While the limiter/compressor will protect the subwoofer from being overdriven on source material, playing continuous test tones (sine waves) at loud volumes is not advised. The voice coil does not have an opportunity to cool-off (like it would with normal source material) and you might damage the subwoofer.
Being a bass reflex subwoofer tuned to about 20 Hz, the PB12-NSD will have linear/clean/usable output from about 18 Hz -up. While the subwoofer does have some clean output at 16 Hz, Josh's test result of 93 dB (CEA-2010 limit) is already about 10 dB below the max clean output of 103 dB @ 20 Hz.
Below system tuning, the woofer decouples from the port (Helmholtz resonator) and starts to unload. As a result, clean output capability will tail-off quickly below system tuning, and while the woofer will certainly respond to a 5 Hz test tone input, the subwoofer really doesn't have any usable output capability at that frequency.
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