US2636943A

US2636943A - Spatial music projecting device

Info

Publication number: US2636943A
Application number: US271335A
Authority: US
Inventors: Pierre H Schaeffer
Original assignee: Individual
Current assignee: Individual
Priority date: 1951-02-26
Filing date: 1952-02-13
Publication date: 1953-04-28
Anticipated expiration: 1970-04-28

Description

April 8, 1953 P. H. SCHAEFFER 2,636,943

SPATIAL MUSIC PROJECTING DEVICE Filed Feb. 15. 1952 2 1 Patented Apr. 28, 1953 ,UNITED STATES PATENT OFFICE v SPATIAL MUSIZfiSifiECTING DEVICE Application February 13, 1952, Serial No. 271,335 In France February 26, 1951 2 Claims. 1

The present invention relates to a device for projecting spatial music and, more particularly, to a device for causing to be described, for a virtuai source of music, which'is'equivalent to four actual stationary sources or" music, which are fed at variable levels by the same musical signal, any desired trajectory in a three-divisional space.

The object of thepresent invention is-to provide a device of this kind in which the means for controlling the levels of'the stationary sources of music, the composition of which determines the location of the virtual source, is a singlemanipulata'ble member; which can be manipulated by an operator, like a conductorof an orchestra, in the manner 'of' a batch for the purpose of-obtain'ing artistic effects from a variable localisation of the said virtual source.

Another object of the .invention is to provide a device of'this kind in which the trajectory described by the virtual source of music is sufficiently similar to the trajectory described by the single means for controlling the levels of the ,stationary sources of music.

The originality of the present invention as compared with the stereophonic methods of the prior art resides in'two essential points. i

The device or the invention does not aim at reinstating, in its three dimensions, an actual source of sound which occupies a certain volume but to create a virtual source of sound which is localised at a point of a three-dimensional space in which the listener is situated, this localisation 'iheing arbitrary and fixed by an operator; this adds .a spatial characteristic, called static spatialisation, to the audition of the music.

In addition, the device of ,t'heinvention enables an operator to apply to the virtual source, arbitrary movements'on selected trajectories; this adds, to the audition oi the music, a new characteristic called cinematic spatialisation.

In the present invention, a correspondence ;between the points or a tetrahedral volume hereinafter called control volume and the points of a tetrahedral volume hereinafter called hearing volume is established, the hearing volume being a tetrahedron defined by four loud-speakers which occupy its apices and the-control volume-being 'a tetrahedron which is approximately similar to the torm-ertetrahedron :and is defined byfour stationary coils that occupy its 'a pices; A movable "coil, which is ,fed by-an alternating current of supersonic frequency, is held in the hand of an operator and can be displaced inside the control volume. It induces "voltages in the stationary coils. These voltages are "detected and serve to control the gain of four amplifiers which respectively feed the loud-speakers in such a. manner that the gain of each amplifier is substantially inversely proportional to the dis- :tance of the said movable coil from the station ary coil, the induced voltage of which controls this amplifier.

Under these conditions a correspondence which is substantially near to homology is obtained between the position of the movable .coil in the control volume and the position .of the virtual sourcelof music in the hearing volume.

()ther objects and features of the invention will appear on reading the following detailed description with the aid of the accompanying drawings, in which:

Fig. 1 represents diagrammatically a concert hail, showing the arrangement in relation to .a "hearing space of four loudespeakers which mark the boundaries of the hearing volume;

Fig. ,2 represents the movable coil, the stationary coils marking the bmmdanies .of the con? trol volume and the electric circuits controlling the sound levels of the loud speakers;

Fig. Step-resents diagrammatically the sound level of a loud'speaker as a function of the distance between the movable coil and the station.- .ary coil, the current induced in which controls the said level; and

Fig. 4 shows the-means tor suspending the movable coil when the lat-teris not :being held in the hand.

Referring to Fig. 1, I represents diagrammatical ly a concert hall, the reverberation time of which is less than 0.5 second and the dimensions of which are-definitely greater than those of the tetrahedral hearing volume. In this hall, there are arranged four loud-speakers .2, -3, 4 and i, the first three of which are arranged at the vertices of an equilateral triangle having a length of side of :a and at a height, from the soundabsorbing floor of the hail, of between 1 and 2 metres. The fourth loud-speaker is arranged on the perpendicular passing through the centre of theegui-lateral triangle tor-med by the three firstrnen-tioned loudspeakers and at a distance from the plane, which passes through the three mentioned loud-speakers, of between and . 3 The listeners are placed inside an almost circular curve 6 which is centred at the foot of the perpendicular 7, and the diameter of which is of the order of The axes of the loud-speakers are all directed to the centre 0 of the tetrahedron the apioes of which they define and they have a polar-radiation curve possessing an angle of opening of the order of 60. through the connections to 29 respectively.

Referring to Fig. 2, 8 denotes a reading system of a recording medium which feeds-a pre-am- These loud-speakers are fed fhaving its middle point connected to earth and the cathodes of which are loaded by the resistance 56 and the potentiometer 51.

