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Touch-Specific Issues - Tactile effects library

Some aspects of tactile interface design are specific to working with touch sensations and the devices that impart them. Here are some things to remember when designing a touchable user interface:

A tactile interface holds the user by the hand. A user experiencing a tactile interface usually navigates the interface with the same hand that feels touch effects. The most obvious implication of this fact is that strong forces applied to the hand of the user will have an equally strong impact on that user's ability to navigate as desired. The interface designer must remember that an interface that holds the user's hand in this way can be helpful and guiding, or frustrating and confounding. Any user encountering the latter type of interface may decide that no feedback is better than unhelpful feedback, and will either unplug the device or leave altogether.

Some effects can be heard as well as felt. On certain devices, particularly force-feedback mice of all descriptions, repeating effects of certain frequencies in the audible range can be heard as well as felt. The human ear can hear frequencies above about 20 Hz (a period of 0.05 seconds). Therefore, any periodic effect with a period shorter than 0.05 seconds will be heard by the user to some extent. Other types of effects can be sufficiently strong that a clicking or banging can be heard from the device. This fact can sometimes be exploited to create both tactile and sound effects from a tactile peripheral device. If it is important that a touch-enabled application or web page be completely silent, however, the tactile interface designer must test all touch effects with an ear for the sounds they might also make when they play.

Effects can cancel each other. You design two strong tactile effects, then play them simultaneously, and feel nothing. Why? Strong pushes in opposite directions add up to no push at all. A stiff spring overlaid with a strong constant force can make the spring disappear. The sum of two periodics slightly out of phase can be a faint, wimpy periodic instead of the intended interesting harmonic. When adding effects together into a single compound effect, it is important to test the result and test again to make sure it feels as planned.

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