You are exactly right in your thinking. The reason you are confused is that you know humans cant hear below 20Hz but if you are tapping something at 5hz you can definitely hear it. How is this possible? It's because when you tap an object (say the desk with your fingernail) you are creating many frequencies at once. You could download a fft waterfall app for your phone and test this for yourself. The reason it contains many frequencies is that the sound wave resembled an impulse function (a sharp spike). When people say humans can't hear below 20hz they mean that they can't hear a pure sine wave below 20hz.

Beats per second and frequency are exactly the same thing. If you tap a 440hz tuning fork 20 times per second you will get a mixture of 20hz and 440hz sound coming from it. The reason for this lies in the mathematics of the fourier transforms. This math tells us that any arbitrary waveform (like the impulse from tapping your desk) can be broken down into a sum of sine waves (like your tuning fork).

You are exactly right in your thinking. The reason you are confused is that you know humans can't hear below 20Hz but if you are tapping something at 5hz you can definitely hear it. How is this possible? It's because when you tap an object (say the desk with your fingernail) you are creating many frequencies at once. You could download a fft waterfall app for your phone and test this for yourself. The reason it contains many frequencies is that the sound wave resembles an impulse function (a sharp spike). When people say humans can't hear below 20hz they mean that they can't hear a pure sine wave below 20hz.

If you have a good subwoofer you could have it output a 10hz sine wave that you couldn't hear, but you could feel. In real life you can actually hear the 10hz from a subwoofer if it is loud enough because other things in the car will start to vibrate and emit sounds at higher frequencies. But if the subwoofer is made well it will not distort the 10hz sine wave (by rattling or emitting sound at other frequencies) and will therefore be silent. You can also make a 10hz sound by tapping on something, but it will have a ton of other frequency content higher than 10hz that makes it possible to easily hear.

Beats per second and frequency are exactly the same thing. If you tap a 440hz tuning fork 20 times per second you will get a mixture of 20hz and 440hz sound coming from it. The reason for this lies in the mathematics of the fourier transforms. This math tells us that any arbitrary waveform (like the impulse from tapping your desk) can be broken down into a sum of sine waves (like your tuning fork).