I always thought that stringed instruments have hole(s) called soundholes so that the produced sounds can be heard.
But the sitar as a stringed instrument doesn't have any soundholes.
So how is the sound heard from a sitar?
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The sound holes on stringed instruments are not there so that the sound can be heard. They are there to tune the resonances of the wooden bodies that amplify the sounds.
It is true that there are certain frequencies that are loudest right near the sound holes, but plugging those sound holes or building those instruments without any holes would not make them silent. The wood itself vibrates and causes the air next to the wood to vibrate and that creates most of the sound we hear from stringed instruments.
Also, one of the sitar’s resonators, the lakadi ka tumba (the one behind the head) does have a hole. It’s just on the back.
A guitar does not need a sound-hole, actually omitting the soundhole eliminates much of the feedback problems that people who play acoustics in loud environments have been plagued with since the birth of rock.
Now, this guitar has a sound-hole at the top, but that is a feature so you can hear yourself play. I'm sure that could have been omitted as well.
For gory details, see this physicsSE answer.
Among other things mentioned there:
--- it's the sound board in any plucked, bowed, or percussion instrument (piano is percussion) which amplifies the vibrations as well as providing better impedance matching.
--- The sound hole makes the box a Helmholtz resonator, which slightly modifies the spectral resonance levels.