Consider these three case studies. Perhaps we can learn how to not to ruin a lecture.
Case 1: Have you ever heard of the term "QSB"? In ham radio lingo, it refers to ionospheric fading.
The other day, I heard this lawyer speaking at seminar. He said many interesting things. He spoke well, he spoke clearly and so his presentation was very much worth the time.
Then his partner took over. For the next half hour, I could not understand one single concept this second fellow was trying to put forth. This is no exaggeration. It was the utter perfection of this experience that made it so memorable.
Most lawyers have good diction, but this second presenter did not. As he went on, it seemed that every tenth to twentieth word, there would emerge a sound which I have to assume was a part of the English language vocabulary, but what word it might have actually been, I simply could not discern. He would repeatedly turn himself back and forth at the lectern, first facing the audience and then facing away, back and forth, back and forth again and when facing away from the audience and the microphone, his voice faded away (QSB) to being almost inaudible.
Of course, the obligatory Powerpoint was in use and up on screen came this very carefully constructed and wonderfully instructive bar chart. It had a horizontal axis, a vertical axis and some isometrically drawn vertical blocks of all different colors and heights. There was writing everywhere. Some letters placed horizontally, some were placed vertically, some were placed at 45 degrees and not one single letter of all of that was legible.
Case 2: Have you ever thought about what else makes something readable and what does not?
At a different recent seminar, an engineering tutorial this time, Powerpoint was also in use. In this case, there were lots of key words and phrases placed inside of darkly shaded and colored rectangles. Most of them I could not read because the colors and shadings were so dark that the lettering could just barely be seen. There was nowhere near enough contrast.
This same speaker also used quite a lot of unfamiliar acronyms and techno-jargon. Some of what the acronyms meant I was able to discern from their context, but many of them I could not.
Case 3: Have you ever contemplated the stupefying effects of extreme monotony?
I was at this third lecture where, yes, Powerpoint was in use. The lecturer put up this one slide in which there were perhaps half a dozen bulleted lines. Then he began his narration and continued and continued and continued and continued and ...... and in all of that time, that one single slide remained up there on the screen. It was up there for about thirty of the longest minutes I can recall to memory. I could see people starting to squirm and fidget. That speaker lost a good part of his audience due to intense visual boredom.
Suggested viewing: http://www.volacci.com/marketing-news/5-steps-compelling-visual-content
Summary: Speak clearly, speak to your audience and do be thoughtful about the visuals.