Classifications

• • H—ELECTRICITY
  • H04—ELECTRIC COMMUNICATION TECHNIQUE
  • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
  • H04R7/00—Diaphragms for electromechanical transducers; Cones
  • H04R7/02—Diaphragms for electromechanical transducers; Cones characterised by the construction
  • H04R7/04—Plane diaphragms
  • H04R7/045—Plane diaphragms using the distributed mode principle, i.e. whereby the acoustic radiation is emanated from uniformly distributed free bending wave vibration induced in a stiff panel and not from pistonic motion

Definitions

• the invention relates to an acoustic device, and particularly to a distributed mode device.
• Flat panel bending wave mode loudspeakers are known, for example from O97/09842. This document discloses loudspeakers having a radiator member and an exciter which excites bending waves m the radiator member.
• the shape of the radiator member and its material properties determine a number of resonant bending wave modes of the radiator merr-oer each with a particular mode shape and frequency. These parameters may be chosen so that the resonant bending wave modes of the radiator member are distributed in frequency over the operative frequency range. By distributing the resonant bending wave modes as evenly as possible m frequency it is possible to make the acoustic response of the acoustic device as even as possible m frequency and to avoid excessive dips and peaks in the response. O97/09842 teaches placing the exciter at a location at which the modes are evenly excited, i.e. at an exciter position that as far as possible couples evenly to all the modes .
• the modes are sparsest: m frequency at the lower end of the operative frequency range, so it is m this frequency region that it s most important to excite the modes evenly.
• the exciter may be located to couple evenly to all the resonant sending wave modes at the lower end of the operative frequency range.
• an acoustic device comprising a panel supporting a plurality of resonant bending wave modes each having a resonant frequency, the resonant bending wave modes being divided into a plurality of groups of modes of overlapping resonant frequencies, and a transducer located so as to couple preferentially with one of the said groups of modes .
• Such exciter locations are inevitably a compromise. Rather, it is only necessary to find exciter positions that couple evenly to a group, i.e.
• the groups of modes are not simply split into groups in different frequency ranges. Rather, the groups of modes have overlapping resonant frequencies. Of course, the exact frequencies of the individual modes will not, except by chance, be identical, but taken as a whole the groups of modes may cover roughly the same frequency range .
• the modes of each group are substantially orthogonal to the modes not in that group.
• the groups of bending wave modes may be odd modes and even modes.
• One way of dividing the modes into odd and even is to number the modes in order of increasing frequency; the odd modes are then those with odd numbers and the even modes with even numbers.
• transducer location can then be optimised for one or other of the groups .
• the odd modes may be the antisymmetric symmetry modes with respect to a predetermined axis, typically one of the symmetry axes of the resonant panel.
• the odd modes may be the antisymmetric modes with respect to the axis parallel to the short side of the rectangle.
• the even modes are then the symmetric modes with respect to the same axis.
• the antisymmetric and symmetric modes can refer to the symmetry about the minor axis of the ellipse. It is important to distinguish between modes antisymmetric about an axis and modes along that axis.
• Some resonant bending wave modes have a mode shape in which the distance of the panel to the median plane of the panel varies significantly along one axis but not along an orthogonal axis. Such modes are essentially one- dimensional and can be sai ⁇ to be modes along the said one axis.
• Modes along an axis are symmetric about that axis, but may be either symmetric or antisymmetric about the orthogonal axis.
• the modes may be said to be antisymmetric or symmetric with respect to each of the symmetry axes .
• the resonant bending wave modes can thus be divided into four groups of modes : antisymmetric/antisymmetric, if they are antisymmetric about each symmetry axis, antisymmetric/symmetric if they are antisymmetric about the long axis and symmetric around the short axis, symmetric/antisymmetric if symmetric around the long axis and antisymmetric around the short axis, or symmetric/symmetric if symmetric about both axes.
• the transducer can be arranged to preferentially couple to some or all of these modes.
• symmetric/symmetric modes may couple much more strongly to the surroundings such as walls or enclosures than antisymmetric/antisymmetric, antisymmetric/symmetric or symmetric/antisymmetric modes.
• a loudspeaker made in such a way may have reduced interaction with its surroundings compared to a loudspeaker m which all modes are equally coupled.
• a further transducer or transducers may be provided to preferentially couple to other groups of modes. Each of the groups of modes may be coupled to one or other of the transducers.
• the odd modes are coupled to one transducer and the even modes to another transducer.
• the location of each of the transducers may be separately optimised for the corresponding group of modes; this may give better results than attempting to optimise each transducer for all modes.
• the acoustic device may be a loudspeaker, the transducer or transducers then being an exciter to excite the modes of the panel to cause an acoustic output from the panel .
• the invention provides a panel in which a transducer is mounted m one of the regions marked with an "X" in Figure 3 or 4.
• a transducer is located in a region marked with an "X" m Figure 3 and one m a region marked with an "X” m Figure 4.
• the invention also provides a method of manufacture of an acoustic device, including the steps of providing a panel having a plurality of bending wave modes, the bending wave modes being divided into a plurality of groups of modes, and attaching a transducer to the panel at a location where it preferentially couples with the modes of one of the groups .
• Figure 1 shows a lowest modes of a resonant panel
• Figure 2 shows the nodal lines of the modes of the panel of Fig. 1
• Figure 3 shows the nodal lines of the antisymmetric modes of the panel of Fig. 1
• Figure 4 shows the nodal lines of the even modes of the panel of Fig. 1.
• Figure 5 shows a loudspeaker m accordance with the invention.
• a rectangular panel 1 is shown, together with x and y axes.
• the mode number m frequency order will be labelled with the integer n.
• the modes calculated from solving the equation are not simple sine waves as they would be for second order equations, but are in fact combinations of trigonometric and hyperbolic trigonometric functions, as is known from classical theory.
• the real modes of an actual panel may depend for example on the mounting of the panel, any clamping provided or on any boxes or baffles located close to the member. However, the same approach may still be used.
• the nodal lines for the lower modes are shown in Figure 2. These can be split into the nodal lines for the odd modes which are shown in Figure 3 and the nodal lines for the even modes shown in Figure 4.
• transducer locations suggested by the aforementioned patent application O97/09842 are those in which all of the lower modes are as far as practicable coupled to the transducer. To cause good coupling to the modes, transducers should be located spaced away from the nodal lines. Various suitable regions are marked with an "X" in Figure 2; some of these correspond to the locations taught to be beneficial in O97/09842. The "X"s are not intended to mark single points, but regions. Indeed, the transducers are of finite size and good mounting positions for transducers are available over a reasonable range of positions away from the nodal lines.
• Suitable regions for preferentially coupling to the odd modes are shown with an "X" in Figure 3.
• the regions are located away from the nodal lines of the odd modes.
• suitable regions for preferentially coupling to the even modes are shown with an "X" in Figure 4.
• a first transducer is attached to a location preferentially coupling to the odd modes and a second transducer to a location preferentially coupling to the even modes .
• Better performance for the whole panel may be achievable m this way by using two exciters each placed at a location suitable for exciting all of the modes.
• Figure 5 shows a schematic diagram of a loudspeaker using this technology. Unlike Figures 2 to 4 , it is not drawn to scale.
• a panel 1 has a first exciter 3 coupled to it at a location that preferentially excites odd modes and a second exciter 5 coupled to it at a location that preferentially excites even modes.
• the nodes can be divided into antisymmetric/antisymmetric , symmetric/antisymmetric, antisymmetric/symmetric and symmetric/symmetric modes then one or more transducers may be provided to excite some or all of these groups. If it is desired to avoid symmetric/symmetric modes, for example to reduce boundary effects, then one, two or three transducers may be provided to preferentially excite the antisymmetric/antisymmetric, symmetric/antisymmetric and antisymmetric/symmetric modes.
• an approach exciting antisymmetric and symmetric modes separately may be particularly suitable to torsionally excite modes using inertial exciters.
• the coupling of transducers preferentially to odd and even modes may also be applied to other applications of a distributed mode panel, for example as a microphone or other acoustic device. In such cases however the transducers will of course need to be appropriate to the application.

