GB2037130A - Single source stereo sound - Google Patents

Abstract

A method of and apparatus for reproducing sound stereophonically from incoming electrical signals including a L (left-hand) and a R (right-hand) component in which respective loudspeakers 115, 117 are arranged with the reference axes of their polar diagrams at right angles, or approximately at right angles, to each other and are fed with signals derived from L and R components of the incoming signals, preferably these signals being L - R and L + R. The loudspeakers on respective axes may each consist of a pair of facing or back-to-back units such as 115, 116 with corresponding pairs of tweeters 115a and 116a outside the enclosures 140, 141. <IMAGE>

Description

SPECIFICATION Single source stereo sound This invention relates to a method of and apparatus for reproducing sound from incom ing electrical signals. Such signals may be transmitted by a radio or television transmitter, or may be transduced from corresponding signals on a record member. The record member may comprise magnetisable material, for example it may be a magnetic tape, or the record member may have the signals recorded thereon by virtue of the configuration of a physical formation such as a groove on a record disc.

Conventional stereophonic sound reproducing apparatus comprises a pair of transducers which are fed with a respective incoming electrical signal and produce respective sound waves corresponding to such signals.

Usually in such conventional apparatus the transducers herein called loudspeakers each comprise a loudspeaker unit (including a diaphragm normally of conical or like form, actuating means responsive to the incoming electrical signal to produce movement of the diaphragm in a vibratory mode, and supporting means normally termed a chassis for supporting the diaphragm while providing freedom for the latter to undergo movement in said vibratory mode) and means enclosing the loudspeaker unit and providing for the radiation of sound waves from an aperture in said enclosing means (normally termed an enclosure).It is to be understood that such loudspeaker may contain one or more loudspeaker units (as is customarily the case) designed for radiating sound waves over different bands of the audible frequency spectrum. the case) designed for radiating sound waves over different bands of the audible frequency spectrum.

Ordinarily such loudspeakers have a single aperture in the enclosure and sound waves are radiated from this aperture, normally at the front of the enclosure, and accordingly the loudspeaker has a directional characteristic represented by a polar diagram in the form of a lobe which is usually of elongate form and symmetrical about a reference axis passing centrally through the aperture at right angles thereto. The lobe ordinarily has its greatest radius vector coincident with the reference axis. There may be sound radiation in directions other than those encompassed by the lobe, for example to the rear of the enclosure, either designedly to a minor extent by provision of an aperture in the wall, or inadvertently by radiation from the enclosure itself in the case of what are termed fully enclosed reflex loudspeakers.

In conventional stereophonic sound reproducing apparatus it is usual to provide two such loudspeakers arranged with the apertures of their enclosures facing in the same general direction, i.e. frontally, and having the reference axes of their polar lobes parallel, or approximately parallel, to each other, the loudspeakers being spaced apart laterally with respect to these reference axes. A listener situated on a median axis parallel to the reference axes of the polar lobes, or bisecting the angle between these if the reference axes intersect at a small acute angle, may hear a sound image which is stereophonic provided that the incoming electrical signals which are fed to the loudspeakers respectively are components of a stereophonic electrical transmission.

The sound source from which such stereophonic components are derived may notionally be conceived as occupying an area such as would be represented by that occupied by performers in a concert hall, theatre stage, or studio, through the centre of which area a median axis can be drawn extending towards the auditorium, that is to say the area in which listeners would normally be situated.

Pick-up transducers for converting sound waves from the sound source into electrical signals are normally positioned at positions spaced apart on opposite sides of this median axis and one of these produces one component termed the L (left-hand) component while the other produces the R (right-hand) component of the stereophonic signal transmission.

A listener positioned on the median axis of the reproducing apparatus is thus exposed to the same, or a somewhat similar, sound image, but if the listener moves to one side of the median axis of the reproducing apparatus the stereophonic sound image which he hears becomes unbalanced, that is no longer corresponds to what would be heard by the listener in the concert hall, studio or theatre when standing on the median axis through the sound source.

