What Are Clicks?

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What's the difference between a click or thump sound (e.g. when you hit something with a stick or hammer a nail) compared to striking a bell or a string in a musical instrument? It's mainly the amount of damping involved.
The string of a piano, when struck by a hammer attached to a key, resonates, as do many objects in nature. This means it vibrates at one or more of its natural frequencies. Generally, shorter, smaller, or tensioned or stiffer objects tend to produce higher frequencies and vice versa. Hence the reason why the pitch of a guitar string increases as it's tensioned or longer organ pipes have lower pitches. The exact pitch also depends on the object's shape, material, and how it is constrained.
A piece of steel or a bell, when struck, rings. That ringing sound can last anywhere from fractions of a second to many seconds (I counted over ten seconds for the church bell in Kilcullen). If there were no damping, the vibrations would continue indefinitely. However, in a real system, vibrational energy is gradually dissipated through internal friction in the material, air resistance, and energy transferred into the surrounding structure. Some of the mechanical energy is converted into heat.
The greater the damping, the faster the note decays. Pianos have damping pedals that can increase damping and shorten the length of time that a string vibrates when a key is released.
A click or thump is actually a very short-lived vibration. If you could see it on an oscilloscope (an electronic test instrument used to display signals as they change over time), you would see a waveform that oscillates up and down, but the number of cycles would be small and the energy would decay rapidly. Because there are so few cycles, our brains perceive it as a click or thump rather than a musical note.
The clicks produced by bats are more sophisticated than those generated by simply hitting an object. Bats can produce frequency-modulated (FM) sounds, where the frequency sweeps during the pulse. This helps them determine range accurately, distinguish fine details of objects, and extract information about moving prey.

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