Question

When does frequency change?

Answer 1

Frequency changes when time period changes because time period and frequency are inversly proportional. Hope this helps u.

Answer 2

→ As we know very well that 'Frequency of vibration of a particle is defined as the number of vibrations completed by particle in one second'.

So, it means that  $n= \frac{1}{T}$.  As the time period will increase or decrease it affects in change of frequency(ν).