This is because any listener detects intensity differences between the sounds reaching his two ears (assumed to be one on each of the opposite sides of the median axis when the listener is facing towards either the sound source or the loudspeakers as the case may be), partly because he is nearer one loudspeaker than to the other, and partly because of the forms of the polar diagrams of the loudspeakers.

The applicant has recognised that the sound image subjectively heard by the listener is affected, not only by these intensity differences, but also by phase differences and by complex interference patterns of the sound waves emanating from the two loudspeakers.

This hypothesis is consistent with the ability of a listener to perceive the direction from which a sound is coming even though the source of the sound is at a considerable distance such that differences of intensity with respect to the left and right ears of the listener are not, or are not likely to be significant.

Thus, a conventional stereophonic sound re,br6tiucing apparatus does not produce a true sound image f6 an appreciable extent because such system relies, for the production of the stereophonic image, upon sound inten sity differences which accurately reproduce those in an auditorium near the source, and these occur only when the listener is stationed on or near the median axis.

A further disadvantage of conventional ster eophonic apparatus is that the loudspeakers have to be positioned within relatively close limits at a particular distance apart from each other. If that distance is exceeded the stereo phonic image appears to have a "hole" in the middle whereas if the loudspeakers are too close the stereophonic. image produced is weak or not even discernible by a listener situated on the median axis.

A still further disadvantage of conventional stereophonic apparatus is that at least two loudspeaker enclosures are required (since the loudspeakers have to be spaced apart one each side of the median axis) and this inevita bly requires a room of not less than a predet ermined size for effective production of a stereophonic image.

The object of the present invention is to provide a new or improved method of stereo phonic sound reproduction and new or im proved apparatus therefor, by means of which one or more of these disadvantages is over come or reduced.

According to one aspect of the present invention 1 provide a method of sound repro duction which comprises radiating transduced stereophonic incoming electrical signals from respective loudspeakers along respective refer ence axes which are transverse to each other.

The axes may be at right angles to each other or approximately so. By "approximately" is meant that a departure from a right angled relationship may be adopted within angular limits such that, so far as a listener is con cerned, there is no significant difference to the quality of the sound image which such listener hears when stationed in any part of the area intended to be served compared with that heard when a right angled relationship is used. It is thus envisaged that the axes may be within 30" of a right angled relationship.

Preferably the loudspeakers are proximate to each other so that the reference axes radi ate at right angles, or approximately at right angles, from a common origin. The term "origin" is used herein in a general sense to denote a point and a moderately sized vol ume, e.g. the volume enclosed by a sphere having a diameter of 1 to 12 metres. While positioning the loudspeakers in proximate rela tionship, stereophonic reproduction can be made effective in rooms of relatively small size. In rooms of any size the distortion of the stereophonic image is caused by the listener moving from place to place within the room, i.e. the area served is much less than occurs by virtue of a corresponding movement of the listener from the median axis of a conventional apparatus.

The stereophonic signals fed to the respective loudspeakers may be derived from L and R components.

While it would be within the scope of the method for the stereophonic signal components fed respectively to the loudspeakers to be simple L and R components, it is a further feature of the invention that the signals fed to the loudspeakers may be derived from L and R components, preferably by addition and/or subtraction operations. Thus, one derived signal may comprise a component represented by wL + xR and the other component may be represented by yL - zH.

The quantities w, x, y, z may have any preselected values (which can possibly be varied according to the acoustic characteristics of the room) but ordinarily will have equal values and, therefore, can be treated as equal to unity.

Also, although it is comtemplated that within the scope of the invention radiation may take place in only one direction along the reference axes, it is preferred that such radiation shall take place either at the same intensity in each direction or at different intensities (again selected with reference to the acoustic characteristics of the room) along both directions of each reference axis, the sound waves being in phase at the origin.

