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Impedance is a concept that shows up in any area of physics concerning waves. In transmission lines, impedance is the ratio of voltage to current. In optics, index of refraction plays a role similar to impedance. Mechanical impedance is the ratio of force to velocity.

What is a general definition of impedance?

What are some examples of "impedance matching" other than in electrical transmission lines?

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up vote 10 down vote accepted

I found a general, qualitative answer in David Blackstock's book Physical Acoustics, on page 46:

Impedance is often described as the ratio of a "push" variable $q_p$ (such as voltage or pressure) to a corresponding "flow" variable $q_f$ (such as current or particle velocity).

I also received a nice answer to this question on another Q&A site which expands a bit on this qualitative statement with a quantitative one. In particular, this answer makes the point that impedance is the ratio (transfer function) of a force applied at a particular point to the velocity at that point.

I suppose what I am looking for next is an intuitive explanation of the phenomenon of impedance matching and maximum power transfer.

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Uh... I liked this question. And a quick look at the Google or Google scholar did not give me much. So is written here more what I grasp people understand about impedance. Like Noldorin was saying I take impedance as how much is impeding power to be transferred from one place to another. In a homogeneous medium power is transferred as wave unimpeded but when it find an interface of another medium then power transfer won't be so straight forward. The point is at the two mediums dynamic equations of the two place does no match. Say, electromagnetically one medium has the Maxwell equations of $(\mu_1,\epsilon_1)$ and the other medium has the equations with $(\mu_2,\epsilon_2)$ At the interface the countour conditions needs to be satisfied \begin{equation}\left(\vec{D_2}-\vec{D_1}\right)\cdot\hat{n}=0\end{equation} \begin{equation}\left(\vec{B_2}-\vec{B_1}\right)\cdot\hat{n}=0\end{equation} \begin{equation}\left(\vec{E_2}-\vec{E_1}\right)\times\hat{n}=0\end{equation} \begin{equation}\left(\vec{H_2}-\vec{H_1}\right)\times\hat{n}=0\end{equation} where $\hat{n}$ is the normal of the interface. Using the above conditions to connect the solutions of wave equations at each medium, will give rise to the Reflections and Transmission Coefficients. In the case of Maxwell's equations \begin{equation}R=\frac{Z_2-Z_1}{Z_2+Z_1}\end{equation}

For a one dimensional wave, with standard solutions \begin{equation}f_i\left(x,t\right)=A e^{k_i x-\omega t}+B e^{k_i x+\omega t}\end{equation}

the conditions that needs to be satisfied are the equality of displacement and the its derivative \begin{equation}f_2(x_0,t)=f_1(x_0,t)\end{equation} \begin{equation}f'_2(x_0,t)=f'_1(x_0,t)\end{equation} where $x_0$ is the position of the interface. Using these conditions with the solutions of the wave equations we will get. \begin{equation}R=\frac{k_2-k_1}{k_2+k_1}\end{equation} So in this case the impedance will be characterized by $k$.

So generalizing what you want is find what is the physical conditions that needs to be matched at the interface and use it to connect the solutions of each side of the interface, and figure how you get the reflections coefficients, which should look something like the reflection coefficients above. The terms should be your impedance for that system. Note that the definition of your impedance in that system is not quite unique. As in the previous example you could normalize the $k_i$ by some arbitrary $k_0$ and call this the impedance. \begin{equation}z_i=\frac{k_i}{k_0}\end{equation} Check the literature to find what is the exact definition for the system you are interested in. I suppose you already went to Wikipedia for other examples.

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A good example of impedance matching outside the world of transmission lines is acoustic horns, which match acoustic impedance between a sound source (like a vibrating string or reed) and the air.

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Mechanical impedance matching does have an application in electrical transmission lines (or any elastic cable/structure vibration) because it helps describe how much of the wave gets through a discontinuity, and how much is being reflected. Mechanical impedance is force over velocity and along the cable it is equal to tension over wave speed. The discontinuity may be either an elastic support (with damping properties), or a change unit weight of the cable (bigger cross section) which changes the wave speed.

For a cable with tension $T$ and unit weight $w$ (weight per length) the wave speed is $c=\sqrt{T/w}$ and the impedance is $z=\frac{T}{c}=\sqrt{T\,w}$. The practical application of this is in the design of stockbridge dampers that go on cables and structures to absorb aeolian vibrations. There is a theory of matching impedances and measuring the "Inverse Standing Wave Ratio" in testing to check the efficiency of the damper. With a good match most of the wave energy does not get reflected back into the cable once it reaches the end near the tower. There is a IEEE standard covering all this.

In case you missed it, mechanical impedance is force over speed. Maybe someone something similar for the electrical, or acoustical impedance.

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A good (if relatively grisly) example of impedance is the transmission of the shock-wave from an exploding bomb into a target. If a bomb detonates over a target, it will create a shock wave in the surrounding air, which will then hit and severely damage any structures it encounters. However, if the target is some sort of reinforced bunker, a large part of the energy will proceed to bounce off it as a reflected shock wave, because the acoustic impedance difference is too great.

This led to the development of earthquake bombs, which are designed to insert themselves into the ground before exploding; this causes the shock wave to travel through the ground and minimizes the impedance difference. A related "device" is the bouncing bomb of Dam Busters fame:

enter image description here

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Impedance is general simply refers to the "amount of impeding (or block)". It's specific meanings in the various branches of physics (mechanical, electrical, wave impedance) varies, but are all loosely based on the concept of the everyday word.

A good example of impedance matching I always think of relates to audio systems. When you want to chain a number of amplifiers in stages, you usually want to match the output impedance of one with the input impedance of the other in order to maximise power transfer.

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Impedance matching is not used in audio electronics. Not in microphones/preamps, nor interconnects, nor speaker amplifiers. In all these cases, the source impedance is (much) smaller than the load, to maximize fidelity, not power. Also, you misunderstand the maximum power theorem, which only applies when the source impedance is fixed and you're trying to get the maximum power out of it. If you have control of the source, you just make its impedance as small as possible to increase power to the load. – endolith Nov 9 '10 at 19:35
@endolith: I'm afraid you're mistaken, at least in part. When you connect power amps in series, you want to impedance match. I was explained this by an electrical engineer, no less. – Noldorin Nov 9 '10 at 19:55
@Noldorin: What do you mean by "connect power amps in series"? – endolith Nov 9 '10 at 20:22
HyperPhysics sounds confused. As far as I know, impedance matching was only used in vacuum tube circuits, if it all. Mic outputs are typically 600 Ω or less. Mic preamp inputs are 1 kΩ or more. Loudspeakers are like 4 Ω, amplifier outputs are <0.5 Ω. They aren't matched. Connect a 4 ohm loudspeaker to an amplifier with variable output impedance. Do you get maximum power transfer when the output impedance is 4 ohms? Nope. You get maximum power when it's 0 ohms. – endolith Nov 12 '10 at 22:31
Impedance matching does maximize power transmission but the "maximum" is over all possible receiver impedances. For example, if the driver impedance is 16 ohms, the maximum power transfer occurs with 16 ohm speakers. But as endolith says, this is not how hifi audio circuits re designed. On the other hand, it's a well known fact among electrical engineers and it has applications in other areas. – Carl Brannen Feb 5 '11 at 0:23

Impedance is the ratio of a force applied at a particular point to the velocity at that point.

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This does not provide an answer to the question. To critique or request clarification from an author, leave a comment below their post - you can always comment on your own posts, and once you have sufficient reputation you will be able to comment on any post. – Sklivvz Dec 9 '12 at 12:01

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