Loop one end of a large rubber band over the peg at one end of the sonometer.  Stretch the rubber band and pluck.  Observe that the frequency is very low.  Stretch the rubber band some more and pluck again.  Observe that the frequency has not changed very much. 

This is one of the characteristics of rubber and rubber bands.  That is, with a guitar string, the pitch becomes higher if it is tightend because the tension is higher but the length and density of the string stay constant.  When a rubber band is stretched, the tension, the length, and the density all change.  The higher tension tends to increase its pitch but is compensted by the increased length which tends to lower its pitch.  Basically this means the frequency doesn't change much when the rubber band is stretched.

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