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[InflatableMouse]

Look what I picked up yesterday for 10 euro's. She even helped me load them in my car. These are the FB 825 with isophase magnetostatic ribbon tweeter. Very rare model and supposed to be the best speaker Philips ever made (I'm skeptical, we'll see how they match up to my current Davis Acoustics M7's).



The foam on all woofers and midrange are rot so they need a fix but that doesn't break the wallet; about 200 euros total. If I don't like them I can easily sell them for 400 euros, the cabinets are in mint condition, not a scratch or dent on them!

[Arindelle]

  I'll buy the tweeters from you for 20 euros and you can keep the rest!!

100% return on your investment

[InflatableMouse]

Ha! I can sell those for 150 each on ebay 

You'll have to do better than that to get a deal out of me.

Those ribbons look beautiful by the way. No discoloration, no wrinkles, they look new.

[Arindelle]

he he I thought you'd say something like that   

yep if they are in good shape, they'd be great for monitors -- no ear fatigue with those

[InflatableMouse]

I took the speakers out and took some pictures. I'm wondering if there are any quick wins I can make modifiying it while I'm at it. Shouldn't the cabinet be stuffed with some dampening material? It's only on the sides.

[mwillems]

I took the speakers out and took some pictures. I'm wondering if there are any quick wins I can make modifiying it while I'm at it. Shouldn't the cabinet be stuffed with some dampening material? It's only on the sides.

Does the cabinet have a port/vent?  If so the custom is not to stuff the cabinet full for fear of potentially restricting airflow to the port (I don't personally agree with that custom, but it's the conventional wisdom).  If the cabinet is completely sealed, then you could safely stuff it full, but it will change the Q of the cabinet, which will result in changes to the speaker's frequency response and or time-domain response.  

It's hard to know going in, but it may only have X amount of stuffing in it because the designers managed to get the FR flatter with that amount of stuffing (i.e. it may have the "right" amount of stuffing).  Or it may have been "built to a price" and they just put in the minimum stuffing needed to avoid gross box reflections and called it a day.  The truth is probably somewhere in the middle.  

Generally, my opinion is that more stuffing is better to a point (in terms of controlling reflections, you almost can't damp a box too much but diminishing returns do set in after a while), but, on the other hand, sometimes when making passive speakers, one has to make compromises to improve the flatness of the frequency response.

My advice: if you're planning to replace the woofers with something different, all bets are off anyway, so I'd say stuff 'em full; then measure/listen when you're done, and adjust the stuffing to taste.  Alternatively, if you're just going to re-foam the existing woofers or replace with new-old-stock, then I'd suggest fixing the woofers and measuring before making alterations to the cabinet; you may find that the existing stuffing is "just right."

BTW, those are some nice window braces inside the cabinet; those are a pain to make, but make a big difference.  

In terms of quick wins, my advice:

Take your knuckle and knock around the outside of the cabinet and listen.  Ideally you'd knock and it would make a solid "thud."  If you knock and hear reverberance, or if the panel vibrates for a bit, then the cabinet could probably use additional wood or metal bracing in the location that you knocked.  One easy way to train your ear to hear what I'm describing is to knock where you know the cabinet has a brace (or at the box edges) and then gradually work your way to the center of a panel.  You should hear a difference, but if it's a big difference, you know the cabinet might need additional bracing.  The rule of thumb is that you don't want to have more than 8 inches in any direction from any point on the panels without some kind of bracing material.  There are ways to add braces without scarring the finish, ask me if you're interested.

[InflatableMouse]

It's a closed/sealed cabinet. The woofers and midrange speakers are getting a refoam, hopefully I'll send them off this week.

I've knocked around the cabinets too, they did sound a bit hollow, especially in the spots between the studs/braces. The Davis Acoustics (my current speakers) have a vent and although they are quite a bit smaller they are just as heavy and sound very solid. When you knock where a brace is it barely makes a noise. With the Philips when you knock towards a stud/brace you hear it change from hollow to less hollow . The difference between the two cabinets is quite big. But I suppose with smaller panels its a lot easier too.

