It depends on what you want or need to measure. Most all common meters don't measure peak noise levels. You get an average over a period of time. This most often is very misleading if you want to measure the sound from an air gun or PB. These have very loud sounds for very short durations. Averaging the sound you are mislead to believing it is much quieter than it was. Ideally you want to be able to capture peaks noises of just 1/2 wave of a 20KHz sound. Then you won't miss anything that a human would hear. Also the meter should have a flat frequency response from 20 to 20KHz and a flat amplitude response. Meters meeting these requirement are not cheap. Also how and where you measure can have a very large effect on the results even if you use and "ideal" meter. Meaningful measurements of sound levels are not easy even if you use the best meter.
A search will turn up the measurement protocols recommended by various gubmint agencies. It can get complicated. Not sure if there's anything about it in the ASTM or 40CFR (the labor code).