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Calibrating Audio Performance

When you set the speaker distance measurements on your receiver/preamp, you're performing a rough first pass at adjusting your system's channel balance. Because other factors affect this relative loudness—items sitting between you and the speaker, reflections caused by room acoustics, and so on—the final channel-volume adjustment must be made manually.

The proper way to adjust speaker loudness is to measure the sound pressure coming from each speaker. This measurement is done by generating a pink noise signal in each channel successively, and then using a sound pressure level (SPL) meter to measure the precise sound level. Most receivers and preamps can generate a test tone that sweeps across each channel, which is perfect for this purpose. Measuring the sound level, however, is a bit more tricky—especially if you don't happen to have an SPL meter handy.

If you don't have an SPL meter, you have to "measure" the relative sound levels by ear. That is, you have to determine in which channels the test tone sounds louder or softer, and then adjust the volume level for that channel accordingly. The desired effect is for the test tone to sound equally loud in each channel; this is the perfect surround sound balance.

Now we come to the frequency adjustment. On simpler systems, this is accomplished by adjusting the bass and treble controls. More sophisticated components may include a built-in graphic equalizer. Ideally, you would use a signal generator (connected to one of your receiver's audio inputs) to generate a series of test signals at different frequencies. You would then use a spectrum analyzer to develop a plot of your system's actual frequency response within your listening room. This frequency response is affected by everything in your room—curtains, furniture, walls, ceilings, you name it. As the sound waves from each speaker bounce around the room, the frequency response you hear at your desired listening position changes accordingly. You use the spectrum analyzer to determine which frequency bands you need to increase or decrease using the graphic equalizer.

If all this sounds really complicated, that's because it is. For most casual listeners, you simply want to adjust the bass and treble controls (or the graphic equalizer, if your receiver/preamp has one) for the most neutral sound possible; you want to get as close as possible to a "flat" frequency response. If you have an SPL meter and spectrum analyzer, then you can get more precise—but this is typically the purveyance of professional installers and technicians.

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