I'm confused on how to determine the tuning frequency of the port when building a box to the specs. As recommended on the spec sheet of a subwoofer, I've always thought that you matched the port (tuning) frequency to the "FS" (in the Thiele Small parameters) on the spec sheet. Is this not correct? If not, how should I calculate the frequency of my ported boxes? I use a box building program called BOXNOTES that allows you to calculate all the necessary dimensions needed to make a custom subwoofer enclosure, including the port size and frequency, but nothing says how you determine the frequency of a subwoofer.Donald Miller
A: The Thiele Small parameters that you find on the specification sheets of subwoofers (and other drivers) are meant to be plugged into a series of formulas or equations that tell you the ideal cabinet that the sub is meant to fit. In the case of a sealed box equation, the two key formulas tell you the optimum box size and the expected f3 point. F3 is the point where the bass energy has dropped by 3dB and anything below that is considered to not be present anymore. One variable of the sealed box equation is "Q" factor, sometimes called "quality" factor. Q represents the nature of the resonance that the box and speaker combination will exhibit. A Q value of 0.707 is considered best for sound quality. A higher Q means the box becomes somewhat smaller and a bit boomier rather than tight. A Q lower than 0.707 means a bigger box, better damping, but less power handling.
For a ported box, there are three key equations. One determines the optimum box size, the second determines f3 point and the third reveals the "ripple" or amount of deviation from flat-frequency response. You would plug the Fs value into the formula to determine the box size and f3 point. Fs is the natural resonant frequency of the speaker, not the lowest bass note produced, but it's critical in telling you the resulting f3. This is critical, since ported cabinets tend to rip apart subs if you feed them energy at frequencies below the f3 point.
One of the formulae results in the "Fb" or "box tuning frequency," which in turn is plugged into another equation that gives the port length based on the preferred diameter.
There are other formulae for ported designs depending on the type of port and cabinet interaction you want. Each of these formulae gives a different sound quality and boominess that can be characteristic of a poorly designed box. Essentially, your box design software takes the Thiele Small parameter in and spits out the answers based on these "alignments."
A couple dozen different Thiele Small parameters make up the electrical and mechanical characteristics of the speaker. Some equations and computer programs use more of them than others and deliver a clearer picture of what the speaker will do when you actually build the cabinet. If you don't have a software package like LEAP, Term or Bass Box Pro, the numbers won't do much for you. With the equations, you can get basic information like cabinet volume and expected bass extension limits (f3). Even if you do plug in the numbers and get a printout, the box and speaker combination that results may still not please your listening tastes.
There's a lot of experimentation that can be fun, if you have the time. If you can imagine this analogy: A sealed box is like juggling two objects - the speaker and the box. A ported cabinet is like juggling three objects - the speaker the box and the port. This is more difficult, with more variables and can lead to a bad mismatch if you don't understand what you're doing. The equations are found in a well-respected publication called The Loudspeaker Design Cookbook by Vance Dickason, available from Old Colony Sound Labs or amazon.com. For computer software, feel free to try my old but still working MobileDesign program for PCs, unless you have the budget for some more exotic stuff. This program is a crude little Thiele Small calculator that still works pretty well. Go to www.mobiledynamics.com/special.html and click on the icon in the "Free Box Design Software" section. Have fun!