From: DPierce@world.std.com (Richard D Pierce)
Subject: Bunk about Re: Speaker vent placement question
Organization: The World Public Access UNIX, Brookline, MA
Date: Wed, 27 Jan 1993 19:01:07 GMT

In article <1993Jan27.074935.1713@nwnexus.WA.COM> Scott Rowin writes:
>
>Porting is best done at the rear, 180 degrees out of phase with the
>cone, 

AAAARRRRGGGGHHHH! 

This has got to rank at the top of the nonsense scale! Okay, class, repeat
after me:

   THE OUTPUT OF THE PORT IS IN-PHASE WITH THE WOOFER AT BOX RESONANCE NO
   MATTER WHERE YOU PUT THE DAMN PORT! 

PERIOD!

>also the port should be moved up slightly so the woofer dosen't have
>direct-firing through the port (middle or higher of the box). 

Nonsense! If that were true, then it would completely refute your claim
that the rear is the best place. The combination of the mass of the air in
the port and the compliance of the air in the enclosure combine to make a
second-order low-pass filter whose cutoff frequency is the box resonance
frequency, typically right above the speaker's lower end cutoff. The
output of the port, therefore, will diminish at a rate of 12 dB per octave
above that point. 

>The reason
>for rear mounting? Front mounting could incur the infamous whisting effect
>where rear mounting can eliminate some of that since it fires away from you.

Why? The "whisting" [sic] effect is due to non-linearities in the port
itself caused by excessive velocities inducing turbulence effects within
the port. It makes no difference where the damn port is. Yeah, putting it
the rear will reduce the amount of high-frequency effects that find their
way to the listening position, but WHY NOT DESIGN THE PORT PROPERLY TO
BEGIN WITH?

>There's been many to argue on it, but I find that if you must front-mount
>at least keep to mounting the port a decent distance from the cone so as
>to eliminat coupling that can cause distortion.

More unsubstantiated nonsense!

Mutual coupling between the port and the woofer DOES NOT cause distortion.
No more than the mutal coupling between multiple woofers or multiple
ports. The mutual coupling is at any reasonable sound pressure level below
the dissassociation of air molecules is a completely linear process and
does not introduce any distortion products. It may have a minor influence
on frequency response, and that influence may or may not be good. but it
DOES NOT cause distortion.

>The experts agree typically on rear-mounting ports

What experts? Care to list the names of these experts and, by enumeration,
prove your assertion the "the experts agree?" If you do, I'd wager you'd
fail. 

>and the "bible" of loudspeaker design (Loudspeaker Cookbook) mentions rear
>mounting as the optimum position.

First, I don't think anyone, save yourself, considers Vance Dickason's
"Loudspeaker Cookbook" the "bible" of loudspeaker design, inclusing Vance
himself. It is a cookbook. Period. It has pre-cooked tables and cormulas
that allow you to have a reasonable shot at getting a home-brew design
working the first time. Period. As a cookbook, specifically targetted at
amateur home builders, it's very good. As the definitive text on
loudspeaker design, NOT! Vance graciously and correctly acknowledges a
wide variety of design and theory articles written by a wide variety of
experts, including me!

Second, where does Dickason mention that rear mounting is optimal? The
only passage I can find says the following in the narrow context of 
pipe-like resonance in vents:

    ".. pipe resonance usually only produce minor changes in driver
    response, and are typically difficult to separate from other
    driver response anomolies. Pipe resonances are extremely 
    unpredictable and are generally dependent upon their location
    on the baffle, vent proximity to enclosure walls, and the
    location of damping material in the immediate vicinity of the
    vent. The rear of the cabinet is an alternative vent location.
    This will cause some change in low-end output, depending upon
    where the speaker is located in the room, but will tend to
    subdue the subjective imprtance of vent-related noise problems."
        V. Dickason, "Loudspeaker Design Cookbook," Fourth Edition,
        pp. 54-55, Audio Amateur Press, Peterborough, NH 1991

Hardly an unambigous, ringing endorsement of rear-mounted vents as
"optimum."

You've made some pretty wild assertions here, unfortunately for you, all
of which are completely testable and falsifiable, in the grand tradition
of the scientific method. I thus challenge you to prove your assertions.
-- 
|                Dick Pierce                |
|    Loudspeaker and Software Consulting    |
| 17 Sartelle Street   Pepperell, MA  01463 |
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