[rustneversleeps]

I am restoring several pairs of speakers, and have a question regarding internal speaker damping.

Inside the AR, KLH, genesis (not the new genesis), they were filled with fiberglass.

Inside the Bozak, the acoustical foam were glued (not filled) to the side, top, bottom, and back panels.

Inside the Advent, there two big pieces of foam.

Inside the Mission 70MKII (the one that Frank recommended years ago), it has no foam damping.

So which one is the most correct internal speaker damping procedure, since they were all designed by the well known, legendary people.

[Wayner]

I would leave the internal damping as it is in each of the speakers. Adding ar subtracting will change the "Q" and affect the voicing of the woofers.

Wayner

[gjs_cds]

So which one is the most correct internal speaker damping procedure, since they were all designed by the well known, legendary people.

I don't consider myself an authority on the matter--but I have read a number of speaker building books (both in theory and design), attended DIY events, and built any number of loudspeakers myself... 

I think the answer to the question is--it depends.  Different types of dampening do different things for different applications.  For example, if you've got a ported design, then a heavy fill (either fiberglass or poly) isn't what you would want.  It would hamper air flow.  Conversely, if you have a sealed design--then foam padding on the side walls won't be enough.  (Foam padding, on its own, is rarely enough.)

The argument that if the speaker was designed by an expert, therefore the internal damping shouldn't be fiddled with--well, that's not an argument in which I subscribe.  It's quite a distance between initial (i.e., optimal) design and actual (i.e., cost-cutting) production.  In the time that I used to sell hi-fi (during college), I would routinely peer inside the cabinets to view the XOs and the dampening---and I was consistently shocked at the 5-and-dime materials that were used.  Minimal damping and cheap components lead to higher profits. 

So in short, I would feel free to play around.  If it's a ported design, then using the three-layer foam on the side-walls with a light fiberglass fill.  (The rule of thumb is that fiberglass may sound a touch better than poly-fill.  On the flip side, poly is a HECK of a lot easier to work with.)  Add or remove fiber/poly fill to taste.

Sealed designs could benefit from the three-layer foam on the back wall.  Add or remove fiber/poly fill to taste--medium to high fill is par for the course.  PartsExpress has been my supplier of choice, although it seems that they have gone downhill (relative to speaker building) since one employee (in charge of SB development) left.

Hope this helps,

[Brett Buck]

I am restoring several pairs of speakers, and have a question regarding internal speaker damping.

Inside the AR, KLH, genesis (not the new genesis), they were filled with fiberglass.

Inside the Bozak, the acoustical foam were glued (not filled) to the side, top, bottom, and back panels.

Inside the Advent, there two big pieces of foam.

Inside the Mission 70MKII (the one that Frank recommended years ago), it has no foam damping.

So which one is the most correct internal speaker damping procedure, since they were all designed by the well known, legendary people.

      I am not the world's authority, but I do know it depends entirely on the intent of the enclosure design. The one thing you can be sure of is that flexing of the enclosure and driver baskets *is not* an intentional design feature. Other than that, what you use depends on which frequencies you want to absorb (or transmit). A piece of paper towel has a huge attenuation at 15khz, but does nothing to 100 hz. Fill a Klipschorn bass horn with loose fiberglass and it probably won't have much effect (it may even be a good idea, from what I have heard), fill it with plasti-clay or foam rubber and it will!   

     And in particular, on the Bozak, the effects of enclosure radiation was probably not well understood at the time, so they were probably only concerned with breaking up the cavity resonance (which, if I did my sums right, are on the order of 500-1khz for the  Concert Grand).

    Point being, other than damping and bracing the enclosure itself, there's almost certainly not one correct answer that is true in all cases. It depends on everything else.

        Brett

[rustneversleeps]

I've always thought that the damping material inside the speaker are for delaying the time for the backwave to reach back to the speaker, so it won't cancel out what the speaker's supposed to put out, therefore the damping material would make the speaker seems larger in volume for the soundwave, because of the delaying effect.

If that's correct, then inside of the Bozak would have less cancelling effect, and I think Mr. Bozak would have known that judging by reading some of the the articles that he wrote about speaker. I'll be doing the "Pat Tobin" mod on this pair since the caps in them are very old, it's a 1963 model B-313.

I'll do the plasti clay mod, or the bracing mod on some of them if condition allows, I don't really want to decrease the internal volume too much. My intention is while I have all these speakers opened up, I'd like to do every right so I wouldn't have to open them up again.

My interest and goal in audio is to get good sound and learn about their historical background, how things were and what have they become.

P.S. I take back about the Mission MKII doesn't have any damping material, I only saw the picture of the inside from a guy who tried to sell them. I'll open mine later.

[gjs_cds]

I've always thought that the damping material inside the speaker are for delaying the time for the backwave to reach back to the speaker, so it won't cancel out what the speaker's supposed to put out, therefore the damping material would make the speaker seems larger in volume for the soundwave, because of the delaying effect.

According to the theory that I've read (and speaker building isn't a science--it's a theory and an art), dampening material reduces acoustic energy by converting it into heat.  It does, indeed, act in such a fashion to functionally increase the box size--but not by slowing down the speed of the wave.

I'll do the plasti clay mod, or the bracing mod on some of them if condition allows, I don't really want to decrease the internal volume too much. My intention is while I have all these speakers opened up, I'd like to do every right so I wouldn't have to open them up again.

Plasti clay is a great little mod that is targeted at reducing box resonance; I also expect that it's use would help sound dispersion too, as it could be applied in a non-unified (i.e., non-flat) way.

My only other thought would be to prepare yourself for some trial and error.  Working with fiber or poly fill is really a "suit-to-taste"--based on room acoustics and speaker placement w/in the room.

[bill huck]

The Bozak (my beloved speakers):  the Tobin re-voicing of the crossover is astonishing.  Mr. Tobin is also a great source of info on the drivers and enclosure.

The Concert series of speakers also have a curtain of damping material hanging down the middle of the cabinet plus the mid-range has a protective plastic cover.  You are correct about not decreasing internal volume; bass response is affected. 

You might check out the Bozak Yahoo group at http://tech.groups.yahoo.com/group/Bozak_speakers/.  They are active and helpful.

Bill