Reverberation equation
As far as I can tell, what you're asking is:

How do α and D affect the output sound?

You stated it in your question


  D is the echo delay (in samples) and α governs the amount of echo fed back.


This does exactly what it sounds like. If you imagine the input being "HEY!", the output would be an echo "HEY! ... hey!", with D being the amount of time between the first "HEY!" and the second "hey!" and α being the intensity of the "hey!" with respect to the "HEY!".

If this isn't what you're asking, please clarify the question.