The invention concerns an electrical transducer system of the type described in the introduction to claim 1, wherein the transducers can be loudspeakers or microphones. Where the system is to be used especially for the stereophonic reproduction of music, the transducers are loudspeakers.

The main features of an electroacoustic transducer system of the type described in the introduction to claim 1 are known from US-PS no. 4 399 328. This transducer system is similar to the present application's 2k+l transducer units, in that the transducer units for the most part have identical directivity patterns and each transducer unit includes a transducer and an amplitude control device in order to regulate the conversion factor to the associated transducer unit, so that the transducer units which are arranged symmetrically relative to a central transducer unit have conversion factors with the same value. Moreover, the phase shift of the transducer units is the same, apart from the fact that the phase shift in one of the transducers in each pair of the transducer units, which, are arranged symmetrically around the central transducer in such a way that each transducer in the pair is situated at a distance from the central transducer unit which is an odd multiple of the distance d___ between each transducer, amounts to a phase difference of 180°. The object of this invention is, by means of a simple network, to obtain a conversion of the sound waves, which is substantially independent of frequency and direction, by means of a suitable choice of conversion factors which can be regulated by the amplitude control device. This is achieved by connecting the transducers in a series-parallel network.

In stereophonic reproduction or recording of sound where two electrical transmission channels are used called the right and left channels respectively, it has proved difficult to achieve a satisfactory sound image. The primary objective of

stereophonic reproduction or recording of sound, and above all music, is to reproduce in a natural way the three-dimensional sound field at the recording site either for storage by a recording medium or by reproduction in an electroacoustic transducer system. However, both in recording and reproduction in a two-channel system, it has proved difficult to obtain a satisfactory reproduction of the sound image, i.e. the three- dimensional sound field.

The object of the present invention is therefore to be able to render the impression of three-dimensionality of the sound field which is to be recorded or reproduced. This object is achieved by an electroacoustic transducer system which is characterized in those features described in the characterising portion of claim 1. Further features and advantages are described in the attached independent claims.

The invention will now be described in more detail in connection with embodiments shown on the accompanying drawing, by way of example only, wherein the electroacoustic transducers in said embodiments are loudspeakers for reproduction of sound, particularly music, without this in any way limiting the scope of the invention.

Fig. 1 is a schematic representation of an electroacoustic transducer system in accordance with the invention, comprising five loudspeakers.

Fig. 2 shows is a schematic wiring diagram for the system in fig. 1, where the loudspeakers are shown as impedances. - Fig. 3 is a schematic wiring diagram in accordance with the invention for seven loudspeakers shown as impedances. Fig. 4 is a schematic wiring diagram in accordance with the invention for nine loudspeakers shown as impedances.

Fig.l shows five loudspeakers 1-5 connected in a parallel network in such a way that loudspeakers 1, 3 and 5 are connected in parallel and loudspeaker 2 is connected in inverse parallel between the negative terminal on loudspeaker 1 and the

positive terminal on loudspeaker 3, i.e. the negative terminal on loudspeaker 2 is connected to the positive terminal on loudspeaker 3 and the positive terminal on loudspeaker 2 to th negative terminal on loudspeaker 1. Similarly, between loudspeaker 3 and loudspeaker 5, a fourth loudspeaker 4 is connected in direct parallel. All the loudspeakers or transducers are arranged in a line at an equal distance d, so that loudspeakers 2 and 4 constitute a symmetrically arranged pair in relation to the central loudspeaker 3, with the pair's transducers connected in such a way that they are in counter- phase or has a mutual phase shift of 180°.

The electrical wiring diagram for the transducer system in fig 1 is shown in fig. 2, in that the individual loudspeakers are schematically described as impedances Z1-Z5. It is a well-know fact that an electroacoustic transducer in the shape of a loudspeaker can be regarded as a combination of a resisitive and a reactive impedance, the latter in the form of an inductance which must be identified with the loudspeaker coil if electrodynamic loudspeakers are involved.

If the loudspeaker is of the electrostatic type, the reactive impedance is of course a capacitance.

In the same way as in fig. 2, fig. 3 shows the connection diagram for a configuration with seven loudspeakers, wherein two counter-phased pairs of loudspeakers, symmetrical in relation to the central loudspeaker, are used, in that each of these pairs has a symmetrical distance in relation to the central loudspeaker which is an odd multiple of the fixed mutual distance d between each of the loudspeakers Z11-Z17. In this case loudspeakers Z13 and Z15 together with Zll and Z17 respectively constitute the counter-phased pair.

Similarly, fig. 4 shows a configuration with 9 loudspeaker elements Z21-Z29, where two symmetrical pairs are used connected in counter-phase, viz. Z24 and Z26,and Z22 and Z28 respectively.

It will be obvious that in order to reproduce a stereophonic source programme, two electroacoustic transducer systems or loudspeaker systems must be used, one for each transmission channel, which in accordance with normal practice are called right and left channels respectively. In practice the impression of the sound image has proved to be one of the most difficult features to measure in stereophonic reproduction of, e.g., music. The sensory perception of a three-dimensional sound field is to a great extent inflenced by psychoacoustic factors which are not easily quantified. In practice, therefore, it is only possible to obtain an impression of the quality of the reproduced sound image by means of comprehensive listening tests with a variety of programme sources and preferably with comparisons of different loudspeaker systems, and this applies particularly to the impression of three- dimensionality and hence space. At the same time consideration must be given to the fact that the listening room has a great influence on the sound image, provided the listening tests are not conducted under acoustically dead or free-field conditions. When testing the electroacoustic transducer system according to the invention, a transducer system comprising five electrodynamic loudspeakers for reproduction of the right and left sound channels respectively, was used. Programme sources with both monophonic and stereophonic signals were used, including sine waves, music, speech, pink noise and square waves. At the same time the electroacoustic transducer system, i.e. the stereophonic loudspeaker system designed in accordance with the invention, was compared with a pair of conventional electrodynamic loudspeakers, with separate units for bass, mid- range and treble and use of cross-over filter. This system was supplied by a recognized producer in the field. The listening tests showed that regardless of the programme material, the reproduction of the sound image emerged with a far better impression of space and three-dimensionality than was the case with the known loudspeaker system. Alterations in the source directivity gave the impression of a steady transition without any concentration on one channel or the other, and monophonic signals were centred exactly between the two channels.

Moreover, the transducer system had a relatively flat dynamic frequency response in the reproduced frequency range, which, however, did not include the 2 or 3 lowest octaves of the audible range. A surprising result was that the impression of the sound image was also maintained when the test subject moved in relation to the loudspeaker units, i.e. that direction and distance from the listening point to the loudspeakers were not critical, in that the impression of space and three- dimensionality was retained when the position in relation to the loudspeakers was highly asymmetrical. In a normal listening room, moreover, with a suitable adjustment of the angle of emission, the loudspeakers could maintain the impression of a coherent sound image even when the distance between the loudspeakers was up to double the distance between the test ^" subject and the centre of the line between the loudspeakers. Thus there was no occurrence of the well-known hole-in-the- iddle effect which is so often heard in conventional systems.

It is a well-known fact that the discrimination of the auditory sense in the time domain is of essential importance in gaining an impression of directivity, besides differences in the level of intensity, which also play a major part in the psycho- acoustic perception of the qualities and position of the sound source. Thus the ear can perceive time discrepancies down to below 0.5 ms. These time discrepancies are reflected as phase information in the acoustic stimuli and it is therefore vital that phase information in the source signal should be reproduced as naturally as possible in order to obtain a three- dimensional sound image of high quality. The listening tests which were performed proved this to be the case when using an electroacoustic transducer system in accordance with the present invention, in that two such transducer systems naturally had to be used in order to reproduce the sound image of the programme source and provide the phase information which was the basis of the stereophonic effect.

If transducers in the form of loudspeakers are used for the reproduction of music, the loudspeakers may well be full

frequency units, i.e. providing a relatively flat dynamic response over the entire audible frequency range. If loud¬ speaker units with a smaller frequency range are used, these should in every case cover the six to seven highest octaves of the audible frequency range, while one or two bass loudspeakers are used simultaneously in order to reproduce the frequency range below 200 Hz. The transducers which are used as loud¬ speakers can either be of the electrodynamic type, ribbon or foil loudspeakers or electrostatic loudspeakers. In the last two cases, the transducers can constitute sections of transducers in the form of foils or ribbons.

The transducer system in accordance with the invention can also be used for recording sound, above all music. In this case the transducers will consist of microphones, which can be either electrodynamic or capacitive.

With the present invention, an electroacoustic transducer system is provided which, when the transducers are in the form of loudspeakers used for the reproduction of music, above all provides the opportunity to achieve an exceptionally high- quality loudspeaker system which can be constructed with relatively small loudspeaker elements and which in a version with, e.g. , only five elements in each system, i.e. with a total of ten elements in a loudspeaker pair for stereophonic music reproduction, can provide a loudspeaker system at a reasonable price and with moderate dimensions, while at the same time the sound reproduction is better than for much more expensive systems. Realized in this manner, therefore, the transducer system in accordance with the invention is particularly suitable for private hi-fi systems for home use.