Frequency of sound waves can be measured DIRECTLY, given that it is easy to make a circuit or computer that can process frequencies on the order of a few hundred Hertz.
For any electromagnetic wave (the same stuff as light) that is a radio frequency, that can also be measured directly. Anything higher in frequency than microwaves cannot easily be measured directly. THEN the wavelength is put to use for a work-around of frequency measurement. Usually via diffraction.
For any electromagnetic wave (the same stuff as light) that is a radio frequency, that can also be measured directly. Anything higher in frequency than microwaves cannot easily be measured directly. THEN the wavelength is put to use for a work-around of frequency measurement. Usually via diffraction.