Our applied math department is beginning to have a serious attendance problem at our colloquium. The appropriate level for our colloquium is that any person with an undergraduate degree in a quantitative/theoretical field should be able to follow the talk as long as they are paying close attention.
In an invitation letter to our speakers, who are mostly faculty members at other Universities in a variety of fields but also include some faculty at our university, we say
"Your talk should be entirely self contained. You can assume the audience is mathematically mature and has a solid understanding of undergraduate real analysis and linear algebra but do not assume any prior knowledge in physics, biology or more specialized mathematics. Of course feel free to talk about such topics, just be sure to explain all basic concepts needed to follow the rest of the presentation as soon as they come up."
Despite this warning, speakers go on, as usual, giving seminar level talks. Graduate students and the few advanced undergraduates that used to attend have basically stopped attending, which looks bad on our department, and lowers graduate student morale. Note the target audience is not undergrads. The target audience is first year grad students (and up) and faculty from many different quantitative fields.
I think part of the problem is that speakers assume applied math means "The math done in my field of research". They don't understand despite us linking to our graduate student's list of research interests in the email that applied math is extremely broad. Some researchers will have never taken probability theory or optimization and some who work in theoretical computer science haven't studied a differential equation since their sophomore years of college. And science backgrounds outside of math will be even broader.
Besides adding the request in our email, that the talk not be too technical, is there anyway to prevent speakers from giving talks that are too specialized or advanced?