The sliding contact 58 of the potentiometer 51 is connected to the grid of the tube 59 of the amplifier H] which, in addition, receives the sigplifier 9, the outlet of which is connected to fouramplifiers H], II, I2 and [3. These four amplifiers comprise a second inlet to which is applied a signal for controlling the gain by means of theconnections l4, l5, l5 and I1 respectively.

I The amplifiers [0 to E3 are followed by expander circuits 8 to 21 which are themselves followed by power amplifiers 22 to 25. The outlets of these power amplifiers are connected to the loud-speakers 2 to 5 by connections 23 to 29 respectively.

The signals for controlling the gain of the amplifiers [0 to I3 are obtained in the following manner:

Mounted on four supports 30 to 33 are four I,

stationary coils 34 to 31, each having a diameter of the order of 50 cms. and comprising about 500 turns of wire. These coils are fixed on their supports by means of a sliding arm 38 so as to form, around an operator, a tetrahedron which i is 'approximately similar to the tetrahedron which is formed by the loud-speakers and the length of a side of which is of the order of 1.50 metres. The normals to the coils converge to the centre 0' of the tetrahedron.

A coil 40 (Fig. 4) may either be held in the hand of an operator or be fixed on an arm 4| which slides on a second arm 42 that ends in a ball-and-socket joint 43 carried by a support 44 and situated nearly at the centre of the tetrahedron formed by the stationary coils. This coil is fed by a source 45 of alternating voltage, which has a frequency of the order of 5000 cycles per second, by means of connections 46. It comprises a divided iron core having the shape of a hollow cylinder 41 with a diameter of 2 cms., and a length of 10 cms. terminated by a substantially spherical widened" portion 48 having a diameter of 5 cms. The cylindrical part carries a coil 49 with about 1000 turns. The voltage at the terminals of the coil is about 1 volt. The coil radiates va magnetic field, the lines of force of which are very markedly radial with respect to the centre of the sphere 48, and it induces, in each stationary coil, a voltage of a magnitude dependingupon its distance in relation to the tationary coil concerned and which is substantially independent of its orientation in relation to this stationary coil.

Connected to the terminals of each stationary coil by connections is a potentiometer 50, the sliding contact 52 of which is connected to the grid of an amplifying tube 5! which is followed by a rectifying stage 53. The outlets of the rectifying stages 53 are connected respectively by means of connections, l5, l5 and I! to the gain-control terminals of the amplifiers 10, ll, 12 and H3. The composition of a rectifying stage 53 is given only for one {of the four stages, the three others being identical, and, in the same nal issuing from the pre-amplifier 9. The tube 59 is a tube having a gain which is inversely proportional to the negative polarisation voltage of its grid and therefore to the potential of the sliding contact 58. There may be used, as the tube 59, a tube of the EF9 type for example or,

failing this, a tube of the BL? type.

The operation of the device for projecting spatial music is as follows:

If only two loud-speakers, 2 and 3 for example, are fed by musical signals of equalintensity, a listener situated inside the space 6 will perceive 'a virtual source of music situated at the centre .of the straight line segment connecting the two loud-speakers. If the sound level of one of the loud-speakers in relation to the other exceeds a certain threshold value, the virtual source will leave the middle point and will be displaced over the straight line segment joining the two loudspeakers towards the one that is emitting the higher intensity of sound. If the unbalance between the levels increases, the virtual source continues to approach the more intense source of sound and, if the unbalance reaches a sufficiently high limiting value, there is an absolute preeminence of the loud-speaker emitting the greater intensity of sound with which the virtual source appears to coincide.

The law of variation of the angular direction of the virtual source for an observer situated approximately on the mediatrix of the segmentjoining the two actual sources is rather complicated. It has been studied by numerous authors in the case of actual sources emitting pure sounds, as a function of the frequency of these sounds. I have found that the threshold from which one would perceive respectively a displacement, which is just perceptible, of the virtual source in relation to the medial plane of the two actual sources and the limit from which a complete pre-eminence of one of the actual sources over the other took place were smaller in the case of musical sounds than in the case of pure sounds. In the case of musical sounds, these thresholds and these limits are respectively of the order of 4 and 8 decibels whilst, for pure sounds of 500 cycles per second, they are approximately 9 and 26 decibels, for pure sounds of 1000 cycles per second, 13 and 30 decibels and, for pure sounds of 2000 cycles per second, 20 and 35 decibels. In practice, in order to be able to keep the possibility of displacing, if not over the whole segment joining the actual sources, at least over a part of that segment, a virtual source that is emitting both pure sounds and complex sounds, it is necessary to allow oneself a margin of variation of gain of the amplifiers [0 to [3 which I have found to be of the order of 20 decibels.

What has hereinbefore been said for a relai reve t en 9 tha p si so m l ns-

j speakers

2 and 3 may be repeated for any couple of loud-speakers and, on extending the reasoning to the set of four loud-speakers, it is seen that, to every set of four sound levels given for the said loud-speakers, there corresponds a virtual source having a given location.

