problem in supressing noise while using ultraound transducer

Hi ! guys
Special thanks to IanP
and thank you A. Anand Srinivasan

Now can I share a new problem,

I am using a ultra sound transducer as input to a ckt as shown in attachment;

The required signal is sine wave of pk-pk volt 50mv;

and output of the circuit is dc of 6-8V as shown in attachment;

Now as you guys know that even noise in the envirionment can have sine wave of pk-pk amplitude 50mV.

Now the problem is I always get output 6-8V.
So,I am finding hard to discriminate the noise even if I am using low noise amplifier LM833.

So how to make output go to minimum DC when there are noise same as signal.

Any ideas guys@!?

Regards,
kushal khanal