Insert the battery into the sound-pressure level meter. Point the microphone of the SPLM at the source and take the measurements of the sound coming out of the speakers.
Choose A-weighted on the SPLM when you want to measure specific frequencies, usually between 500Hz and 10,000Hz. Choose C-weighted on the SPLM when you want to get a measurement of the entire frequency range from 32Hz to 10,000Hz. Choose the response time, either fast or slow. Use slow to measure average sound levels and fast to measure peak levels.
Mount the SPLM on a tripod by screwing it into the mount threads on the bottom of the SPLM. Using a tripod will get you the most accurate measurements. Place the tripod at the listening position, generally where your ears are.
Calibrate your speakers by playing pink noise from a test CD, like Stereophile's Test CD (see References for link) through your speakers. You can take measurements on the SPLM to figure out whether you are getting sharp peaks or higher than average sound levels with the speakers placed in different parts of the room. Turn the volume up on your receiver until the SPLM reads 0.
Play the test tones on the Test CD and write down the measurements on the SPLM. Be sure to stand off to the side of the SPLM while doing this to ensure accuracy. Your goal should be to get a reading as close to 0 for each of the test tones. Fiddle with the speaker placement to get this. Some subwoofers, such as Infinity, use this system in conjunction with an equalizer to get the best bass response in your room.