Acoustic impedance

Acoustic impedance and specific acoustic impedance are measures of the opposition that a system presents to the acoustic flow resulting from an acoustic pressure applied to the system. The SI unit of acoustic impedance is the pascal second per cubic metre () or the rayl per square metre (), while that of specific acoustic impedance is the pascal second per metre () or the rayl. In this article the symbol rayl denotes the MKS rayl. There is a close analogy with electrical impedance, which measures the opposition that a system presents to the electrical flow resulting from an electrical voltage applied to the system.
Mathematical definitions.
Acoustic impedance.
For a linear time-invariant system, the relationship between the acoustic pressure applied to the system and the resulting acoustic volume flow rate through a surface perpendicular to the direction of that pressure at its point of application is given by:
or equivalently by
where
Acoustic impedance, denoted "Z", is the Laplace transform, or the Fourier transform, or the analytic representation of "time domain" acoustic resistance:
where
Acoustic resistance, denoted "R", and acoustic reactance, denoted "X", are the real part and imaginary part of acoustic impedance respectively:
where
Inductive acoustic reactance, denoted "X""L", and capacitive acoustic reactance, denoted "X""C", are the positive part and negative part of acoustic reactance respectively:
Acoustic admittance, denoted "Y", is the Laplace transform, or the Fourier transform, or the analytic representation of "time domain" acoustic conductance:
where
Acoustic conductance, denoted "G", and acoustic susceptance, denoted "B", are the real part and imaginary part of acoustic admittance respectively:
where
Acoustic resistance represents the energy transfer of an acoustic wave. The pressure and motion are in phase, so work is done on the medium ahead of the wave; as well, it represents the pressure that is out of phase with the motion and causes no average energy transfer. For example, a closed bulb connected to an organ pipe will have air moving into it and pressure, but they are out of phase so no net energy is transmitted into it. While the pressure rises, air moves in, and while it falls, it moves out, but the average pressure when the air moves in is the same as that when it moves out, so the power flows back and forth but with no time averaged energy transfer. A further electrical analogy is a capacitor connected across a power line: current flows through the capacitor but it is out of phase with the voltage, so no net power is transmitted into it.
Specific acoustic impedance.
For a linear time-invariant system, the relationship between the acoustic pressure applied to the system and the resulting particle velocity in the direction of that pressure at its point of application is given by
or equivalently by:
where
Specific acoustic impedance, denoted "z" is the Laplace transform, or the Fourier transform, or the analytic representation of "time domain" specific acoustic resistance:
where "v" −1 is the convolution inverse of "v".
Specific acoustic resistance, denoted "r", and specific acoustic reactance, denoted "x", are the real part and imaginary part of specific acoustic impedance respectively:
where
Specific inductive acoustic reactance, denoted "x""L", and specific capacitive acoustic reactance, denoted "x""C", are the positive part and negative part of specific acoustic reactance respectively:
Specific acoustic admittance, denoted "y", is the Laplace transform, or the Fourier transform, or the analytic representation of "time domain" specific acoustic conductance:
where
Specific acoustic conductance, denoted "g", and specific acoustic susceptance, denoted "b", are the real part and imaginary part of specific acoustic admittance respectively:
where
Specific acoustic impedance "z" is an intensive property of a particular "medium" (e.g., the "z" of air or water can be specified); on the other hand, acoustic impedance "Z" is an extensive property of a particular "medium and geometry" (e.g., the "Z" of a particular duct filled with air can be specified).
Relationship.
For a "one dimensional" wave passing through an aperture with area "A", the acoustic volume flow rate "Q" is the volume of medium passing per second through the aperture; if the acoustic flow moves a distance d"x" = "v" d"t", then the volume of medium passing through is d"V" = "A" d"x", so:
Provided that the wave is only one-dimensional, it yields
Characteristic acoustic impedance.
Characteristic specific acoustic impedance.
The constitutive law of nondispersive linear acoustics in one dimension gives a relation between stress and strain:
where
This equation is valid both for fluids and solids. In
Newton's second law applied locally in the medium gives:
Combining this equation with the previous one yields the one-dimensional wave equation:
The "plane waves"
that are solutions of this wave equation are composed of the sum of "two progressive plane waves" traveling along "x" with the same speed and "in opposite ways":
from which can be derived
For "progressive" plane waves:
or
Finally, the specific acoustic impedance "z" is
The absolute value of this specific acoustic impedance is often called characteristic specific acoustic impedance and denoted "z"0:
The equations also show that
Effect of temperature.
Temperature acts on speed of sound and mass density and thus on specific acoustic impedance.
Characteristic acoustic impedance.
For a "one dimensional" wave passing through an aperture with area "A", "Z" = "z"/"A", so if the wave is a progressive plane wave, then:
The absolute value of this acoustic impedance is often called characteristic acoustic impedance and denoted "Z"0:
and the characteristic specific acoustic impedance is
If the aperture with area "A" is the start of a pipe and a plane wave is sent into the pipe, the wave passing through the aperture is a progressive plane wave in the absence of reflections, and the usually reflections from the other end of the pipe, whether open or closed, are the sum of waves travelling from one end to the other. (It is possible to have no reflections when the pipe is very long, because of the long time taken for the reflected waves to return, and their attenuation through losses at the pipe wall.) Such reflections and resultant standing waves are very important in the design and operation of musical wind instruments.