Impedance is defined for mechanical systems as force divided by velocity, while the inverse (velocity/force) is called an admittance. For dynamic systems, the impedance of a ``driving point'' is defined for each frequency , so that the ``force'' in the definition of impedance is best thought of as the peak amplitude of a sinusoidal applied force, and similarly for the velocity. Thus, if denotes the Fourier transform of the applied force at a driving point, and is the Fourier transform of the resulting velocity of the driving point, then the driving-point impedance is given by
In acoustics [317,318], force takes the form of pressure (e.g., in physical units of newtons per meter squared), and velocity may be either particle velocity in open air (meters per second) or volume velocity in acoustic tubes (meters cubed per second) (see §B.7.1 for definitions). The wave impedance (also called the characteristic impedance) in open air is the ratio of pressure to particle velocity in a sound wave traveling through air, and it is given by , where is the density (mass per unit volume) of air, is the speed of sound propagation, is ambient pressure, and is the ratio of the specific heat of air at constant pressure to that at constant volume. In a vibrating string, the wave impedance is given by , where is string density (mass per unit length) and is the tension of the string (stretching force), as discussed further in §C.1 and §B.5.2.
In circuit theory , force takes the form of electric potential in volts, and velocity manifests as electric current in amperes (coulombs per second). In an electric transmission line, the characteristic impedance is given by where and are the inductance and capacitance, respectively, per unit length along the transmission line. In free space, the wave impedance for light is , where and are the permeability and permittivity, respectively, of free space. One might be led from this to believe that there must exist a medium, or `ether', which sustains wave propagation in free space; however, this is one instance in which ``obvious'' predictions from theory turn out to be wrong.
The elementary impedance element in mechanics is the dashpot which may be approximated mechanically by a plunger in a cylinder of air or liquid, analogous to a shock absorber for a car. A constant impedance means that the velocity produced is always linearly proportional to the force applied, or , where is the dashpot impedance, is the applied force at time , and is the velocity. A diagram is shown in Fig. 7.1.
In circuit theory, the element analogous to the dashpot is the resistor , characterized by , where is voltage and is current. In an analog equivalent circuit, a dashpot can be represented using a resistor .
Over a specific velocity range, friction force can also be characterized by the relation . However, friction is very complicated in general , and as the velocity goes to zero, the coefficient of friction may become much larger. The simple model often presented is to use a static coefficient of friction when starting at rest () and a dynamic coefficient of friction when in motion ( ). However, these models are too simplified for many practical situations in musical acoustics, e.g., the frictional force between the bow and string of a violin [308,549], or the internal friction losses in a vibrating string .
The concept of impedance extends also to masses and springs. Figure 7.2 illustrates an ideal mass of kilograms sliding on a frictionless surface. From Newton's second law of motion, we know force equals mass times acceleration, or
The impulse response of a mass, for a force input and velocity output, is defined as the inverse Laplace transform of the transfer function:
Once the input and output signal are defined, a transfer function is defined, and therefore a frequency response is defined . The frequency response is given by the transfer function evaluated on the axis in the plane, i.e., for . For the ideal mass, the force-to-velocity frequency response isamplitude response rolls off dB per octave, and the phase shift is radians at all frequencies.
Figure 7.4 depicts the ideal spring.
The frequency response of the ideal spring, given the applied force as input and resulting velocity as output, isamplitude response grows dB per octave, and the phase shift is radians for all . Clearly, there is no such thing as an ideal spring which can produce arbitrarily large gain as frequency goes to infinity; there is always some mass in a real spring.
We call the compression velocity of the spring. In more complicated configurations, the compression velocity is defined as the difference between the velocity of the two spring endpoints, with positive velocity corresponding to spring compression.
In circuit theory, the element analogous to the spring is the capacitor, characterized by , or . In an equivalent analog circuit, we can use the value . The inverse of the spring stiffness is sometimes called the compliance of the spring.
Don't forget that the definition of impedance requires zero initial conditions for elements with ``memory'' (masses and springs). This means we can only use impedance descriptions for steady state analysis. For a complete analysis of a particular system, including the transient response, we must go back to full scale Laplace transform analysis.
One-Port Network Theory