Single-sided deafness (SSD) is a condition which does not present a readily visible impairment. (How many of you know that Rob Lowe is profoundly deaf on his right side)? For this reason, it is easy for a person with normal hearing to be unable to understand how noisy environments affect a person with unilateral hearing. You have to realize that while the person is perfectly capable of carrying on a normal conversation in a quiet environment (even with their back turned), once the level of ambient background noise meets, exceeds, or even comes close to the volume of speech which is directed at him, he is essentially deaf as a rock. All he can see is your lips moving. In addition to the inability to localize sound, this is the major difficulty that SSD presents.
~Taken from alldeaf.com~