USE OF TWO-POrt NETWORKS By Ross Wilson
What is a Two-Port Network? 
A two port network is a four terminal unit on an electrical circuit of a device. The circuit has 2 pairs of terminals that are connected internally via an electrical network. The two terminals will form one port if they meet the requirement which is the current that enters and leaves the port must be equal, A good example of this is a transistor.
This is a picture of a Two-Port Network, The two terminals will form one port if they meet the requirement which is the current that enters and leaves the port must be equal.
What is Impedance Matching? 
Impedance Matching is created to maximise power transfer, this is by making both the Input Impedance (Zl) and the Output Impedance (Zs) equal, this is commonly used in telephone lines where the impedances are equal to minimise echoes and interference in long distance phone calls. Matching is obtained when Zs=Zl where Zl is the normal input impedance into the load and Zs is the output impedance from the load.
What is Transmission Lines? 
Transmission Lines are used to get the maximum amount of energy to the other end of the line or send information with minimal errors, this is done by matching the impedances. Impedances can be unmatched by different variations in geometry along the cable length which can be caused by a manufacturing fault or faulty installation. Telephone Systems use matching impedances to minimise echoes in the long distance phone lines as explained earlier, this is to transmission lines theory. Matching Impedances can also enable the telephone hybrid coil (2 to 4 wire Conversion) to operate correctly.
What is Loudspeaker Impedances? 
Loudspeaker Impedances are kept low when compared with the other audio components so that the required power can be transmitted without using high voltages. Loudspeakers are now designed to operate at frequencies above their normal resonance and for this reason it is found that to define normal impedance at minimum and then round to the nearest standard value.
References Two-Port Networks
http://fourier.eng.hmc.edu/e84/lectures/ch2/node4.html
http://web.tiscali.it/decartes/phd_html/img248.png
Impedance Matching
http://hyperphysics.phy-astr.gsu.edu/hbase/audio/imped.html
Transmission Lines
http://www.antennatheory.com/tutorial/txline/transmissionline.php#txline
Loudspeaker Impedance
http://www.soundonsound.com/sos/jan03/articles/impedanceworkshop.asp