It is not rude unless you are in a group (class, community, culture) that sets that agreement (acknowledgement must be verbal) up. Essentially, acknowledging a communication can be done any way that gets across as long as the person knows they've been acknowledged. The thing that is rude, whatever the vocabulary of signals used may be, is not acknowledging a communication, leaving the person wondering whether they are being not heard, snubbed, or dismissed in some way.

A