Dismiss Notice

Psst... Ready to join TalkBass and start posting, make new friends, sell your gear, and more?  Register your free account in 30 seconds.

Ampeg 8 X 10 Speaker Cab...Help!

Discussion in 'Amps and Cabs [BG]' started by Zooberwerx, Jun 21, 2004.


  1. Zooberwerx

    Zooberwerx Gold Supporting Member

    Dec 21, 2002
    Virginia Beach, VA
    I'm restoring a cab which is badly in need of some TLC. The internal damping material is in sorry shape and will definitely need replacing. What material(s) would be appropriate? I appreciate any input!
     
  2. Rockbobmel

    Rockbobmel Supporting Member

    Thick fiberglass insulation amout a 2' x 2' in each folded in half. That's what they use. Don't get the polyfill crap.
     
  3. Sidecar666

    Sidecar666

    Mar 27, 2004
    There's a company called Parts Express that sells all different kinds of speaker insulation (and other stuff for cab making). I believe they are at www.partsexpress.com. They have some insulation thats in sheets...not all fibery and messy...plus spray on glue to hold it in there!!
     
  4. Rockbobmel

    Rockbobmel Supporting Member

    I tried the polyfill in my mesa cab and yanked it out. Fiberglass was the only thing that sounded good. My 8-10 has 2' x2' pieces about 8" thick stuffed to the back of the compartments (4 of them).
     
  5. Dampening materials affect the volume of the cab in different ways, so you need to replace like with like, otherwise you will affect the cab tuning.

    Fibreglass slows down sound waves, so the cab behaves as if it is larger (sound takes longer to get to the cab wall and back)

    Polyester (and foam I think) do not slow down the sound waves, so they just reduce the volume of the box.

    I don't have the links to hand, but most DIY HiFi speaker sites will get you to the information.
     
  6. Rockbobmel

    Rockbobmel Supporting Member

    That makes sense....
     
  7. Zooberwerx

    Zooberwerx Gold Supporting Member

    Dec 21, 2002
    Virginia Beach, VA
    I actually have a free and abundant supply of "eggcrate" foam material in both 2" and 4" thicknesses. It's probably the same as the acoustic eggcrate that I see advertised in www.partsexpress.com. I believe it is intended for studio installation. Fiberglass is also a cost-effective alternative. The existing material is the weirdest sh** I've ever seen, almost like a dense mattress filler.
     
  8. Rockbobmel

    Rockbobmel Supporting Member

    It sounds like you are hell bent on "other than fiberglass"
     
  9. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    Actually, the filling in a sealed cabinet like that (or a ported cab for that matter) isn't to slow down the sound.

    What it does is to absorb heat! Yes, heat.

    When you compress air, even a little, it heats up. Has to.

    Heated air expands. So the heating causes the air to "push back" harder than it would if it didn't heat up. The total heating in degrees is small, but it does occur.

    The stuffing acts as a heatsink and keeps the air from heating as much by absorbing some heat. So the "push back" isn't quite as much as it would be.

    The reverse occurs when the speaker moves out. There would be a "pull" on the speaker as it expands tha air volume slightly. But as the air expands the filling gives up a little heat to the air. The slight heating of the air relieves the "pull" on the speaker a little, acting like a somewhat bigger cabinet.

    It is the difference between what the physics folks call "adiabatic" and "isothermal" compression. Don't worry about the names, though, it works.

    The polyfill probably wouldn't work as well, since plastic doesn't conduct heat well. That means the heat may only get into (or out of) the surface of the fibers in the very short time available, so it won't "hold" as much heat. Then it will not absorb as much heat, and the effect should be smaller.

    The actual fiberglass fibers conduct heat pretty well, compared to plastic, although the stuff is actually insulation. So more of the fiber is available to hold heat. Think of a glass window vs a plastic one in the winter, which one feels colder?
     
  10. Zooberwerx

    Zooberwerx Gold Supporting Member

    Dec 21, 2002
    Virginia Beach, VA
    Thanks for the explanation. I'll try the 8" fiberglass. Is it necessary to line the entire chamber or just the back surface? I think I'll finish it off with some nice asphalt shingles.
     
