My A7-500s. A journey

[Mustang Marvin]

Hi Pano,
I'm following closely here with the port size. I'll be the first to admit I don't know a Q from a V when it comes to audio lingo. I do know enough to listen to the people that do though. I had my 416's reconed by GPA so they are new. I had GPA advise me on the port size relative to the 825/828 box. This is where I came up with the 26"X3 1/4" port size. There was no math required by me except the actual measurement of the plywood. The bass sounds pretty good to me. I'm sure it could be better though. Like GM said cabinet sealing is needed. I also understand bracing should be added as well as more Roxul for damping.

I'm trying to go one step at a time here. I have some disadvantages here to start with. My cabinets are made of 5/8" plywood instead of 3/4". I understand with proper bracing I can deal with that, but I have not found any suggestions on that so far. I suppose my biggest complaint so far is I have to play them too loud for them to sound really good, which they do IMHO. I'm still open to suggestions to improve them, especially the bass.

Please keep me informed as to your progress. Someday I hope to build another set.

[Panomaniac]

Well sorry that I did not get more info posted as promised. The holidays sort of got in the way. You probably did not miss me. =)

I've done some further tuning and am now pretty darn happy with what I have.
Based on G.M.'s suggestion of an 8" diameter port, I went with a 50 sq in. rectangular port. Same surface area. This does not bring the tuning down quite as far as I had hoped, but it doesn't seem to matter.

At my listening position, the F3 is about 22Hz. Yep, that's right, flat down to 25Hz out of an A7. Glory be! I really don't need any more.

To get there - I have a port that is 50 sq inches and 6.5" deep. I also need to pull out a lot of energy around 220Hz with EQ. It's a notch of -9dB and a Q of 2.2. Without that EQ there is no way I can get the box flat. But pull it out and tune the port right and the box plays right on down to where a big box should play.

If I measure the box 1 meter from the front, it does not play so low. But back where I sit it does, and is nice and flat. Must be room gain making up for it.
I don't know if this 220Hz peak in common in this box, but both mine suffer from it. If you have some method of EQ, I suggest you try this.

Below is an impedance plot showing my old tuning and the new tuning. New tuning is in orange. The old tuning was a simple slot 26" wide by 3" tall. That tuning plays down to about 70Hz and rolls off under that.

On the plot you will also see some kinks between 100-300Hz common to both. I assume these are panel resonances. Time to chase them down.
(sorry about the small graph. I made a nice clear gif, but the forum shrinks it down to an ugly jpeg)

[Altec Best]

Hi Pano, See I knew you would get there pretty fast too.A few weeks would have been sooner if the holidays weren't in the middle of it.Great I knew you would get that smile there it is . Have you tried Photobucket I'm going to start using my account there because I have to keep going back to old posts and delete the photos to make room for more attachments so I think I'll try that.Do you have some final pics of everything and did you finnish dampening the horn flares everything or is that smile premature yet.I just would like to say that you did a Real Nice Job and your documentation of it will help alot of others Top-Notch Thanks for all the info and time you put into posting it. Regards ~ AB

[Old Guy]

It would be interesting if those A7's could write their life story.

The original owner was a muso who new a lot of national acts. His heyday was before mine...I remember as a teenager going in an seeing autographed pics on the walls.

He was acoustic oriented.

I did get that one of his sons won a grammy a couple years back. I think it was as a writer not performer though.

I only knew him in passing...he had left town by the time I got involved in the store. Different ownership.

[Panomaniac]

You're right O.G. These speakers have had quite a life! Would love to know what they have been used for - and who played thru them.

A.B. Yep, looks like I got there. I'm very happy with the in-room result. I would have been happy getting down to just 35Hz. Not bad for a very efficient speaker. But 25Hz? Never thought it possible. Of course I have thrown away some efficiency with my 220Hz dip. I'll need to measure to find out just what I get at 1 watt. In theory 98dB/1 watt - but I doubt I'm close.

Now comes the hard part. Bracing and damping. Just not sure what to do here.
I could brace only and hope for the best. Or I could double up some of the panels and then brace. Maybe constrained layer damping? Just not sure yet which is the best approach. There are so many techniques and materials it's hard to know what to do.

My two suggestions (so far) to A7 owners:
1: Brace and damp those flares.
2: Tune that port!

Both of these will transform the speaker.

[Mustang Marvin]

I believe I'm in desperate need of bracing in my 5/8 in plywood A7's also. I have not done anything since I want to hear suggestions before I try. Pano seems to be having good luck with his adjustments. I would like to know more details. I have heard many ideas from 2X4's screwed and glued to the inside walls to completely inclosing the box with another box glued to the outside.

It would appear many of those A7 owners have their own opinions on bracing. Has anyone taken any measurements after bracing to determine the best method? Details here would be greatly appreciated.
Thanks,
Marvin

[GM]

FWIW, my measurement is that a square edge coin such as a fresh nickel must remain standing while the speaker reproduces the lowest frequency content at the highest expected SPL since this is an excellent indicator that the cab's stiffness/damping is sufficient for both acoustical and mechanical efficiency. Note that this also takes the speaker/floor coupling into consideration, so if you have a floating (suspended) floor construction as I do the speaker must either be extremely massive (heavy) and/or setting on a tuned damping system. That, or do as I did and add bracing/support to the underside of the floor when even my ~245 lb (calc'd) cabs proved too light for my house's crappy Korean War era construction.

GM

[Panomaniac]

Has anyone taken any measurements after bracing to determine the best method?

