A unit of specific acoustic impedance and of characteristic impedance, based on the ratio of sound pressure to particle velocity at some point. In the centimeter-gram-second system of units, the specific acoustic impedance is 1 rayl (cgs) when a sound pressure of 1 microbar produces a linear velocity of 1 centimeter per second, = 10 pascal-seconds per meter.

Beranek (1954)¹ distinguishes between the “mks rayl”, defined as “newton-sec/m³” and the “rayl”, which is in the cgs system and defined as “dyne-sec/cm³”. In other words, in both the MKS and cgs systems, the rayl is the ratio of unit pressure to unit velocity.

Forty-eight years later, the American Institute of Physics Desk Reference² defined the rayl only as a unit of specific acoustic impedance, with the definition first given above.

The rayl is not one of the special units approved by the CGPM for use with SI.

The unit is named for the English physicist John William Strutt, third Lord Rayleigh (1842-1919). In Britain, “Rayleigh” is pronounced ray-lee, not rawl-ee, as Americans pronounce Raleigh, the city in North Carolina. The name of the unit is also pronounced with a long a, like the “rail” in “railroad.”

1. Leo L. Beranek.
New York: McGraw-Hill, 1954.
Page 11.

2. E. Richard Cohen, David R. Lide, George L. Trigg, editors.
AIP Physics Desk Reference. Third Edition.
New York: Springer-Verlag, AIP Press, 2003.



[ρc] This is called the characteristic impedence¹⁵ of the medium.

15. The CGS unit for characteristic impedence is the “rayl” (g/cm²/sec). To obtain the MKS unit for ρc, multiply the number of rayls by 10.

Theodor F. Hueter and Richard H. Bolt.
Sonics. Techniques for the Use of Sound and Ultrasound in Engineering and Science.
New York: Wiley, 1955.
Pages 33-34.


Rayl. The rayl is the magnitude of a specific acoustic resistance, reactance or impedence for which a sound pressure of one microbar produces a linear velocity of one centimeter per second (dyne-sec/cm³). When expressed in newton-sec/m³ it is called the mks rayl.

Dwight E. Gray, Coordinating Editor.
American Institute of Physics Handbook. 3rd ed. McGraw-Hill, 1972.
Page 3-17.


Each tissue has its own acoustic impedence (Z) that is equal to the density of the tissue multiplied by the speed of sound, i.e Z = rc … where 1 rayl = 1 kg∙m⁻²∙s⁻¹.

These are the acoustic impedence values for body tissues:

Graham Arthurs.
Chapter 1. The Physics of Ultrasound.
Page 7 in Ian Johnston, William Harrop-Griffiths and Leslie Gemmell, eds.
AAGBI Core Topics in Anaesthesia 2011.
Wiley-Blackwell, 2012.

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