Calculating Xmax of Loudspeaker

[lubbie]

Anyone know of a formula for calculating Xmax from all the other speaker peramaters??? Like Vas, Diam.

Ralph

[kelticwizard]

To my knowledge, you can't get it from Vas, diameter,or any Thiele-Small parameters.

You can get it from length of voice coil and length of magnetic gap, (air gap). These measurements are frequently listed.

The formula: Excursion(+ or -) = (voice coil length - magnetic gap length) / 2.

Example: A speaker with a voice coild length of 17 mm has a 6 mm air gap, (magnetic gap). What is the excursion?

Answer: (17mm - 6mm) / 2 = 11mm / 2 = 5.5 mm.

the excursion is 5.5 mm.

The DuMax method is sometimes used to measure the excursion, and it sometimes comes out just a little bit longer than this method. But the difference is not significant.

This method will give you the answer you need.

[kelticwizard]

By the way, this is for linear excursion. The max excursion, for hi fi speakers, is usually 50% to 100% longer.

The max excursion for PA speakers is frequently even longer than that.

[kelticwizard]

PPS: Sometimes a manufacturer will publish the voice coil length, but not the air gap, (magnetic gap), length.

In that case, it probably is a fair guess that for most midbasses and woofers, unless these are special speakers, will have a magnetic gap between 6 and 8 mm. You are probably reasonably safe with that.