According to a further aspect of the invention, there is provided apparatus for stereophonic reproduction comprising at least two loudspeakers, means for mounting said loudspeakers to provide for sound radiation along respective reference axes at right angles to each other or approximately so and with a common origin for the polar diagrams of the loudspeakers, and an input circuit providing for feeding of respective components of a stereophonic signal to respective ones of the loudspeakers.

The expressions "approximately" and "origin" have the meanings previously defined.

The mounting means could comprise, within the scope of the invention, a respective enclosure for each loudspeaker (and, as previously mentioned, such loudspeaker may com prise more than one loudspeaker unit whether designed or radiate predominantly over differ ent frequency bands of the audible spectrum or not), the enclosures including means for assembling or locating them in a relationship such as to provide the required right-angled or approximately right-angled relationship between their reference axes. Each loudspeaker would preferably radiate in both directions along its respective axis or a pair of loud speakers, one pair for each axis, would be arranged to radiate in this manner.

The angled relationship between the reference axes is ordinarily the angular relationship as viewed in plan, i.e. towards a horizontal reference plane, but it will be understood that it would be within the scope of the invention for the reference axes to be in or parallel to an inclined plane, for example where the loudspeakers are mounted in the upper part or in a lower part of the room. The reference axes need not necessarily be coplanar.

It will further be understood that the apparatus provided in accordance with the invention may include an input circuit for feeding derived signals to the loudspeakers respectively in accordance with any of the relationships referred to in defining the method of the invention.

The invention will now be described, by way of example, with reference to the accompanying drawings wherein: Figure 1 is a plan view of one embodiment of apparatus in accordance with the invention; Figure 2 is a plan view of a somewhat more elaborate embodiment utilising four loudspeakers; Figure 3 is a perspective view of a third embodiment also utilising four loudspeakers; and Figure 4 is a view in front elevation of a fourth embodiment of the invention utilising an arrangement of loudspeakers similar to that of Fig. 2 and showing one form of input circuit.

Referring to Fig. 1, sound reproducing apparatus in accordance with the invention for performing the method thereof comprises loudspeaker units 11 and 1 2 having their reference axes disposed at right angles to each other and lying in a common horizontal plane.

The loudspeaker units may be of the conventional form having conical diaphragms, the actuating means being coils carried by the diaphragms and disposed in the gaps of respective pot magnets and separate terminals being provided for each loudspeaker unit for the reception of components of an electrical incoming stereophonic signal. The reference axis is an each case that which passes through the maximum of the polar lobe, and is usually coincident with the axis of the conical diaphragm.

The loudspeaker units may be contained in separate respective enclosures (not shown).

Ordinarily, acoustic absorbent material is provided in the interior of such enclosures to prevent or reduce unwanted resonances, and in the case of fully enclosed loudspeakers to prevent or reduce radiation of sound from the rear, i.e. except from an aperture in the enclosure in front of the larger end of the loudspeaker cone. In the present form of apparatus the form of the enclosure may be such as to provide radiating apertures at the front, i.e. in coincidence with the front 1 3 of each speaker chassis, and an aperture facing rearwardly (not shown) to permit of rearward radiation from the same diaphragm as that providing the forward radiation.

The stereophonic signals fed respectively to the two loudspeaker units may be derived from a L signal and a R signal and may comprise respectively (L + R) for one speaker, namely 11, and (L - R) for the speaker 1 2.

The conventional stereophonic receiver and amplifier may be used for producing the L and R signals and an input circuit (not shown) may be provided to perform addition and subtraction.

In the input circuit attenuator means may be provided to vary the magnitude of the L and R signals in each derived component, if desired, independently. Such attenuator means may be provided with operator's controls external to the speaker enclosures, or the attenuators may be adjustable by way of preset controls. Instead of attenuators variable gain amplifiers may be provided for amplifying the L and R component in each derived signal (L + R) or (L - R).

