Which of the following is NOT a principal type of audible signal?

The correct answer highlights that sensors do not fall under the category of audible signals. Audible signals are sound-based warnings or alerts designed to attract attention and convey important information or instructions. They include devices specifically designed to produce sound, such as bells, horns, and sirens, all of which emit loud noises to signal alarms or alerts.

In contrast, sensors are typically detection devices that respond to physical stimuli, such as heat, motion, or light, and they do not inherently generate sound. Instead, sensors often trigger audible signals or alarms when certain conditions are met, but they do not produce sound themselves. This distinct characteristic makes sensors fundamentally different from the other listed options, which are all sound-producing devices, thus confirming that sensors are not a principal type of audible signal.