sound dampening insulation?

[bigstupool]

hello just had my altec 620a cabinets built and was wondering exacly how much sound dampening insulation is needed for the 604-8gs in thease factory spec. boxes? any opinions would be a great help thanks stu

[Jim Butler]

HI Stu,
I have several original Altec 620A cabs. They all have the same amount of insulation and it is the yellow fiberglass Altec always used in its 70's cabs. In the 620 it completely covers all interior surfaces except the front baffle.
JIm

[Todd W. White]

Hi Stu,

The standard Altec method was to cover three non-parallel sides with fiberglas.

One of these sides must be the rear (I hope you had it and the sides, top, and bottom well braced internally), plus two others. I usually go for one side and the top.

You can use standard fiberglas found at the lumberyard, or use the stuff that they sell at RadioShaft...just be sure if you use the stuff from the lumberyard that you remove the Kraft paper backing - it will buzz when you get the bass notes going and generate a sympathetic vibration...much like tissue paper on a comb - VERY annoying!

I prefer to glue the stuff in place. Stick the piece you're planning to use inside and make sure it fits. I usually make each piece 1" smaller all the way around than where it's going. Get the piece inside, then apply the glue.

I use standard carpenter's wood glue. Turn the cabinet so that the side you're going to glue to is face up (you might want to lay it on carpet or a blanket so the cabinet won't get scratched up), then put glue on the side of the fiberglas that will go against the wood. Carefully position the fiberglas onto the wood. Allow to dry. Repeat this process for the other two sides.

Some people like to staple the fiberglas in place with a staplegun, but the fiberglas tends to pull away from the staples over time. PLUS you DON'T want any of those staples to get loose and get into your speakers!

Hope this helps!

[Steve M]

Hello Todd,

I am curious to know what the downside would be do you think of using fiberglas on both sides and also the top and bottom?

Also just wondering, is there a reason for choosing the top as a preference and not the bottom?

Many thanks for all the wonderful information on this site.

[GM]

As insulation resistance increases, the system Q goes down, rolling off the LF output. if the cab is well designed/built, you only need one side of parallel surfaces covered to damp the mids/HF standing waves so they won't be reflected back through the driver and vent. Normally you never put insulation on the surface nearest the vent as this affects output at Fb.

Really, with what we now know about cab design, once you move away from a golden ratio cab you should ideally put the vent at the bottom and only loosely distribute damping material from the top down to the bottom of the driver, leaving the rest bare to maximize 1/4WL standing wave pipe action. I much prefer this way to the old 'just stick a vent anywhere' and line the cab to damp the acoustic mess it makes typical of early reflex designs. Where in this 'pipe' the driver is positioned also has an audible impact on performance, so there's more gains to be had in overall performance.

GM