Handbook for Sound Engineers

Surround Sound 1599

Dolby later expanded Pro Logic II technology to
derive 7.1 surround (Pro Logic IIx, above), while
competing systems from DTS, SRS, and others along
with Dolby’s have made surround sound from stereo
content a standard feature in home theater systems and
an increasing number of automobile sound systems.
Many of these companies have also developed surround
virtualizing technologies to provide a surround effect
via just two stereo speakers. These technologies vary
widely in their cost and effectiveness, but the best can
provide quite startling results, albeit only in a limited
listening “sweet spot,” while the less sophisticated can
provide a pleasant broadening of the stereo image over
the stereo speakers built into TV sets.

45.8 The Question of Playback Level

When Dolby introduced surround on optical movie

soundtracks in the late 1970s, it also introduced the con-

cept of a reference playback level for both cinemas and

the dubbing theaters where soundtracks are mixed. By

calibrating both to the same reference level, the movie-

goer hears in the cinema the same level as the director

and sound designers heard when mixing the soundtrack.

The objective—further supported by the introduction by

Lucasfilm of its rigorous THX standards for playback

quality—was for the moviegoer to experience the film-

makers’ original intentions.

Over the past three decades, the standardization of

playback level and other characteristics has rivaled

surround itself as a significant improvement to the

cinema experience. However, what about home play-

back of these same movie soundtracks via disc, tape, or

broadcast?

Theoretically, to reproduce the cinema experience

exactly, movies at home should be played at the same

level as in the cinema. That level in the home, however,

is both difficult to achieve and far too loud for most

viewers. Most home movie viewers choose a level they

find comfortable for dialogue intelligibility, which is

substantially lower than cinema reference level. This

results in a loss of impact compared to the cinema due

to one of the peculiarities of human hearing: a loss of

sensitivity to low and high frequencies that increases as

playback level decreases. Indeed at lower playback

levels, low-level detail, such as ambience in the

surround channels, can disappear altogether.

So-called loudness controls that attempt to compen-

sate for this effect have been incorporated in home play-

back equipment for many years. They boost low and

high frequencies, usually based in some way on the

famous equal loudness curves originally published by

Fletcher and Munson in 1933 and updated over the

years, Fig. 45-9. However, until recently, there has been

no practical way to relate the actions of these controls to

the actual playback level the listener has chosen. For the

most part, therefore, they have been largely ineffective.

Figure 45-8. Speaker configuration for 7.1 home theater.

Figure 45-9. The equal-loudness contours. Illustration

courtesy Syn-Aud-Con.

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Frequency–Hz

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0

Sound Pressure Level–dB ref. 20 

10

MPa

MAF

120 Loudness level(Phon)

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10