X (1/ If the tunnel is infinitely long, your voice will prop- 1000) seconds or 4 milliseconds. Phase delay can be used agate along the tunnel until it is totally attenuated or to measure time because there are 2 radians in a period absorbed. If the tunnel is not infinitely long, your voice time (T). For the example of a 1000 Hz signal, a phase will be reflected when it reaches the end wall of the delay of (4 X 2 radians is equivalent to four periodic tunnel and it will return to you as an echo or reflected times (T) or 4 milliseconds. Phase delay per unit length wave. The ratio reflected wave/incident wave is called is usually represented by the symbol (B). It is measured the reflection coefficient. You can prevent this re- in radians per metre. flection if it were physically possible to put some good Hence if we were to sum up propagation properties, sound absorption material at the end of the tunnel there would be at least three properties which are which absorbs all the incident sound. In other words, obvious: you would be creating a matching termination or attenuation of the signal as it travels along the line; matched load impedance (ZL) which matches the propagation characteristics of an infinitely long tunnel the time or phase delay as the signal travels along the in a tunnel of finite length. The ratio of the received line; sound relative to the incident sound is known as the dispersion which is the different attenuation experi- transmission coefficient. enced by different frequencies as it travels along A signal travelling from a point A to another point B a line. takes time to reach point B. This time delay is known as Finally, if you walked along a tunnel which produces propagation time delay for the signal to travel from point echoes, while a friend whistled at a constant amplitude A to point B. In fact, any signal travelling over any dis- and pitch, you would notice the reflected sound in- tance undergoes a propagation time delay. terfering with the incident sound. In some places, the CHAPTER 2.2 Tra