To be honest it does worry me a little bit and I think the cabinets are not as sturdy as they could (or rather should) have been, especially since they are rated for 300 watt (350 peak max). I was planning to give those mono power amps a real workout once those Philips are fixed .

The rather hollow sound is actually the reason why I'm asking about the stuffing but maybe you have a better idea?

Thanks!

[mwillems]

Stuffing will help a little with the "boxy" sound, but won't help with the hollow-sounding panels, it will only soak up some of the rear-radiating energy of the speakers converting it into heat (and will make the box behave as though it were a slightly bigger box for Q purposes).  To put it in simpler terms, the stuffing helps reduce the "supply" of excess energy reaching the panels.  

The problem with just using stuffing is that some energy will reach the panels and once they are energized they will vibrate or ring, which creates a boxy sound and hurts precision of sound (because of the ringing continuing after the initial impulse). The idea is to mechanically damp the panels themselves to make them 1) harder to energize at step one and 2) quicker to stop ringing at step two.  So adding braces/damping material to the panels will help.  

1) Braces:  The cheapest and most effective way by far is just to buy some 1x2 hardwood boards, cut them to appropriate lengths, and affix them to the interior of the cabinet wherever the ringing is worst.  The more sides of the cabinet that are in contact with a brace, the better it will work (which is why window braces are so nice because they touch four sides of the cabinet).  But even with just a straight length of board, you can easily get it in contact with three sides of the cabinet if you make sure it's the exact interior length of the side it's on.

Affixing the braces to the interior of a finished cabinet is the hard part. If you have a right-angle drill and are good with it, you can screw them to the interior of the cabinet, just make sure to use a screw length that's short enough so that it can't pierce the exterior of the cabinet.  I'd suggest using adhesive along with the screws for maximum damping.  

The alternative (which is safer for the cabinet's finish) is to just use adhesive by itself.  If you have clamps and can get them in position, I recommend using some kind of polyurethane construction adhesive (loctite, liquid nails, etc.) as it fills gaps, adds mass, and has tremendous adhesive strength.  If you don't have clamps and are relying on gravity to hold the brace in place while the adhesive dries, use wood glue instead and a brick (or something suitably heavy) on top of the brace as it dries.  Don't try to use polyurethane without clamps, you'll probably regret it.

If you're comfortable working with metal, you can also add quite a bit of stiffness with less added weight using u-channel aluminum, but it's much harder to work with and rules out using wood glue.

2) Another alternative is to add mass or damping material to the inside of the panels directly.  For most of these solutions you'd need to take out the stuffing temporarily.  You can buy acoustic paint that will add a certain amount of damping (quite a bit with extra coats), and you can also get spray can based damping solutions designed for use in cars.  The spray can solutions stink to high heaven for a week or two, but they do add significant damping.  You can also get mass loaded vinyl and glue it to the interior of the cabinet walls which will add quite a bit of mass.  I've tried (or been in the room while someone else tried) each of these at different times, and I can say that they're not necessarily a good replacement for proper bracing, but are much better than nothing.

Some extremely hardcore speaker builders have basically mixed sand into glue and painted the interior of their cabinets with the mixture.  My understanding is that, done properly, that can be as good or better than conventional bracing, and costs very little, but is very hard to work with.  And it makes sense, as that's basically what commercial polyurethane adhesive is (sand and talc in an adhesive suspension), but it would cost a fortune to coat the interior of a speaker in polyurethane, which is why the DIY crowd just use sand.  

Bottom line, my recommendation is to add braces if possible and, if not, to find other ways to add mass.  

[InflatableMouse]

I think I can go with the braces or the u-channel aluminium.

The problem I see is, how do I get the braces in there if its supposed to make contact with 3 sides? I asume I would make that from a single piece, or could I make it out of 3 pieces and fix them together once they are inside? I could use wood glue between them and drill some small holes for screws to go in. The size would have to be exact though which doesn't make it very easy (but doable).

