WO1993014607A1

WO1993014607A1 - Loudspeaker system

Info

Publication number: WO1993014607A1
Application number: PCT/US1993/000154
Authority: WO
Inventors: Christopher Noel Blair; Hal Perry Greenberger; Bradley Mark Starobin
Original assignee: Thomson Consumer Electronics, Inc.
Priority date: 1992-01-08
Filing date: 1993-01-08
Publication date: 1993-07-22
Also published as: GB9200301D0

Abstract

Loudspeakers for both of the surround sound channels of a Dolby Pro Logic system are mounted on a common loudspeaker enclosure. The loudspeaker enclosure (10) is elongated with one of a pair of woofer loudspeakers (12L, 12R), one for each channel, being mounted on each end (13). An array of higher frequency loudspeakers (14L, 14R), one array for each of the two surround channels, are mounted on one of the enclosure sides (15) between the ends (13). The woofer loudspeakers (12L, 12R) of the two channels are wired out-of-phase. Each of the higher frequency loudspeakers within an array (14L, 14R) are wired in-phase with each other, with each of the two arrays (14L, 14R) being wired out-of-phase with each other. The higher frequency drivers (14L, 14R) are positioned closer together than the lower frequency drivers (12L, 12R) to optimize radiation efficiency and directivity patterns over their operating range.

Description

LOUDSPEAKER SYSTEM

BACKGROUND The present invention relates to loudspeaker systems, and more particularly to the surround channel loudspeaker for a DOLBY PRO LOGIC HOME THEATER system.

Briefly, the Dolby system is a stereophonic system which includes surround sound encoding based upon 4-2-4 matrix 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 recording, the center channel signal is added to the left and right channels as a monophonic in-phase signal and the surround channel signal is added to the left and right channel signals 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 center channel signal while the L-R subtraction extracts the out- of-phase surround channel signal. The Dolby Pro Logic system provides a monophonic surround sound signal of a nominal frequency range of 100 Hz to 7 KHz to the surround loudspeakers. The surround channel signal is delayed by about 20 milliseconds to prevent unwanted location of frontal sounds in the surround channel loudspeakers. Additionally, to improve separation, Dolby Pro-Logic decoders have steering circuits to detect which channel is loudest at each moment and to raise the volume of that channel while momentarily reducing the level of the other channels. Surround sound is used to provide certain audio effects such as: 1) to create a sense of envelopment and spaciousness, and 2) to provide directional enhancement of certain special effects. For maximum spatial effect, surround sound should be diffuse and arrive from all directions. Most conventional surround systems use two direct radiating loudspeakers usually mounted on the side of the room or against the rear wall. In either case, the two separate loudspeakers are aimed directly at the listing area. When two direct radiating loudspeakers, pointed at the listening area, reproduce the same signal, as would be the case if they were reproducing Dolby Pro Logic surround program material, the listeners will perceive the sound as coming directly from the loudspeaker closest to them. In such a case, the sense of spaciousness will be reduced or lost, regardless of where the loudspeakers are positioned in the room. While placement on the sides of the room would increase the lateral energy arriving at the listener's position and improve the sensation of spaciousness, the directional cues would be wrong for special effects. Should the loudspeakers be mounted at the rear of the room, the directional cues would be correct for special effects designed to come from behind, but the sense of spaciousness would be degraded.

One solution for this problem would be to use of a pair of loudspeakers that have a dipolar radiation pattern. In the ideal situation, dipole speakers radiate sound in a "figure 8" pattern over their entire operating range. However, the radiation pattern of most small dipole radiating loudspeakers collapses to monopolar below 200Hz. Thus, the inherent inefficiency of a dipolar sound source requires monopolar reinforcement at low frequencies. The recommended placement for such dipolar loudspeakers is on the sides of the room so that the loudspeakers radiate most of their sound to the front and rear of the room. Dipolar loudspeakers reduce the amount of direct sound that radiates into the listing area and localized sound is received only after reflecting off the walls. This effectively reduces the interaural correlation, and provides a greater sensation of spaciousness. However, for side mounted loudspeakers, the reflections arrive from the front and rear of the room. This configuration therefore generates only a low amount of lateral reflected sound and directional information becomes confused. Since these systems radiate to both the front and back of the room, they cannot provide the proper directional cues in the case when an effect is intended to come from behind the listener except in a limited number of of specially configured room layouts. In many cases, strong reflections from the front of the room confuse the auditory system, making proper localization difficult. Accordingly, it is desirable to provide a surround sound loudspeaker system which simultaneously provides the appropriate acoustic cues for rear and lateral directionality as well as for providing spacious surround effects.

SUMMARY OF THE INVENTION

Briefly, loudspeakers for both of the surround sound channels of a Dolby Pro Logic system are mounted on a common loudspeaker enclosure. The loudspeaker enclosure is elongated with one of a pair of woofer loudspeakers, one for each channel, being mounted on each end. An array of higher frequency loudspeakers, one array for each of the two surround channels, are mounted on the top of the enclosure between the ends. The woofer loudspeakers of the two channels are wired out-of-phase. Each of the higher frequency loudspeakers within an array are wired in-phase with each other, with each of the two arrays being wired out-of-phase with each other and spaced closer together than the woofer loudspeakers.

