I have a new decibel meter on my smart phone that I like. It has a good graphic interface and it very easy to use. The graphic I really like is the chart which shows several seconds of activity, including spikes. The name of this app is Decibel 10th, and the download is FREE. You can find it in the Apple or Android Stores or just Google it for more information.
If you are in a noisy restaurant it is best to tell the manager something, and you can underscore and prove your point by pausing the screen, then later showing the data to the manager.
Though there are 2 or 3 phone decibel meter apps that are admired by the scientific community, I found them to hard to use, since I lack that degree. The easier to use apps should be looked at as entertainment devices, but can give a good indication of what is going on. Since I do not plan on testifying in court, or writing a thesis, I think Decibel 10th will work fine for me