In movie theaters and professional studios, audio systems are calibrated using sophisticated measuring equipment to make sure the sound is being replicated in a precise manner. One of the calibrations they do involves balancing the volume output of every channel in the system. Today's home theater receivers give you one of the tools necessary to do the same for your home theater system.
Recorded sound material is mixed to certain standards. During the mastering process, a Sound Engineer adjusts the sound level of every sound that goes into the recording. When played back on a home audio system, the sound pressure levels of those same sounds will vary from how they were heard by the Sound Engineer due to many factors.
For example: A Sound Engineer may sit at a distance of 5 feet from each speaker during recording. The studio audio system is setup to make sure that the volume level of each channel is the same for every speaker at the position of the Sound Engineer. In your living room, you may sit 5 feet from the surrounds, 10 feet from the center, 9 feet from the front left and right, and 6 feet from the subwoofer. Since the speakers are at different distances from you, all of the speakers will have different volume levels relative to your position when sound is played back through them.
Proper calibration ensures that you hear everything with the same intensity as the Sound Engineer when he recorded the audio material. Without proper calibration, you may not hear subtle details in the surround channels that you should, or some details may be overwhelmingly exaggerated if the levels are too high. You also may be hearing more volume from the left speaker than the right. Worst of all, the center channel may become difficult to hear because the fronts are set so high that the center can't overcome them. In other words, improper calibration destroys the vision that the Sound Engineers had while recording, and you're not experiencing the sound as it was intended.
Although calibrating the channel levels on your receiver is actually pretty easy, you'll first need to have a general idea of how an SPL meter works.
SPL meters come in two varieties. There are models with an analog meter and ones with a digital display. Most experts recommend the analog version because it's easier to work with. There are several places you can obtain a SPL meter. For most, a model from Radio Shack is usually good enough for home use. There are other models available that may take more accurate measurements, but they are usually much more expensive. There are five major components on a SPL meter. Depending on the model you own, your meter may have more functions than what's listed below.
With your speakers in their permanent locations, move to your primary listening position. If you frequently sit in multiple locations, find an average location between your listening positions. For example: If you will be sitting on both the left and right sides of a sofa, the middle will be the average position. Hold the meter in a position close to what your ears would be. In other words, don't calibrate your system while standing unless you're standing when you listen to your speakers.
There is mixed advice about how to hold your SPL meter to take measurements. Some will tell you to aim the SPL meter's microphone at the speaker you're measuring. If you consistently do it like this for every speaker, you'll be fine. Others will tell you to hold the meter straight up for every channel. Most enthusiasts feel that holding the meter straight up provides more consistent results between channels. (Think about it. Your ears are not directly aimed at your speakers when you listen to them).
Check the battery on your SPL meter. A weak battery will skew your results, so replace it if it's low. Set the range on the SPL meter to 70, C-weighting, and slow response.
Engage the test tone, or pink noise generator, on your receiver. You will hear a static sound in one of the speakers. Depending on your receiver, this static may automatically move from speaker to speaker. If you have a choice, choose manual channel selection. This way the tone won't change speakers while you're looking at the meter. Increase or decrease the main volume of the receiver and subwoofer (if it's a powered sub) to a "reference" position. Make it an easy position to remember, such as straight up on knobs, 0 on relative volume controls, and 75 on actual volume controls.
While holding the meter straight up from your listening position, check the SPL reading on the meter for that channel. Adjust that channel's level (not the main volume control) so that the needle reads +5 while the range is set to 70, or 75 on digital diplays. This means the calibrated level for that channel is 75db at your reference volume and listening position. Repeat this process for each speaker in your system. I recommend calibrating your subwoofer to the same level, but if you like a little extra kick in your bass, calibrate it 5db to 10db higher than the rest of your speakers.
Once all of the speakers read the same SPL level, your system is calibrated. The next time you watch a film, if you set the volume control to your "reference" position, you will be hearing the film in the exact balance and sound pressure levels that the sound engineers intended. Reference volume levels can be really loud. If you choose not to listen at these levels, the sound will still be balanced at lower volumes.
Another use for SPL meters is making sure your speakers are operating at safe listening levels. While playing music, hold the meter up and take a reading with the meter set to A-weighting. Below is a table from the U.S. Department of Labor regarding safe sound pressure level exposure.
Hours per day = Sound Level(dB, A-weighting, SLOW response)
After you've completed your calibration, you'll be ready to listen to movies and music as it was intended. It's recommended to occasionally verify your calibrated settings with your SPL meter to ensure everything is still perfect. Anytime you move or change a speaker you'll need to recalibrate your audio system to reflect the changes.
Keeping a finely tuned audio system is similar to keeping your car in tip-top shape. You'll need to perform a little maintenance every once in a while to make sure everything is proper working order. When watching movies, sound is 60% of the experience, so getting the sound right is one of the most rewarding steps you can take to maximize your movie experience.