The Rolls SLM305 is a Digital sound level meter with a back lit display. The SLM305 operates off a single 9 volt battery. It features a clean elegant design with high quality construction throughout. It features a MAX button to turn on and off a maximum level reading feature making it easy to get a good reliable maximum dB reading rather than just guessing it yourself off of an erratic display. The SLM305 also has an automatic OFF function that will turn the unit off after 10 minutes to save your battery.