How does changing length and mass affect the frequency of a vibrating string?

[Syzygy271]

I found that if you have a stretched wire under tension that if you

a) change the length the change in frequency is directly proportional to the change in length.

b) change the mass the change in frequency is inversely proportional to the change in mass.

Does anyone know how to back this up with relevant physics? Any useful links?

Thanks.

 

[nicksauce]

Do you know the formula for the frequency of a vibrating string? If not, try http://en.wikipedia.org/wiki/Vibrating_string