wash-out effect
 n.— «In the early 1970s, manufacturers introduced sirens with different patterns and frequencies, to address a growing problem: Officers in different police cars using the same frequency often could not hear each other when approaching the same intersection, a dreaded phenomenon known as the wash-out effect that is a recipe for a crash. The yelp, the wail, the fast and the hi-lo sirens were born.» —“Earsplitting Symphony, With the Maestro in Blue” by Cara Buckley New York Times June 15, 2007. (source: Double-Tongued Dictionary)

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