8
Multi-Channel Music & Cinema Systems
Theory and Design
Lexicon
When two full range speakers (or two symmetrically placed subwoofers)
are used in the front, the level can become more uniform, but the low
frequencies do not externalize, that is, they sound as if they are inside the
listener’s head. This “in-the-head” bass impression is very common in high-
end recording studios but is decidedly unnatural. Our research has shown
that if frequencies below 150Hz are to be reproduced through more than one
loudspeaker, the sound can be externalized and made more uniform by
applying a constant phase shift of 60° to 90° between the loudspeakers on
the left side and the right side of the listener.
Both theoretical models and room measurements show that the uniformity
of the low frequencies (and the externalization) is further increased by using
loudspeakers at the sides of the listeners which are capable of response to
about 50Hz or lower. Note that many smaller speakers can reproduce
usable bass below their rated frequency response.
The Music Logic, Logic 7 and TV Matrix algorithms include the necessary
phase shift networks for the Bass Enhance effect. The difference between
optimal low frequency bass and that produced by a standard single
subwoofer can be easily compared by changing the speaker configuration
of the processor. For un-enhanced bass reproduction, turn the subwoofer
on, and set all other speakers to Small. For bass reproduction that takes
advantage of the Bass Enhance effect, set the crossover to the subwoofer at
40Hz and set the center channel to Small with a crossover setting of 120Hz.
Set all of the other speakers to Large, or to Small with a 40Hz crossover.
Make sure Bass Enhance is engaged in the Effect Adjust menu. Invariably,
the sense of fluidity in the low frequencies will increase and there will be an
increased sense of spaciousness.
Bass Split Operation for the Center Speaker
The Music Logic, Logic 7 and TV Matrix algorithms deal with a small center
speaker differently than the other matrix effects. Rather than taking the low
frequencies which would ordinarily be sent to the center speaker and
splitting them between the left and the right front speakers, the new
algorithms split these low frequencies between all the side speakers —
including the sides and the rears. This is important because in most popular
music the low frequencies are monaural, and much of the higher frequency
material is also centered. As a result, there is a large amount of low
frequency information in the center channel. However, these recordings
typically have a strong monaural low frequency component in the left and
right channels, and low frequencies reproduced from the center loud-
speaker are always perceived as coming from inside the head. Thus, the
optimal system uses a Small center speaker setting — whether the speaker
is capable of full range operation or not. Ideally, the center speaker should
be set to 120Hz crossover.
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