Certain types of information are better transmitted as sound signals rather than visuals. This is the case in the following situations:
1. When the signal is originally acoustic.
2. Warning signals. The advantage is that the operator does not need to see the signal in order to detect it; that is, he does not need to look at the instrument constantly.
3. Where the operator lacks training and experience of coded messages.
4. Where two-way communication is required.
5. Where the message concerns something that will occur in the future, e. g., the countdown to the start-up of a process.
6. In stressful situations, where there is a possibility that the operator would forget what a coded message meant.
Tones are preferable in the following situations:
1. For simplicity.
2. Where the operator is trained to understand coded messages.
3. Where rapid action is required.
4. In situations where it is difficult to hear speech (tones can be heard in situations where speech is inaudible).
5. Where it is undesirable or unnecessary for others to understand the message.
6. If the operator’s job involves constant talking.
7. In those cases where speech could interfere with other speech messages.
Warning signals are without doubt the most common form of sound information. Sound is an information source that is little used, with the exception of warning signals. Sound signals/information could probably be used to advantage in giving spoken instructions in disaster and other acute situations, for example, (1) that there is a fire at a particular location, and (2) what action should be taken. In this type of situation one can use a calm voice to give further instructions on necessary actions, such as clear and simple instructions on how to evacuate the building. Various forms of acoustic alarms should be able to be complemented with tones that would give preliminary information on the type of fault. If there is a hierarchy of alarms, different tones can be used for different groups of alarms. If detailed information is required about the alarms, questions can be entered via the keyboard or directly using the voice. Section 3.6 includes a description of the design of computer-generated speech and computers that understand speech.