FR2791802A1 - MECHANICAL FILTER FOR ACOUSTIC SYSTEMS AND TELEVISION EQUIPPED WITH SUCH FILTERS - Google Patents

Abstract

The invention relates to a mechanical filter for acoustic systems comprising at least one loudspeaker (6) and an acoustic horn (5) associated with the loudspeaker, one open end (7) of the horn being made integral with the edge (8) of the diaphragm (9) of the loudspeaker, and the other open end (4) of the horn constituting the acoustic output. The horn has rigid walls, part of which (10, 11) is made flexible, the flexible part of the horn being able to vibrate under acoustic pressure inside the horn so as to filter or absorb the pick (s). resonance in the acoustic response of the rigid horn. The invention is particularly interesting for improving the acoustic system for televisions.

Description

MECHANICAL FILTER FOR ACOUSTIC SYSTEMS AND

TELEVISION EQUIPPED WITH SUCH FILTERS

  The present invention relates to a mechanical filter for acoustic systems. Among the possible applications of such a filter, one can

cite the case of televisions.

  Nowadays, consumers are more and more demanding on the criteria for choosing televisions on the one hand, the technical quality of images and sound, and on the other hand, the aesthetic visual aspect of the television set. To ensure good sound quality, especially in the medium frequencies (between 300Hz and 5kHz) and low frequencies (between 20Hz and 300Hz), we need loudspeakers with large diaphragms, which is rather bulky . To ensure a functional aesthetic appearance, the dimensions of the part surrounding the television screen on the front face are reduced. Such a requirement means that the sound columns on each side of the screen are discreet and narrow, especially for

16: 9 format TVs.

  As a result, it is necessary to locate the speakers set back relative to the front face to take advantage of the spaces available between the electronic tube and the side walls of the television cabinet. As the sound in the mid and high frequencies is propagated in a directional mode, it is preferable that the sound coming from the speakers leads into the sound columns on the front of the television. To meet these constraints, sound guides are used which are conduits connecting the loudspeakers and the sound columns. The conduits have a "cornet" shape with one open end more flared than the other

  open end, and which will be called subsequently "acoustic horn".

  The acoustic horn is generally produced by molding rigid plastic material. The flared end of the horn is made integral with the edge of the membrane of a loudspeaker, by mechanical fixing, and the other end of the horn is of oblong section parallel to a sound column

  on the front of the TV and opening into this column.

  Such acoustic horns are certainly satisfactory mechanically and aesthetically, but have distortions in the rendering of the

its noticeable by the listener.

  The object of the present invention is to reduce such distortions

  thanks to a simple and effective solution.

  The invention also relates to a mechanical filter for acoustic systems which removes the resonance peak (s) without introducing disturbance in the acoustic response of the system in the

  rest of the acoustic frequency spectrum.

  The mechanical filter for acoustic systems according to the invention satisfies the following conditions. The acoustic system comprises at least one loudspeaker and an acoustic horn associated with the loudspeaker, one of the open ends of the horn being made integral with the edge of the diaphragm of the loudspeaker and the other open part of the horn constituting the acoustic output. The horn has rigid walls, part of which is made flexible. The flexible part of the horn can vibrate under the acoustic pressure inside the horn, so as to filter or absorb the pick (s) of

  resonance in the acoustic response of the rigid horn.

  The invention will be better understood and advantages of the invention

  will appear on studying the detailed description of an embodiment taken

  by way of nonlimiting example and illustrated in the appended drawings, in which: - Figure 1 is a schematic representation of a television set as the application environment of the invention, - Figure 2 is a rigid acoustic horn, - FIG. 3 is a rigid acoustic horn, part of the walls of which is made flexible according to the invention, and - FIG. 4 is a comparison of the response curves

  acoustic horns illustrated in Figures 2 and 3 respectively.

  Figure 1 shows a television with a cabinet or box 1, enclosing an electronic tube (not shown) whose display screen (not shown) opens into the opening 2 of the front or front of the TV. On either side of the opening 2 there are two sound or acoustic columns 3 in each of them opens the oblong open end 4 of an acoustic horn 5. The other flared open end of the horn 5 is fixed to a loudspeaker 6. The role of the horn 5 is to guide the sound produced by the loudspeaker 6 towards the sound column 3

corresponding.

  The acoustic horn 5, as shown in more detail in FIG. 2, has a part with constant oblong section 5a and a flared part 5b. The flared open end 7 of the flared part 5b is mounted on the edge or periphery 8 of the diaphragm 9 of the loudspeaker 6, using mechanical screws or clips. The horn 5 is made by molding plastics, for example, polystyrene, with a wall thickness of the order of 2.5mm to 3mm. The structure of the horn thus obtained is rigid. The inventor has observed distortions in the acoustic response curve of such a rigid comrnet 5. In fact, for a horn length of approximately 180mm, the acoustic response curve has a resonance peak around 440 Hz (see figure 4, curve A). This means that at the open end 4 of the acoustic horn 5, the sound around the frequency 440 Hz is

  excessively amplified to the detriment of the surrounding frequencies (150Hz -

  300Hz and 600Hz - 11kHz). This creates a perceptible acoustic distortion for the listener who can also be a viewer. This inevitable defect is due to the acoustic resonance of the "organ pipe" formed by the horn 5 according to which the resonance peak corresponds to a wavelength which is 4 times the length of the pipe (or the length of the pipe is equal has a

quarter of the wavelength).

