all happen at the same points, that was easy to feel with my hand. All the bad stuff seems to be between about 120-200Hz. In fact just pressing my hand on the flare at 200Hz made the fundamental rise a good 6dB! The horn flare must have been vibrating in opposite phase, canceling the fundamental. Super easy to hear. So you A7 owners, get those flares under control!

Sounding pretty decent now. Good port and the 811/806 combo running at 800Hz, 3rd order. Still the dip at ~2K to pull back the midrange. Getting nice. And can only get better!

[Altec Best]

Hi Mike,

Well that didn't take to long I can see a half smile almost there good deal....

[Panomaniac]

You're right John.
Just a little work and they already sound much better. In many ways better than my home made Big Ugly Speakers.

The right crossover, the right horn and the right port take them a long way. Very promising for the future improvements.