Two speakers are linked so that they always emit the same frequency sound. An observer stands next to one speaker, which is motionless. Meanwhile, the second speaker moves slowly towards the observer(slow enough that you may disregard the doppler effect). The frequency of both speakers change with time, such that the observer cannot hear the sound from either of the two speakers no matter the location of the moving speaker.
Which of the following could be an equation for the frequency $f$ of each speaker as a function of the distance $d$ between the distant speaker and the observer?
The speed of sound in air is 350 m/s.