APPARATUS COMPRISING A BAFFLE AND A LOUDSPEAKER, AND LOUDSPEAKER FOR USE IN THE APPARATUS

Abstract: An apparatus comprises a baffle (41) and a loudspeaker (1), the stiffness of the loudspeaker chassis being dimensioned so as to reduce the transmission of vibrations from the loudspeaker to the baffle.

Description: Apparatus comprising a baffle and a loudspeaker, and loudspeaker for use in the apparatus. The invention relates to an apparatus comprising a baffle and a loudspeaker provided with a mounting element for mounting the loudspeaker on the baffle, a diaphragm having a first mass M t , which is movable in a direction of excursion and is connected to the mounting element by a first suspension having a first stiffness K t in the direction of excursion, and a magnet system having a second mass M 2 , which is connected to the mounting element by a second suspension having a second stiffness K 2 in the direction of excursion. The stiffness of a suspension is to be understood to mean the ratio between the force exerted on a suspension point and the excursion of the suspension point as a result of this force. The invention also relates to a loudspeaker for use in the apparatus defined in the opening paragraph. Such a loudspeaker is known from EP-A-0,516,471. The known loudspeaker comprises a chassis carrying a magnet system and a diaphragm, by means of which chassis the loudspeaker can be mounted in an apparatus. A coil connected to the diaphragm cooperates with die magnet system to drive the diaphragm. When the diaphragm is driven the magnet system is subjected to reaction forces. By means of the chassis these forces are transmitted to the apparatus in which d e loudspeaker is mounted. In order to minimise this transmission of forces the magnet system is connected to the chassis via a rubber suspension. By giving this rubber suspension a very low stiffness the movements of the magnet system are isolated from the chassis. A drawback of the known loudspeaker is that the isolation is only effective for higher frequencies. Another drawback of the known loudspeaker is that vibrations of the diaphragm are transmitted to the apparatus via the suspension of the diaphragm, which may still give rise to undesired vibrations in the apparatus. Besides, the rubber suspension requires additional parts and additional assembly operations during the manufacture of the loudspeaker. It is an object of the invention to improve an apparatus of the type defined in the opening paragraph so as reduce the transmission of vibrations from the loudspeaker to the baffle of the apparatus. To this end the apparatus in accordance with die invention is characterised in that The stiffnesses K, and K 2 are generally expressed in N/m and the masses M, and M 2 in kg. The invention is based on the recognition of the fact that when the loudspeaker is driven both the diaphragm and the magnet system are set into vibration and that both parts transmit these vibrations as dynamic forces to the mounting element of the loudspeaker. By means of a balancing process in accordance with die above relationship it can be achieved that the dynamic forces exerted on die mounting element by the magnet system and die dynamic forces exerted on the mounting element by me diaphragm compensate for each other. If the proposed measure is applied this compensation will be effective over a large frequency range. It has been found that this measure leads to loudspeakers which can be implemented very well in practice. Owing to this recognition it has proved diat a substantial reduction of d e transmission of vibrations from the loudspeaker to d e baffle of an apparatus can be achieved even in the case of very large tolerances in the manufacture of me loudspeaker. An embodiment of die apparatus in accordance with the invention is characterised in that This measure enables the transmission of vibrations from the loudspeaker to d e apparatus in the centre of the frequency range of interest to be reduced by approximately 20 dB in comparison with an apparatus with a loudspeaker widiout any measures. For audio and television equipment this frequency range of interest ranges from 0 to 600 Hz because in this range resonances of the housings of such equipment may occur. These resonances are excited by the loudspeaker and may therefore give rise to annoying noises. Moreover, the shadow mask in a picture tube of a television apparatus may be set into vibration as a result of the transmission of vibrations of the loudspeaker to die shadow mask. These vibrations of the shadow mask result in a deterioration of die picture quality of die television apparatus. As a result of the proposed measure, die vibrations in the apparatus are reduced to an