Loudspeaker System Having A Cardioid Directional Response Pattern

Abstract

A loudspeaker system, preferably in the form of a straight or concavely bent column in the or each lateral wall of which at least one slit has been formed which acts only as an acoustic resistor. This ensures that the cardioid directional response pattern remains constant over a wide frequency range. The directional response pattern may be influenced by varying the location and/or the width of the or each slit.

Description

This invention relates to a loudspeaker system having a cardioid directional response pattern and comprising at least one loudspeaker built in at the front face of a substantially closed enclosure, at least one of the lateral walls of which contains a slit-shaped opening the direction of length of which is substantially at right angles to the axial direction of the loudspeaker.

Such a loudspeaker system is described in the published German Pat. application No. 1,270,117. In the said system a number of loudspeakers are arranged in the form of a column in the front wall of an enclosure. The enclosure has the shape of a prism, the lateral walls being at angles of 120.degree. to the front wall. In each of the lateral walls several slits have been made which, at their rear, i.e., internally of the enclosure, have been provided with rectangular tubes, the cross-sectional area of each tube corresponding to that of the associated slit.

This loudspeaker system has a directional response pattern the cardioid shape of which is not maintained constant as a function of the frequency owing to the tubular slits acting as acoustic masses.

The invention is characterized in that each slit has been covered by acoustic damping material, the shortest distance of the slit from the most effectively active part of the diaphragm of the loudspeaker being about equal to the diameter of this diaphragm, while the upper limit of the frequency at which the cardioid effect still holds is about equal to the frequency associated with a wavelength equal to the circumference of the loudspeaker.

This provides the advantage of ensuring a cardioid directional response pattern which is maintained substantially constant over a very wide frequency range. This requires the use of an acoustic resistance as a component of the acoustic elements in order to ensure that for each frequency the phase shift of the acoustic network of the enclosure is equal to the phase shift imparted to the sound by way of the outside of the system owing to the round-about path taken. The latter phase shift varies linearly with the frequency.

When the acoustic resistance of the slits made in the enclosure is maintained constant, the above holds true for the resonant frequency of the enclosure. If this resonant frequency is made to conform with that associated with a wavelength of the order of magnitude of the loudspeaker or loudspeakers used, the directional effect of the enclosure is optimally utilized, since starting from this frequency the loudspeaker or loudspeakers ensure the directional effect for the higher frequencies.

An embodiment of a loudspeaker system according to the invention is characterized in that behind and parallel to each opening there has been formed a second opening of the same shape. Alternatively, each opening may be shifted in the direction of its width, or its width may be adjustable. In all these cases the said steps influence the cardioid shape.

As a further alternative, each opening may be replaced by a large number of adjoining holes.

Loudspeaker systems according to the invention in the shape of a column may advantageously be mounted within a pulpit. In this case, the column may have a concave shape in accordance with the dimensions of the hall.

Embodiments of the invention will now be described, by way of example, with reference to the accompanying diagrammatic drawings, in which:

FIG. 1 is a perspective view of a pulpit provided with a loudspeaker system according to the invention,

FIG. 2 is a sectional view of the said loudspeaker system,

FIG. 3 is a sectional view of a similar pulpit in which the loudspeaker column is vertically arranged,

FIG. 4a is a sectional view of a loudspeaker enclosure according to the invention, placed in front of a wall,

FIG. 4b is a sectional view of the same enclosure placed in a corner,

FIG. 5 is a perspective view of a loudspeaker projector according to the invention, and

FIGS. 6a and 6b show directional response patterns.

Referring now to FIG. 1, there is shown a pulpit which mainly comprises a desk 1 and a loudspeaker enclosure 2. Four loudspeakers (not shown) are mounted in the shape of a column in a rear wall 3 of the enclosure. In order to achieve a cardioid directional response pattern, slits 5 have been made in the upper face (lateral wall) of the enclosure. The slits 5 have been covered by gauze 5a so that they behave as acoustic resistors.

Various buttons and switches are accommodated in a front panel 6 of the pulpit. The desk part 1 is provided with a microphone 8 on a stand 7. A supply voltage connection 9 is provided in a side panel. The pulpit may further be mounted on removable legs 10.

