EP1070437B1 - Akustisches gerät - Google Patents

Akustisches gerät Download PDF

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Publication number
EP1070437B1
EP1070437B1 EP99914672A EP99914672A EP1070437B1 EP 1070437 B1 EP1070437 B1 EP 1070437B1 EP 99914672 A EP99914672 A EP 99914672A EP 99914672 A EP99914672 A EP 99914672A EP 1070437 B1 EP1070437 B1 EP 1070437B1
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EP
European Patent Office
Prior art keywords
panel
enclosure
cavity
dml
velocity
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Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Expired - Lifetime
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EP99914672A
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English (en)
French (fr)
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EP1070437A1 (de
Inventor
Henry Azima
Joerg Panzer
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NVF Tech Ltd
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New Transducers Ltd
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Publication of EP1070437A1 publication Critical patent/EP1070437A1/de
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    • 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
    • H—ELECTRICITY
    • H04—ELECTRIC COMMUNICATION TECHNIQUE
    • H04R—LOUDSPEAKERS, MICROPHONES, GRAMOPHONE PICK-UPS OR LIKE ACOUSTIC ELECTROMECHANICAL TRANSDUCERS; DEAF-AID SETS; PUBLIC ADDRESS SYSTEMS
    • H04R1/00—Details of transducers, loudspeakers or microphones
    • H04R1/20—Arrangements for obtaining desired frequency or directional characteristics
    • H04R1/22—Arrangements for obtaining desired frequency or directional characteristics for obtaining desired frequency characteristic only 
    • H04R1/24—Structural combinations of separate transducers or of two parts of the same transducer and responsive respectively to two or more frequency ranges

Definitions

  • the invention relates to acoustic devices and more particularly, but not exclusively, to loudspeakers incorporating resonant multi-mode panel acoustic radiators, e.g. of the kind described in our International application WO97/09842. Loudspeakers as described in WO97/09842 have become known as distributed mode (DM) loudspeakers.
  • DM distributed mode
  • DML Distributed mode loudspeakers
  • the product may with advantage be light, thin and unobtrusive.
  • a method of modifying the modal behaviour of a resonant, bending wave, multi-mode panel acoustic device is characterised by the step of bringing the resonant panel (5) into close proximity with a boundary surface (2,3,14) to define a cavity therebetween, said cavity (13) enclosing at least a portion of one face of the panel (5) and being arranged to contain acoustic radiation from the said portion of the panel face, said close proximity being such that the boundary surface of the cavity facing said one panel face causes fluid coupling to the panel (5) and defines a resonant cavity (13) in which x and y cross modes are dominant.
  • the method may also comprise the step of sealing said cavity.
  • the ratio of the cavity volume to the enclosed panel face area may be in the range 10:1 to 0-2:1.
  • the method may also comprise the step of mounting the panel in and sealing the panel to the cavity defining means by a peripheral surround.
  • the surround may be resilient.
  • the panel may be terminated at its edges by a generally conventional resilient surround.
  • the surround may resemble the roll surround of a conventional pistonic drive unit and may comprise one or more corrugations.
  • the resilient surround may comprise foam rubber strips.
  • edges of the panel may be clamped in the enclosure, e.g. as described in our co-pending PCT patent application PCT/GB99/00848 dated 30 March 1999.
  • Such an enclosure may be considered as a shallow tray containing a fluid whose surface may be considered to have wave-like behaviour and whose specific properties depend on both the fluid (air) and the dimensional or volume box geometry.
  • the panel is placed in coupled contact with this active wave surface and the surface wave excitation of the panel excites the fluid. Conversely the natural wave properties of the fluid interact with the panel, so modifying its behaviour. This is a complex coupled system with new acoustic properties in the field.
  • Subtle variations in the modal behaviour of the panel may be achieved by providing baffling, e.g. a simple baffle, in the enclosure and/or by providing frequency selective absorption in the enclosure.
  • baffling e.g. a simple baffle
  • a sealed box loudspeaker 1 comprises a box-like enclosure 2 closed at its front by a resonant panel-form acoustic radiator 5 of the kind described in WO97/09842 to define a cavity 13.
  • the radiator 5 is energised by a vibration exciter 4 and is sealed to the enclosure round its periphery by a resilient suspension 6.
  • the suspension 6 comprises opposed resilient strips 7, e.g. of foam rubber mounted in respective L-section frame members 9,10 which are held together by fasteners 11 to form a frame 8.
  • the interior face 14 of the back wall 3 of the enclosure 2 is formed with stiffening ribs 12 to minimise vibration of the back wall.
  • the enclosure may be a plastics moulding or a casting incorporating the stiffening ribs.
  • the panel in this embodiment may be of A2 size and the depth of the cavity 13 may be 90mm.
  • the loudspeaker embodiment of Figure 3 is generally similar to that of Figures 1 and 2, but here the radiator panel 5 is mounted on a single resilient strip suspension 6, e.g. of foam rubber, interposed between the edge of the radiator 5 and the enclosure to seal the cavity.
