1.5 nV/√HzDue to the low intensity of the acoustic signals and high noise floor of in-situ environments the PA is usually detected with lock in amplifiers. These can be had used on ebay for ~$200-400 dollars, I'm choosing to make one because:

1) unit price - while $200 is arguably in price range, but is more than I would like to spend...

2) physical size - I live in a shoebox

3) I'm interested in having more than one experiment at a time, and willing to trade NRE for unit price

4) desire to gain more analog experience - 'cus I'm a masochist evidently.

Part selection:

ADA2200 - synchronous demodulator

The source carrier wave is available, so synchronous demodulation is viable, and seems to be the standard technique for this field. I did not find many components available other than the ADA2200 and AD630. The ADA2200 is the active active component. Selected on active status, ability to do internal filtering, and availability of development board. I did not find alternative components from other manufacturers prior to ordering development kit.

I found datasheets but not suppliers for the lm1596.

I have ordered the evaluation board for the ADA2200 so I can start vetting the detection scheme immediately.

AD8428 - low noise, high gain, high CMRR, precision inst. Amp.

1.5 nV/√Hz, 140 dB CMRR, gain of 66 dB in the audio/ultrasonic range. It's a tad pricy at $15/ea in singles, but is also the front-end LNA, and due to the fact that the frequency of interest is in the audio spectrum, most RF LNAs aren't viable.

I also am considering the AD8429 - a pin compatible version with adjustable gain from unity to ~66 dB. this will involve a little more noise due to the gain resistor, but will all me to select a lower gain if saturation/gain compression is an issue.

OP1177 - Mid-span voltage reference driver 

ADA2200 requires the DC component of the input signal to be at VDD+/2.OP1177 is recommended in AD8428 datasheet as the reference voltage driver. It's not expensive enough for me to care to find an alternative at this stage.