The reference axes of the speaker units may be at right angles to each other. However, it is within the scope of the invention to vary this angle from a right-angled relationship by reference to the quality of the sound image judged by a listener at a number of positions within the area intended to be served. It is contemplated that the variation from a right angled relationship will usually be within the limits of + 30". Also the axes may be coplanar or one above the other if desired.

It has been found that by producing the sound image in this manner, the image produced closely resembles that which is heard when standing adjacent to a sound source such as in the auditorium of a threatre, concert hall or studio. Further, such sound image is reproduced satisfactorily to a listener standing only a short distance from the loudspeakers or standing at a larger distance away from the speakers.

Referring now to Fig. 2, two pairs of loudspeaker units are provided 15, 1 6 and 17, 1 8 mounted with their pot magnets back to back and with the reference axis of the loudspeakers of each pair coincident, and the coincident reference axis of one pair being at right angles to the coincident reference axes of the other pair.

These loudspeaker units may be contained in an enclosure (not shown) common to all four, any acoustic absorbent material provided in the enclosure being positioned to inhibit internal resonances but not to inhibit radiation of sound from an aperture provided in the enclosure opposite to the front side of each speaker chassis. Thus sound will be radiated along the respective reference axes as indicated by the arrows 19, 20, 21, 22. The phasing would be such that the diaphragms of speakers 1 5 and 16 move in the same direc- tion at any instant, this being also the case for speakers 17, 18.

Referring now to the embodiment shown in Fig. 3, two loudspeaker units 23 and 24 having their reference axes aligned are mounted face to face in assembled relation with a common baffle 25, and another pair of loudspeaker units 26, 27 are mounted face to face in assembled relation with a common baffle 28 with their reference axes aligned and at right angles to the reference axes of the loudspeakers 23, 24. In this arrangement, as in that of Figs. 1 and 2, derived (L + R) and (L - R) may be fed respectively to one pair of speakers 23, 24 and to the other pair of speakers 26, 27, sound waves being radiated from the rear of each speaker as indicated by the arrows 30, 31, 32, 33.

In the arrangement of Fig. 2 and in the arrangement of Fig. 3, it is preferred that for each pair of loudspeakers which have their axes coincident with each other, one signal (L + R) fed to one of these loudspeakers should be 180 out of phase with the same signal fed to the other speaker of the pair so that the diaphragms of the speakers of this pair move simultaneously in the same direction. Likewise, the signal (L -- R) fed to each of the other speakers in the other pair should provide for movement of the diaphragm of each of these speakers in the same direction by appropriately phased connection.

It will, of course, be understood that instead of single loudspeaker units 11 and 1 2 being used, a plurality of loudspeaker units may be used in place of each such single unit, e.g.

three, connected in an input circuit having a conventional cross-over unit to provide for application of respective bands of the frequency spectrum predominantly to respective loudspeaker diaphragms designed to reproduce those frequency bands optimally. For example, if three loudspeaker diaphragms were employed, these would operate over the base band of the spectrum, the mid range band of the spectrum and the high frequency band of the spectrum. Each diaphragm would ordinarily have its own actuating means but parts of the supporting means may be common to all such units.

The invention has so far been described with reference to use with conventional stereophonic incoming signal components, namely L R. At present the L and R signals are made in two ways.

Firstly, two or more transducers are spaced apart to pick up sound at different places, e.g.

on opposite sides of a median axis with respect to a source. The signal produced by one transducer is designated L and the other is designated R. More recently, however, stereophonic signal components have been derived by placing the two transducers with their reference axes at right angles to each other but at the same location, the reference axes however having an angular relationship of 45" or approximately so to the median axis formed through the sound source. Again, one of these transducers produces the L signal and the other the R signal.It will be noted that this latter arrangement more closely resembles the relationship of the loudspeakers in the stereophonic reproduction apparatus of the present invention, and the latter is therefore well suited to effect reproduction from L and R signals without necessarily producing the derived signals (L + R) and (L - R).

In any of the foregoing embodiments the reference axes which are at right angles to each other may alternatively have a transverse relationship which departs from a right angle of up to 30 consistent with quality of the sound image not being significantly impaired.

