Which factor does NOT affect the speed of sound in a medium?

The speed of sound in a medium is determined by several physical properties, but frequency is not one of them. The speed of sound is primarily influenced by the medium's density and temperature.

Density affects how tightly molecules are packed in a medium; generally, sound travels faster in denser mediums, such as in solids compared to gases. Temperature also has a significant effect; as the temperature increases, the speed of sound usually increases in gases because the molecules move more rapidly and can transmit the sound waves more efficiently.

Viscosity, which measures a fluid's resistance to flow, can subtly influence acoustic propagation but is less significant compared to density and temperature. However, frequency refers to the number of oscillations of the wave per second, which does not affect how fast sound travels through the medium. Therefore, frequency remains constant regardless of the speed of sound in any given medium, making it the correct choice as the factor that does not affect the speed of sound.