Two cars A and B are moving away from each other in opposite directions. Both the cars are moving with speed of $20 \mathrm{m}\mathrm{ }{\mathrm{s}}^{-1}$ with respect to the ground. If an observer in car A detects a frequency $2000 \mathrm{H}\mathrm{z}$ of the sound coming from car B, what is the natural frequency of the sound source in car B?
(speed of sound in air $=340 \mathrm{m}\mathrm{ }{\mathrm{s}}^{-1}$ )