Frequency is the number of wavelengths that fit into one unit of time. Remember that a wavelength is equal to one compression and one rarefaction. Even though the singer sang the same note, because the sounds had different frequencies, we heard them as different. Frequencies are measured in Hertz. One Hertz is equal to one cycle of compression and rarefaction, per second.
High sounds have high frequencies and low sounds have low frequencies.
Thunder has a frequency of only 50 Hertz, while a whistle can have a frequency of 1000 Hertz. The human ear is able to hear frequencies of 20 to 20,000 Hertz. Some animals can hear sounds at even higher frequencies. The reason we cannot hear dog s whistles, while they can, is because the frequency of the whistle is too high be processed by our ears. Sounds that are too high for us to hear are called ultrasonic .