Audio volume limit (12/5/2002). (See also Noise! Unnecessary Noise!). Many people do not think about the disturbance they may be causing with unnecessary volume levels. This noise polution could easily be limited by providing feedback control from a sensor external to the car/house, which would not allow the external volume to go above a set level and therefore affect other people.
Another application of this feedback technology would be in car audio systems, where the audio volume needs to be constantly changed as the car speeds up and slows down. An automatic system could control the audio volume so that it remains a set level above the background noise level.
Update (2/6/2003). Recently I saw a wonderful device connected to the main circuit breaker in a village hall. If the noise volume went over a set level it switched the mains power off. This could easily be fitted to houses of known noise offenders! It could even be joined with a time switch so that the volume level required to switch the power off is reduced at preset periods, for example at night.