  • the radiator panel size may be A5 and the cavity depth around 3 or 4 mm.
  • Figures 1 to 3 relate to loudspeakers, it would equally be possible to produce an acoustic resonator for modifying the acoustic behaviour of a space, e.g. a meeting room or auditorium, using devices of the general kind of Figures 1 to 3, but which omit the vibration exciter 4.
  • a panel in this form of deployment can provide a very useful bandwidth with quite a small enclosure volume with respect to the diaphragm size, as compared with piston speakers.
  • the mechanisms responsible for the minimal interaction of this boundary with the distributed mode action are examined and it is further shown that in general a simple passive equalisation network may be all that is required to produce a flat power response. It is also demonstrated that in such a manifestation, a DML can produce a near-ideal hemispherical directivity pattern over its working frequency range into a 2Pi space.
  • a closed form solution is presented which is the result of solving the bending wave equations for the coupled system of the panel and enclosure combination.
  • the system acoustic impedance function is derived and is in turn used to calculate the effect of the coupled enclosure on the eigen-frequencies, and predicting the relevant shifts and additions to the plate modes.
  • Figure 4 illustrates a typical polar response of a free DML. Note that the reduction of pressure in the plane of the panel is due to the cancellation effect of acoustic radiation at or near the edges.
  • a free DML is brought near a boundary, in particular parallel with the boundary surface, acoustic interference starts to take place as the distance to the surface is reduced below about 15cm, for a panel of approximately 500 cm 2 surface area.
  • the effect varies in its severity and nature with the distance to the boundary as well as the panel size.
  • the result nonetheless is invariably a reduction of low frequency extension, peaking of response in the lower midrange region, and some aberration in the midrange and lower treble registers as shown in the example of Figure 5. Because of this, and despite the fact that the peak can easily be compensated for, application of a 'free' DML near a boundary becomes rather restrictive.
  • FIG. 7 The system under analysis is shown schematically in Figure 7.
  • the front side of the panel radiates into free space, whilst the other side is loaded with an enclosure.
  • This coupled system may be treated as a network of velocities and pressures as shown in the block diagram of Figure 8.
  • the components are, from left to right; the electromechanical driving section, the modal system of the panel, and the acoustical systems.
  • the normal velocity of the bending-wave field across a vibrating panel is responsible for its acoustic radiation.
  • This radiation in turn leads to a reacting force which modifies the panel vibration.
  • the radiation impedance which is the reacting element, is normally insignificant as compared with the mechanical impedance of the panel.
  • the effect of acoustic impedance due to its rear radiation is no longer small, and in fact it will modify and add to the scale of the modality of the panel.
  • This coupling is equivalent to a mechanoacoustical closed loop system in which the reacting sound pressure is due to the velocity of the panel itself.
  • This pressure modifies the modal distribution of the bending wave field which in turn has an effect on the sound pressure response and directivity of the panel.
  • L B is the bending rigidity differential operator of fourth order in x and y, v is the normal component of the bending wave velocity. ⁇ is the mass per unit area and w is the driving frequency.
  • the panel is disturbed by the mechanical driving pressure, p m , and the acoustic reacting sound pressure field, p a , Figure 7.
  • Each term of the series in equation (1) is called a modal velocity, or, a "mode" in short.
  • the model decomposition is a generalised Fourier transform whose eigen-functions ⁇ pi share the orthogonality property with the sine and cosine functions associated with Fourier transformation.
  • the orthogonality property of ⁇ pi is a necessary condition to allow appropriate solutions to the differential equation (2).
  • the set of eigen-functions and their parameters are found from the homogenous version of equation (2) i.e. after switching off the driving forces. In this case the panel can only vibrate at its natural frequencies or the so-called eigen-frequencies, ⁇ i , in order to satisfy the boundary conditions.
  • ⁇ pi(x,y) is the value of the i th plate eigen-function at the position where the velocity is observed.
  • ⁇ pi (xo,yo) is the eigen-function at the position where the driving force F pi (j ⁇ ) is applied to the panel.
  • the driving force includes the transfer functions of the electromechanical components associated with the driving actuator at (x o ,y o ), as for example exciters, suspensions, etc. Since the driving force depends on the panel velocity at the driving point, a similar feedback situation as with the mechanoacoustical coupling exists at the drive point(s), albeit the effect is quite small in practice.
  • Figure 10 gives an example of the velocity magnitude distribution of a single eigen-function across a DML panel.
  • the black lines are the nodal lines where the velocity is zero. With increasing mode index the velocity pattern becomes increasingly more complex. For a medium sized panel approximately 200 modes must be summed in order to cover the audio range.
  • the modal admittance, Y pi(j ⁇ ) is the weighting function of the modes and determines with which amplitude and in which phase the i th mode takes part in the sum of equation (1).
  • Y pi as described in equation (3), depends on the driving frequency, the plate eigen-value and, most important in the context of this paper, on the acoustic impedance of the enclosure together with the impedance due to the free field radiation.
