The way we call for attention can have a huge influence on the classroom atmosphere. I like to save my voice for instruction and avoid the unpleasant interruption that loudly calling for attention can be. Instead, I rely on a calm, soothing tone to signal that it is time for a transition.
Tuned wind chimes are my favorite (these have a more beautiful tone than some of the tinny-sounding, inexpensive models). They can be found in many garden shops. By placing a few sets in strategic locations, I can get students’ attention whenever I need to without having to walk very far.
Other options are music wands which come in a variety of shapes and are very affordable; this simple desk chime; or this harmony bell (which I like to keep with me so I don't even have to walk to a set of chimes). And I just heard of one teacher who uses this doorbell in her room because the remote lets her ring the soft tone from anywhere.
If you keep your phone or a device with you while teaching, search for “chimes” in the app store. You’ll find a wide variety of choices, and you’ll always be ready to calmly signal your students at transition time.
Whatever you decide, the goal is to use a quiet and calming sound that doesn’t disrupt the atmosphere of the room. Once you’ve introduced the sound and communicated expectations, students will learn that chimes mean they need to pay attention and make a quick, calm, safe, and quiet transition.