LOUDSPEAKER

Abstract: A loudspeaker which comprises a loudspeaker element (1) and a box (2), the loudspeaker element being installed on one wall (3) of the said box. The objective is to provide good directivity of low frequencies without the drawbacks of a dipole radiator. At least one wall of the loudspeaker box, e.g. the rear wall (4) or both side walls, is designed such that it allows the passage of the background radiation (9) of the loudspeaker element into the listening space as attenuated and as of opposite phase, compared with the frontal radiation of the loudspeaker element (1).

Description: Loudspeaker The present invention is concerned with a loudspeaker which comprises a loudspeaker element and a box, the loudspeaker element being installed on one wall of the said box. The solution most commonly used is a dynamic loudspeaker. It has several applications. It can be said that a feature common of hi-fi loudspeakers pro- vided with dynamic loudspeaker elements is the use of two or more elements in order that a sufficiently wide frequency range could be covered by means of the loud¬ speaker. Subjects of the most active product develop¬ ment in recent years have been uniform frequency response, high efficiency, low harmonic and intermodular dis¬ tortion, wide radiation properties, and lack of phase distortion. These goals have been achieved by improving the quality of loudspeaker components, by means of more precise tolerances of manufacture, by means of the newest materials (Bextreni and Polypropyl¬ ene cones, Alnico-Samariu cobalt magnets) , and by creating better models of computing in particular for precise dimensioning of the box system of a bass loudspeaker. Moreover, up to 5-way principles have been introduced to replace traditional 2- and 3-way solutions. In spite of these attempts, it has not been possible to improve the quality of the loudspeaker sufficiently. On the other hand, on the basis of absolute listening tests performed in a good acoustic environment or even in an echo-free room, the conclusion has been drawn that a modern hi-fi loudspeaker that has been manufactured correctly is a sufficiently good component in a sound-reproduction chain when the deteriorating effect of the room is not present to spoil the result. This has resulted in a laborious improvement of the acoustics of the room, on which several publications and papers have been issued. In them, better and better OMPI methods and attenuation materials are suggested by means of which it is possible to regulate the reverbe¬ ration time of the room and the homogeneity of the- sound field. Since it is almost impossible to imagine that every buyer of a hi-fi loudspeaker should start modifying his living-room only in order to obtain good acoustic properties, the object of the present invention is to provide a novel loudspeaker whose sound reproduc- tion can be affected by the properties of the room as little as possible. In an ordinary loudspeaker solution with rigid walls, the ratio of power spectrum and frequency response is unfavourable, because the directivity of the loud- speaker is affected almost exclusively by the diameter of the radiator. If such a loudspeaker is designed so that it has a uniform frequency response, its power spectrum emphasises the bass frequencies strongly. Such a loudspeaker reproduces relatively well in a softly acousticized room, but in an ordinary living room the reproduction of sound is dark and indistinct owing to the emphasis on bass. If a loudspeaker has been designed so that it has a uniform power spectrum, its free-field response unavoidably emphasizes the high frequencies, owing to the properties of directivity of the piston radiator. Such a loudspeaker sounds reasonably agreeably in a hard room, but in a room with soft acoustics it is annoyingly sharp with the missing bass and emphasized middle and treble ranges. The loudspeaker in accordance with the present invention is mainly characterized in that at least one wall of the loudspeaker box is made of such a material as permits passage of the background radiation of the loudspeaker element as attenuated through the wall into the listening space so that the said radiation is of at least substantially opposite phase (phase difference 180°) , as compared with the frontal radiation of the loudspeaker element, within a wide frequency range. Depending on the situation, the wall con¬ cerned may be the rear wall of the box, both of the side walls, or even all the walls of the box. One suitable wall material is polystyrene plastic whose density is about 25 to 40 kg/m 3 , but it is also possible to use other so-called lossy materials, e.g. cardboard or thermoplast. The sound penetration properties (vibration properties) depend on several factors, such as thickness, density, area and internal structure of wall. Correct dimensioning is obtained by experimenting. By means of the invention, it is possible to improve the directivity of the loudspeaker at low frequencies within the range of 40 to 800 Hz, and by its means it is possible to control the power spectrum. From this, two important circumstances result, in view of the reproduction. Firstly, since the method increases the directivity at low frequencies considerably, it at the same time dramatically reduces the power radiated by the loudspeaker at these frequencies. In this way it is possible to reach an ideal ratio between