Close-Field Monitoring

An idea that sounds immediately plausible is "near-field monitoring," perhaps better termed close-field monitoring. The idea is that small loudspeakers, located close to the listener, put the listener in a sound field where direct sound predominates over reflected sound and reverberation. Thus such loudspeakers and positioning are allegedly affected less by the room acoustics than are conventional loudspeakers at a greater distance, and have special qualities to offer, such as flatter response (Fig. 2-8).

Unfortunately, in real-world situations, most of the advantages of the theory of near-field listening are unavailable, sotheterm close-field monitoring is probably more apt.The loudspeakers used are small compared to the range of wavelengths being radiated, and thus broadly radiate sound. The broadly radiating sound interacts with nearby surfaces strongly. For instance, "near-field" monitors on top of a console meter

Fig. 2-8 So

called "near-field" monitoring suffers from the same problem as the elevated monitor, splashing excessive sound off the control surface. Moving the speaker to behind the console can be an improvement.

Near-field monitor also splashes sound off the control surface

Loudspeaker mounted behind console uses it as a barrier, though sound diffracts over this console and still reflects, it is at a lower level than when mounted on the top

bridge reflect sound strongly off the console's operating surface, at levels well above audibility. Also, the console surfaces act to extend the baffle face of the monitor loudspeaker placed on top of the meter bridge, and this changes the mid-bass response. At mid-bass frequencies, the idea that the near-field monitor gets us out of trouble from interaction with the room is wrong. There is still just one transfer function (frequency response) associated with one source location (the loudspeaker) and one receiver location (the listener's head). The effects of standing waves occur at the speed of sound, which produces effects quickly in small rooms, so the theory that we are out of trouble because the speaker is close to the listener is wrong.

Another difficulty is with the crossover region between woofer and tweeter. When used at close spacing to the listener, the exact listening position can become highly critical in many designs, as the crossover strongly affects the output of the loudspeaker spatially. Only a 6 in. move on the part of the listener located a few feet away can make a dramatic shift in the monitor's direct sound frequency response, and be audible.

The close-field monitor arose as a replacement for the simple, cheap loudspeakers, usually located on top of the console that provided

a "real-world" check on the large and professional built-in control room loudspeakers. In multichannel monitoring, the same theories could lead to having two systems, one huge and powerful, and the other close up and with lesser range and level capacity. This would be a mistake:

• The built-in type of control room monitors are often too high, which splashes sound off the console at levels above audibility.

• Close-field monitors suffer from the problems discussed above. They may not play loudly enough to operate at reference levels.

The best system is probably one somewhere in between the two extremes, that can play loudly enough without audible distortion to achieve reference level calibration and adequate headroom to handle the headroom on the source medium, with smooth and wide frequency range response, and otherwise meets the requirements shown inTable 2-2.

Time Adjustment of the Loudspeaker Feeds

Some high-end home controllers provide a function that is very useful when loudspeakers cannot be set up perfectly. They have adjustable time controls for each of the channels so that, for example, if the center loudspeaker has to be in line with the left and right ones for mounting reasons, then the center can be delayed by a small amount to place the loudspeaker effectively at the same distance from the listener as left and right, despite being physically closer.

The main advantage of getting the timing matched among the channels has to do with the phantom images that lie between the channels. If you pan a sound precisely halfway between left and center, and if the center loudspeaker is closer to you than the left one, you will hear the sound closer to the center than in the chosen panned position midway between left and center. The stereo sound field seems to flatten out around the center; that is, as the sound is just panned off left towards center, it will snap rather quickly to center then stay there as the pan continues through center until it is panned nearly to the right extreme, when it will snap to the right loudspeaker. The reason for this is the precedence effect. The earlier arriving center sound has precedence over the later left and right loudspeakers.This may be one reason that some music producers have problems with the center channel. Setting the timing correctly solves this problem.

Sound travels 1128 ft/s at room temperature and sea level. If the center is 1 ft closer to you than left and right, the center time will be 1.1 ms early. Some controllers allow you to delay center in 1 ms steps, and

1 ms is sufficiently close to 1.1 ms to be effective. A similar circumstance occurs with the surround loudspeakers, which are often farther away than the fronts, in many practical situations. Although less critical than for imaging across the front, setting delay on the fronts to match up to the surrounds, and applying the same principle to the subwoofer, can be useful.

The amounts of such delay are much smaller typically than the amounts necessary to have an effect on lip sync for dialog, for which errors of 20ms are visible for the most sensitive listener/viewers. Note, however, that video pictures are often delayed through signal processing of the video, without a corresponding delay applied to the audio. It is best if audio-video timing can be kept to within 20ms. Note that motion pictures are deliberately printed so that the sound is emitted by the screen speakers one frame (42ms) early, and the sound is actually in sync 47 ft from the screen, a typical viewing distance.

Low-Frequency Enhancement—The 0.1 Channel

The 0.1 or LFE channel is completely different from any other sound channel that has ever existed before. It provides for more low-frequency headroom than on traditional media, just at those frequencies where the ear is less sensitive to absolute level, but more sensitive to changes in level.The production and monitoring problems associated with this channel potentially include the end user not hearing some low-frequency content at all, up through giving him so much bass that his subwoofers blow up.

With adoption by film, television, and digital video media already, the 0.1 channel has come into prominence over the past few years. With music-only formats, how will it affect them? Just what is the "0.1" channel? How do we get just part of a channel? Where did it come from, and where is it going? And most importantly of all, how does a professional apply the standards that have been established to their application, from film through television to music and games?

Film Roots

The beginnings of the idea came up in 1977 from the requirements of Gary Kurtz, producer of a then little-known film called Star Wars. At the time, it seemed likely that there would be inadequate low-frequency headroom in the three front channels in theaters to produce the amount of bass that seemed right for a war in outer space (waged in a vacuum!). A problem grew out of the fact that by the middle 1970s, multichannel film production used only three front channels—left, center, and

right (two "extra" loudspeaker channels called left extra and right extra, in between left and center, and center and right, were used in the 1950s Todd AO and Cinerama formats). The loudspeakers employed in most theaters were various models of Altec-Lansing Voice of the Theater. A problem with these loudspeakers was that although they use horn-loaded operation across much of the woofer's operating range for high efficiency, and thus high-level capability, below about 80 Hz the small size of the short horn became ineffective, and the speaker reverted to "bass reflex" operation with lower efficiency. In addition, the stiff-suspension, short-excursion drivers could not produce much level at lowerfrequencies, and the "wing walls" surrounding the loudspeakers that supported the bass had often been removed in theaters. For this combination of reasons, both low frequency response and headroom were quite limited.

loan Alien and Steve Katz of Dolby Labs knew that many older theaters were still equipped with five-front channels, left over from the original 70mm format. So, their idea was to put the "unused" channels back into service just to carry added low-frequency content, something that would help distinguish the 70 mm theater experience from the more ordinary one expected of a 35 mm release at the time. Gary Kurtz asked for a reel of Capricorn! to be prepared in several formats and played at the Academy theater. The format using left extra and right extra loudspeakers for added low-frequency level capability won. Thus was invented the "Baby Boom" channel, as it was affectionately named, born of necessity. Just 6 months after Star Wars, Close Encounters of the Third Kind was the first picture to use dedicated subwoofers, installed just for the purpose of playing the Baby Boom channel.

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