I enjoy tracking air traffic at my local KORD. I listen on LiveATC and use my private virtual radar setup to get "real-time" traffic info.
I understand which instructions need to be read back by the pilots per this question however on more than one occasion I don't hear read back on critical vector info on departure, despite the visual confirmation of instruction (pilot making proper vector and speed adjustments).
I tend to notice this with bigger birds (777,747,340), however smaller regional jets almost always promptly read back.
Is there an alternative way of ATC instruction acknowledgement? (Other than read back?)
Is it possible that the reply is somehow on a different frequency?
Is this just a problem with LiveATC? (One theory is that A/C leaves receiver coverage area and that's why I don't hear reply, however on approach side much bigger distances are heard in my area)
I did verify that indeed the aircraft that I don't hear read back from receives special departure frequency by tower.
Once tower clears aircraft for take off and hands them off to departure controller they use "departure" frequency to read back instructions. It is typically different from standard departure frequency.
The only thing that is not clear is if there is any rhyme or reason for which aircraft gets this special departure frequency and which doesn't.
As pointed out in the question for some reason mostly it is "big" aircraft that gets this preferential treatment, but I am not 100% sure why.