Is road buzz something that happens due to my speed or is a road surface issue?

I plan on test riding 3 similar fitness road bikes tomorrow. The only difference between them is the frame materials. One is all aluminum, aluminum with carbon rear stays and full carbon. I am not familiar with the roads around the LBS but I will assume they are all smooth asphalt.

I'm just trying to figure out what I need to do to get the aluminum frame to buzz so I can feel the difference between the frames.