Open baffle distortion in woofer

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#21
john k... said:
I don't post much any more but one thing I have always taken issue with is the idea that the compliance of a sealed box is a linearizing factor. It can be argued that IF the force applied to the driver by the box compliance is linear, then the total force applied to the drive has a greater linear component and therefore the nonlinear components will be less. However, the issue is that the force due to the box compliance is not linear. It goes like

F = X /[Cab *(1 + Sd * X/Vo)^2.4]

where X is the driver displacement, Sd = cone area, Vo is the box volume when the driver is at rest, and Cab is the conventional box compliance. The factor (1 + Sd * X/Vo)^2.4 shows that Cab is not actually constant but varies nonlinearly with driver displacement. If Sd * X is small compared to Vo (small drive in big box, very small displacement, etc) then Cab can be considered constant and there is little nonlinear contribution. But, for a large woofer in a smallish box the nonlinearity of the box compliance can be significant and an increase in nonlinear distortion results.
Click to expand...

Excellent point. Siegfried Linkwitz put together an Excel spreadsheet that lets you calculate the percent distortion contribution from the air-spring of the box (you read that right: the air in a small box CAUSES distortion) and I seem to recall that this was on the order of 1-10% for typically "small" boxes (Vb<<Vab) that are popular with today's low Vas, low Fs "subwoofer" drivers.

The spreadsheet is available here:
http://www.linkwitzlab.com/closed-box1.xls
and is discussed on his THOR subwoofer page here:
Subwoofer design
 
#23
Hi,

If the air in a box is more linear than the driver it will reduce distortion.
However air is not linear and a driver can be designed to be suitably
non-linear in the opposite sense for some distortion cancellation.

rgds, sreten.
 
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