FINE-TUNING Your Audio System, Room Reflections

FINE-TUNING YOUR AUDIO SYSTEM

ACOUSTICAL PROBLEMS IN LISTENING ROOMS

A home THX audio system, such as the Synthesis Four system, addresses many of the problems common to high-quality reproduction of music or soundtracks in a home environment. For example, the dispersion pattern of the front LCR speakers minimizes the effects of floor and ceiling reflections. Still, the single most important variable in any sound system is the listening environment. Room reflections create spurious false images and “comb filter” interference effects which alter the tonality of the system while degrading the localization of specific sounds. Larger rooms sustain echoes that degrade dialogue intelligibility and detail. All rooms have standing waves that emphasize certain frequencies at the expense of others, based on the dimensions of the room.

Other concerns include environmental noise, which is often greater than people realize. Although they might become accustomed to its presence and “tune it out,” it still reduces the perceived low-level resolution of the system. In addition, the profound bass capabilities of a home THX audio system can create distracting rattles that lesser systems might never evoke.

This section of the manual contains a variety of suggestions for addressing some of the peculiarities of the listening environment to improve the accuracy of your Synthesis Four system’s sound reproduction.

ROOM REFLECTIONS

The most troublesome room reflections are usually the early reflections of the front LCR speakers off the floor, ceiling and side walls. These reflections reach the listener’s ears after a delay, with respect to direct sounds, and blur the perceived image. They can also degrade dialogue intelligibility through the same mechanisms (see Figures 21 and 22).

The design of the Synthesis Four speakers minimizes the floor and ceiling reflections. As an extra enhancement, it is often a good idea to place a thick, absorptive carpet between the front speakers and

t1 t2

Figure 21.

Direct

An Additive

Reflected Frequency

Result: (+6dB)

Direct

A Canceling

Reflected Frequency

Result: (–∞dB)

the listening position, just to further reduce this pri- mary reflection from floors with hard surfaces. A rug made of wool will generally have more uniform absorption characteristics than one made from syn- thetic fibers.

JBL Synthesis Four speakers have broad dispersion in the horizontal plane in order to ensure a wide usable listening area. This design choice can induce reflec- tions off the side walls, especially in installations where they are relatively close to the front speakers. With conventional speakers, these reflections can be reduced simply by angling the left and right speakers inward somewhat.

Resultant Frequency Response From Reflection

Sound Level

+6dB

0dB

Frequency

Peak and dips at various frequencies

Figure 22.