TacT Audio RCS 2.0 digital equalizer/preamplifier Speakers, Rooms, & Flat Response

Sidebar 2: Speakers, Rooms, & Flat Response

At first glance, it seems reasonable to regard the tonal balance of a real speaker in a real room as the sum of the performance characteristics of speaker and room. If there's something amiss in the sound, just find the offending frequencies, by ear or meter, and adjust the incoming signals with an equalizer.

Unfortunately, rarely will the result be the desired flat response. There will be a change, but, depending on how and from where you listen, there may or may not be an improvement. The reason is that the interaction of speakers, room, and listener is more complex and, significantly, not steady-state; it changes dynamically.

In a typical listening environment, you hear both a direct sound from the drivers aimed at you, and an agglomeration of indirect sounds reaching your ears via a nearly infinite number of reflections of the sound off of the room's surfaces: walls, floor, ceiling, furniture, etc. The direct sound reaches your ears and brain first, while the reflected sound is spread across a wide span of time, depending on your room's dimensions and the number of reflections. Moreover, the frequency balance of each reflected sound is typically different due to three major factors.

First, almost all speakers exhibit variation in their frequency responses off-axis. You need only look at the lateral and vertical response curves in John Atkinson's speaker-measurement sidebars to know that the sound directed at room objects and boundaries is not identical to that directed, on-axis, at you, the listener. Second, the spectral balance of sound reflected from each surface is modified by each surface's particular characteristics of reflection, absorption, and diffusion. Third, the room's very dimensions create resonances and antiresonances that result in peaks and nulls at certain frequencies at certain locations. For a great demonstration of this, try CARA's "movie" 3D mode and see how the peaks and nulls move about the room at different frequencies.

These factors interact in many ways. Very simply, they fall into early and late effects. The brain cannot distinguish low-order reflections off of nearby surfaces from direct sounds, which reach the ear almost simultaneously. Consequently, because these reflections might have a different frequency balance due to the surface's construction and materials, they affect the speaker's perceived sound. Higher-order reflections, which are spread out over time, are distinguished from direct sound by the brain, and impart the sense of a space's ambience. However, this is the ambience of the listening room, and it is superimposed on whatever ambient cues of the original recording venue are contained on the recording.

To reveal all the musical and environmental details of the original recording, these confusing effects should be eliminated or minimized. Early reflections should match the on-axis response in spectral balance, and late reflections should roll off smoothly in time and frequency.

In such a complex situation, what can any tone control or equalizer do? It can't correct colorations due to aberrations in the off-axis response without changing the on-axis direct radiation. It can't change the delayed room/object influences on ambience without also modifying the frequency balance of the direct radiation. And it can't affect room resonances without corrupting the directly perceived sounds.

This leaves us with two alternatives. The traditional one is to change the arrangement, content, or even the dimensions of the room, using absorption/reflection/diffusion materials and resonators (eg, bass traps). Many tools—such as CARA, Room Optimizer, and ETF—are available to help in this task. A newer way is to use electronic devices that work in the time and frequency domains, and thus can distinguish between direct and delayed sounds. Such devices have been offered for more than a decade, with varying degrees of success; it is likely that they will continue to rapidly evolve.

I wish I could say that such room-correction systems, or even acoustically perfect rooms, are the final solutions, but real-world recordings are still mixed and balanced according to producers' assumptions about the listening situation. These include typical "good" rooms, with their added boost in the bass and midbass due to their dimensions, as well as poor rooms, boomboxes, automobiles, etc. Until room-correction systems are ubiquitous—a not too unreasonable prospect, given the rise of DSP in consumer electronics—most recordings will continue to be made with the expectation of substantial acoustic contributions from the playback room. Until then, tweaking and shaping those contributions, as the TacT Audio RCS can do, will be more important than their elimination.—Kalman Rubinson