Start Up Idea: Noise Warden
Suppose someone created a stereo with a decibel sensor which would detect when sound crossed a lower threshold, turning on the stereo. The decibel sensor would poll for ambient noise and increase the radio's volume until it matched it decibel for decibel; if the external noise is lowered, the stereo's volume is lowered, and vice versa.
Further, suppose this is a smart device that plays generated music. A bass sensor tracks any rhythm in the external noise and purposely staggers the beat to clash. A pitch sensor tracks any predominant tones in the noise and overlays a dissonant minor second. Finally, a sensor detects movement of the device, and if disturbed while on, sounds an alarm within a set number of seconds. A keypad allows one to enter a security number to disable the alarm.
Being automated, the device scans a section of one's property, acting as a "noise warden". If the belligerent neighbour turns up the volume, their music becomes a wash of reciprocal dissonance. If the neighbour turns down their music shortly after calling the police, the warden follows suit and the police have nothing to cite. After several noise tantrums the neighbour realizes that the response is coldly efficient and reciprocal.
Using technology, a single person is now capable of making more noise, effortlessly, than at any other time in history. Likewise, through technology communities are more densely populated than ever before. Perhaps with technology one can also make the world a more peaceful place.
Furthermore, I'd buy it. Ycombinator has enough copycat startups. Try a project like the above. Many people are the victims of misused technology, and would love to have technology fight for them for a change.