Altec 620 cabs

[ljungberg]

Hi,

I have just finnished my new 620 cabs that will house my 416-8a woofers.

How about internal damping on these cabs? After looking around I have seen that folks using isolation on one of the side internal walls and half of the back. Is this something to laborate with or a standard that Altec used on this type of cabs?

Br
J

[Elitopus1]

In my Hillcraft 620 cabs there is yellow fiberglass insulation, about 3-4" thick, on the all inside walls except the baffle side. There is bracing inside but no divider between the top and bottom section

[GM]

The goal is to sufficiently damp down any reflections from bouncing off the backside of the woofer[s] diaphragm[s], hopefully without too much loss of acoustic efficiency, so the amount will vary depending on the material used, ergo unless response measurements are used, the typical DIYer damps by ear and only the woofer[s] should be connected, i.e. disconnect any horn[s], etc. from the test signal.

Altec's rule-of-thumb for a vented alignment was to line only one of each parallel wall [none on the baffle or down in the vent area], so the top and one side, back down to the top of the vent area with either 1" or 2" thick acoustic fiberglass insulation [O.C. 703 'duct board'] depending on how big the cab was and only add more thickness and/or the other side wall if required; whereas many folks prefer to line both side walls initially and call it 'sufficent'.

IOW, once the 'hollowness' [under-damped] is sufficiently damped down, this is typically all that's required and any more begins to over-damp both the woofer's HF BW as well as rolling off the bass [over-damped], which in some rooms/locations and/or personal preference is desirable, though normally better to damp just the vent as required [or even seal it off in extreme cases] as it just impacts its [mid] bass output.

Nowadays, a lot of folks have issues with using FG, with nothing being close WRT its damping properties AFAIK, though in recent years they've been happy with the inexpensive shredded cotton [jeans] as a'good enough' substitute.

GM

[ljungberg]

Wow,

Thanks. That is what i call a detailed reply. Much appreciated.

[endeeinn]

GM Did you once talk about Cotton or Polyester Batting as a sub for Fiber Glass? Philmore Upholstery Supply :: Cotton :: Cotton/ Polyester Blend Batting Rolls 15-16 lbs.

[Elitopus1]

I wonder if they sell that cotton batting at a local craft store. It probably wouldn't be cheap to ship that 20lbs roll.
I like the idea of using something else than fiberglass. Especially in ported cabs

[GM]

Thanks.

You're welcome!

GM

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GM Did you once talk about Cotton or Polyester Batting as a sub for Fiber Glass?

Other than the treated stuff I mentioned, no cotton. Polyfil has been the most popular with DIYers since I got on-line in '96, but beyond a bit of experimenting, FG has always been my recommendation/choice except for when Miraflex [~itch free FG] was available. This was great stuff except for needing more thickness to match it to the FG duct board or un-backed house insulation.

GM

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I will use acoustic wool.

Or just use wool felt premium carpet underlayment, a once popular substitute for FG, which at one time [at least in my locale] was free for the taking when a large [or at least profitable] business office got new carpet. I lined a bunch of various size tower, bookshelf, PA speakers with just the hallway 'runner' from a law firm near me at the time. The rest made my bedroom's cheap carpet feel very plush/expensive and still does nearly 40 yrs later!

Unfortunately, every carpet layer I know now use foam or its cheap 're-bond' variant, but may still be available in some locales.

GM

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It may not be an issue but many natural fibre fabrics are susceptible to insect infestation.

Moisture absorption may be a problem with some natural fibres in humid locations and can lead to mold and mildew presence.

Yeah, down here in the Deep South, these can be a real problem if you don't have an on-going good pest control contract and central heat/AC year round climate control. Even then, mold/mildew can still be a problem if there's any roof and/or foundation leaks into an unsealed dirt 'crawl space', which caused me a near $6800 repair last year.

GM