.. I had to hand were very similar to those shown, being 10 mm in diameter by 6 mm high. Some experiments to check their frequency response practical details later showed a steady 6 dB/octave roll-off below about 15 Hz, implying that ...
.. (usually bulky, static, and power-hungry) is too obvious to mention, so we won’t. A cheap 50 0 50 µA meter connected between the output and common via a suitable resistor worked, but its response was 50% down at ~2 Hz. ...