The Bruel & Kjaer 2231 Modular Precision Sound Level Meter 2 Hz - 70 kHz is a type 1 precision instrument. It is ideal for measurements according to the most stringent standards because of the comprehensive construction and extreme accuracy of the instrument. A system of interchangeable modules to make different measurements is possible by using the 2231 sound meter. The instrument is compatible with almost any Bruel & Kjaer microphone and extends the measurement capabilities. The 2231 Modular Precision Sound Level Meter derives its many measurement capabilities from a series of interchangeable application modules. Each inserted module loads software that defines the functions of the buttons of the sound level meter. The software loaded still remains in tact when the sound meter is turned off.