- For electrical systems, impedance is voltage divided by current:
- For transverse traveling waves on a vibrating string,
the
*wave impedance*is given by- transverse force wave
- transverse velocity wave
- string density (mass per unit length)
- string tension (stretching force)

- For longitudinal plane-waves in air, the
*wave impedance*is given by*pressure*divided by*particle velocity*:*adiabatic gas constant*for air (ratio of the specific heat of air at constant pressure to that at constant volume) - For longitudinal plane-wave sections in an
*acoustic tube*, the*wave impedance*is given by*pressure*divided by*volume velocity*: - Typical physical units used in practice are the Standard International (SI) units:
*force*in*Newtons*(kilograms times meters per second squared)*pressure*in*Newtons per meter squared**velocity*in*meters per second**mass*in*kilograms*

- Related terms:
*admittance*=*reactance*= purely imaginary impedance*susceptance*= purely imaginary admittance*immittance*= either impedance or admittance

- For transverse electromagnetic (TEM) waves in a
*transmission line*, the*characteristic impedance*(wave impedance) is given by electric potential in volts divided by electric current in amperes (coulombs per second): - In a vacuum, the wave impedance for light (also a TEM wave) is
*permeability*and*permittivity*, respectively, of the vacuumIt is odd and interesting that waves in the vacuum are subject to the special theory of relativity (speed of light always measured to be the same, irrespective of one's velocity)

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