My A7-500s. A journey

**Panomaniac** · August 9th, 2011, 02:12 PM

I'm still working with the sealed box, but may try to go ported soon. And hope the port will boost the bass response some. As it is, I have a pretty big shelf that drops under about 140Hz. I can make up for it in my crossover (active or passive) but it would be nice to make up for it acoustically. I suppose a combo of sealed box, baffle step and horn loaded woofer conspire to create the shelf. With proper crossover EQ, it all sums well and sounds good, but takes a bit of power.

The main problem I'm having now is a lot of energy around 220Hz, or a few notes below middle C. Can't see it on the frequency response chart , but sure can hear it. And it shows up pretty as you please on waterfall and sonograph charts. "Ringing" right around there. I tried Dominique's curtain trick by putting a big, fluffy pillow hanging down below the woofer. It had some effect, mostly smearing the ringing or reducing its Q. That makes it a little bit less noticeable. Once I figure out where the ringing is coming from and fix it, I'll be super happy. The system should then be super clean.

GM · August 9th, 2011, 03:37 PM

Again, this is the horn 'ringing'. It's a half size ~55 Hz expo, so a ~110 Hz 1/4 WL F0 and since a horn must be a 1/2 WL long before it gets up 'on the pipe' [to use 2 stroke tuning jargon], this means ~220 Hz; but with too small a mouth this sends huge initial reflections back to the throat, so either the mouth must be larger [wings] and or some damping around its mouth perimeter and/or adding an acoustic band stop filter and/or pre-loading the driver more by tuning the cab much higher. IIRC both baffle boards have to be off, making it a crude ~80 Hz BLH, then an acoustic screen is required to critically damp it/attenuate unwanted harmonics off the back of the driver. The latter of course severely rolls off its bass output.

Another option that might help, though never tried it on an 825/828 is to tightly cover the rear of the driver with damping material, experimenting with increasing thickness to find the best compromise. The key here is ?tightly? since what you want it to do is acoustically lower Qms/Qts, i.e. make it more suitable as a horn driver like the cab was originally designed around.

Anyway, good luck with it, I've got to get back to trying to salvage as much molding, siding as I can since pricing has gone ballistic since the housing construction slowdown. My 34 yr. inflation adjusted materials budget has proven woefully short of reality.

GM

**Panomaniac** · August 9th, 2011, 07:29 PM

Thanks GM, I'll experiment with wings. I tried a pillow in the gap between the top of the bass horn and the bottom of the 1005 horn - all that did was give me more junk below 200Hz. It's easy to see the results in the measurement. Real wings on the sides should be a different story.

Good luck with the house stuff!

**Steve Mac** · August 9th, 2011, 08:45 PM

Originally Posted by Panomaniac

Thanks GM, I'll experiment with wings. I tried a pillow in the gap between the top of the bass horn and the bottom of the 1005 horn - all that did was give me more junk below 200Hz. It's easy to see the results in the measurement. Real wings on the sides should be a different story.

Good luck with the house stuff!

With that big mid range horn you really need to splurge and just build the big mid bass horn.
That will be the only way to make that big altec mid range horn happy.
Two altec 515s and a mouth bigger than Rosie O'Donnell's.

**Panomaniac** · August 9th, 2011, 10:58 PM

I've thought about, I really have. But for the moment, I'm tired of building speakers!

**dominique-tanguy** · August 10th, 2011, 09:57 AM

Hi Panomaniac,

I am convinced that you cannot get the best of a VOT without using the port.

a) The effect on the bass extension is significant
b) The effect on the quality of the medium is also very noticeable

I am very impressed by the overal result I have obtained so far. I will now concentrate on the room response issue in order to continue optilisation, but the actual result is already excellent, with a very well balanced sound.

I understand that one of the issue is that these cabinets can be fitted with very different loudspeakers (416-8A and 515-8G for example) and the tuning cannot be the same... Considering the datasheet, it is even very surprising that both could give good result in the same cabinet.

Regards,

Dominique

**Panomaniac** · August 11th, 2011, 05:50 PM

Yes, port and wings for sure.

I did a little port work today. Thought I'd make a "slot port". I.E. a rectangular port right at the bottom with the box bottom being one wall of the port. So now I have a 4" tall hole at the bottom the full width of the cabinet. Could not get it to tune worth a fart no matter how deep or shallow it was, but it does add a very nice 6dB or more at the bottom. Just what I needed! At 1M, the response has its problems (floor bounce, I suspect), but there is certainly more bass. Back at the listening sofa, it's more a more even response and that's great. Now I gotta bring the horns up.

Experimented with one wing, it does smooth things out a bit in the 200-300Hz range, so I will probably go with two. Progress.

GM · August 11th, 2011, 06:56 PM

If you're still in an experimenting mood, take off the lower panels and make one to slope back down to a full width 4" slot at the bottom back to create a horn loaded vent.

GM