Question

I believe that sound is a physical quantity and independent of any other quantity. I want to know why is it not placed in any of the systems (like SI, MKS, FPS, CGS).

Answer 1

Sound is a phenomena, but it has no single unit. It's not like "distance", it carries various properties. Sound can be described as a wave in which case it has wavelength (lambda expressed in the SI unit meter), frequency (Hertz equal to cycles per second, which is a rate but directly based on the SI unit 'second') and velocity (m/s, although this velocity could mean both horizontal velocity of a pattern in the wave or the actual velocity of a single particle within the wave as sound is actually a standing wave). You may have heard of the units dB (decibel) and sone to describe the intensity of sound. However, neither are part of the SI units or SI derived units. Though dB is part of the International System ofQuanitities(that is different from the SI). Technically, the best way to describe sound 'loudness' would probably be by energy over area (described in Joules per square meters in SI units, J m^-2). This is however not used as it is a difficult unit to work with for sound, moreover to measure it. But in acoustics I suppose it is useful. High school physics mostly limits the units of sound to meter, Hz and m/s (respectively wavelength, frequency and velocity).