Note: Descriptions are shown in the official language in which they were submitted.
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LOUDSPE_YSTEM
BA~GROVND
5The present invention relates to loudspeaker systems,
and more particularly to a center channel loudspeaker system for
a DOLBY PRO LOG~C home theater system. ,.
Briefly, the Dolby system is a stereophonic system ~,
which includes surround sound encoding based upon 4-2-4 matrix
10 methods for four-channel recording devised in the early 1970's.
The four original signals are mixed to make a two channel stereo ~ . -
recording which is decoded upon playback to recover an
approximation of the original four channels, which are left, center,
right, and surround. To produce a two-channel Dolby stereo
1~ recording, the center channel signal is added to the left and right
channels as a monophonic in-phase signal. The surround channel
signal is added to the left and right channel signals but as an out~
of-phase signal. For playback, the total left and total right
channels (L+R) are added~together in a decoder to recover the
20 center channel signal while the L-R subtraction extracts the out-
of-phase surround channel signal. Logic steering circuitry is used
upon decoding to increase the apparent separation of the left,
center, right, and surround signal. The ~surround channel signal is
delayed by about 20 milliseconds to prevent unwanted location of
2 ~ frontal sounds in the surround channel loudspeakers.
The center channel provides most of the dialogue for a
motion picture or a television ~ program, and carries music and
effècts sounds ~as wcll. The~ purpose of the center channel is to
insure that volces and other sounds originating from on-screen~
30~ sources, will appear to come froml the screen even when viewers `~
are seated off-center. Thus, the requirements for the center
loudspeaker system are different from the left and right channel
loudspeakers which are concerned mainly with music and the .
directionality of stereophonic music.
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The center channel loudspeaker system must be
capable of generating the same acoustic output levels as the left `~
and right loudspeakers over its operating frequency range. These
5 output requirements demand that the center channel loudspeaker
system have a minimum volume velocity capability, which
translates into a minimum sound radiating area. Additionally, the ~center channel loudspeaker system should be located as ; -
physically close to the television screen as possible to maintain
10 the fusion of the visual and auditory images. The mo3t practical
location to locate the center channel loudspeaker system in a
typical living room is either on top of or underneath the television
receiver. This requires that the center channel loudspeaker ~
system be compact. Thus, the need for compactness, along ~,vith a
15 minimum radiating area requirement, are important design
considerations for a center channel loudspeaker system.
A problem arises in the off axis behavior of two ~ ~;
closely spaced loudspeakers that are radiating the same signal. At
certain frequencies related to the spacing between the two
20 sourcesl the acoustic outputs from the two sources will interfere
destructively causing large notches in the frequency response of
the system. This "notching", similar to a comb filter, causes a
perceptible degradation in the quality of the sound. It is desirable
to minimize this destructive interference so that off axis listeners
2 5 will not suffer from degraded sound quality.
The destructive interference that occurs due to the
two woofers displaced in space having the same acoustic radiation
can als o occur in the crossover frequency range where jthe
~woofers and tweeter are ~oth oper?ting. It is desirable to ~ -
30 minimize this source of destructive interference.
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SUMMARY OF THE rNVENTlON -
Briefly, a center channel loudspeaker system for use ~ ~-
with a Dolby four channel sound system is presented. A first ~ ~ ~
3 5 loudspeaker is mounted at the front of an enclosure. Second and - ~ :
third loudspeakers, each having a lower frequency range than ~he , ~
.
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first loudspeaker, are mounted at the front of the enclosure, one
on each side of the first loudspealcer, at an angle of approximately
40 degrees in the vertical plane back from the first loudspeaker.
S With this arrangement, destructive cancellation of the sound
emanating from the closely spaced center channel loudspeakers is
minimized. ;
BREF DESCRIP~QN OF THE DRAWING~ `
FIG. 1 shows a representation of a prior art center
loudspeaker system with the listeners being exposed to
cancellation of sound due to destructive interference.
FIG. 2 shows a representation of a center loudspealcer
system, according to aspects of the present invention, showing
15 how cancellation of the sound due to destructive interference is
minimized. ;
DETAII ED DESCRIP~ON OF T~ PREFl~RRED EMBODIMENT
FIG. 1 shows loudspeaker enclosure arrangement
20 according to the prior art. The loudspeaker enclosures shown are
for three cbannels, i.e., left, right, and center, for a Dolb)~ Pro-Logic
system, with the surround loudspeakers not shown. The left and
right stereophonic loudspeakers are any appropriate loudspeakers
suitable for the purpose, and receive standard left and right
25 signals. The structure and operation of the left and right i ~ ;
stereophonic sound radiating loudspeakers form no part of the
present invention and will not be discussed further.
;; FIG. 1 shows a prior art center loudspeaker system ~ `
comprised of a pair of 5.25 inch drivers 12 and 14 mounted- '
30 within a common enclosure 16. Drivers 12 and 14 cover the low
and middle frequency ranges. A horn tweeter 18, covering the
high frequency range, is mounted between drivers 12, 14. For the
shown prior art system where the loudspeakers are mounted to
radiate straight ahead, i.e., mounted on a common planar front
3 5 baMe, the loudspeaker radiation would produce nulls in the
combined response of the two drivers 12, 14. Only in regions A, B,
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and C would the response be reasonably accurate. In regions D
and E, the radiation from both drivers 12, 14 would be sufficiently
strong to cause cancellation effects, since the path length
S differences between a listener in these regions and the two
drivers is a significant fraction of a wavelength, or even multiple
wavelengths within the similar passbands of the two drivers. The
polar response for non-optimized loudspeakers includes wide
angular spaces in which destructive interference between the
10 acoustic radiation of drivers 12, 14 occurs.
Referring now to FIG. 2, there it is shown a center
channel loudspeaker arrangement for overcoming the cancellation
effects shown in FIG. 1. As shown in FIG. 2, each of the 5.25 inch
drivers 12, 14 of the exemplary embodiment are mounted at a
15 backward angle of approximately 40 degrees, e.g., 37.~ degrees,
within the vertical plane, with respect to loudspeaker 18. The
optimum angle will be different for different sized and different
frequency range loudspeakers. With such a mounting angle, as
one moves o~f-axis to center loudspeaker 18, one moves on-axis to
20 one of drivers 12, 14, and further off-axis to the other one of -;
drivers 12, 14. Thus, in the frequency range where cancellation ~ -due to destructive interference would otherwise occur, the output
from the off-axis loudspeaker is reduced by its own directivity
and the interference cancellation is reduced and/or minimized. ;~
2 5 In the exemplary ennbodiment, horn loudspealcer 18 is
mounted so that its acoustic center is approximately 4.5 cm
behind the front panel of the speaker (not shown) with the
optimal displacement depending on the characteristics of the , -
particular loudspeakers used. This places the acoustic center of
30 loudspeaker 18 in close but not in exact alignment with the
acoustic centers of loudspeakers 12 and 14. This displaced ~ `
alignment is designed to further minimize destructive ' ~;
interference effects by minimizing the path length variations -
between the tweeter loudspeaker 18 and each woofer
35 loudspeaker 12, 14 in the range from zero degrees up to 4~
degrees off axis, which are the normal television viewing angles.
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FIG. 2 sbows the approximate coverage of the two
mid-woofers 12, 14 just below the frequency at which the
crossover frequency for the tweeter 18 occurs in the exemplary ~: :
S center channel loudspeaker system. In region A, the acoustic
outputs of drivers 12, 14 arrive essentially in phase and no ::
cancellations occur. In regions B and C, the acoustic outputs of
loudspeakers 12 and 14 will be shifted in phase with respect to
each other. However, even if a large phase difference were to
10 oc~ur, the net effect on the system is small because the mounting
angle of loudspeakers 12, 14 significantly reduces the sound level
from the further loudspeaker with respect to the nearer
loudspeaker.
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