Vehicle Speaker System Provided With a Passive Radiator

This invention relates to a speaker system provided with a passive radiator to be mounted in a vehicle. Such a speaker system is typically mounted on the rear window tray or shelf of an automobile. Modern automobiles are frequently equipped with stereo FM cassette players or other hi-fi sound systems. However, in view of space constraints large-full range speakers cannot be used. Hence, small speakers are employed with attendant deficiencies in frequency response.

In the case of a speaker cabinet in which a compact speaker unit is mounted, the reproduction of the lower sound spectrum tends generally to be deficient over the acoustic reproduction frequency range due to various restrictions of the cabinet design. Such speakers tend to sound "thin" because of a bias to reproduce high frequencies of the audio spectrum. In order to improve the deficiency of the lower sound spectrum, a passive radiator (or drone cone) type speaker system has been proposed. As shown in FIG. 1, a speaker unit B and a passive radiator C are provided in the same housing A. The passive radiator C does not have a magnetic circuit. The passive radiator C is adapted to resonate at frequencies lower than those reproduced in the speaker unit A, so that the reproduction range of the lower frequency spectrum can be broadened. The resonance of the passive radiator C takes place via air in the same phase as the speaker unit B.

Referring again to the speaker system of the type mounted on the rear window tray or shelf of the automobile, the size of the speaker cabinet and the speaker unit are restricted due to the area of the mounting place of the automobile. Modern automobiles generally have sloping rear windows and narrow rear shelves or trays. For most cases, the available speaker system is such that the volume of the speaker cabinet is at most about 3 liters. In such a size, the passive radiator type speaker system is preferable in terms of cost for reproducing the lower sound spectrum.

In FIG. 1, if a horn (not shown) is attached to the opening portion of the passive radiator C, the radiating sound energy can be effectively diffused in a certain direction to establish directionality of sound emission. As a result, the lower sound spectrum can be further strengthened. In the rear window tray or shelf of the automobile, however, it is extremely difficult to attach the horn to establish this effect because of the above noted positional or shelf restriction.

Accordingly, an object of this invention is to provide a speaker system provided with a passive radiator in the interior thereof so that the lower frequency reproducing range of the speaker system can be broadened.

It is another object of this invention to provide an improved speaker structure for automobiles.

Yet another object of this invention is to provide for an improved automobile speaker that is low in cost yet highly efficient and compatible with modern automobile sound reproduction systems.

Briefly stated, according to this invention, due to the fact that the rear glass of a vehicle is inclined the radiating surface of the passive radiator is positional to face the rear glass to thereby produce a horn effect. Due to the horn effect, the lower frequency reproduction range is broadened.

In the accompanying drawings:

FIG. 1 is a sectional view showing a conventional speaker cabinet provided with a passive radiator;

FIG. 2 is a perspective view showing a speaker system to be mounted in a vehicle according to this invention;

FIG. 3 is a longtitudinal sectional view of the speaker system shown in FIG. 2;

FIG. 4 is a sectional view showing the condition where the speaker system according to this invention is mounted in the vehicle; and

FIG. 5 is a graphical indication showing frequency to output sound pressure level characteristics according to a conventional and the present speaker system;

FIG. 6 is a sectional view showing an embodiment of the speaker system of the present invention in which the ceiling board of the speaker cabinet is installed parallel to the surface of the rear window tray of the vehicle in which the speaker system is mounted.

A preferred embodiment according to this invention will now be described with reference to the accompanying drawings.

In FIGS. 2 through 4, reference numeral 1 designates a speaker cabinet having a front board or wall 2 to which a speaker unit 3 is attached. An opening 2a is on the front board 2, and a top wall or ceiling board 4 slopes downward from the front board 2 to a rear part 1a of the speaker cabinet 1. As shown, the ceiling board 4 has an opening portion 4a . A passive radiator (or drone cone) 5 is attached to the opening portion 4a from the interior of the speaker cabinet 1.

In order to obtain a horn effect, it is necessary that the acute angle .alpha. of the ceiling board 4 with respect to a bottom board or wall 6 of the speaker cabinet 1 should be made smaller than the angle .beta. formed by the rear window 8 of an automobile with respect to a rear shelf or tray 7 thereof. This is shown in FIG. 4 where .beta. is greater than .alpha.. The ceiling board 4, however, need not be inclined but it may be installed to be in parallel to the surface of the rear tray 7 when the speaker cabinet 1 is positioned on the rear tray of the automobile. This embodiment of the speaker system of the present invention is shown in FIG. 6. In FIG. 4, reference numeral 9 designates a rear deck of the automobile; 10 is the roof thereof; and 11 a seat back of the automobile.

As shown therein, if the speaker cabinet 1 is positioned on the rear window tray or shelf 7 in such a manner that the rear part 1a of the speaker cabinet 1 is in the rearmost position of the rear tray 7, due to the difference in the acute angles .alpha. and .beta. between the ceiling board and the rear window 8, horn region 12 is formed therebetween. Hence the divergent opening of the horn region 12 faces the front side of the automobile and the passive radiator 5 is situated in the throat portion of that horn region. Consequently, the radiating sound of the passive radiator 5 is propagated through the horn region 12 to the front side of the automobile.

As can be appreciated from the graph, FIG. 5, the frequency to output sound pressure level characteristic II of the speaker system according to this invention is superior to the characteristic I of the conventional speaker system. This is practically true in the lower frequency reproducing region which is significantly broadened. In order to enhance the horn effect owing to the horn region 12, it is preferable that the passive radiator 5 be provided as close as possible to the rear part 1a of the speaker cabinet 1.

As described, according to this invention, the radiating surface of the passive radiator confronts the rear glass of the automobile so as to produce a horn effect. As a result the lower frequency reproducing region can be broadened due to the position of the passive radiator 5 and the horn effect even if the inner volume of the speaker cabinet is maintained rather small.