From: DPierce@world.std.com
Subject: re: Celestion SL6S port recommended
Originator: tjk@csd4.csd.uwm.edu
Organization: University of Wisconsin - Milwaukee
Date: Mon, 14 Sep 1992 12:58:48 GMT


Brent Fry write at length...
>
>I have owned Celestion SL6S speakers for some 6 years now.  Never liked
>the sound, and especially agree with comments many have made that they
>are dull, and lack bass extension.  Aluminum tweeter and about 8"
>mid-bass unit, two way design, British stuff.
>
>Well, years ago I removed the squidgy gray "acoustic" foam which stuffed
>them full, and used more sensible damping materials like ordinary ceiling
>acoustic tile on one surface, and a dollup of quilting inner lining material.
>Minimum damping to achieve excellent Result: much improved clarity!

Minimum damping is an understatement indeed! "ordinary ceiling acoustic tile"
is probably one of the most ineffective materials imaginable for this
application. A sheet of similarily sized plywood has characteristics not
unlike it (no smileys!)

>But (shock, horror to Celestion engineers) what I've done now (to this
>sealed box design) is *ported* it !!  The port is located in the front
>lower quadrant, some downspout pipe roughly 3 inches in diameter and 7
>inches long.
>
>Anyone owning SL6S's who has an interest in more details, is welcome
>to mail me.  I would describe myself as a very discriminating listener,
>and the result is truly magic in the areas of clean bass extension, and
>overall increased clarity throughout the midrange.

I reply here at the very high risk that Brent will take great offense at
whatever criticisms are advanced, but here goes anyway.

No, I don't think the Celestion engineers will fall into uncontrolled shock
and horror at what you've done. Maybe a bit of mild bemusement, but hardly
shock.

The design requirements for a sealed box and vented box systems are very
different. The people at Celestion are no dummies, they know how to design
a proper sealed box. And they know how to design a vented box. The chances
of the driver parameters for the woofer being suitable for both vented and
sealed box usage are low, but not non-existant. The chances of a given box
volume making a suitable enclosure for both sealed and vented use are, though
precisely 0.

If a driver was designed to be usable for both (we'll skip the math for the
moment), the optimum volume for a vented box implementation would be nearly
twice the size of one for sealed box use. Once that's determined, how do
you come up with the port size and length? Well, that's intimately tied to
the driver parameters and the box volume and the desired response function
of the system.

The chances of you having achieved a system that has REAL bass extension,
smooth low frequency response AND appropriate mechanical power handling
capability is exceedingly low. However, that's not to preclude that the
system you ended up with (however strange) is something you like.

>I'd never again touch another "acoustic suspension" (sealed box) design
>as a result of my long experience tweaking the Celestions.

I'd never touch a mod like this (converting a sealed box to a bass reflex)
as a result of my (slightly-less-than-humbly-admitted) very long experience
in loudspeaker design.

>An interesting side effect is that, even though I am not a head-banger,
>prior to the porting I had to replace the bass drivers twice, due to radial
>cracking of the surrounds.  With the port, those nasty instantaneous pressure
>peaks just aren't there any more.

This is bunk. Precisely what "nasty instantaneous pressure peaks" are you
talking about. If you would care to explore the theory or measure the reality,
the forces on the surrounds are no greater in a sealed box than in a reflex.
IF the reflex system is properly designed, you can achive a significant 
reduction in cone excursion near the box cutoff frequency, but above and 
below that frequency, the cone excursion will be exactly the same as a
sealed box. And especially much below, they are greater:

>The speakers move like mad (subsonically), and the sound is great.

No shit!

>Highly recommended.

Doubtful, in an objective sense.

Bass reflex (vented box) loudspeaker systems have clear advantages over
sealed box systems. The advantages include about twice the efficiency-
bandwidth-box volume constant, reduced excursion at a particularily narrow
band of frequencies near box cutoff with reduced distortion and somewhat
reduced mechanical excursion requirements.

The chances of achieving these goals simply by pounding a chunk of pipe
the front baffle are vanishingly low. The particular set of advantages I
just outlined are achieved over only a fairly narrow range of "alignments",
the integrated combination of driver characteristics, box size, vent size
and internal absorbtion characteristics.

Without any doubt, this exercise produced a system that sounds radically
different than it originally did. I will be the first to endorse the theory
that you've made an audible change. That you made an improvement in the
true frequency response/transient response/power handling/efficiency sense,
well, I will merely partake in the bemusement of the original engineers :-)


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