I have an electric screwdriver thats small and easy to handle, I think I'll manage to use that inside the cabinet. Or I can use a short hand screwdriver and end up with some blisters .

I think I prefer the u-channel though. It sounds easy. Drill a couple of holes and screw it in there. Would I place them diagonally on a panel? I made a quick drawing, see the attachment. The dots inside the aluminum would be screws.

Is using braces that much better?

[mwillems]

I think I can go with the braces or the u-channel aluminium.

The problem I see is, how do I get the braces in there if its supposed to make contact with 3 sides? I asume I would make that from a single piece, or could I make it out of 3 pieces and fix them together once they are inside? I could use wood glue between them and drill some small holes for screws to go in. The size would have to be exact though which doesn't make it very easy (but doable).

 

If the brace is running from the front of the cabinet to the back, you should be able to slide it in through the speaker hole and then move it to the left or right without rotation.  If you're planning to run the braces vertically, that's harder, obviously and might require multiple pieces.  One easy solution for vertical braces is to just run them in segments: one from the bottom to the first window brace, one between the window braces, etc.  The next best thing to connecting sides of the cabinet is connecting existing braces and sides of the cabinet.

The sizing/fit issue is one of the reasons I recommend using polyurethane when possible; polyurethane expands as it dries and will fill gaps up to a quarter inch!  That expanding property is why I didn't recommend using polyurethane without something to hold the brace in place; the adhesive will expand and dislodge the piece.  If you're using screws to hold everything though there's no problem; with screws polyurethane would be recommended (the screws would be serving the same role as the clamp).  

If you want to use wood glue, you basically have to sand the pieces to fit as closely as possible because wood glue shrinks as it dries so you need a very good fit to start.  

I think I prefer the u-channel though. It sounds easy. Drill a couple of holes and screw it in there.

You can do the exact same thing with wooden braces (drill holes and screw it in); you just need longer dry wall screws.  Whether you use wood or aluminum I'd definitely recommend using screws for retention if you can do it.  I only mentioned clamps, etc. because it can be hard to get a drill inside a cabinet.  Whether you use wood or aluminum, as noted above, I'd recommend using some kind of gap-filling adhesive between the brace and the cabinet for maximum damping.  

Would I place them diagonally on a panel? I made a quick drawing, see the attachment. The dots inside the aluminum would be screws.

You can place them diagonally and some folks think that's the way to go, but IMO it's easier to get good solid contact with more sides of the cabinet if they're lateral or vertical.  If you were going to make them diagonal, I'd strongly recommend making them longer than illustrated so that they make contact with the sides of the cabinet and/or the existing window braces.  The more parts of the existing structure that you can connect by a brace, the more effectively that brace will damp the cabinet.  Ideally you want to create a network of interconnected sides and braces without consuming too much interior space, so make sure each brace touches/is glued to as many walls of cabinet and/or other braces as possible.  

Having the braces positioned as in your illustration will not damp the panel very much because they're only connected to one side of the cabinet and no braces (Or maybe two sides? I'm not sure what the wavy lines represent).

Is using braces that much better?

I assume you mean wood braces? The aluminum u-channel is also being used as a brace, so I assume you mean wood vs. aluminum?

The main advantages of wood over aluminum are: price and ease of work.  

1) Hardwood 1x2s in my part of the world are less than a quarter of the price of aluminum U-channel inch for inch.  
2) Making and securing a wooden brace is super easy and can be done in a few minutes with a hand saw and an electric drill; customizing the fit can be done by hand with sandpaper or an electric palm sander. By contrast cutting aluminum requires a hacksaw and some serious effort, and drilling it requires stronger bits, more time, and a deburring tool to handle the nasty jagged edges left by the drill.  And if you got the dimensions slightly wrong, it's very hard to remove exactly 1/8 inch from the end of the u-channel (unless you have an angle grinder on hand).  

The main advantages of the aluminum are:
1) Much lighter for the same stiffness (which can be a plus or a minus if you need more mass)
2) Occupies much less internal space for equivalent stiffness.

