When sound waves pass through any physical substance the pressure of the waves causes the particles of the substance to move. The sound
specific impedance is the ratio between the
sound pressure and the
particle velocity it produces.
where and are the specific acoustic impedance, pressure and
particle velocityphasors, is the position and is the frequency.
Characteristic acoustic impedance
The rayl is also used for the characteristic (acoustic) impedance of a medium, which is an inherent property of a medium:[6]
Here, is the characteristic impedance, and and are the density and speed of sound in the unperturbed medium (i.e. when there are no sound waves travelling in it).
In a viscous medium, there will be a phase difference between the pressure and velocity, so the specific acoustic impedance will be different from the characteristic acoustic impedance .
MKS and CGS units
Subscripts are used in this section to distinguish identically named units. Texts often refer to "the MKS rayl" to ensure clarity.
The
MKS unit of specific acoustic impedance is the
pascal-
second per
meter,[7] and is often called the rayl (MKS: 1 Rayl = 1 Pa·s·m−1).
The MKS unit and the
CGS unit confusingly have the same name, but are not the same quantity (or unit):
As an MKS unit, one rayl equals one
pascal-
second per
meter (Pa·s·m−1), or equivalently one
newton-second per cubic meter (N·s·m−3). Expressed in
SI base units, that is kg·s−1·m−2:[6]
1 RaylMKS = 1 N⋅s/m3 = 1 Pa⋅s/m = 1 kg/(s⋅m2)
As a CGS unit, one rayl equals one
barye-
second per
centimeter (ba·s·cm−1), or equivalently one
dyne-second per cubic centimeter (dyn·s·cm−3). Expressed in CGS base units, that is g·s−1·cm−2:
1 RaylCGS = 1 dyn⋅s/cm3 = 1 ba⋅s/cm = 1 g/(s⋅cm2)
The CGS unit rayl is ten times larger than the MKS unit rayl: