Home-Theater Audio

Straightforward answers to basic questions about home-theater audio setup and optimization

THX’s timbre matching circuit compensates for the differences in the way we perceive timbre between sounds arriving from the front and the sides. Try this experiment: snap your fingers in front of your head and then again at the side of your head. The finger-snap’s timbre is “sharper” when arriving from the side. THX’s timbre matching ensures that, as sounds move from in front of to behind the listener (or vice versa), their timbres remain constant.

Finally, the THX subwoofer crossover separates bass frequencies from the midrange and treble. The bass is reproduced by a subwoofer and the midrange and treble by the main loudspeakers. Although all AVRs and controllers contain a crossover, the THX crossover is standardized with regard to frequency (80Hz) and steepness of roll-off (24dB/octave low-pass and 12dB/octave high-pass).

In addition to incorporating those four core signal-processing technologies, a THX Certified receiver must meet certain performance criteria, including output power, dynamic headroom (the ability to reproduce peaks without distortion), noise levels, and the ability to drive lowimpedance loudspeakers, among other factors. (Specifically, a THX Certified receiver must be able to deliver a minimum of 211Wpc into a 3.2-ohm load.)

Products with the THX Select designation incorporate the four technologies described above, but have somewhat relaxed performance standards with regard to output power. This allows lower-priced receivers to employ THX processing. The original THX certification is now called THX Ultra to differentiate it from THX Select.

Finally, the relatively new THX Ultra2 designation is a combination of new performance criteria for the video-switching circuits, as well as new signal-processing algorithms for creating 7.1-channel playback from 5.1-channel sources. THX Ultra2 Certified receivers contain sevenchannels of amplification, as well as a Boundary Gain Compensation circuit to reduce boominess caused by speakers being placed close to walls. In my experience, THX improves the sound quality of films and is worth having in a receiver or controller.

How can I get the best possible sound quality from my theater system for music listening?

First, choose an AVR or controller with an “analog bypass” mode. This feature provides a pure analog path through the receiver that doesn’t subject the signal to sonically degrading analog-to-digital and digital-to-analog conversions. Second, take sound quality into consideration when buying a DVD player. High-quality video processing has become incredibly cheap recently, resulting in fabulouslooking video performance in sub-$100 DVD players. But there’s been no parallel trend in audio performance. To get good sound, you’ll still need high-quality parts in the audio circuitry, along with a large and clean power supply—all relatively expensive items.

An alternative is to select a receiver with first-rate digital- to-analog conversion and analog-output circuits, which obviates the need for a high-quality DVD player. In this setup, you’re not even using the digital-to-analog converters in the DVD player. Instead, you make a digital connection between the DVD player and receiver, and the job of converting the signal to analog falls to the receiver. If you choose this configuration, be sure to connect the DVD player to the receiver with a coaxial cable, not the optical TosLink connection. TosLink introduces timing variations (called “jitter”) in the clock that degrade sound quality.

Finally, be sure to turn off any unwanted signal processing in the receiver or controller.

What can I do to improve my room’s acoustics?

This topic could fill an entire book, but a few simple techniques will greatly improve your system’s sound. First, avoid having acoustically reflective objects near the front loudspeakers. That’s difficult if you have a television between the left and right speakers. You can, however, reduce the amount of reflected energy by moving the speakers forward of the television. This placement also has the advantage of positioning the three front speakers in an arc, which puts the listener the same distance from each of the front speakers. If your center-channel speaker is on top of your television, position the front baffle (the surface on which the drive units are mounted) in the same plane as the television screen so that there’s a continuous surface. This simple trick confers a large increase in dialogue intelligibility.

If possible, avoid hard and flat surfaces on the side walls between you and the loudspeakers. These surfaces reflect the speaker’s energy to the listening position, degrading fidelity. Hanging rugs, bookcases, CD and DVD storage racks, and any other common items will absorb or diffuse that reflected energy and improve the sound. If possible, use similar material on the left and right walls to maintain acoustic symmetry between the left and right channels. Similarly, the rear wall behind the listening position is a good place for a large bookcase. This diffuses the reflected energy and prevents it from interacting coherently with the direct sound.