phone will automatically provide a superior transmission performance. This is not the case. To explain this it is necessary to know how a noise cancelling microphone works. The definition of a noise cancelling microphone is a microphone which functions according to the pressure gradient principle. It has two sound ports which give direct access to the front and the back of the microphone diaphragm. The sound ports are symmetrical around the diaphragm. The noise cancelling microphone reacts to the difference of sound pressure on the diaphragm. It is necessary to distinguish between the sound field originating from a close sound source (called 'near field') and the sound field originating from a distant sound source ('far field'). Noise is a typical distant sound source. The microphone reacts differently to the two types of sound source. The near field frequency response closely resembles a 'normal' pressure micro- Figure 58.14 Over-the-head headset phone characteristic, while the far field frequency response shows a distinct proportional dependence on the frequency. The directional characteristic of noise cancelling microphones is such that greater attenuation of microphone signals takes place when the sound activates the microphone simultaneously on each side of the diaphragm. In a polar form the characteristic would be shaped as a figure of eight. This means there are two noise reducing effects present in a noise cancelling microphone, partly the effect of direction and partly a strong dampening of low frequencies from the far field sound source. The near field measurement less the far field measurement is termed noise cancellation. A good noise cancelling microphone has a noise cancellation of not less than 11dB. Experience with use of noise cancelling microphones is that there is a very minimal real requirement for this type of microphone except in specialised uses such as military, heavy industrial, air traffic control etc. The main objection from other users being the distance dependence of the microphone i.