Tim McCulloch wrote on Mon, 19 July 2010 13:03 |
The driver excursion encroaches the "nominal" cubic volume of the rear chamber as you play louder and lower. The non-linearity of loading creates the distortion.
It changes the loading ratio more dramatically for a small chamber than a large one.
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Tim,
At what driver excursion to chamber size percentage does it “encroach” the cubic volume of the rear chamber ?
What aspect of the loading is non-linear ?
Why does the “non-linearity of loading” create distortion ?
In the example of the Growler driver, the maximum linear displacement is .04 cubic foot.
If the compression chamber was only one half cubic foot (a very small box for a 12” driver) that would be a ratio of .04/.5.
8% does not normally qualify as a “large percentage”.
Of course, Jeff said the Growler is not near Xmax at 29 volts at 45 Hz, so the percentage would be smaller if that is true, and I would be quite surprised to find the compression chamber to be only one half cubic foot, it probably is closer to 3 times that size, putting the % of displacement to chamber size below 3% when driven to Xmax.
Below the cut off frequency of the horn, the acoustic resistance of the horn is zero, so the driver acts as it would in a small sealed chamber. As the driver is driven at frequencies below the horn cut off, excursion rises rapidly, while output reduces rapidly. Excursion above Xmax results in higher distortion, which increases rapidly with small increments of excursion past Xmax.
In each case of measured distortion in horn loaded cabinets, the distortion increased below the horn cut off. The bigger horns, with lower cutoff frequencies, have less distortion to lower frequencies.
Inquiring minds await you or Jeff's alternative explanations of the causes of low frequency distortion in straight horns.
Art Welter