Free The Design of Future Things by Don Norman

the car is traveling, how can the driver know what speed is safe? Sounds and vibrations provide natural indicators, implicit signals of important conditions. In electrically driven vehicles, the engine can be so silent that even the driver might be unaware that it is operating. Pedestrians subconsciously rely upon the implicit sounds of automobiles to keep them informed of nearby vehicles; as a result, they have on occasion been taken unawares by the silent, electrically propelled ones (or by any quiet vehicle, a bicycle, for example). It has become necessary to add a signal inside the automobile to remind the driver that the engine is running (alas, one manufacturer does this most unnaturally by using a beeping sound). It is even more important to add some naturalistic sounds outside the vehicle. The Federation for the Blind, an organization whose members have already been affected by the silence of these vehicles, has suggested adding something in the car’s wheel well or on the axle that would make a sound when the car was moving. If done properly, this could produce a natural-sounding cue that would vary with the speed of the vehicle, a desirable attribute.
    Because sound can be both informative and annoying, this raises the difficult design problem of understanding how to enhance its value while minimizing its annoyance. In some cases, this can be done by trying to minimize distasteful sounds, lowering their intensity, minimizing the use of rapid transients, and trying to create a pleasant ambience. Subtle variations in this background ambience might yield effective communication.One designer, Richard Sapper, created a kettle whose whistle produced a pleasant musical chord: the musical notes E and B. Note that even annoyance has its virtues: emergency signals, such as those of ambulances, fire trucks, and alarms for fire, smoke, or other potential disasters, are deliberately loud and annoying, the better to attract attention.
    Sound should still be used where it appears to be a natural outgrowth of the interaction, but arbitrary, meaningless sounds are almost always annoying. Because sound, even when cleverly used, can be so irritating, in many cases its use should be avoided. Sound is not the only alternative: sight and touch provide alternative modalities.
    Mechanical knobs can contain tactile cues, a kind of implicit communication, for their preferred settings. For example, in some rotating tone controls you can feel a little “blip” as you rotate it past the preferred, neutral position. The controls in some showers will not go above a preset temperature unless the user manipulates a button that enables higher temperatures. The “blip” in the tone control allows someone to set it to the neutral position rapidly and efficiently. The stop in the shower serves as a warning that higher temperatures might be uncomfortable, or even dangerous, for some people. Some commercial airplanes use a similar stop on their throttles: when the throttles are pushed forward, they stop at the point where higher throttle setting might damage the engines. In an emergency, however, if the pilot believes it is necessary to go beyond in order to avoid a crash, the pilot can force the throttle beyond the first stopping point. In such a case, damage to the engine is clearly of secondary importance.
    Physical marks provide another possible direction. When we read paper books and magazines, we may leave marks of our progress, whether through normal wear and tear or by deliberate folding of pages, insertion of sticky notes, highlighting, underlining, and margin notes. In electronic documents, all of these cues don’t have to be lost. After all, the computer knows what has been read, what pages have been scrolled to, which sections have been read. Why not make wear marks on the software, letting the reader discover which sections have been edited, commented upon, or read the most? The research team of Will Hill, Jim Hollan,