sound pressure and power. On the other hand, most recent studies have indicated that, among reflections in a room, the so-called early reflections are the most detrimental ones. As early reflections are considered reflections arriving at delays of about 0.1 to 2 ms. These reflections are produced from faces placed in immediate proximity of the loudspeaker, i.e. from the rear wall, the floor, and from the side walls. Accord¬ ing to the studies, the early reflections cause the following distortions of reproduction: colouring of sound, attenuation of transients and dynamics, unstable stereo picture, weakened resolution power. In the loudspeaker accomplished by means of the principle of the present invention, it has been OMPI WIPO possible to eliminate the early reflections almost completely. The loudspeaker radiates rearwards and to the sides as attenuated by up to 20 dB, as compared with the radiation in the direction of the main axis. The acoustic advantages obtained can be presented as follows, as condensed: - uniform free-field response - uniform power spectrum - clear and bright reproduction (as usually in the case of electrostatic speakers) - precise stereo picture - excellent transient reproduction - precise and strict bass (no standing waves) - the effect of surrounding acoustics on the reproduction is considerably less than with loudspeakers in general. Moreover, structural advantages are obtained: - it is possible to use less expensive materi¬ als for the box system, whereby the product is also less expensive - loudspeakers of lower weight are obtained - it is possible to use the casting technique, whereby the design is completely free (which lacking feature is a real problem in the present-day loudspeaker manuf cture) . As a result of good directivity, the effici¬ ency remains relatively low, which is, on the other hand, also typical of traditional hi-fi loudspeakers. As compared with the advantages obtained, this drawback is, however, of very little significance. Below, the invention will be described with reference to the attached drawing. Figures 1 and 2 show situations in which the loudspeaker has a wide and a narrow, respectively, directional cone. Figure 3 shows a dipole radiator, Figure 4 the radiation pattern produced by it. Figure 5 shows an embodiment of a loudspeaker in accordance with the invention, Figure 6 shows its radiation pattern. Figure 7 shows a second embodiment of a loudspeaker in accordance with the invention, Figure 8 shows its radiation pattern. Figure 1 shows a normal situation in which the loudspeaker has a wide directional cone. In such a case, an abundance of early reflections 11 can be generated from the walls 10 and from the floor, the intensity of the said early reflections being high. Moreover, they come almost straight from the direction of the sound (L, R) , whereby their effect deteriorating the sound is emphasized further. Figure 2 shows a loudspeaker whose directional cone is narrow. In such a case, the amount of early reflections 12 is little and their intensity is low. In a theoretical examination, a dynamic loud¬ speaker can be considered as a piston radiator whose properties of directivity almost exclusively depend on the ratio of the diameter of the radiator to the fre¬ quency to be reproduced. Under these circumstances, it is clear that a good directivity of the bass range, i.e. a narrow directional cone, requires a large area of the radiator. This is, of course, not possible in all constructions. Another possibility to obtain good directivity is the use of a dipole radiator. A dipole radiator con¬ sists, e.g., of a loudspeaker element placed in a finite plate. Figure 3 shows a dipole radiator, and Figure 4 the radiation pattern 15 generated by same. A drawback of a dipole radiator is very poor ability to reproduce low sounds, for when the distance of the loudspeaker 13 from the edge of the plate 14 is less than X/4, below the corresponding frequency, the reproduction starts being attenuated by about 12 dB/oct. In order that a sufficient lower-limit frequency could be attained by means of a dipole radiator, an unreasonably large front panel is required or, correspondingly, the power to be fed into the radiator must be increased considerably at the low frequencies in order to eli i- nate the effect of an acoustic short-circuit. Figures 5 and 7 are schematical sectional views of two embodiments in accordance with the invention seen from above. Reference numeral 1 denotes the loud¬ speaker element, 2 denotes the loudspeaker box as a whole, 3 denotes the front wall of the box, into which the loudspeaker element has been mounted. Reference numerals 4 , 5 and 6 denote such walls penetrable by sound as (by vibrating) allow passage of the background radiation 9 of the loudspeaker element 1 , as attenuated, into the listening space so that the background radiation is at an opposite phase, phase difference 180 , as compared with the frontal radiation of the loudspeaker element 1 , within the frequency range of about 40 to about 800 Hz. Numeral 7 (in Fig. 7) denotes an additional attenuating part for bass sounds, attached to the middle portions of the walls 5 and 6, which said attenuating part may consist of a piece of wood and which is not in con¬ tact with the rigid walls 2 and 3. Numeral 8 denotes an ordinary insulation, e.g. mineral wool. In Fig. 5, the rear wall of the loudspeaker