Marvin, here you have run into the crux of the problem. We'd like to see direct comparisons of different methods of damping and bracing an A7 so that we could compare. But who has several sets of cabinets to do this testing with?

I could try perhaps 4 different methods on the walls of my pair, but how could the bad modifications ones be undone when the best is found? I'd want the best on all the walls,of course. Wouldn't want to leave what I now know to be inferior.

That's the impasse where I know find myself. What to do? Gluing and screwing damping and bracing isn't likely to be reversible. So all I can do is read about what has worked for others, go on my own past experiences and hope for the best.

My suspicions are that doubling much of the panel area inside with OSB and then bracing will deaden things a huge amount. Better than bracing alone? I don't know....

Any other ideas are welcome!

[Mustang Marvin]

Ha, What a dilema. This is what I have experienced as well. I would presume the area where there is the greatest amount of unbraced plywood would be where you would need to start. I have read so many opinions on methods and needed areas for bracing I am thoroughly confused. I, like Pano, would like to hear more from the experienced members who have found actual and not perceived improvement in performance. Also like Pano, I am reluctant to screw and especially glue braces to my A7's with out feeling confident that I haven't ruined my original Altec cabinets. I would rather do nothing than that.

Still, I feel improvements can be made. I just want to hear as much as possible before I make any additional changes. I kind of like GM's method of detecting loss in structural integrity with the nickel. Assuming you still have a nickel after restoring the all the various components of your A7's. Ha...

[GM]

Ignoring the costly/labor intensive constrained layer damping (CLD) method, there's two basic ways to deaden a speaker cab, either make it so dense that it pushes its Fs down to below its pass-band or make it stiff enough to push it up above it and since we're talking about cabs with < ~40 Hz pass-bands it will take a concrete, marble, slate or similar construction to push it low enough. That, or many inches thick MDF. Often you see folks posting that their MDF cab passes the 'knuckle' test since it makes a dull thud. Well, all this does is prove that its Fs is in its pass-band, so I tell them to smack their knuckles against a concrete pad to hear what it must sound like.

A worst case scenario for pushing it above its pass-band is 400-500 Hz and since acoustic energy falls at 1/f it takes much less material to get the job done, so if you start with a high MOE (modulus of elasticity) material such as 11-13 lams (3/4"(19 mm)) no void BB, Apple or some marine grades of plywood you're already there if the cab is a typical size rectangular box common today.

It makes sense then that if a cab with large panels is broken up with bracing to mimic a collection of smaller size rectangular boxes that you've created an adequate amount of stiffness. Breaking them up into triangular spaces is even better, reducing the amount of bracing required; just look at bridge, etc. construction to see why 'X' braced 'window frame' braces are so popular.

For high aspect ratio panels, offset brace(s) spaced along its length at the golden or some acoustic ratio is sufficient and since stiffness increases at the cube of thickness, simply laminating a layer of rigid material of the same type (or with a higher MOE) to all the outside walls with a hard setting adhesive can be sufficient.

Unfortunately, most 825/828 cabs are either made of relatively thin, so-so quality plywood or MDF or as I learned here awhile ago, a combination of the two apparently during the transition from all plywood to all MDF construction with a relative being made from 3/4" plywood, so it gets far more math intensive than I'm able to wade through trying to calc what's the best way to get sufficient structural efficiency with the least amount of bracing.

Regardless, from experience me and many others have found that with an all MDF cab you're kind of stuck since you can't raise its Fs much at all with bracing, so laminating another 3/8" of MDF to it will make it slightly less rigid than if the cab was made of 19 mm BB, etc. plywood and if 3/4" is used it should be right up there with a well braced one, so about the best one can practically do since this will nearly double the empty cab's already considerable weight.

Again, the quick, easy way to stiffen any cab is by increasing its panel thickness, but with the high price of premium plywood, not a viable option for the $$ and weight conscious among us, so do basic large panel bracing, then use a cheap mechanics stethoscope to search for any hot spots, then glue a piece of scrap at least 2.5" wide on edge to knock it down. Note that it's not necessary to run braces all the way from joint to joint and some contend that it's in theory detrimental due to creating HF hot spots. FWIW, I've found that making the bracing ~0.7x the span works well. Also, screws, nails reduces an engineered wood's structural integrity, so the fewer of them you use, the better. Indeed, my intense speaker building 'career' began just as mil spec adhesive technology began trickling down to the consumer, so quickly began a 'love affair' with PL brand construction adhesives that gave me the quick/easy application and the fast/hard setting times that allowed me to quickly assemble a cab with only a handful of screws for a pair Vs a gross or more I used on earlier builds.

In all cases, if a screwed down HF horn sled isn't used, then unless the cab's inverted and setting on a rigid/damped surface, a laminated top plate is required to adequately stiffen this side of the horn.

WRT the cab's backside, any with the factory rails are already stiff enough, so mimicing them on the older two piece designs works for me, though offset their break point so that the removable cover pieces overlap the fixed bottom cover enough to add some screw mounting to form a ~one piece cover bracing wise. That, or add a tie piece to the ones on the removable cover to clamp them together.

Putting panels under tension to raise both the panel's/cab's Fs is another option some tout, but all of the typical speaker building materials will take a set, ergo require periodic tuning, so not an option per se. 'Window frame' or cross braces to tie all sides together to keep the cab from potentially 'breathing' is a proven alternative with the latter being my preference since it's quick/easy and you can use up scraps. Note that they should all tie together to effectively create one or more 'window frame' braces in both the horizontal and vertical plane.

GM