If you want to experiment with how weak signals  (tones and CW) sound
in accurately defined level of  noise ratio,  download the utility
below which is still in beta test stage.

It allows you to generate either a CW message of your choice, at any
speed, or a plain tone.  Gaussian noise is added to this at a level
determined by the S/N ratio requested.   S/N is calculated based on
any bandwidth of choice.  The output is then applied to a filter whose
bandwidth and centre frequency can be arbitrarily chosen.
The results are stored as .WAV files      Conditions of clipping or
too low a generated level are detected and you can adjust the ouput
scaling to compensate
.WAV files are stored in the same directory the software is run from.

There are no instructions, user manual or anything else, just the
programme itself which I hope is self explanatory

http://www.g4jnt.com/DownLoad/SNDemo.exe
or, if your browser or nanny-software objects to .EXE files, save
http://www.g4jnt.com/DownLoad/SNDemo.eex    and change the file type
to  .EXE

I've tested the files generated with Spectran and Gram to make sure
the S/N ratios generated look correct, based on the spectrum display;
all seem to agree within a dB or two.   (Exact figures depend on
things like effective bin size due to windowing in the  FFT the
spectrum analyser)

Enjoy

Andy
G4JNT