Hello I am a beginner in electronics and eager to expand my knowledge in it. Earlier I was trying to build an Ultrasonic receiver using a transducer and some op amps. I ran searches in the Internet and found this: 
I figured that the signal from the transducer is amplified by the two stages of non inverting amplifiers and fed to comparator for triggering. But why is the voltage divider of resistors $R_1$ and $R_2$ used here? Why can’t we just feed the input signal directly in to the + terminal of the op amp?