Vented Enclosure: Overview, Background
Andy Lewis rev 6/27/2011
Part 1: How Does A vented Enclosure Work?
Once again, we rely on Richard H. Small (of Thiele-Small fame). He figured out the vented system, just as he did the sealed system. I can't cite the exact issues of the Journal of he Audio Engineering Society in which his work appeared. Maybe you can clue me in. Mine went the way of my "Wheels of Fire" album.
One of them was the June, 1981 issue, in which he published a piece with Margolis, entitled "Personal Calculator Programs...Approximate Vented-Box and closed-Box Loudspeaker System Design." I also used D. B. Keele's interpretation of Small as published in David B. Weems' important summary of cutting-edge design at the time, "How to Design, Build, and Test Complete Speaker Systems" in 1978. Great stuff. (I was using Small's work before I understood who he was!)
But at any rate, the relationships Small laid out for us can be presented in such a way as to graphically maximize our understanding an a very real-world way. This has been my approach. I believe the sheets I present to be the most useful illustrations I can envision, and I've never seen anything quite like them.
So again, we start with the most abstract of examples, the General Vented Enclosure. This is a spreadsheet which allows you, much as was the case with the General Sealed Enclosure, to invent out of thin air, an infinite number of vented systems, based on parameters you choose.
Specifically, I have chosen to start by modeling a simple sealed enclosure, and then to write a sequence which would allow us to add a port of choice to the once-sealed system,
There is, of course, another parameter, the tuning frequency, known as fb, which doesn't exist in a sealed system.
So, the allows us to create a vented loudspeaker based on four parameters:
Sensitivity, expressed in dB.
fc = in-box resonance
Qtc = in-box system damping
fb = vented tuning frequency
That's it. Four variables.
If you experimented with the spreadsheets pertaining to sealed enclosures, you know the drill. There are instructions on the spreadsheet. So here it is:
Introduction: Model General Vented System
Part 2: How to Apply this to a Specific Woofer.
Here is a link to the worksheet which predicts the performance of a specific woofer in a vented enclosure: Vented Enclosure Specific Woofer.
The Model General Vented System spreadsheet obviously can't be used to design a vented enclosure, only to illustrate how one works. When using a specific woofer, there are far fewer possibilities. The characteristics and parameters of the driver define what can and can't be done with it.
The Sealed Enclosure Background page gives an explanation of the interconnectedness of enclosure size, damping, and resonant frequency. No need to repeat it here.
To use this sheet, the parameters of the specific woofer must be entered.
As is the case of the Sealed Enclosure Specific Woofer, we enter the Qtc of the sealed enclosure, after which the computer calculates the effective enclosure volume and sealed-system damping, Qtc.
Again, what remains is to enter the tuning frequency of the vented enclosure, fb.
You can then model then performance of your woofer in any size enclosure you want.
Remember, enter the parameters of your woofer, first.