how do you calculate the frequency when...when you do not know the wavelength...and same about finding wavelength when frequency is given????
You have to know the speed that the waves travel. For sound and light, this speed is well known. It is also easy to measure.
Once you know the wave speed V, you can use the relationship
(wavelength)x(frequency) = V
With this, you can calculate the frequency from the wavelength, or vice versa