BFO Detector. Substituting capacitors. Output too loud.

Polyester capacitors are quite popular in audio circuits. I assume the above circuit can be labeled as an audio circuit? What is the benefit of polyester capacitors over ceramic disc capacitors in audio circuits? And what would one expect if he or she were to exchange them.

The schematic parts list requires an 8 ohm speaker. I will be taking this to Mexico in a few weeks (assuming I can get it through the airport) and I'm afraid the ocean will drown out the beat note. It is important that I am able to hear subtle changes in pitch. I tried wiring up some ear phones to the output of the above circuit and ran the search coil over an altoids canister. My ears rang so bad I blinked.

Wire a resistor either in series or in parallel with the output?

I need to tune this circuit in such a way that I am able to hear subtle changes in pitch, yet be able to run this over metallic objects (like treasure!) without damaging my ear drums.

The beat note of this circuit is set around 100hz. Why 100hz? Wouldn't 50hz be better (assuming I can discriminate between changes in pitch from 50hz & up)?

Lastly, why amplifying the beat note will not make for a better detector. If I can hear the beat note just fine, then amplifying it shouldn't make for a better detector, correct? The idea here is discrimination in pitch, ie NOT discrimination in dB. However, if I were to use an ordinary loudspeaker for the output, it would be nice to get a real LOUD beat note so the "road noise" from the world around me doesn't drown it out.

Thanks!

1. Polyester capacitors are more stable then ceramic, their value changes less for a change in temperature. They have a lower loss component, so are more suitable for precision circuits and their tolerance on their actual value is much tighter.
The circuit really needs a volume control, but a resistor wired in series with the loudspeaker would work.
The circuits has a fixed oscillator and one that has its coil exposed to the buried metal. When the coil goes over the metal, the inductance of the coil falls, so the frequency of that oscillator rises, changing the beat note. The 100 HZ is just there you give you the satisfaction of knwing the circuit is working, it could be set to anything.
Frank