I used U-channel extensively on my big speakers, but that was because I knew they would be pushing 200lbs when I was done, and wanted to be able to move them    Using wood for all the bracing would have added another 20 or 30lbs most likely.  When I make smaller speakers, I always use wood because it's cheaper, easier, and massier.  IMHO I'd only suggest using aluminum if you're worried about adding too much weight/using too much space (or if by some fluke of international pricing it's cheaper for you than wood).  

[InflatableMouse]

Thanks for taking the time, appreciate it.

I'll go with some hardwood, I was thinking maybe it had to be a single piece like that window brace and the only way to get that in there is to take the cabinet apart which is not what I'm going to be doing . Using 3 pieces and making them touch is easy. Some polyurethane, some screws to hold it in place, put the stuffing back in ... piece of cake  .

I want to make sure I don't misunderstand, The braces in this story will be 4 pieces of hardwood, rectangular (I asume 1x2s means 1 by 2 inch?), going horizontally on the panels to create something similar to the window brace in there, but with 4 pieces, all touching on each end, effectively making a ring like a window brace, touching all four panels, correct?

There are 2 of those window braces in there, one right above and another right below the top woofer. The lower part with the bottom woofer has no extra window brace below it and its the part of the panels that reverb the most when I knock it. I would place the braces below the lower woofer.

PS. the wavy line was to indicate the panels I drew were incomplete.

[mwillems]

I'll go with some hardwood, I was thinking maybe it had to be a single piece like that window brace and the only way to get that in there is to take the cabinet apart which is not what I'm going to be doing . Using 3 pieces and making them touch is easy. Some polyurethane, some screws to hold it in place, put the stuffing back in ... piece of cake  .

Exactly 

I want to make sure I don't misunderstand, The braces in this story will be 4 pieces of hardwood, rectangular (I asume 1x2s means 1 by 2 inch?)

Yes, I meant one x two inches; that sort of nomencalture is how planks are sold/described here in the states (you'll hear a plank referred to as a two-by-four or a one-by-two, etc.), I forgot those designations wouldn't travel well... Anything similarly sized would work.

going horizontally on the panels to create something similar to the window brace in there, but with 4 pieces, all touching on each end, effectively making a ring like a window brace, touching all four panels, correct?

Yes, that's exactly it.  The goal is to make sure they're well anchored to each other and the box and polyurethane makes that job much easier.

There are 2 of those window braces in there, one right above and another right below the top woofer. The lower part with the bottom woofer has no extra window brace below it and its the part of the panels that reverb the most when I knock it. I would place the braces below the lower woofer.

That sounds like a good plan; if you're still feeling energetic after putting in your new window brace, you might want to think about running a few pieces vertically up the back connecting the window braces to each other and to the top and bottom of the cabinet.  The backs of cabinets are where the highest concentration of energy is and they're often under-braced, and "anchoring" the window braces to the two remaining sides probably isn't a bad idea.   

But you can install the four-piece window brace, give it a day or so to dry and redo your knock test to see if any of that is even necessary.  Let your knuckles be your guide 

[InflatableMouse]

Thanks! 

I'll make the braces but I'm gonna wait for the speakers to return before I put them in so I can have a listen how they sound now to have some reference. I can then experiment with more stuffing for instance or put the vertical braces in as well.

[InflatableMouse]

All my contacts able to refoam were too busy (or not interested I guess). I finally found someone else I trust to do it. I'm getting my speakers back this weekend. I'll have a listen and decide whether I want to put in the extra braces. I'll post some pics when the new speakers are in.

[InflatableMouse]

Ugh .. they sound so bad its not worth the effort. Dissapointing!

I guess I'll just sell them at least I'll make a nice profit out of them .

[mwillems]

Ugh .. they sound so bad its not worth the effort. Dissapointing!

I guess I'll just sell them at least I'll make a nice profit out of them .

I'm sorry to hear that.  What do they sound like?

