Sound levels and decibels are usually measured with a device called a sound-level meter. However, a computer with a microphone and some simple recording software can also serve as a makeshift sound-level meter. Most audio-recording software has a dB or decibel level that measures the volume of the recording. Depending on the quality of the measurement microphone, the computer version of a sound-level meter may be more accurate.
Connect a measurement microphone to the computer's "Microphone" port. The microphone will have a 3.5mm jack that fits into the computer's microphone port.
Download and install an audio software program. Adobe Audition, Adobe Soundbooth, CoolEdit Pro, Cubase and Logic all offer fairly sophisticated audio-capture capabilities. Most of these programs offer a free trial.
Open the program and configure the microphone under the program's options. Most programs should do this automatically. If the program does not, select which port on the computer is designated for audio capture/input and select the microphone port.
Open a new file.
Click "Capture" or "Record." The program will begin capturing audio from the microphone. Remain still and do not make any noise while capturing the audio.
Click "Stop" to end the capture once significant sample data is collected. Review the recording using the decibel meter in the program's spectrum analyzer.