acceptable level without any special steps for die purpose of vibration damping being required in the apparatus. An embodiment of d e apparatus in accordance with the invention is characterised in that the loudspeaker comprises a chassis carrying the magnet system and die mounting element, the chassis has a such a shape that it is compliant in the direction of excursion, and die stiffness K 2 of the second suspension is mainly determined by die compliance of die chassis. It has been found that the required stiffness for the suspension of the magnet system can be obtained by giving the chassis a suitable shape. This shape can be obtained by a mere adaptation of the die by means of which the chassis is manufacmred. An embodiment of the apparatus in accordance wid the invention is characterised in that the compliance of the chassis is obtained by means of a meander shape. By means of this measure it is achieved that the stiffness in the direction of excursion of die diaphragm has the required value while the stiffness of the chassis in other directions can remain high. A high stiffness in the other directions is desirable in order to keep a coil, which is secured to the diaphragm and situated in a gap of d e magnet system, correctly centred in this gap. An embodiment of me apparatus in accordance with the invention is characterised in that the chassis is made of a plastics. This plastics may be, for example, polyvinyl chloride or polystyrene, if desired reinforced widi fibres such as carbon fibres or glass fibres. An advantage of this embodiment is that intricate shapes such a meander shape can be moulded at low cost by means of an injection-moulding die. Embodiments of die loudspeaker in accordance widi die invention are constructed as described above. The invention will now be described in more detail, by way of example, wid reference to the drawings, in which Figure 1 shows an embodiment of an apparatus in accordance widi d e invention, Figure 2 is a sectional view of a first embodiment of die loudspeaker in accordance wid d e invention for use in an apparatus in accordance wid d e invention, Figure 3 is a diagrammatic representation of the relevant masses and stiffnesses in die loudspeaker in accordance widi d e invention for the frequency range of 0- 600 Hz, Figure 4 shows die transmission of vibrations from d e loudspeaker in accordance widi die invention to the baffle as a function of d e frequency for a number of different suspensions of die magnet system to the mounting element, Figure 5 shows an second embodiment of die loudspeaker in accordance widi die invention for use in an apparatus in accordance widi d e invention, and Figure 6 shows a diird embodiment of me loudspeaker in accordance with the invention. It is to be noted that the embodiments are shown diagrammatically and die Figures are shown to an arbitrary and not always die same scale. Figure 1 shows an apparatus in accordance widi die invention, in the present case a television apparatus 40 accommodating loudspeakers 1. The loudspeakers 1 are mounted on a baffle 41. The television apparatus 40 further comprises a picture tube 43, in which a shadow mask 45 is mounted. In modern television sets much attention is paid to the sound quality produced by these sets. Moreover, the output power to be delivered by die loudspeakers 1 is constandy raised. Since die loudspeakers 1 vibrate with die frequency of die sound to be produced these vibrations are transmitted to die baffle 41 of die apparatus 40 and, via this baffle 41, they are further transmitted to other parts of the apparatus such as for example die shadow mask 45. If the amplitude of die vibrations in the shadow mask 45 exceed a given value this will clearly manifest itself as a deterioration of die picture quality produced by die picture tube 43. In addition, vibrations of the baffle 41 lead to undesirable noises. Figure 2 shows a detail of an apparatus 40 widi die baffle 41 and die loudspeaker 1. The loudspeaker 1 has a mounting element 3 for mounting die loudspeaker 1 onto die baffle 41. The loudspeaker 1 further comprises a diaphragm 10, which is movable in a direction of excursion 5 in order to radiate sound. The diaphragm 10 is connected to die mounting element 3 by means of a first suspension, in d e present case a compliant surround 11 having a first stiffness K, in the direction of excursion 5. In addition, d e loudspeaker 1 comprises a magnet system 20 having a second mass M 2 and connected to the mounting element 3 by a second suspension, in the present case formed by a chassis 21. The chassis 21 has such a shape that it is compliant in the direction of excursion 5. Thus, die magnet system 20 is suspended to mounting element 3 via the