> Re: EQ recordings with parabola
> Posted by: "Tim Nielsen"
> What I notice more than the high frequency emphasis on my Telinga, is
> a very noticable peak or resonant around 500-600hz. This is I'm
> assuming about where a 20" dish would start to steeply roll off? But
> I almost always do a notch around there to remove the sound of the
> dish. Wondering if anyone else notices it, it's kind of a gentle hum,
> tonal in nature, that is almost always there.
It does not roll off, there is a interference between the incoming wave
and the standing wave reflected off the dish. The frequency you report
is about right for that. It's a notch in the gain and both above and
below that frequency follow the main gain curve. Note the amount of
interference notch you get is related to the location of the focus
relative to the dish opening. You will get the most with dishes with the
focus outside the dish, the least with the focus 1/4 of the dish depth.
It's a compromise to have the focus just inside the dish as it is with
the Telinga. At that point the notch in the gain curve is not very big,
ie you still have plenty of gain there.
Dish size does not have a lot to do with it, dish shape does.
Think about it carefully, sound is vibration of air molecules along the
direction of travel. If it were transverse then dish size would make a
big difference, as it does with radio waves. But it's not.
Walt
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