The first thing that comes to mind when I think of a sound measuring instrument is a decibel meter. It is a small, portable device that tells the volume of a sound in decibels (or hertz depending on the device). This is the type of device that is used to test speaker levels at concerts or volume levels from industrial equipment. Ear plugs are also rated in decibels and a decibel meter can tell what type of ear plug is needed.
Due to the nature of sound waves, two sounds that are equal is decibels might sound different in volume to the human ear. New technologies are being invented and perfected that allow a meter to measure sound the way human hear it. Currently, the decibel meter is still the most commonly used device.