chassis 21 having a second stiffness K 2 . The construction of die chassis 21 is such that the ratio between the second stiffness K 2 and die mass M 2 of die magnet system and d e ratio between die first stiffness K, of the first suspension and die mass M, of the diaphragm are substantially equal to one another. Figure 3 shows diagrammatically me forces exerted by d e various parts on one anod er. By energising die coil 15 shown in Figure 2 a dynamic force F 0 is exerted on d e diaphragm 10, shown as a block marked M, in Figure 3, and an equal but opposite force -F 0 is exerted on die magnet system 20, shown as a block marked M 2 in Figure 3. The diaphragm 10 is connected to die mounting element 3 by means of a suspension 11 having a first stiffness K,. A force F 0 exerted on die diaphragm 10 results in the mounting element 3 being subjected to a force F,. The magnet system 20 is connected to die mounting element 3 by means of a suspension 21 having a second stiffness K 2 . A force -F 0 on the magnet system 10 results in a force -F 2 on the mounting element 3. Moreover, the diaphragm 10 and die magnet system 20 are connected by a third suspension 13 having a third stiffness K 3 and exerting a force -F 3 on the cone and a force F 3 on the magnet system. This third suspension corresponds to a centring diaphragm 13 in Figure 2. Such a centring diaphragm 13 serves to centre the coil 27 of the magnet system 20. To allow this function to be performed correctly tiiis centring diaphragm 13 is generally arranged near die magnet system 30. In the case that the centring diaphragm 13 is connected to die mounting element 3 and die diaphragm 10 the stiffness K, should first be calculated from the sum of die stiffness of die surround 11 and die centring diaphragm 13. The invention is based on die recognition of the fact that the transmission of vibrations from the loudspeaker to d e baffle 41 can be prevented if d e sum of die dynamic forces F, and -F 2 exerted on die mounting element is zero. This requirement can be met by selecting the parameters K,, K 2 , M, and M 2 in a given proportion. How this proportion should be may be understood as follows. The sum of die forces on the mass M, results, via die admittance Y m of die mass M„ in a velocity V, which depends upon die frequency f of the impressed force F 0 . This velocity V, results in a force F, on the mounting element 3 via d e admittance Y K1 of the first stiffness. Expressed as a formula this is as follows: V, = (F 0 -F 3 -F,)Y M1 F - Λ 1 γ κ , Likewise: V 2 = (F O ^-F^Y M , F = * K2 Since the situation is described in which no vibrations are transmitted to die mounting element die mounting element is assumed to be stationary. The desired proportion can be derived by assuming that the force F, and d e force F 2 are equal to one anotiier. This yields d e following formula: The admittance of a mass M is given by Y M = - — - M jωM The admittance of a stiffness K is given by Y κ = -^ , where ω = 2τf. K K, Together widi equation (1) diis yields -_L = -22.. (2) M, M 2 When die requirement (2) is met the sum of the forces on die mounting element 3 is equal to 0 regardless of d e frequency of die impressed force F 0 . This precludes die transmission of vibrations from the loudspeaker 1 to die baffle 41 so that no undesirable noises or disturbances of die picture quality will arise. Figure 4 shows to a logarithmic scale the transmission of the impressed force F 0 to the baffle 41 as function of die frequency f for a number of systems (the transmission is die ratio between the sum F F 2 of the forces exerted on die mounting element 3 and die impressed force F 0 ). The curve 50 represents the transmission of a known system for a customary stiffness K 2 of the second suspension. The curves 53 and 55 represent this transmission for systems with a second stiffness K 2 which deviates minus and plus 20% from the optimum stiffness K 2 , d e optimum stiffness K 2 being given by the above equation (2). A deviation from die optimum stiffness results in a peak in the transmissions 53 and 55, so that the transmission of vibrations from the loudspeaker 1 to die baffle 41 is even amplified in certain frequency bands. However, for the major part of die relevant frequency range the transmission of vibrations is reduced substantially owing to the measure in accordance widi die invention. In this case the suspension 11 of die diaphragm 10 exhibits a damping, resulting in a finite quality factor of a first resonant system formed by die diaphragm 10 and die suspension 11. In the present case a second resonant system comprising the magnet system 20 and die second suspension 21 also has a finite quality factor because die second suspension also exhibits