Don Norman states that "the term signifier refers to any mark or sound, any perceivable indicator that communicates appropriate behavior to a person." Adding signifiers is a great way to enhance discoverability of affordances and make sure that the feedback is understood clearly by the user but sometimes everything that is needed cannot be made visible. Sound can provide information available in no other way. Sound can tell us that things are working properly or that they need maintenance or repair. It can even save our lives from accidents.
Consider the information provided by:
The click when the bolt on a door slides home
The tinny sound when a door doesn't shut right
The roaring sound when a car muffler gets a hole
The sound of a notification on the phone
The whistle of a kettle when the water boils
Many devices simply beep and burp. These are not naturalistic sounds; they do not convey hidden information. When used properly, a beep can assure you that you've pressed a button, but the sound is as annoying as informative. Sounds are generated so as to give knowledge about the source. They convey something about the actions that are taking place, actions that matter to the user but that would otherwise not be visible. The buzzes, clicks, and hums that you hear while a telephone call is being completed are one good example: take out those noises and you are less certain that the connection is being made.
Real, natural sound is as essential as visual information because sounds tells us about things we can't see, and it does so while our eyes are occupied elsewhere. Natural sounds reflect the complex interaction of natural objects: the way one part moves against another; the material of which the parts are made - hollow or solid, metal or wood, soft or hard, rough or smooth. Sounds are generated when materials interact, and the sound tells us whether they are hitting, sliding, breaking, tearing, crumbling, or bouncing. Experienced mechanics can diagnosis the condition of machinery just by listening. When sounds are generated artificially, if intelligently created using a rich auditory spectrum, with care to provide the subtle cues that are informative without being annoying, they can be as useful as sounds in the real world. Sound is tricky. It can annoy and distract as easily as it can aid. Sounds that at one's first encounter are pleasant or cute easily become annoying rather than useful. One of the virtues of sounds is that they can be detected even when attention is applied elsewhere. But this virtue is also a deficit, for sounds are often intrusive. Sounds are difficult to keep private unless the intensity is low or earphones are used. This means both that neighbors may be annoyed and that others can monitor your activities.
The use of sound to convey knowledge is a powerful and important idea. Just as the presence of sound can serve a useful role in providing feedback about events, the absence of sound can lead to the same kinds of difficulties we have already encountered from a lack of feedback. The absence of sound can mean an absence of knowledge, and if feedback from an action is expected to come from sound, silence can lead to problems.
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