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• Engineering & Computer Science (AREA)
• Multimedia (AREA)
• Physics & Mathematics (AREA)
• Acoustics & Sound (AREA)
• Signal Processing (AREA)
• Diaphragms For Electromechanical Transducers (AREA)
• Surface Acoustic Wave Elements And Circuit Networks Thereof (AREA)

Applications Claiming Priority (3)

Publications (1)

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• 1999
  • 1999-05-15 GB GBGB9911271.6A patent/GB9911271D0/en not_active Ceased
• 2000
  • 2000-05-12 AU AU45933/00A patent/AU4593300A/en not_active Abandoned
  • 2000-05-12 EP EP00927539A patent/EP1179284A2/de not_active Withdrawn
  • 2000-05-12 WO PCT/GB2000/001751 patent/WO2000070909A2/en active Search and Examination
  • 2000-05-12 JP JP2000619235A patent/JP2003500915A/ja active Pending
  • 2000-05-12 CN CN00807604.9A patent/CN1350765A/zh active Pending
  • 2000-05-12 NZ NZ514593A patent/NZ514593A/xx unknown
  • 2000-05-12 IL IL14598500A patent/IL145985A0/xx unknown
  • 2000-05-17 TW TW089109467A patent/TW479438B/zh active

Non-Patent Citations (1)

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