CONCEPT: - Frequency (v): The number of waves that pass a given point per second is called frequency. - The unit of frequency is vibration per second or Hertz. - Time period (T): The time taken to complete one full oscillation or cycle by the Wave is called the time period. - The SI unit of the time period is second (s). - The relationship between time period and frequency is given as: Time period (T)=1∕v EXPLANATION: - A sound's loudness is measured in decibels (dB). Therefore option 1 can't be the unit of frequency of a sound wave. - The loudness of sound is determined by the intensity of the sound waves. - Intensity is a measure of the amount of energy in sound waves. The unit of intensity is the decibel (dB), Phon, and Bel. Therefore option 3 is correct. - s−1, Hertz (Hz), and min−1 is the unit of frequency.