Voice too "authoritative?" As part of my graduate program, I am required to give presentations to a group of attendees and a program panel about various topics including but not limited to research, involvement, and case studies. 

The audience pays attention but rarely ask questions. A survey was released about my effectiveness as a lecturer and nearly 65% of attendees stated my voice was "too authoritative."  A common problem was my voice being authoritative and causing the audience to be too timid to ask serious questions related to the topic. Talking in an "enthusiastic" tone (in my own opinion) yielded no results. My voice is very deep and, in order to project well, I tend to speak loudly (no microphones).

What are some tactics to get the attendees engaged and not appear "too authoritative?"

EDIT:

Let me add some details that I find relevant that I missed adding. The survey is handed out as attendees walk in. Between each lecturer there is a 10 minute "intermission" to allow attendees to exit or enter. The attendees are required to hand in surveys as they leave. The uni hosting the lecture combines common themes among the surveys. It is unknown how similarities are determined except to the staff reviewing the surveys. The results are then emailed to the lecturer with good and bad things about the lecture. I am then required to email to my supervisor.
The solution will depend on what the survey respondents meant by "too authoritative". For example, it might mean any of the following:


your talk is pitched at too high a level, so people feel dumb when
they don't understand, and are too intimidated to ask questions 
your style is argumentative 
one or two people did ask questions, which you didn't answer clearly 
your style is pompous 
you sound overconfident
you state your opinions as fact
...and so on


I assume the survey was confidential, so you can't go back to people and ask them what they meant. So I suggest you ask a few colleagues who are good speakers what they think you need to improve on.


  My voice is very deep and, in order to project well, I tend to speak
  loudly (no microphones).


A deep (low-pitched, bass-y) voice projects farther than a high voice. You may simply be speaking too loudly for the room.