Sound

Sound is a wave that propagates through a physical medium by pushing on other particles in the medium. Every wave has a frequency and a wavelength, which are related by the speed of the wave. Since waves propagate by pushing on neighbouring particles, the speed of the wave is dependent on the elastic and inertial properties of the medium. Yes, I’m also considering light as a wave that propagates by pushing on particles in the vacuum, making it a form of sound. The vacuum can be thought of as being embedded with point particles constituting the photon field, or the electromagnetic field. I believe quantum field theory has the same understanding.

Speed of Sound = $\sqrt{\frac{\text{elastic properties}}{\text{inertial properties}}}$

Speed of sound in various media

• For a string: $v=F_T/\mu$, where $F_T$ is the tension in the string and $\mu$ is the linear density.

• For a solid medium: $v=Y/\rho$, where $Y$ is the Young’s modulus and $\rho$ is the density.

• For a liquid medium: $v=B/\rho$, where $B$ is the Bulk modulus and $\rho$ is the density.

• For an ideal gas: $v=\gamma RT/M$, where $\gamma$ is the adiabatic index, $R$ is the gas constant, $T$ is the temperature and $M$ is the molar mass.

• For speed of light: $v=\sqrt{\mu ϵ}$, where $\mu$ is the relative permeability and $ϵ$ is the relative permittivity.