Buy a decibel meter. When loud parties are occuring, use a video camera to record the decibel meter at different locations (e.g., inside your house, outside their house, on the sidewalk across the street, etc.). Take this evidence to the police station and/or to a legal firm to sue your neighbors in a small claims court.
http://www.millslawfirm.org/PracticeAreas/Excessive-Noise.asp

You have a legal right to pursue action against your neighbors if their noise exceeds New Haven's noise ordinance: 55dB between 7am and 10pm, and 45dB between 10pm and 7am.
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"The City adopted a noise ordinance that would control noise at a level not to exceed 55 decibels – roughly the sound of a clothes dryer or microwave between the hours of 7am and 10pm Monday through Saturday (or 9am through 9pm on Sundays) and a maximum 45 decibels from 10pm until 7am throughout the week. "
http://www.cityofnewhaven.com/Mayor/ReadMore.asp?ID={41FD0022-E2D7-498A-8B72-000D39BBBD4B}
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