[InflatableMouse]

The mids sound a little bit like its coming out of hollow tin cans. The bottom end is way too heavy and muddy and completely saturates the room. The cabin vibrates and resonates so I suppose the muddiness can be helped by the braces, but the rest isn't good enough to bother. They just sound very boxed, narrow soundstage and just not open at all. The only really good thing is the high end, it has a very nice sound to it, clear and crisp without being harsh. However, I do feel there's a bit of a gap between midrange and high, but maybe that's because the midrange isn't very good to begin with.

So while they aren't bad, they are no match for my current set.

While I was careful not to connect a unit out of phase I am going to double check today.

[InflatableMouse]

But now I'm not so sure anymore.

After a few minutes of listening to one I think its pretty good and I switch to the other set and its kinda dull and bass heavy. But after a few minutes it's not so bad and then when I switch back, the one I thought was pretty good is suddenly shrill and thin sounding.

Neither are perfect and both have some aspects that I like but now I can't decide which ones to keep.

[mwillems]

But now I'm not so sure anymore.

After a few minutes of listening to one I think its pretty good and I switch to the other set and its kinda dull and bass heavy. But after a few minutes it's not so bad and then when I switch back, the one I thought was pretty good is suddenly shrill and thin sounding.

Neither are perfect and both have some aspects that I like but now I can't decide which ones to keep.

If you just refoamed them, I'd say give them a little time to wear back in.  I don't believe in "wear in" for electronic components, but a certain amount of mechanical wear/break in for moving cone speakers is actually a real thing.  It's a recognized thing in the industry, and I've personally measured differences in bass response from drivers between when they're new and a few hours later (not huge ones, but definitely present). 

Hollowness and boxiness are usually signs of either phase problems or inadequate cabinet damping.  The box resonates at certain frequencies changing frequency response, or reflected sound from inside the cabinet radiates back out through the drivers after echoing around in the box, etc.   A good test of whether bracing will help:  Put some very heavy books on top of the speakers, and listen again.  Also try having someone hold onto the cabinet sides during playback and see if it sounds any different.  Basically you can easily see if the sound problems you're hearing are box damping related by just damping the panels in a temporary way and seeing what happens to the sound. 

Other than that, measurement is recommended, of course 

[InflatableMouse]

Thanks for the advise.

I'm sure you're familiar with this and I've been through this before. When you start switching between 2 components eventually you loose track of what actually sounds good. I had a fresh pair of ears coming over for a 2nd opinion and they confirmed exactly what I thought from the start. But I started to confuse myself after that.

The Philips are not worth the bother, there's too much wrong with them. If it were just the low end I might try modifying them but the difference is just too big.

[mwillems]

Thanks for the advise.

I'm sure you're familiar with this and I've been through this before. When you start switching between 2 components eventually you loose track of what actually sounds good. I had a fresh pair of ears coming over for a 2nd opinion and they confirmed exactly what I thought from the start. But I started to confuse myself after that.

I know exactly what you mean.  After a certain point in system tuning I just started letting the measurements do the listening for me because it was easier than endlessly going around in circles about what sounded "right."  Once I figured out the response curve that sounds best to me, I just replicate that curve and call it a day.

The Philips are not worth the bother, there's too much wrong with them. If it were just the low end I might try modifying them but the difference is just too big.

Sorry to hear it, hopefully you can find a buyer.

[6233638]

But now I'm not so sure anymore.
After a few minutes of listening to one I think its pretty good and I switch to the other set and its kinda dull and bass heavy. But after a few minutes it's not so bad and then when I switch back, the one I thought was pretty good is suddenly shrill and thin sounding.
Neither are perfect and both have some aspects that I like but now I can't decide which ones to keep.

Human perception is relative - that's why we have measurement instruments to get an objective look at how a device performs.
 
That's why I consider any audio/video hardware review which does not include measured performance to be pretty worthless - it's too subjective.
 
Of course, it still requires that the correct measurements are made, and the correct procedures taken for measured results to be valid.