Two cars are approaching each other at an equal speed of \( 7.2 \, \text{km/hr} \). When they see each other, both blow horns having frequency of \( 676 \, \text{Hz} \). The beat frequency heard by each driver will be ________ \( \text{Hz} \). \([\text{velocity of sound in air is } 340 \, \text{m/s}]\)