damping. It has been found tiiat even a deviation from the optimum value for K 2 by a factor of two still yields a substantial reduction of die transmission of vibrations from the loudspeaker 1 to die baffle 41. Figure 5 shows a further embodiment of a loudspeaker for use in the apparatus in accordance widi d e invention. In this embodiment die compliance of the chassis 21 is obtained by means of a meander shape 25. This meander shape is obtained by providing die chassis 21 with circumferential rows of slots 26, which slots extend perpendicularly to the direction of excursion 5 of die diaphragm 10, d e rows being offset from one anotiier. Such a meander shape 25 is particularly suitable because it enables die required stiffness in die direction of excursion 5 to be obtained, whereas a very high stiffness can be achieved in directions perpendicular thereto. This is desired because the coil 15 is disposed in a narrow gap 27 of the magnet system 20 (see Figure 2). A low stiffness of the chassis 21 in directions perpendicular to die direction of excursion 5 could result in the coil 15 colliding widi die magnet system 20, thereby impairing a correct operation of the loudspeaker 1. The chassis 21 is preferably made of a plastics. This plastics may be, for example, polyvinyl chloride or polystyrene reinforced, if required, widi fibres such as carbon fibres or glass fibres. In the loudspeaker in accordance widi die invention shown in Figure 6 die chassis 21 comprises a plurality of, in die present example three, integral hinge elements 65, which are uniformly spaced in a circumferential direction of die loudspeaker. (Figure 6 shows only one and one half hinge element.) The hinge elements 65 each have two rod- shaped hinge bodies 65a and integral hinges 65b, which connect die hinge bodies 65a to one anotiier and, at one end, to a chassis portion 21a and, at die other end, to a chassis portion 21b, in a manner as shown in die drawing. In die present example, die chassis 21 and d e hinge elements 65 are made of a plastics and may be constructed as a single injection- moulded product. The present construction is very suitable to achieve a low stiffness in the main direction of d e loudspeaker, i.e. the direction of excursion 5, and to oppose translations in the other directions and rotations about axes in all directions. The present hinge elements 65 can be used in general to interconnect two members with desired stiffnesses. It is to be noted that the invention is not limited to die embodiments shown herein. Several other embodiments are possible without departing from the scope of die invention. For example, the diaphragm 10 and d e magnet system 20 may be connected to one another or to the mounting element 3 by a plurality of suspensions. However, by means of the inventive concept it is always simple to determine die requirements to be met in such a situation in order to reduce die transmission of vibrations from the loudspeaker to a baffle 41 in the apparatus 40.

Claims: CLAIM?: 1. An apparatus (40) comprising a baffle (41) and a loudspeaker (1) provided wid a mounting element (3) for mounting die loudspeaker on die baffle, a diaphragm (10) having a first mass M lt which is movable in a direction of excursion (5) and is connected to die mounting element by a first suspension (11) having a first stiffness K, in the direction of excursion, and a magnet system (20) having a second mass M 2 , which is connected to die mounting element by a second suspension (21) having a second stiffness K 2 in the direction of excursion, characterised in that 2. An apparams (40) as claimed in Claim 1, characterised in that 3. An apparams (40) as claimed in any one of the preceding Claims, characterised in that the loudspeaker (1) comprises a chassis (21) carrying the magnet system (20) and die mounting element (3), die chassis has a such a shape that it is compliant in the direction of excursion (5), and die stiffness K 2 of die second suspension (21) is mainly determined by die compliance of the chassis. 4. An apparams as claimed in Claim 3, characterised in that die compliance of the chassis (21) is obtained by means of a meander shape (25). 5. An apparams (40) as claimed in Claim 3 or 4, characterised in that d e chassis (21) is made of a plastics. 6. A loudspeaker (1) as defined in any one of die preceding Claims and suitable for use in an apparams as claimed in any one of the preceding Claims.

Inventors: DE WIT MARCUS GERARDUS MARIA, SCHALLIG MICHIEL ALLAN AURELIU, DE WIT, MARCUS, GERARDUS, MARIA, SCHALLIG, MICHIEL, ALLAN, AURELIUS
Applicants: PHILIPS ELECTRONICS NV, PHILIPS NORDEN AB, PHILIPS ELECTRONICS N.V.
Application year: 1995
Main classification: H04R1
IPC codes: H04R1, H04R9