  11. I note that you work for Ampeg, and I admit that I am not a speaker design expert, but I have some concerns about your explanation.

    It was not my intention to suggest that the fill in a speaker cab was to slow down the sound. I think we agree that its primary purpose is to absorb energy so that the sound does not flex the cab walls, or reflect back to the speaker cone and make it honk or boom. I also think that we agree that speakers cones behave differently when they move out, and stretch the air in the cab, to when they move in, and compress the air in the cab.

    My understanding is that speaker units are less than 10 % efficient in converting electrical watts into sound watts. That means that your 300 watt head is dumping 270 Watts of heat into the speaker coil, and this will be radiated into the speaker enclosure. Compared with that, it seems unlikely that each fiber is being heated up and cooled down 40 times a second, to a degree (no pun intended) that would be audible.

    Another factor that does not seem to fit with what you said is with transmission line speaker designs. These work on the principle that there is no enclosure pressure, and you delay the sound wave from the back of the speaker until it is in phase with the sound from the front of the speaker. You need to make the resonant frequency of the tube which gives the delay, lower than the lowest note you want to reproduce. You do this by selection of the tube length, and by the type and quantity of the filling you put in. Raw wool and fibreglass work well, polyester doesn't work well.
     
  12. Zooberwerx

    Zooberwerx Gold Supporting Member

    Dec 21, 2002
    Virginia Beach, VA
    Boy oh boy, this is getting interesting! What material does Ampeg use in the manufacturing process? I'm not sure how stock or original my 810 really is. The stuff actually looks like compressed wool filler. I saw an article in Bass Player detailing the differences amongst sealed, ported, and transmission line cabinets. Even the builders can't agree on much!
     
  13. Jerrold Tiers

    Jerrold Tiers

    Nov 14, 2003
    St Louis
    It does that also. Depends on how much you put in. Line the walls and you damp reflections with minimal effect on volume. Stuff the cabinet and you change effective volume AND damp reflections.

    not really, those are the same operation with a different "sign" on the value (+ or -). One does indeed have a limit on its value......there isn't any limit on pressure, but you get to a vacuum and that's it for expansion. For small percent changes it is approximately linear.

    The magnet heating "signal" you refer to is "DC" if you want to call it that, just a change in ambient temperature. Density of the air changes, and tuning can vary as a result, of course. (the amount of air inside the speaker is not fixed long term, it can leak in or out).

    The "AC" signal of the slight heating and cooling is not really affected by the absolute temp. Air is fairly compliant with the "perfect gas law", in which pressure, volume, and temperature are related in a linear proportion. So the effect remains, but the center temperature is simply shifted up.

    Transmission lines don't need filling to work in the most basic sense. However, the filling does affect the amount of high frequency back wave that comes out the tube. Less of that is better.

    The filling also has the effect of frictionally lowering the "Q" of the resonance, making the resonance of the tube less apparent. If you are seeking a wide band delay, you may want a low "Q" to avoid the abrupt phase shift and amplitude change through resonance.

    Whether or not packing affects the effective length, I am not sure. It should, because it would tend to slow the wave to some degree by mechanical interference, if nothing else. By the end of a relatively long tube that might add up to something. And effective tube length increase it is a form of volume increase, which could conceivably be accounted for on the "heat theory", of course.

    The usual figure I recall for packing and volume increase is somewhere around 25% effective volume increase. That is fairly substantial.

    I don't have figures on it, but it does not make intuitive sense to me that the speed of sound would be dropped anything like that much by some fairly loose material. I don't see a clear mechanism for that.

    The polyester theory I suggested need not be the correct one. It makes some sense, however, and provides a possible reason why that material should not work as well. The fact that polyester in fact does not work as well tends (in a somewhat circular form of reasoning after the fact) to support the theory.

    Most of our cabinets are merely lined to prevent reflections.
     
  14. I think that part of my confusion is due my limited working knowledge of reflex cab design, not being wholly relevant to a sealed box or a transmission line, so I have learned something.

    I still struggle with the concept of a glass fibre hair heating up and cooling down 42 times a second. Pre-warming my thermos flask takes several minutes, and water is a better heat conductor than air. I can't argue with it, I just struggle to visualise it.