rodent proofing a garage

A couple years ago I bought 4 of these on sale at Menards for $10 each. It doesn't look like they sell them anymore. They make quite a bit of noise (sound like a quieter version of a burgler alarm) but you can just barely hear them outside of a house or garage they are in. I put one in our little place up north and its garage and our pole barn. All three had mouse problems prior to putting them in and haven't since. Our pole barn back up to a corn field and used to get pretty over run especially in the fall.
[ame="http://www.amazon.com/Black-Decker-EP1100-A-Ultrasonic-Repeller/dp/B0007CXT8G"]Amazon.com: Black & Decker EP1100-A Ultrasonic Pest Repeller, Black (Discontinued by Manufacturer): Patio, Lawn & Garden[/ame]