## Impedance

*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

*reactance*. A purely imaginary admittance is called a

*susceptance*. The term

*immittance*refers to either an impedance or an admittance [35]. In mechanics, force is typically in units of newtons (kilograms times meters per second squared) and velocity is in meters per second.

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 [110], 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.

### Dashpot

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 [419], 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 [73].

### Ideal Mass

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

Since impedance is defined in terms of force and velocity, we will prefer the
form
. By the differentiation theorem for Laplace transforms
[284],^{8.1}we have

*integrator*. Thus, an ideal mass integrates the applied force (divided by ) to produce the output velocity. This is just a ``linear systems'' way of saying force equals mass times acceleration.

Since we normally think of an applied force as an *input* and the resulting
velocity as an *output*, the corresponding *transfer function* is
. The system diagram for this view
is shown in Fig. 7.3.

The *impulse response* of a mass, for a force input and velocity output,
is defined as the inverse Laplace transform of the transfer function:

*unit momentum*to the mass at time 0. (Recall that momentum is the integral of force with respect to time.) Since momentum is also equal to mass times its velocity , it is clear that the unit-momentum velocity must be .

Once the input and output signal are defined, a transfer function is
defined, and therefore a *frequency response* is defined [485].
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 is

In circuit theory, the element analogous to the mass is the *inductor*,
characterized by
, or
. In an analog
equivalent circuit, a mass can be represented using an inductor with value
.

### Ideal Spring

Figure 7.4 depicts the ideal spring.

From Hooke's law, we have that the applied force is proportional to the
*displacement* of the spring:

*stiffness*of the spring. Taking the Laplace transform gives

*differentiator*. We can say that the ideal spring differentiates the applied force (divided by ) to produce the output velocity.

The *frequency response* of the ideal spring, given the applied force
as input and resulting velocity as output, is

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.

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Nonlinear Elements