Sound level meters are required to measure noise of different levels, spectra and waveforms under widely varying conditions of sound source distribution and reflections at the sound field boundaries. Usually the purpose of these measurements, whether they are estimating hearing damage risk, annoyance, acoustical insulation efficiency, acceptability of manufactured products or any other factor, is to collect data which will improve our understanding of the problem and also help in solving it. Obviously an instrument giving readings that can be related to subjective impressions of loudness would be desirable. Attempts have been made in the past to design such an instrument, but in view of the difficulties involved in simulating the human hearing system for all types of noise, the International Electrotechnical Commission (IEG), has decided that the most practical solution is simply to standardize an apparatus by which sound pressure can be measured under closely defined conditions, so that results obtained by different users can be compared. The Type 2203 Sound Level Meter is an instrument with a practical combination of characteristics that will achieve a high degree of stability and accuracy. The accuracy and validity of the results are, however, determined by the manner of use, which must be chosen to suit the situation. In particular, care must be taken so that the presence of the observer does not invalidate the calibration. The instrument is not intended for measuring sounds of very short duration or discontinuous sounds. Some of the main requirements in the IEG specification are quoted here: A precision sound level meter shall include at least one of the three weighting networks called A, B or C, and should cover the frequency range 20 to 20ooo Hz (c/s) within certain tolerances. (Table 5.6). The microphone shall be of the omnidirectional pressure type. Permissible tolerances on the variation of sensitivity with angle are given in Table 2.1, and it is suggested that the diffuse sound field sensitivity (i.e. the root-mean-square of the sensitivities for all orientations) should by some means be brought within the tolerances already mentioned for the specified incidence. A square law indication instrument is specified, i.e. it must be capable of correctly summing two pure tones according to the root-mean-square law. The Precision Sound Level Meter Type 2203 is a highly accurate instrument designed for outdoor use as well as for precise laboratory measurements. It is easily portable, battery driven and completely self-contained for ordinary sound level and vibration measurements. Used in conjunction with a suitable filter set e. g. the B & K Octave Filter Set Type 1613, the instrument becomes a handy and easily operated frequency analyzer. There are no requirements stated in the IEG Draft Specification regarding dynamic range, but the B & K Type 2203 covers the range 22 to 134 dB (or 40 to 148 dB using a l/27/ microphone) and as will be seen from Table 1.1 this covers most sound levels which need to be measured. All three weighting networks (A, B and G) are included in the instrument as well as a linear characteristic and means for connecting external filter circuits for further shaping of the frequency characteristic if necessary. From instrument serial No. 82939 the Sotind Level Meter complies with the American Standard for General Purpose Sound Level Meters, ASA S 1.4-1961.