If desired the sound reproducing apparatus may include a loudspeaker unit arranged with its reference axis mutually perpendicular to the existing reference axes of the speakers 11, 12, or the pair of speakers 15, 1 6 and 17, 18, or the pairs of speakers 23, 24 and 26, 27.

The third speaker or third pair of speakers may be energised by an appropriate signal such as one derived from a pick-up transducer having a diaphragm, or a pair of transducers having diaphragms, movable along a vertical axis at a pick-up station in the region of the sound source.

Referring now to the embodiment illustrated in Fig. 4, which utilises a loudspeaker arrangement somewhat similar to that illustrated in Fig. 2, parts corresponding to those already described with reference to Fig. 2 are designated by corresponding references with a prefix 100 and the preceding description is to be deemed to apply to these parts.

In a fourth embodiment the loudspeaker units 11 5 and 116, which are of a size and design appropriate to reproduction of sound waves in the base or lower part of the frequency spectrum are supplemented by a pair of loudspeaker units 11 spa, 11 6a of a size and design to be suitable for reproduction of sound waves in the upper part of the frequency spectrum, these units 11 spa, 11 6 a being commonly known as tweeters.

Similarly, the other pair of base loudspeaker units, of which one is seen at 11 7 is supplemented by another pair of tweeters, of which one is seen at 11 7a.

The two pairs of base reproduction loudspeakers have their axes at right angles to each other and are housed in respective enclosures 140, 141 each of square form as viewed in plan and rectangular form as viewed in side and front elevation, these enclosures being mounted one on top of the other.

The base loudspeaker units have their chas sis secured to appropriate walls of the enclosure in alignment with respective apertures therein of a size approximating to the size of the larger diameter of the cone-shaped diaphragms of the loudspeaker units.

The tweeter loudspeaker units may be mounted at the top upper end of the enclosure 141 on a supporting post 142.

The two tweeter units 115a, 1 16a are mounted so that the aligned axes along which their diaphragms move are prallel to the aligned axes 119, 1 20 of the functionally associated base loudspeaker units 11 5, 11 6 and similarly the axes of the other pair of tweeter units are aligned with each other and are parallel to the aligned axes of the functionally associated pair of base speakers such as 117.

In the input circuit, electrical connections are denoted by chain lines. A stereophonic L signal is applied to the conductors presented at a terminal T1 and a stereophonic R signal is applied to the conductors presented at terminal T2.

The L channel conductors feed two variable gain amplifiers 143, 144 and the R channel conductors feed two variable gain amplifiers 145, 146.

The amplifiers 1 43, 1 45 feed a subtracting means comprising a non-inverting (x1) amplifier 148, the resulting output L R being connected to the input of a cross-over unit 149.

Amplifiers 144 and 146 feed an adding means comprising a (x1) non-inverting amplifier a second (x1) non-inverting amplifier 151 connected to provide an output signal L + R which is fed to cross-over unit 1 52.

The cross-over unit 1 49 distributes the upper part of the frequency spectrum to the pair of tweeters such as 11 7a and the lower part of the frequency spectrum to the base loudspeaker units such as 117, in each case the feed channel preferably containing a variable gain amplifier as indicated at 1 53, 1 54.

Similarly the cross-over unit distributes the upper and lower parts of the frequency spectrum to tweeter units 11 1 16a and base loudspeaker units 11 5, 11 6 via variable gain amplifiers 1 55, 1 56.

The phasing of the signals apply to the base loudspeakers 11 5, 11 6 and to the functionally associated tweeter units 115a, 116a is such that the diaphragms of all these units move at any given instant in the same direction, and the same phasing relationships are true of the other pair of base loudspeaker units and the other pair of tweeter units.

The variable gain amplifiers 1 43, 1 45 enable a signal to be fed to the cross-over unit 149 of the general form yL - zR as previously mentioned, and the variable gain amplifiers 144, 146 enable a signal to be fed to the cross-over unit 1 52 of the general form wL + xR.

