The sound emitted from the siren of an ambulance has frequency of $1500 \mathrm{Hz}$. The speed of sound is $340 \mathrm{m}/\mathrm{s}$. Calculate the difference in frequencies heard by a stationary observer, when the ambulance initially, travels towards and then away from the observer at a speed of $30 \mathrm{m}/\mathrm{s}$.