has been replaced by a wall 4 penetrable by sound. Figure 6 shows the directional pattern 16 generated as well as, shown by broken line 17, the corresponding situation if the rear wall were traditionally rigid. On the other hand, in Fig. 7, both of the side walls have been replaced by walls 5 and 6 penetrable by sound, and Fig. 8 shows the corresponding directional pattern 18 as well as the directional pattern of a conventional loudspeaker, dotted broken line 19. The apparatus in accordance with the invention operates as follows: the sound material to be repro¬ duced is passed into the loudspeaker 1. The background radiation 9 of the loudspeaker has access, being partly attenuated, through the wall 4 (or walls 5. and 6) pene¬ trable by sound into the listening space. Thereby a dipole effect is produced, wherein the back vibra- tion of opposite phase partly annuls the frontal radi¬ ation. Thereby, a radiation pattern in accordance with Fig. 6 (or Fig. 8) results, in which pattern the inten¬ sity of the radiation in the lateral direction of the loudspeaker is strongly reduced. This results in an efficient attenuation of early reflections caused by the side walls of the room. By selecting the wall penetrable by sound such that it attenuates low frequencies more than it attenuates high frequencies, it is possible to prevent an otherwise detrimental excessive attenuation of lower bass frequencies. By appropriate dimensioning and choice of material of the wall penetrable by air, it is possible to affect the directivity in the desired way, and in this way a controlled directivity, i.e. independence of the directivity from the frequency, is achieved. Below, an embodiment in accordance with the solution of principle of Fig. 7 is illustrated in practice, whereat this illustration is, of course, not supposed to restrict the invention. Principle of 2-way S P (Soft Wall Principle, operation: means the principle of the present invention) Output: 50 W continuous 60 music Loudspeaker 215 mm bass (part 1) elements: 25 mm tweeter (not shown in Fig. 7) Frequency 40 to 20,000 Hz ! 3 dB range: 35 Hz - 10 dB Sensitivity: 90 dB/1 W/1 m IImmppeeddaannccee:: 8 Ohm Net volume: 38 litres OMPI Dimensions (WixHexDe) : 280 x 770 x 285 mm Connector: Push-pull coupling The front wall 3 and the rear wall are made of rigid particle board, thickness, e.g., 18 mm; the side walls 5 and 6 are made of polystyrene board, density about 40 kg/πr 3 and thickness about 25 mm; the attenuation parts 7 are made of pine wood, length about 220 to 230 mm, height about 35 to 40 mm, and thickness about 25 mm. The wood parts 7 can be sub¬ stituted for, e.g., by appropriately dimensioned alu¬ minium profiles. The walls 5 and 6 are preferably lined with fabric, underneath the loudspeaker 1 , in a way known in prior art, a stationary support between the front wall and the rear wall is fitted, which support is not shown in Fig. 7. OMPI A 10

Claims: WHAT IS CLAIMED IS: 1. Loudspeaker which comprises a loud¬ speaker element (1) and a box (2), the loudspeaker element being installed on one wall (3) of the said box, c h a r a c t e r i z e d in that at least one wall of the loudspeaker box is made of such a material as permits passage of the background radiation of the loudspeaker element as attenuated through the wall into the listening space so that the said radiation is of at least substantially opposite phase (phase difference 180°) , as compared with the frontal radiation of the loudspeaker element (1) , within a wide frequency range. 2. Loudspeaker as claimed in claim 1, c h a r a c t e r i z e d in that the rear wall (4) of the loudspeaker box (2) has been made penetrable by sound in the said way. 3. Loudspeaker as claimed in claim 1, c h a r a c t e r i z e d in that both side walls (5, 6) of the loudspeaker box (2) have been made penetrable by sound in the said way. 4. Loudspeaker as claimed in claim 1 , c h a r a c t e r i z e d in that all the walls of the loudspeaker box (2) have been made penetrable by sound in the said way, 5. Loudspeaker as claimed in any of the preceding claims, c h a r a c t e r i z e d in that the wall (4; 5, 6) penetrable by sound in the said way is made of polystyrene board. 6. Loudspeaker as claimed in claim 5, c h a r a c t e r i z e d in that the thickness of the polystyrene board is about 25 mm and its density about 25 to 40 kg/m 3 , preferably about 40 kg/πt 3 . 7. Loudspeaker as claimed in any of claims 1 to 4, c h a r a c t e r i z e d in that the wall (4; 5, 6) penetrable by sound in the said way is made of cardboard. OMPI 8. Loudspeaker as claimed in any of claims 1 to 4, c h a r a c t e r i z e d in that the wall (4; 5, 6) penetrable by sound in the said way is made of thermoplast, e.g. of acrylics. 9. Loudspeaker as claimed in any of the preceding claims, c h a r a c t e r i z e d in that an attenuating part (7) , preferably a piece of wood, which does not contact the other walls, has been attached to the middle of the wall (4; 5, 6) penetrable by sound in the said way, on the inside face of the said wall, in view of attenuating the bass sounds. 10. Loudspeaker as claimed in claim 1, c h a r a c t e r i z e d in that the frequency range is about 40 Hz to about 800 Hz. OMPI

Inventors: LESKINEN KIMMO, SAASTAMOINEN PEKKA, LESKINEN, KIMMO, SAASTAMOINEN, PEKKA
Applicants: POLARPRO OY
Application year: 1983
Main classification: H04R1
IPC codes: H04R1