What is the difference between the fourth harmonic and the nth harmonic?
The fourth harmonic has frequency f4= v/λ4= 4v/2L = 4f1, and, to generalise, The nthharmonic has frequency fn= v/λn= nv/2L = nf1. All waves in a string travel with the same speed, so these waves with different wavelengths have different frequencies as shown.
