The frequency of electromagnetic (EM) waves increases as we move down the EM spectrum, while the frequency of sound waves decreases as we move to lower pitches. This difference is due to the fundamental nature of these two types of waves and the medium through which they propagate.
EM waves are produced by oscillating electric and magnetic fields, and they can travel through a vacuum (such as outer space) or various mediums, including air, water, and solids. The EM spectrum spans a wide range of frequencies, from extremely low frequencies (ELF) to gamma rays. As you move down the EM spectrum from radio waves to microwaves, infrared, visible light, ultraviolet, X-rays, and gamma rays, the frequency of the waves increases. This is because the waves in the EM spectrum are characterized by the oscillation of electric and magnetic fields, and higher frequencies correspond to more oscillations per second.
On the other hand, sound waves are mechanical waves that require a medium, such as air, water, or solids, to propagate. In the case of sound, the frequency refers to the number of vibrations or cycles of compression and rarefaction that occur in the medium per second. When you move to lower pitches or lower frequencies in the audible range of sound, such as moving from a high-pitched note to a low-pitched note on a musical instrument, the frequency of the sound waves decreases. This is because lower-pitched sounds have fewer cycles of compression and rarefaction per second compared to higher-pitched sounds.
In summary, the difference in the behavior of frequency as we move down the EM spectrum and for sound waves is primarily due to the nature of the waves themselves (electromagnetic vs. mechanical) and the medium through which they propagate (vacuum vs. medium-dependent).