To deliver alerts to the user, three primary modalities are used — visual alerts, sound alerts and tactile alerts.
Visual alerts are the most general method. Depending on the needs, the alerts on the UIs or HMIs can be more annoying, with the animated blinking effect until they are acknowledged by the user, or just more emphasized compared to other controls of the user interface.
The key issue with visual alerts is how to attract the attention of the operator if he is not looking at the display. It can be impossible without other modalities (sound), or special separate blinking bulbs may attract attention if they are bright enough to give reflections of the objects around.
Sound alerts are the secondary method, and typically they attract enough attention.
But there are plenty problems with sounds:
- It can be hard to understand the source of the sound if there are multiple devices in a single area that transfer sound alerts.
- It is also hard to distinguish one sound alert from another if it is a complex situation with multiple sounds played simultaneously.
- The noise that is created by sound alerts can easily overwhelm the user if he is not able to acknowledge them in a normal way. It can seriously reduce attention, which will lead to mental overload and loose of situation awareness.
- Also, sound alerts can affect the communications between personnel in the control room, aircraft cockpit, or vessel bridge.
All this means that sound alerts require much more design efforts if there is a need to use them.
This type of alerts is the feature of most mobile phones, but in complex systems is used rarely — for instance, the virbrating steering wheel may indicate the stall danger for the airplane. Similar behavior can be used in automotive industry when car autopilot is off.