FIG. 3 shows a modified embodiment of the pulpit of FIG. 1 in which the column of loudspeakers is vertically arranged. The column contains loudspeakers 13 and associated slits 12. A supply voltage connection 9 is provided in a base 14.

FIGS. 4a and 4b show diagrammatically the manner in which a loudspeaker enclosure according to the invention may be placed. The loudspeaker enclosure may contain either one loudspeaker or several loudspeakers mounted in the form of a column. A loudspeaker enclosure 20 contains a loudspeaker 21 and has slits 22 and 23 formed in either lateral wall. Obviously, when the enclosure contains several loudspeakers a corresponding number of aligned slits will be provided. The distance L of these slits from the most active part of the loudspeaker must be about equal to the diameter D of the loudspeaker. It is desirable for the depth H to be as small as possible.

In the situation shown in FIG. 4a the loudspeaker enclosure is placed with its rear face close to and parallel to a wall 18. The slits 22 and 23 are equal in width.

If the loudspeaker enclosure is placed in a corner, the wall 19 will greatly reduce the effect of the slit 23. If, in this situation, the directional response pattern is to be substantially equal to that obtained in the aforementioned situation, the width of the slit 22 must be doubled, the slit 23 being closed.

FIG. 5 shows a sound projector which comprises a shallow closed cylindrical enclosure 30 which is closed at its front face by a single loudspeaker 31. Two parallel rows of slits 32 and 33 of equal size have been formed around the cylindrical wall.

If the slits 33 are covered by an adjustable slide member 34, the projector will have a directional response pattern as shown in FIG. 6a, i.e., a cardioid pattern, FIG. 6b shows the hyper-cardioid shape which the pattern will assume when the slits 33 are not covered.

Obviously, these and other intermediate shapes of the directional response pattern are obtainable if the width of the slits is adjustable. This may be effected by means of simple slide mechanisms.

Claims

What is claimed is:

1. A loudspeaker system having a cardioid directional response pattern comprising, at least one loudspeaker mounted at the front face of a substantially closed enclosure having lateral walls, at least one slit-shaped opening located in at least one of the lateral walls, the direction of length of said opening being substantially at right angles to the direction of the axis of the loudspeaker, acoustic damping material covering each slit, each slit being located so that the shortest distance of the slit from the most effectively active part of the diaphragm of the loudspeaker is about equal to the diameter of said diaphragm, whereby the upper limit of the frequency at which the cardioid effect still holds is about equal to the frequency associated with a wavelength equal to the circumference of the loudspeaker.

2. A loudspeaker system as claimed in claim 1, characterized in that there is formed in said one lateral wall behind and parallel to each slit a second slit of equal shape.

3. A loudspeaker system as claimed in claim 1 wherein each slit may be displaced in the direction of its width.

4. A loudspeaker system as claimed in claim 1 wherein the width of each slit is adjustable.

5. A loudspeaker system as claimed in claim 1 characterized in that each slit comprises a large number of linearly disposed adjoining holes.

6. A loudspeaker system as claimed in claim 1 further comprising a plurality of other loudspeakers mounted at said front face in a line with said one loudspeaker, and a plurality of other slit-shaped openings located in said one lateral wall each with its direction of length substantially at right angles to the axial direction of its respective loudspeaker, acoustic damping material covering at least some of said other openings, and wherein the shortest distance of said other slits from the most effectively active part of the diaphragm of its respective loudspeaker is about equal to the diameter of said diaphragm.

7. A loudspeaker system as claimed in claim 6 wherein a like number of slit-shaped openings are located in a second lateral wall of the enclosure opposite said one lateral wall and in a mirror relationship to the openings in said one wall and to their respective loudspeakers, and acoustic damping material covering at least some of the openings in said second lateral wall.

8. A loudspeaker system as claimed in claim 6 wherein all of said openings are positioned with their longitudinal axes arranged in a line.

9. A loudspeaker system as claimed in claim 1 wherein said enclosure comprises a hollow cylinder with the loudspeaker mounted at one end wall and further comprising a plurality of other slit-shaped openings which, along with said one opening, are linearly aligned in a circle about the cylinder wall.