What are we hearing below 20hz?

It is often said that the human ear can hear 20hz to 20khz in range. This varies from one person to another and usually diminishes as we get older. So..........I listened to a test record the other day. It had tones from 20 hz down to 15 hz if I remember right. Anyway, at 20 hz, I could hear it very well. Each note after was down in 1 hz steps. I could hear down to 17 but each below 20 became more faint. This would be due in part to the speakers which are rated down to 18 hz. If I cannot hear below 20 hz, what am I hearing?

Likewise, a pipe organ can reach 8 hz from a 64 foot pipe. Few of these even exist but they do exist. 16 hz is produced from a 32 foot pipe. Most pipe organs probably don't have much more than the 32 foot pipes and some don't even have that. At any rate, the sounds are clearly heard, at least something is heard. The latest pianos are now producing 16 hz tones. We are able to hear the notes. If we can't hear below 20 hz, what are we hearing?