Each of the stationary coils 34 to 3l has current induced therein by the magnetic field produced by the movable coil 40. The induced voltages are rectified by the rectifier stages 53 and there is obtained, between the earth and the sliding contact 58 of each stage, a D. C. voltage which is a function of the distance of the movable coil 40 from the corresponding stationary coil. This voltage is negative and increases when this distance decreases. The amplifier I!) has a gain which is proportional to the D. C. polarisation voltage of the grid in relation to the cut-off voltage and, consequently, inversely proportional to the distance between the stationary coil and the movable coil.

The positions of the sliding contacts 52 and 58 and the values of the resistances of the potentiometers 50 and 5! are adjusted so that the gain of the amplifier corresponding to a given stationary coil is at the maximum when the movable coil is in the immediate vicinity of this stationary coil cms. for example) and is at the minimum and nearly zero when the movable coil is near another stationary coil, that is to say at a distance of the order of 1.50 metres from the stationary coil concerned.

The sound level L of a loud-speaker, as a function of the distance in metres between the movable coil and the stationary coil controlling this loud-speaker, is given by Fig. 3.

The amplifiers ill to Hi have an hyperbolical characteristic and the result of this is that the dynamics of the amplified signal vary with the actual gain of the amplifiers in relation to the dynamics of the input signal. The expander circuits [8 to 2| are also controlled by the rectified signals in the rectifier stages 53 and their object is to make the dynamics of the amplified signals equal to the dynamics of the input signals.

The sound levels of the four loud-speakers are thus almost inversely proportional to the mutual distances between the movable coil and the four stationary coils. As the location of the virtual sound source inside the tetrahedron formed by the loud-speakers is a function of these sound levels and is nearer a given loudspeaker the higher the sound level of the latter,

there is a biunivocal correspondence between the location of the movable coil and the virtual source of sound. In particular, if the movable coil is at the centre of the tetrahedron formed by the stationary coils, the loud-speakers 2 to 5 will emit equal intensities and the virtual source will be located at the centre of the audience space 6 and, in practice, owing to a psychological efiect, the source will appear to surround completely a listener located in the audience space.

If the movable coil 40 is held in the hand of the operator, the latter will be able to displace it and his gestures will have the effect of causing the virtual source to describe any desired trajectory and thus to add a spatial stereophonic element to the audition of the music.

If the virtual source of music is to be localised at a fixed point of the hearing volume, the position of the movable coil 40 will be adjusted on the arm 32 by means of the clamping screw GI and the orientation of the movable arm 42 will be adjusted by means of the clamping band 39 of the ball-and-socket joint 43.

Although the invention has been described in respect to a complete example of embodiment, it is to be understood that modifications are possible and that the scope of these modifications is limited only by the contents or" the appended claims.

What I claim is:

l. A device for projecting spatial music, com prising four loud-speakers situated in a concert hall at the apices of a tetrahedron, four amplifiers feeding the said loud-speakers, four stationary coils situated at the apices of a smaller tetrahedron similar to the tetrahedron formed by the loud-speakers, a coil fed by an alternating current, movable inside the tetrahedron formed by the four stationary coils and inducing a current in the said stationary coils, rectifiers connected to the outlet of the stationary coils, and means for controlling the gain of the amplifiers proportionally to the D. C. voltages rectified by the said rectifiers, whereby the sound levels of the loud-speakers are substantially inversely proportional to the distances of the movable coil from the stationary coils and the location of the virtual sound source, which is equivalent to the four loud-speakers, in relation to the tetrahedron formed by the loud-speakers is substantially homologous to the location of the movable coil in relation to the tetrahedron formed by the stationary coils.

2. A device for projecting spatial music, comprising four loud-speakers situated in a concert hall at the apices of a tetrahedron, four amplifiers feeding the said loud-speakers, four stationary coils situated at the apices of a smaller tetrahedron similar to the tetrahedron formed by the loud-speakers, a spherical polar termination cylindrical core movable inside the tetrahedron formed by the four stationary coils, a. coil mounted upon said core, fed by an alternating current and inducing a current in the said stationary coils, rectifiers connected to the outlet of the stationary coils, and means for controlling the gain of the amplifiers proportionally to the D. C. voltages rectified by the said rectifiers, whereby the voltage induced by the coil mounted upon the movable core in the stationary coils depends solely on the distance between said movable and stationary coils and is substantially independent from the relative orientation of the two coils, the sound levels of the loudspeakers are substantially inversely proportional to the distances of the movable coil from the stationary coils and the location of the virtual sound source, which is equivalent to the four loud-speakers, in relation to the tetrahedron formed by the loud-speakers is substantially homologous to the location of the movable coil in relation to the tetrahedron formed by the stationary coils.

PIERRE H. SCI-IAEFFER.

Name Date De Boer Sept. 13, 1949 Number