Speakers
Five Easy Steps to Superior Home Theater Sound
Jedi-master THX offers simple audio calibration tips for your theater.
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June 12, 2009 by Robert Archer

People are fixated on video. After all, it’s the most obvious part of the home theater. But it’s a huge oversight if your system’s audio doesn’t match the quality of its video.

Why this happens is obvious: The video guys have done a better job of educating consumers on the benefits of a calibrated video system. Led by the efforts of people like Joel Silver of the Imaging Science Foundation (ISF) and Joe Kane of Joe Kane Productions (JKP), video experts and manufacturers have enlightened and educated consumers and professional installers on why a video system needs calibration.

Conversely, the audio community has had a difficult time making the transition from two-channel to home theater, and its educational efforts have been primarily equipment-centric, which has limited the public’s knowledge on the topic of audio calibration.

The effort to educate installers on the benefits of calibrated audio have been led by Jerry Lemay of the Home Acoustics Alliance (HAA). Lemay’s curriculum emphasizes the fundamentals of acoustics and best practices on how to deal with the problems associated with reproducing audio within a small-room environment. 

Certified Standards from the Jedi Master
One company that has educated consumers and professionals on the benefits of calibrated audio and video is San Francisco-based THX, which was founded by Star Wars director George Lucas to improve the audio and video quality of the movie industry and to provide a standardized method of delivering better and more consistent audio and video.

According to Warren Mansfield, director of consumer technology for THX, the process of calibrating audio in a home theater is important because it enables homeowners to reproduce movie soundtracks in their homes the way the audio engineers intended.

“Calibrating receivers/preamps not only balances each channel so that you get matched sound levels from each speaker, but also calibrates the overall level so that 0 dB on the volume control represents the industry standard ‘Reference Level’ in the listening position,” he says.  “This is the same level at which the movie was originally mixed and played theatrically. Only THX Certified products do this in the home.”

How to Calibrate

1. Start by opening the menu of the preamplifier/processor or A/V receiver and find the audio menu. This is where the speaker/and or audio functions should be located. Choose the speaker size option and select large or small for the left and right speakers. This choice is based on the size and frequency range of a system’s speakers. If you own full-range or near full-range speakers, choose “large.” If the center and rear channels are smaller, choose “small” for those channels.

2. Next, determine the speakers’ distances from the main seating position. Installers call this the “money seat,” and it’s where the primary user often sits. Input these distances into the receiver or pre/pro. This enables the receiver or pre/pro to calculate the arrival times of sound to the money seat, in order to produce a synchronized soundfield.

3.
The next step is to set the system’s crossover frequency. This determines how the system will divide the sound spectrum from low-end bass, to midrange to high frequency or treble. THX’s specification is 80Hz, which means that all frequencies 80Hz and lower will be sent to the LFE (low-frequency effects) channel, which is reproduced by the system’s subwoofer. 

4. If possible, use the test tones that are incorporated in the receiver or preamplifier/processor when finishing the sound output leveling process. “For the average user, we do not recommend using an external calibration disc for calibrating audio channels, especially the LFE channel,” Mansfield warns.

“There are a lot of variables in the decoding of a test disc, which could lead to incorrect calibration. For instance, when decoding the LFE test signal from an external disc, the A/V receiver may level-match, and this action is hidden from the user. This risks setting the LFE incorrectly. In addition, today’s technology-rich A/V receivers and DVD/BD players feature numerous modes, filters and EQ settings that may be turned on during the calibration, also throwing off the correct calibration settings.”

A measurement device will be needed, and the most common product used is a sound pressure level (SPL) meter. A lot of custom installers use more advanced devices like real-time analyzers (RTAs) and fast fourier transform (FFT) analyzers for precise measurements.

These products are available through a number of channels like Radio Shack and Guitar Center. There’s an iPod Touch/iPhone app from Studio Six Digital. And Windows Media Center has a built-in app for audio calibration.

The pros use more advanced products from companies like Sencore.

THX guidelines call for each channel’s (or speaker’s) output level to be set at 75dB.  Mansfield says this figure is derived from its commercial theater specifications.  “The THX Reference Level is 85dB [the same as studio Reference Level]. The 75dB volume level is used for the pink noise generated from THX Certified receivers and pre/pros. [We found the] pink noise at 85dB proved to be too loud for the average consumer. So the pink noise generator is padded down -10 dB, and the receiver accounts for this during calibration. This is why THX recommends the user calibrate with the internal test tone generator.”

After a system’s front, center and surround channels are established, set the output level of the subwoofer.

Adjusting a subwoofer is tricky, due to the science behind sound reproduction and the size of low-frequency soundwaves and how the human ear hears low frequencies.

The simplest way to calibrate a subwoofer is to do it by ear. This may not be the most scientific method, but it avoids the problems that could result from using a device that can’t provide correct measurements. “Many low-cost SPL meters are not as accurate for measuring low frequencies,” notes Mansfield.

“If you are using a more sophisticated SPL meter, make sure you take each LFE measurement for at least 20 seconds, and be sure to take measurements at each seating position, then set the output for the average measurement.”

5. Once the calibration is complete, it’s a good idea to write the settings down and to go through the process again to be sure everything is correct.

Some receivers and pre/pros also offer advanced equalization functions that require some familiarity with acoustics, sound frequencies and sound measurement equipment. These additional functions can deliver an added level of performance to provide smooth and responsive audio that goes beyond the performance of the basic calibration process.

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Robert Archer - Senior Editor, CE Pro
Bob is a dedicated audiophile who has been writing about A/V for Electronic House sister publication CE Pro since 2000.

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