Help with detecting 25-50Hz audio signal

Why not use a vibration meter? **broken link removed** or accelerometer?

attach the rim of the coil to a large as possible glass window centre

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Nice idea, but I can't see 'er indoors being too impressed!

No need for a window pane, a decent 12" woofer will be sensitive enough at 25Hz. And you could always put in some sort of bass reflex enclosure if you need a bit lower Hz or tune for a specific freq.