Inability to hear
sound  at "normal" volume. 

Provide for control of sound by user in order to ensure that the loudness level can accommodate all users regardless of their sensitivity.

When the impact of the comunication is critical, relate the volume of sound to the urgency of the cue, and provide both visual and audio cuing.

Minimize the ambient sound produced by the product or the enviroment so that confusing audio signals are eliminated.

PROBLEM   

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Keep all sound indicators in the mid-range frequencies to avoid problems associated with both the high and low fequency ranges.

If full-range sound indicators must be offered, provide the user with sound modulating devices.

Provide sound that is in distinct contrast to the sound environment (e.g., voice indicator for non-voice environments; non-voice indicator for voice environments).

When sound is used, relate the "friendliness" of the sound to the urgency of the alarm (e.g., the greater the urgency, the less friendly the sound).

1. Minimize the use of problem consonants.

As far as practicable, avoid using such problem consonents as "c", "ch", "f", "s", "sh", and "z."

If such consonants must be used, emphasize correct diction and enunciation in the voice messages.

Ambient background noise exacerbates the problem of distinguishing problem consonants.

Provide for means to keep word messages at lest 10 db above the ambient noise level.

Redundant visual cuing will help ensure communication in case the audio message fails.

Rapid speech becomes hard to understand.

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Consider the clarity of the voice cue by presenting speech messages with deliberation.

Emphasize clear and proper enunciation, and consider the environmental background noise.

1. Avoid excessive use of sound for communication purposes.  

In applications where it is necessary or desirable to help locate the source of the sound, combine a visual device with sound, preferable at the source.

Where sound is used to communicate different functions, differentiate each function through the use of a different sound.

Use clear and unambiguous sound cues for critical information or warnings.

When voice messages are necessary, use an attention-getting sound prior to the voice message.

Wherever practicable, focus the sound message toward user and design the product and its environment to minimize the sound's deflection.

Sound messages are desirable and appropriate when:

• visual messages may not be readily perceived (e.g., the user is mobile)
• an excess of complex visual messages already exist
• a quick response time is required
• immediate, short-message signals are needed without further reference to the display