The AURA tweeter is used in PLUTO and
Pluto-2 because of its very wide frequency coverage and small baffle size
which facilitates the construction of a monopolar source. By using a low
crossover frequency of 1 kHz to the small Seas L16RN-SL woofer and
mounting the two drivers at the end of tubes the combination maintains
omni-directional dispersion.
Here is my setup for testing the variability of the
tweeters. Each driver is dropped into the opening of a long cardboard
tube. The tube is stuffed to avoid resonances. The microphone capsule is
approximately centered above the driver. Its distance from the cone is
such that it would just touch a flat sheet of paper that is laid onto the tweeter's roll surround.
Electrical drive levels are around 2V to stay far away from thermal
compression.
The close-up frequency response of the
two drivers that I use in Pluto-2. I have had these drivers for well over
a year. They have small printed circuit boards for soldering the leads to.
Three more drivers of about the same
vintage have been added to the graph.
Their frequency responses track well up to about 8
kHz.
There is about 1 dB maximum variation in sensitivity between the 5
drivers.
Here are 4 drivers of recent vintage.
They have lugs for connecting the leads.
Their frequency response tracks reasonably close
with the older units.
Their sensitivity is slightly higher.
There is considerably more response variation above 10 kHz.
Here all 9 drivers are plotted on the
same graph.
Between 700 Hz and 10 kHz the dominant variation is
in their sensitivity. Thus individual tweeter level adjustment by
comparing left and right loudspeakers - when placed adjacent to each other
and playing pink noise - becomes an essential step before adjusting the
tweeter level relative to the woofer.
Only driver selection can reduce the variability
above 10 kHz.
What you hear is not the air pressure
variation in itself
but what has drawn your attention
in the two streams of superimposed air
pressure variations at your eardrums