  According to numerous tests and many possible solutions envisaged, the inventor arrived at a very simple solution and

  effective, which will be explained using Figures 3 and 4.

  The invention uses a mechanical vibratory phenomenon, obtained by exciting a thin plate by the acoustic pressure present in the horn 5 and generated by the loudspeaker 6, so as to take advantage of its first mechanical eigen modes to absorb the annoying peak of the response. acoustics of the comrnet and improve its acoustic performance. The invention can be applied to any acoustic system (closed or bass reflex enclosures, labyrinths, other acoustic guides,

  .), since it uses the overpressure..DTD: inside a volume (closed or not) created by a loudspeaker.

  One embodiment of the invention (Fig. 3), which gives a satisfactory solution, consists in making an opening 10 in the rigid walls of the flared part 5b of the horn 5 and in covering the opening 10 with a thin plate or flexible 11, so that the first natural mode of mechanical vibration of this plate 11 (considered to be embedded at the limits of the opening 10) is adjusted to the resonance frequency to be attenuated. Thus, the desired filtering is carried out without modifying the acoustic response of the system in the rest of the acoustic frequency spectrum. By playing on the material of the plate 11 (stiffness, damping coefficient, thickness, etc.), on the dimensions, the shape and the position of the opening 10 relative to the loudspeaker 6, this phenomenon can be optimized. vibro-acoustic without generating noise

parasitic mechanical vibrations.

  To determine the dimensions of the opening 10 and the characteristics of the plate 11, the following formulas can be used

D (3 +2 +3

  , 99 a4 a2b2 b4 (1) F - 2 - x Eh3 D Eh- (2) 12 (1 - i2) o F0 is the frequency of the first natural vibration mode of the plate, D is the stiffness factor of the plate, E is the Young's modulus of the material, p is the density of the material, p is the Poisson's ratio, h is the thickness of the plate,

  a and b are the sides of the rectangle formed by the plate.

  From formulas (1) and (2) above, we can calculate the frequency of the first natural vibration mode F0 of a polystyrene plate 0.5mm thick and 57mm x 33mm. The result gives an F0 value around 457 Hz. Curve B in Figure 4 shows the result on the acoustic response of horn 5 with an opening

  covered by a flexible plate 11 with the above characteristics.

  It can be seen that the resonance peak is greatly reduced, which results in better sound quality, because with less distortion, compared to curve A. A simple embodiment consists in making the opening 10 on the horn 5 directly to the molding of this part and to use polystyrene plates provided with adhesives. The vibrating function of the plate 11 is ensured by the polystyrene plate and the adhesive is used to fix the plate to the horn 5 and to add damping to the plate, so as to reduce the possible undesirable effects of the harmonics and

  other plate vibration modes 11.

  According to the dimensions of the opening 10 and the thickness of the plate 11 and in the case of acoustic systems manufactured by plastic injection (such as the horn 5); plate 11 or vibrating membrane can be included

  directly on the part during molding.

  The dimensions of the system to be treated and the importance of faults to be eliminated can impose the multiplicity of this type of system, that is to say several openings 10 covered by several flexible plates 11. These flexible plates can have the same dimensions to reinforce the filtering in a narrow band of frequencies, or of different dimensions

  to widen the frequency band to be filtered.

  In general, when the flexible plate 11 is made of the same material as the rigid horn 5, the thickness of the plate is preferably chosen to be between 10% and 30% of the thickness of the walls

  rigid surrounding the corresponding opening 10.

Claims (5)

  1- Mechanical filter for acoustic systems comprising at least one loudspeaker (6) and an acoustic horn (5) associated with the loudspeaker, the open end (7) of the horn being made integral with the edge (8) of the membrane ( 9) of the loudspeaker, and the other open end (4) of the horn constituting the acoustic output, characterized in that the horn (5) has rigid walls of which a part (10,11) is made flexible, the flexible part horn that can vibrate under acoustic pressure inside the horn so as to filter or absorb the resonance peak (s)   in the acoustic response of the rigid horn.   2- Mechanical filter for acoustic systems according to claim 1, characterized in that the flexible part of the horn is formed by at least one opening (10) in the rigid walls of the horn (5),   the opening being covered by a flexible plate (11).   3- Mechanical filter for acoustic systems according to claim 2, characterized in that the flexible plate (11) is bonded to the rigid walls of the horn.   4- Mechanical filter for acoustic systems according to claim 2, characterized in that the flexible plate (11) is obtained directly by molding the cone.   - Mechanical filter for acoustic systems according to one of the   Claims 2 to 4, characterized in that the flexible plate (11) has   a thickness between 10% and 30% of the thickness of the rigid walls surrounding the opening (10).   6- Television comprising a cabinet (1) framing a display screen, at least one acoustic column (3) adjacent to the screen, and at the   minus a mechanical filter according to one of the preceding claims, the   acoustic output (4) from the horn (5) opening into the acoustic column.