Already got an angry parent email on the first day. Did I go too hard?

At the start of the year I always am a bit "meaner" to pull the rowdy students in line, but this year I starting to think I may have taken it a bit too far. So in the morning I had a year 9 class, and this year level last year was hell, so I was ready for it. I made them line up in silence before going in, and then enforced a no talking rule, which worked for about ten minutes, until the (first) incident happned.

I was walking around the room towards the front with a ruler and the room was quiet, when a student began whispering to another student. I hit the ruler on her desk (far from her) which startled her a bit and said "What about no talking don't you understand?". The class was quiet after that.

Towards the end of class they were handing up worksheets as they were finishing them, when Johnny (not real name ofc) came up to my desk with his sheet and hesitated and sorta turned around halfway. Now, I probably could have dialed down the tone of my voice, but I smacked the desk and shouted "Here!".

The rest of the class proceeded smoothly. Well tonight I received an angry email from a parent of the girl about how I was "invoking terror" in the students, mentioning the ruler and smacking incident, and that they would be informing the principal. I haven't responded yet.

I didn't think my actions were that harsh, let alone "invoking terror". What should I do?