"Semi Inductance".

It's a term that most of you might not be familiar with, but you're likely going to find it being used more and more often, particularly with subwoofer design :-).

Traditionally, designing an "alignment" (combination of box size, tuning, etc) for a particular subwoofer driver involved using parameters that basically described the driver's characteristics, these being the ones that most of us are familiar with - Vas (equivalent air compliance), Fs (resonance frequency) Qts (basically a measure of the driver's tendency to resonate at Fs), Re (DC impedance), and Le (inductance at a specified frequency, usually 1kHz for subwoofers).

It's that last one, Le, that's of interest here. The fact that it's specified at a particular frequency suggests that perhaps it does not actually remain constant, and that's usually the case. In fact, with today's high performance subwoofer drivers, Le can vary quite significantly. So here's the thing - if the modeling program that you're using assumes that Le remains constant (and most of them do), then there's likely to be a noticeable difference between the simulated results and the actual measured results when you build what you simulated, and if Le changes significantly, that difference is going to be more significant, to the point of where the measured response does not look anything like what the simulation said it should be.

Enter "semi inductance". Without getting into the nitty-gritty details, the "semi-inductance" model is an extension of the basic t/s param model, and it uses several additional parameters that "describe" how the driver's inductance changes with frequency, the end result being a simulation that predicts performance significantly more accurately. To put it in simple terms, using a simple t/s parameter based box modeling program will put you in the ballpark; using a modeling program that uses the semi-inductance model can put you on first base, or at least very near it.

Those extra parameters? Well, they are not typically quoted by speaker manufacturers (yet!), but they can be derived from the driver's impedance curve. Thankfully Parts Express does publish the ZMA files for many of the drivers it sells, so that makes getting hold of the "semi-inductance" parameters for those drivers at lot easier. Otherwise you'll need to use a tool like DATS, measure the driver's impedance curve, and then use that to determine those additional parameters.

Anyway, a few links that might be of interest:

Thread on DIYAudio where the semi-inductance parameters for many drivers are being published:

http://www.diyaudio.com/forums/multi...nductance.html

A worked example, using Hornresp

https://www.diysubwoofers.org/misc/semi-inductance.html

It's a term that most of you might not be familiar with, but you're likely going to find it being used more and more often, particularly with subwoofer design :-).

Traditionally, designing an "alignment" (combination of box size, tuning, etc) for a particular subwoofer driver involved using parameters that basically described the driver's characteristics, these being the ones that most of us are familiar with - Vas (equivalent air compliance), Fs (resonance frequency) Qts (basically a measure of the driver's tendency to resonate at Fs), Re (DC impedance), and Le (inductance at a specified frequency, usually 1kHz for subwoofers).

It's that last one, Le, that's of interest here. The fact that it's specified at a particular frequency suggests that perhaps it does not actually remain constant, and that's usually the case. In fact, with today's high performance subwoofer drivers, Le can vary quite significantly. So here's the thing - if the modeling program that you're using assumes that Le remains constant (and most of them do), then there's likely to be a noticeable difference between the simulated results and the actual measured results when you build what you simulated, and if Le changes significantly, that difference is going to be more significant, to the point of where the measured response does not look anything like what the simulation said it should be.

Enter "semi inductance". Without getting into the nitty-gritty details, the "semi-inductance" model is an extension of the basic t/s param model, and it uses several additional parameters that "describe" how the driver's inductance changes with frequency, the end result being a simulation that predicts performance significantly more accurately. To put it in simple terms, using a simple t/s parameter based box modeling program will put you in the ballpark; using a modeling program that uses the semi-inductance model can put you on first base, or at least very near it.

Those extra parameters? Well, they are not typically quoted by speaker manufacturers (yet!), but they can be derived from the driver's impedance curve. Thankfully Parts Express does publish the ZMA files for many of the drivers it sells, so that makes getting hold of the "semi-inductance" parameters for those drivers at lot easier. Otherwise you'll need to use a tool like DATS, measure the driver's impedance curve, and then use that to determine those additional parameters.

Anyway, a few links that might be of interest:

Thread on DIYAudio where the semi-inductance parameters for many drivers are being published:

http://www.diyaudio.com/forums/multi...nductance.html

A worked example, using Hornresp

https://www.diysubwoofers.org/misc/semi-inductance.html

## Comment