In the exemplary embodiment, the specific geometrical spacing of the various drivers optimizes their lateral radiation pattern while broadening the on axis "null" region for preferred listing positions. An additional benefit for enhanced spaciousness is provided by the steering of different frequency components in different directions. As the frequency content of the surround sound signal changes, the localized acoustic image changes position. BRIEF DESCRIPTION OF THE DRAWINGS The sole figure shows a top view representation of a three dimensional surround sound loudspeaker enclosure, according to aspects of the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring now to the drawing, there is shown an elongated surround sound loudspeaker enclosure 10 wherein woofer loudspeakers 12L for the left channel surround signal, and 12R for right channel surround signal, are mounted on diametrically opposite ends 13. Sub-arrays of mid/high frequency drivers 14L for the left channel surround signal, and 14R for the right channel surround signal, are mounted on one of the elongated sides 15. Surround sound enclosure 10 is intended to be located behind the listening area, with woofers 12L and 12R radiating towards their respective side walls, and drivers 14L and 14R radiating upwards towards the ceiling. Thus, both the left surround loudspeaker arrangement and the right surround loudspeaker arrangement are housed within a single enclosure which produces a spaciousness over its operating range.

Typical consumer product Dolby Pro Logic decoder/amplifier products assign less power to the surround channel than the left, right, and center channel. This can create problems for the use of doplar loudspeakers which are inherently inefficient in that they rely on cancellation of the radiated sound in the on-axis direction through destructive interference. One artifact of this cancellation is that frequencies above that frequency, which corresponds to driver separation equalling a quarter wavelength. By selection of driver spacings for low and high frequency drivers, the radiation pattern of the loudspeakers can be controlled while improving the efficiency of the loudspeakers. In the configuration of the exemplary embodiment, the spacings are optimized for the general frequency range of the Dolby Pro Logic surround signals. Loudspeakers 12L and 12R are 11.43 cm woofer loudspeakers, spaced apart 61 cm, which is a quarter wavelength for 140 Hz to provide a desired diffuse soundfield. Decreasing the separation distance would increase acoustic cancellation and increasing the distance would just produce a longer dimension speaker box without appreciably enhancing performance for a Pro Logic surround signal.

The left designated loudspeakers L and the right designated loudspeakers R are driven by their respective surround channel signals as provided by the Dolby Pro Logic sound system. In practice, the Dolby Pro Logic L and R surround signals are identical. In prior art sound system, great care is taken that woofers be wired in-phase with each other. However, the woofer loudspeakers 12L and 12R herein are wired out-of- phase with each other for providing an unusual sound radiation pattern at low frequencies. The usual surround loudspeakers, even those of the dipole variety, have radiation patterns which collapse to an easily localized monopolar pattern at low frequencies. However, according to aspects of the present invention, the present system with the low frequency woofer loudspeakers wired out-of-phase with appropriate spacing provides a broad, diffuse soundfield over the bass frequency range. The middle and high frequencies for each surround sound channel are provided by a pair of 3.5 inch drivers 14L, 14R spaced apart 30.5 cm, which is a quarter wavelength for 280 Hz. Each of the drivers of sub-arrays 14L and 14R operate in-phase with its companion and are directed upwardly towards the ceiling. The sub-arrays 14L and 14R, through a cross-over network (not shown), receive a higher frequency portion of the signal provided to loudspeakers 121, 12R, and are wired out-of-phase with the other sub-array, thereby providing acoustic cancellation of any sound directly radiating into the listening area. The combined sound of the sub-arrays 14L and 14R radiates sound primarily towards the ceiling and side walls, and comparatively little is radiated directly into the listening area.

The present surround sound loudspeaker system is intended to be positioned along a rear wall of the listening area while centered between the side walls. Since loudspeaker system 10 relies upon wall reflections for proper performance, loudspeaker system 10 also includes a high frequency level control (not shown) which allows the listener to compensate for different wall high-frequency absorption characteristics.

Claims

CLAIMS:

1. A loudspeaker system comprising: an enclosure (10) having longitudinal length with elongated sides (15) and diametrically opposite ends (13), a . pair of first and second loudspeakers (12L,12R) mounted one on each of the ends (13), the first and second loudspeakers (12L,12R) being wired out of phase with each other, and a first and second plurality of loudspeakers (14L,14R) mounted on one of the elongated sides (15) which is mountable in an upward direction, each of the first and second plurality of loudspeakers (14L,14R) being smaller in size than each of the pair of first and second loudspeakers (12L, 12R), and receiving a higher frequency portion of an audio signal than the respective first and second loudspeakers (12L,12R).

2. The loudspeaker system of claim 1 wherein the system is a surround loudspeaker system for use with a Dolby Pro-Logic sound system.

3. A surround sound loudspeaker system wherein both left and right surround sound loudspeakers are disposed within a single enclosure.

4. A loudspeaker system comprising: an enclosure (10) having longitudinal length with elongated sides (15) and diametrically opposite ends (13), and a pair of first and second loudspeakers (12L,12R) mounted one on each of the ends (13).

5. The loudspeaker system of claim 4 wherein the first and second loudspeakers (12L,12R) are wired out of phase with each other.

6. The loudspeaker system of claim 5 wherein a first and second plurality of loudspeakers (14L,14R) are mounted on one of the elongated sides (15) which is mountable in an upward direction, each of the first and second plurality of loudspeakers (14L,14R) being smaller in size than each of the pair of first and second loudspeakers (12L,12R), and receiving a higher frequency portion of an audio signal than the respective first and second loudspeakers (12L,12R).