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 in the MKS system the rayl per square metre, while that of specific acoustic impedance is the pascal-second per metre, or in the MKS system the rayl. There is a close analogy with electrical impedance, which measures the opposition that a system presents to the electric current resulting from a 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

p is the acoustic pressure;
Q is the acoustic volume flow rate;
is the convolution operator;
R is the acoustic resistance in the time domain;
G'' = R is the acoustic conductance in the time domain.

Acoustic impedance, denoted Z'', is the Laplace transform, or the Fourier transform, or the analytic representation of time domain acoustic resistance:
where

is the Laplace transform operator;
is the Fourier transform operator;
subscript "a" is the analytic representation operator;
Q is the convolution inverse of Q.

Acoustic resistance, denoted R, and acoustic reactance, denoted X, are the real part and imaginary part of acoustic impedance respectively:
where

i is the imaginary unit;
in Z, R is not the Laplace transform of the time domain acoustic resistance R, Z is;
in Z, R is not the Fourier transform of the time domain acoustic resistance R, Z is;
in Z, R is the time domain acoustic resistance and X is the Hilbert transform of the time domain acoustic resistance R, according to the definition of the analytic representation.

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

Z is the convolution inverse of Z;
p is the convolution inverse of p.

Acoustic conductance, denoted G, and acoustic susceptance, denoted B, are the real part and imaginary part of acoustic admittance respectively:
where

in Y, G is not the Laplace transform of the time domain acoustic conductance G, Y is;
in Y, G is not the Fourier transform of the time domain acoustic conductance G, Y is;
in Y, G is the time domain acoustic conductance and B is the Hilbert transform of the time domain acoustic conductance G, according to the definition of the analytic representation.

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. Acoustic reactance 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

p is the acoustic pressure;
v is the particle velocity;
r is the specific acoustic resistance in the time domain;
g'' = r is the specific acoustic conductance in the time domain.

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 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

in z, r is not the Laplace transform of the time domain specific acoustic resistance r, z is;
in z, r is not the Fourier transform of the time domain specific acoustic resistance r, z is;
in z, r is the time domain specific acoustic resistance and x is the Hilbert transform of the time domain specific acoustic resistance r, according to the definition of the analytic representation.

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

z is the convolution inverse of z;
p is the convolution inverse of p.

Specific acoustic conductance, denoted g, and specific acoustic susceptance, denoted b, are the real part and imaginary part of specific acoustic admittance respectively:
where

in y, g is not the Laplace transform of the time domain acoustic conductance g, y is;
in y, g is not the Fourier transform of the time domain acoustic conductance g, y is;
in y, g is the time domain acoustic conductance and b is the Hilbert transform of the time domain acoustic conductance g, according to the definition of the analytic representation.

Specific acoustic impedance z is an intensive property of a particular medium ; on the other hand, acoustic impedance Z is an extensive property of a particular medium and geometry.

Acoustic ohm

The acoustic ohm is a unit of measurement of acoustic impedance. The SI unit of pressure is the pascal and of flow is cubic metres per second, so the acoustic ohm is equal to 1 Pa·s/m³.
The acoustic ohm can be applied to fluid flow outside the domain of acoustics. For such applications a hydraulic ohm with an identical definition may be used. A hydraulic ohm measurement would be the ratio of hydraulic pressure to hydraulic volume flow.

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, then the volume of medium passing through is, so:
If the wave is 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

p is the acoustic pressure in the medium;
ρ is the volumetric mass density of the medium;
c is the speed of the sound waves traveling in the medium;
δ is the particle displacement;
x is the space variable along the direction of propagation of the sound waves.

This equation is valid both for fluids and solids. In

fluids, ;
solids, for longitudinal waves and for transverse waves.

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 z₀:
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,, 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 Z₀:
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. Such reflections and resultant standing waves are very important in the design and operation of musical wind instruments.