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In article .com writes:

I was looking though the manual for an Elextrix Repeater and toward the
end it has a diagram for building a 12dB attenuation cable for plugging
it into a guitar amp. It's three resistors arranged like this:

tip-------------3.9k----------tip
| |
2.2k 2.2k
| |
sleeve------------------------sleeve

From my math, this gives about -8.9dB, not -12dB.

Your math is fine as far as the voltage divider between the 3.9K and
the 2.2K resistor on the ouptut (right) side. The couple of megohms
input impedance of the guitar amplifier in parallel with the 2.2K on
the output won't make a measurable difference.

There's another voltage divider that we don't know about, though, and
that's the one formed by the source impedance of the Repeater and the
2.2K resistor on the input (left) side. If the intent is to drop the
output of the Repeater by 12 dB, that divider would need to have an
attenuation of about 3 dB. That would make the source impedance about
900 ohms, higher than I would have expected but not unreasonable,
considering the application.

Or maybe they just assumed the output impedance of the Repeater was
1K, used easily obtainable resistor values, and hoped that some
smartass kid wouldn't try to check their math or even measure the
actual attenuation.

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