veloped by Western Electric Company. It uses a reference of frequency as well as magnitude and may be the combination of a frequency of 1000Hz and a reference power level of -85 dBm. The number of dissimilar forms of noise. unit of noise measurement for FIA weighting is dBa (dB adjusted). The results of many experiments involving telephone users have FIA line weighting is being phased out. The second line weighting, which is now the preferred standard in shown that, within the normal telephone frequency range of 300Hz North America, is C-message line weighting which is based upon a to 3400Hz lower frequencies cause less annoyance than higher frequencies. It has also been shown that if simultaneous noises are more sensitive handset. It uses a reference frequency of 1000Hz and a reference power level of -90dBm. The unit of noise measurement present then the effect to the user is additive on a power basis, i.e. if used for C-message weighting is dBrnC (dB above reference noise tones of an equal interfering effect are used a meter indication C-message). would be 3dB higher. A further effect to be noted is that it has been shown by experiment that the human ear does not fully respond to sounds shorter than 200ms and therefore does not fully appreciate the true power. 10.9.8 Noise measurement instruments Instruments for measuring noise are usually built specially for the 10.9.5 purpose because of the random nature and considerable fluctuations Line weighting for channel noise of the noise voltage. In order to conduct measurements of channel noise it is necessary to recognise the effects of annoyance to the telephone user of 10.9.8. Voltmeters frequency. The results of experiments have produced a number of line weighting curves with respect to frequency. This weighting The use of an a.c. voltmeter to measure noise voltage is possible must take account of the relative annoyance of tones, both in the under limited conditions. However, a large amount of fluctuation presence of speech and in its absence, with respect to a t