Why do strings feel soft or hard?
There are two types of perceived hardness:
The Static hardness...
...seems quite clear to anyone: You press a string down to the fingerboard and you can feel it under your finger in the form of a hard or soft feeling. Strings with a small modulus of elasticity (gut strings, certain synthetic strings) feel softer with the same initial string tension as strings with a high modulus of elasticity (steel strings).
This is easier to understand using the example of a clothes line. If the clothes line is made of steel wire, you can only press it down a certain degree despite applying a high force. However, if it is made of rubber, you only need a little bit of force to press it down a lot – even though the tension of the clothes line is the same with both materials. If you’re looking for a string that feels either harder or softer, this cannot be achieved simply by using a higher or lower string tension, contrary to expectations. This is because you cannot feel string tension!
The dynamic string hardness...
...stems from the vibration of the string. This means that a string with less string tension can feel just as hard under the left hand and/or bow hand because harmonic reflections occur on the bridge due to the overtones. What does that mean? The bridge cannot absorb all the overtones. As a result, these are reflected back into the string and return to the fingers and bow. Due to these so-called secondary vibrations, a string can also feel harder or softer when vibrating, even if the string tension is low.
Very highly tensioned strings can also feel soft because the instrument is under too much stress, meaning less overtones are reflected.