Furthermore, the variable gain amplifiers 153, 1 54 enable the relative amplitude of the upper and lower parts of the frequency spectrum to be present or adjusted relatively to each other, as similarly do the variable gain amplifiers 155, 1 56.

It is contemplated that where the embodiment of Fig. 4 is in use in premises which do not provide any, or any appreciable, difference in acoustic characteristics at all points of the compass around the apparatus, the values of the quantities w, x, y, z would be equal or nearly so. However, when the apparatus is in use in premises where the acoustic characteristics differ at respective points of the compass, these quantities w, x, y, z would be preset or adjusted by the user subjectively to achieve optimum performance.

Claims (22)

1. A method of sound reproduction which comprises radiating transduced stereophonic incoming electrical signals from respective loudspeakers along respective reference axes which are transverse to each other.

2. A method according to claim 1 wherein the axes are at right angles to each other or approximately so as herein defined.

3. A method according to any one of the preceding claims wherein the loudspeakers are proximate to each other with their respective reference axes radiating from a common origin as herein defined.

4. A method according to either of claims 1 and 2 wherein the stereophonic signals fed to the respective loudspeakers are derived from L (left-hand) and R (right-hand) components.

5. A method according to any one of the preceding claims wherein the signal components fed to the loudspeakers respectively are derived from L and R components by addition or subtraction operations.

6. A method according to claim 5 wherein one of the components is represented by the expression wL + xR and the other component is represented by yL - zR where w, x, y, z have any preselected values.

7. A method according to claim 6 wherein w, x, y, z are each equal to unity.

8. A method according to any one of the preceding claims wherein, in respect of each of said reference axes, radiation takes place therealong in one direction with the same intensity as it takes place in the other direction.

9. Apparatus for stereophonic reproduction comprising at least two loudspeakers, means for mounting said loudspeakers to provide for sound radiation along respective reference axes at right angles to each other or approximately so (as herein defined) and with a common origin (as herein defined) for the polar diagrams of the loudspeakers, and an input circuit providing for feeding of respec tive components of a stereophonic signal to respective ones of the loudspeakers.

10. Apparatus according to claim 9 wherein the reference axes are within 30" of a right angled relationship.

11. Apparatus according to either of claims 9 and 10 wherein the mounting means for the loudspeakers comprises an enclosure common to them.

12. Apparatus according to either of claims 9 and 10 wherein said loudspeakers have separate respective enclosures.

13. Apparatus according to any one of claims 9 to 1 2 wherein along each of said reference axes the apparatus includes two loudspeaker units mounted on an intervening baffle common to such units.

14. Apparatus according to any one of claims 9 to 1 3 wherein the input circuit is of a configuration to feed signals to the loudspeakers respectively derived from L (lefthand) and R (right-hand) signal components of the stereophonic signal.

15. Apparatus according to claim 14 wherein one of the derived signals is represented by the expression wL + xR and the other signal is represented by yL - zR where w, x, y, z have any preselected values.

1 6. Apparatus according to claim 1 5 wherein the quantities w, x, y, z each have a value of unity.

1 7. Apparatus according to any one of claims 9 to 1 6 wherein the loudspeakers provide, along the reference axis concerned, radiation intensity in one direction along said reference axis substantially equal to that provided along the other direction of said reference axis.

1 8. A method of sound reproduction substantially as hereinbefore described.

1 9. Apparatus for sound reproduction substantially as herein before described with reference to and as shown in Fig. 1 of the accompanying drawings.

20. Apparatus for sound reproduction substantially as hereinbefore described with reference to and as shown in Fig. 2 of the accompanying drawings.

21. Apparatus for sound reproduction substantially as hereinbefore described with reference to and as shown in Fig. 3 of the accompanying drawings.

22. Apparatus for sound reproduction substantially as hereinbefore described with reference to and as shown in Fig. 4 of the accompanying drawings.