Most people recommend mounting a woofer back to front (magnet outwards) when doing the closed box method for this reason. Much easier to measure and calculate the cone shaped extra volume introduced in the front of the cone than work out how much volume to subtract for the complex frame/magnet structure on the back...
Having said that I prefer the added mass method.
Thanks, that's the sort of thing I need to worry about.Vas seems really hard to measure, but I've never used the sealed box method. Maybe time to build one for testing.
Have you abandoned the added mass measurements, or have you found any way to get good results?
It is not that difficult. Come on now. I use both mass displacement and sealed box. You can do ported too. I do two methods enough times until I get pretty consistent results. When I get within 10%, I know I am doing it correctly. Old by hand, (generator and VTVM) or with WT-2. Same results, but you will really like the WT-2 for ease.
Be aware, you get very different results depending on power. If you measure with miliwatts or with several watts. Again, see the good DR. Joe's book on this subject.
Now what IS hard to deal with is Le. Even if you glue the voice coil, it is still tricky. I have questioned several manufactures and test equipment makers. None have definitive answers.
Why would you want to glue the voice coil to measure Le ?
Given that Le is frequency dependant and also dependent on the motion of the cone (modulation of the solenoid effect of the centre pole for one) it makes no sense to attempt to measure it with cone movement artificially constrained. What matters is how it measures when the cone is free to move in normal conditions.
Besides, Le is of little use in practice - the only real use for an Le figure is an approximate first pass attempt at calculating a zobel network - which will usually be in error anyway since Le varies with frequency.
For accurate calculation of a zobel network or the crossover driving the driver a full impedance curve is needed.
I wouldn't lose any sleep over not having an accurate Le figure (since no single figure accurately describes the situation) provided I had a full impedance curve to work with.
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I use added mass method as well because its the most convenient at amateur level but most manufacturers i've seen use the closed box method.
I find Le pretty important, specially if you don't have a set of full data on the driver. You certainly need it to construct an impedance model, Le value tells a bit of story on the freq range of the driver and most important if you put it aside with some physical parameters you can tell a few things about the construction. e.g. if it has a faraday ring or not... things like that.
I find Le pretty important, specially if you don't have a set of full data on the driver. You certainly need it to construct an impedance model, Le value tells a bit of story on the freq range of the driver and most important if you put it aside with some physical parameters you can tell a few things about the construction. e.g. if it has a faraday ring or not... things like that.
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If you don't have a full set of data on the driver then you're not going to be able to design a crossover for it, Le or not. Once you have the driver in your hands and have measured the impedance curve you no longer need the Le value because the impedance curve tells you everything the Le value would have (and more) and far more accurately.
Yes but any impedance model constructed from the Re and Le values is going to be wrong because Le is not a "pure" inductance, but varies with frequency and other parameters. At best it is only a very rough approximation, unsuitable for designing a crossover against, it's far better to simply measure the impedance curve and import that data.
Not really, for a given driver if you have different versions of the driver with different inductance (shorting ring versus no shorting ring etc) you can predict a change in frequency response, (better to just measure it though) however if you are comparing two completely different design drivers and looking at their Le values it really tells you nothing useful about their frequency response.
I'll give you that one
Looking at Le is a good way to see if the driver has a shorting ring or not, for a given size and class of driver it's easy to see if it has a "normal" Le or a very low one. It's about the only thing I use the Le value for.Other than that I still say its not a very useful parameter, and obtaining a highly accurate value is both not necessary and not possible. As long as you measure the impedance curve you have what you need to know far more accurately than trying to model it based on Re and Le.
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