Trying to understand very sensitive preamplifiers...

Hi. Trying to learn about amplification of very low level signals...

An echo from the bottom of the sea is I believe, one of the faintest signals to be picked and processed. But echosounders and sonars perform reliably well.

Looking at piezo-disc preamplifier circuits, they still call for contact-microphony applications, or near field use. Do I have the wrong perception of the 'loudness' a sonar transducer emits and the level of the echo received ?

What is the circuitry involved to pick echoes with a piezo disc transducer ? Or, what is the circuitry involved in receiving voice-range faint signals with hydrophones ?
Any example of professional circuits for whale recording, and considerations for success ?

How does a piezo disc microphone and an electret condenser microphone compare in air, related to sensitivity ?

A piezo mic has a high output level with many high resonant peaks. It is used as a contact vibration sensor on a drum or guitar. Some ultrasonic transducers resonate at 40kHz and are used for distance measuring.
An electret mic has a low level with a flat smooth frequency response. They are used for wideband audio.

Thanks.
Am concerned on voice range and not contact. In free air, a piezo disc seems not very sensitive as an electret condenser microphone. Why ?
Agree, it may be useable as microphone at its resonant frequencies.
But in humble hydrophone applications, is useable in all the audio spectrum. Reception of faint echoes and noises is actually decent.

Jfet preamplifiers can cope with such piezo input. Gains of 100 are often enough. If a piezo disc is glued to something, it gains sensitivity, but only if the something is not stiff and of larger surface, is that right ?
Does a piezo disc glued to a 'something' 100 times its area suspended in air will increase sensitivity accordingly ?

In free air, sounds at the rear of a piezo disc cancel sounds at the front. If you enclose its rear then the enclosure will cause a resonance. Most electret mics have their rear enclosed without a resonance so they are sensitive with no cancellation.
If you glue a mic to a large object then the rear sound will still cancel the front sound, but only low frequencies. The large object will resonate or damp vibration of some frequencies which is bad.

I doubt that the problem can be successfully discussed without understanding the nature of sound waves, the role of medium density and compressibility, the respective acoustical impedance of gases and liquids, the relation of wave length and microphone dimensions. Also getting an idea of sound pressure and related particle velocity of typical acoustical signals.

Although the question title is primarily referring to preamplifiers, it's almost pointless to discuss amplifiers without the acoustical properties and electrical characteristics of connected microphones.

Some applications (marine echosounders, air ultrasonic distance sensors) are using resonant transducers with relative small bandwidth. Comparing them with audio microphones (e.g. dynamic or electrete type) won't bring you far.

I do not know if an ultrasonic frequency is used for sonar but sounds from whales was mentioned. On the internet there are recordings of whale sounds and they are at low audio frequencies.
A hydrophone manufacturer lists the minimum frequency of 1Hz and a maximum frequency of 150kHz for each of its hydrophones but does not say the deviation (+/- ?dB).