Sound level meters (figure 2/12b) record and process sound. The most common devices react to sound in a similar way as the human ear. The structure of all sound level meters is basically the same. Each system has a microphone as receiver, followed by processing units as well as an output unit. The basic structure of a sound level meter is illustrated in figure 2/12a.

Incoming sound waves are converted to electrical signals in the microphone. Very precise condenser microphones are usually used for this purpose. As the measuring signal of the microphone is very small, it is amplified before further processing (pre-amplifier).

Even simple devices are equipped with filters allowing for aurally compensated measurements. Different filters (A), (B), (C) and (D) have been developed in order to account for the various reactions of the human ear to different ranges of sound pressure (Fig. 2/3). The most commonly used is the A-weighting filter, which is defined in international standards.

After further amplification of the signal, the sound pressure is squared, resulting in the sound energy or power. The next step is a time-weighted averaging adapted to the sound signal (time weighting). The usual time weightings are "Fast", "Slow", "Impulse" and "Peak".

The different time weightings allow for an adaptation of the measurements to the particularities of the sound. "Fast" time weighting is mostly used for traffic noises. The current result is displayed at the measurement device in a digital or analogue way (indication of the current level). As mentioned before (section 2.4.2), an energetic average sound level is usually produced to assess sound signals. This level (Leq) can also be displayed at the measurement device.

The devices for official measurements must comply with DIN EN 60651 or DIN EN 60804 and be authorized by the Federal Institute of Physics and Metrology (Physikalisch-Technische Bundesanstalt). Furthermore, these sound level meters must be calibrated at regular intervals (including the application of a calibration certificate). They can then be designated precision sound level meters. But even these devices have to be controlled by a portable acoustic calibrator (e.g. a pistonphone, fig. 2/13) and, if necessary, adjusted for each measurement if the results are expected to be precise.

The available sound level meters can be classified into three classes:

The costs for a sound level meter range from about 200 to 20 000 €.

The selection of the measurement device should be adapted to the particular purpose. Important aspects which have to be considered when purchasing a sound level meter are for example the lower level limit, the level range, sound level averaging, the pause button for the suppression of extraneous sounds during the measurement, stability, humidity resistance and possibly calibration capability.