A source and a detector move away from each other in absence of wind with a speed of \( 20 \, \text{m /s}^{} \) with respect to the ground. If the detector detects a frequency of \( 1800 \, \text{Hz} \) of the sound coming from the source, then the original frequency of source considering speed of sound in air \( 340 \, \text{m /s}^{} \) will be ______ \( \text{Hz} \).
