I don't know about the length of the sound. It should be pretty short.
As you say, you wouldn't want it to interfere with ambient sounds around
you. However, the sound also needs to be somewhat sharp. It needs to
have some upper frequency components to give you good echos and sound
location. If the sound was a dull thud, it wouldn't bounce well, or at
least you wouldn't hear it well. An advantage of the metal tip is that
it adds a sharpness.
Hope this makes sense. I am not an Engineer, but explain it the best I can.
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