I want to know how can we change frequency of wave, both sound and light, and what is the nature of "changing frequency"

Such as, any material could absorb and re-emit wave in another wavelength. Or any kind of field could make an interference in a way that making new wave in difference frequency

p.s. please exclude refraction (which is not really change frequency, just change wavelength and speed) and Doppler shift

I want to know how can we change the frequency of wave, both sound and light, and what is the nature of "changing frequency".

Such as, any material could absorb and re-emit wave in another wavelength. Or any kind of field could make an interference in a way that making new wave in difference frequency.

PS please exclude refraction (which is not really change frequency, just a change in wavelength and speed) and Doppler shift.