MMario, in open air you don't have a feedback, this annoys the singer and make him feel tense and unsure of what people hear. It is the worst situation for the singer, unless.. you use a mic and power amplifiers and they are properly balanced to give you that feedback. I know many singers enjoy this form of performance, and in many cases it is the only way to perform, however.. people don´t hear you. They hear the loudspeakers. I was referring to informal performances.
That´s why I don't attend Pavarotti's big shows in parks, but I respect the musical preferences of many people that like them.
Un abrazo - Andrés (an anti-PA maniac)