The popular Mallory Sonalert is a good example of a piezo-ceramic audible signal. It is simple to operate because it is self-contained, with the drive circuitry supplied within the device. In addition, it is effectively sealed for use in harsh environments. It depends entirely on your application and the sound you anticipate it will take to accomplish your objective. Nailing down all the facts in advance will help determine what sort of sounds should be considered.
Intermittent tones are generally more discernible than continuous tones.
Most devices are available in loud, medium, or soft sounds. The sound level selected should match the application and the surrounding environment. Care should be taken to make certain the device's tone doesn't melt into the background noise and not be heard. If the product is responsible for signifying an emergency, you will probably want it to emit a loud, very discernible, audible signal. Audible signals must be supplied with an actuating signal.