From: DPierce@world.std.com (Richard D Pierce)
Subject: Re: Port Frequencies
Organization: The World Public Access UNIX, Brookline, MA
Date: Wed, 23 Dec 1992 05:25:22 GMT

In article <1992Dec23.034027.10359@udel.edu> bower@pecan.cns.udel.edu (Ty Bower) writes:
>Does the tuning frequency of the port/box depend solely on the three
>parameters which Dick mentioned? (port length, port diameter, 
>enclosure volume)

Essentially, yes. But more specifically, it depends upon enclosure volume
and port acoustic mass. You can achieve this port mass through a wide
continuum of port diameters and lengths.

>Does the shape of the enclosure play any role?  Is there a "best"
>location to place the port?

Over a reasonable range of proportions, the enclosure shape plays a very
minor role if at all.

The best location is the dependent upon many things. Probably closest to
the woofer is best (enhances port-driver mutual coupling, which some feel
may not be good). Also, any end should be at least 1 port diameter away
from any obstructions.

>Why do some speaker manufacturers choose to place the port on the
>front of the box, and some choose the back?  Does this affect the
>sound?  

Because some people are smart, and others are stupid! The idiots who make
bookshelf speakers with the ports in the rear must have their heads up
their asses (BIC is one manufacturer who comes to mind in this respect).
As soon as you shove that speaker up against the wall, you can rest
assured your speaker ain't working anything like it was designed to!

>Is it possible to substitute the conventional "throated" port
>(i.e., a tube sticking into the box) with a simple tubeless
>vent, or would it have to be of such a small diameter to make
>up for it's lack of length that it would "whistle" as air passed
>through it?

Yes, but even an aperture in an infinitely thin enclosure wall has an
appreciable acoustic mass, so that the diameter, while smaller, may not be
so small as you are concerned about.

>Am I asking too many questions?  Should I just read a book on
>the theory of Helmholz resonators?  Where can I find this book?
>What is it called?  Or is the whole thing mostly voo-doo?

No, don't read a book on Helmholtz resonators. It will tell you a lot
about Helmholtz resonators (which a ported cabinet certainly is), but it
won't tell you diddly-squat about loudspeakers.

>If it's not voo-doo, why can't they _accurately_ calculate the
>ideal length of a port for a given enclosure?  I've been told
>that the only way to do it is to make a port tube that is too
>long, then start shaving it down until it "sounds (measures)
>right."

This is nonsense. The theories covering the operation of bass reflex
systems is quite complete and agrees astoundingly well with the real
world. The main reasons authors recommend the above is that most people at
home do not have the facilities to accurately determine the actual
operating parameters of the drivers, so they can only guess at what the
actual enclosure parameters must be. Or they don't know what they are
talking about.

If you can measure your drivers Thiele-Small parameters accurately, then
you can design your enclosure just as accurately. The people who are
advocating the cut-and-try method are certainly well-intentioned, but
after you get done perfectly tuning your enclosure in your damp, unheated
workshop where the relative humidity might be 75% will find that all your
careful tuning to the nth degree is gouing to fly right out the window
when you carry your near-perfect speakers into your centrally heated house
where the winter R.H. might be 10% and the paper cone in the woofer looses
its moisture and the cone mass now drops 10% as a result (not uncommon)
and the resonance goes up and the Qts goes down and so on.

Yeah but you said it's all well understood, Dick! Yeah, it is well
understood. I didn't say it was easy to control, though (actually, this
particular problem is real easy to solve: don't buy drivers with cones that
are hygroscopic!).

But, no, there is absolutely no voodoo needed whatsoever. Just a good
understanding of the principles involved is all. I've designed many
enclosure for clients, and I pretty much get it right on the first shot,
including adjusting for expected driver production variations.

-- 
|                Dick Pierce                |
|    Loudspeaker and Software Consulting    |
| 17 Sartelle Street   Pepperell, MA  01463 |
|       (508) 433-9183 (Voice and FAX)      |
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