Hi Andrea,
I cannot say for sure what may be affecting your setup, but I can
give some suggestions for factors that may be causing some of the
differences you can hear.
The sound strength and quality can be degraded by the connecting
string in a variety of ways, and these will in general depend on the
tension in the string and the length of the string.
One is that the string can be "lossy". If the string stretches, it
should be able to recover its original length and position very quickly
without "oozing" back into place. The times which are important are the
frequency of the human voice, around 300 Hz. Some of the plastics might
perform less well than other materials. If you flex a material and it
heats up instead of springing back into place, you can lose energy in
the transmission process.
The string should not be too "dispersive". Some materials will
transmit high frequencies at a much faster rate than low frequencies.
Take a metal slinky, for example, and hook one end over a piece of
cardboard. Stretch it out and tap the other end with your fingernail.
You will hear a chirping sound which starts off with high frequencies
and ends up with low frequencies. Using a material that does this in
the transmission of human voices will distort the sound, making it
unintelligible.
The string should "couple well" to the can. If you tie a knot in a
piece of fishing line to hold it in place in a hole in the can, the
knot itself can flex and dissipate energy instead of transmitting it to
the wall of the can.
Tom
(published on 10/22/2007)