  • ⁇ pi is a scaling factor and is a function of the i th plate eigen-value ⁇ pi and the total radiation impedance Z mai as described in equation (4).
  • ⁇ pi ( s ) ⁇ pi 4 + s p ⁇ Z ma i ( j ⁇ ) ⁇ 1 K p ⁇ M p
  • the frequency response graphs of Figure 13 show the effect of the enclosure on the panel velocity spectrum.
  • the two frequency response curves are calculated under identical drive condition, however, the left-hand graph displays the in-vacuum case, whilst the right hand graph shows the velocity when both sides of the panel are loaded with an enclosure.
  • a double enclosure was used in this example in order to exclude the radiation impedance of air.
  • the observation point is at the drive point of the exciter.
  • Clearly visible is the effect of the panel eigen-frequency shift to higher frequencies in the right diagram, which was also seen in Figure 12. It is noteworthy that as a result of the enclosure influence, and the subsequent increase in the number and density of modes, a more evenly distributed curve describing the velocity spectrum is obtained.
  • the mechanical radiation impedance is the ratio of the reacting force, due to radiation, and the panel velocity.
  • the radiation impedance can be regarded as constant across the panel area and may be expressed in terms of the acoustical radiated power P ai of a single mode.
  • the modal radiation impedance of the i th mode may be described by equation (5).
  • Z ma i 2 ⁇ P ai ⁇ v i > 2
  • ⁇ v i > is the mean velocity across the panel associated with the i th mode. Since this value is squared and therefore always positive and real, the properties of the radiation impedance Z mai are directly related to the properties of the acoustical power, which is in general a complex value.
  • the real part of P ai is equal to the radiated far-field power, which contributes to the resistive part of Z mai , causing damping of the velocity field of the panel.
  • the imaginary part of P ai is caused by energy storing mechanisms of the coupled system, yielding to a positive or negative value for the reactance of Z mai .
  • a positive reactance is caused by the presence of an acoustical mass. This is typical, for example, of radiation into free space.
  • a negative reactance of Z mai is indicative of the presence of a sealed enclosure with its equivalent stiffness.
  • a 'mass' type radiation impedance is caused by a movement of air without compression, whereas a 'spring' type impedance exists when air is compressed without shifting it.
  • the principal effect of the imaginary part of the radiation impedance is a shift of the in-vacuum eigen-frequencies of the panel.
  • a positive reactance of Z mai causes a down-shift of the plate eigen-frequencies, whereas a negative reactance (stiffness) shifts the eigen-frequencies up.
  • the panel mode itself dictates which effect will be dominating. This phenomenon is clarified by the diagram of Figure 14, which shows that symmetrical mode shapes cause compression of air, 'spring' behaviour, whereas asymmetrical mode shapes shift the air side to side, yielding an acoustical 'mass' behaviour. New modes, which are not present in either system when they are apart, are created by the interaction of the panel and enclosure reactances.
  • Figure 15 shows the frequency response of the imaginary part of the enclosure radiation impedance.
  • the left-hand graph displays a 'spring-type' reactance, typically produced by a symmetrical panel-mode. Up to the first enclosure eigen-frequency the reactance is mostly negative. In-vacuum eigen-frequencies of the panel, which are within this frequency region, are shifted up.
  • the right diagram displays a 'mass-type' reactance behaviour, typically produced by an asymmetrical panel mode.
  • the mechanical radiation impedance for the i th -plate mode is (5):
  • Z mai ⁇ j ⁇ ⁇ ⁇ ⁇ a ⁇ A 0 2 A d ⁇ ⁇ k , l ⁇ ( i , k , l ) 2 k z ( k , l ) ⁇ tan ( k z ( k , l ) ⁇ L dz )
  • ⁇ (i, k, l) is the coupling integral which takes into account the cross-sectional boundary conditions and involves the plate and enclosure eigen-functions.
  • the index, i, in equation (6) is the plate mode-number; L dz is the depth of the enclosure; and k z is the modal wave-number component in the z-direction (normal to the panel).
  • the indices, k and 1 are the enclosure cross-mode numbers in x and y direction, where L dx and L dy are enclosure dimensions in this plane.
  • a 0 is the area of the panel and
  • a d is cross-sectional area of the enclosure in the x and y plane.
  • Equation (6) is a complicated function, which describes the interaction of the panel modes and the enclosure modes in detail.
  • Z ma 0 ⁇ j ⁇ Z a ⁇ A 0 2 A d ⁇ cot ( k z ⁇ L dz )
  • the first set 'A' was selected as a small A5 size panel of 149mm x 210mm with three different bulk mechanical properties. These were A5-1, polycarbonate skin on polycarbonate honeycomb; A5-2 carbon fibre on Rohacell; and A5-3, Rohacell without skin. Set 'B' was chosen to be eight times larger, approximately to A2 size of 420mm x 592mm.
  • A2-1 was constructed with glass fibre skin on polycarbonate honeycomb core, whilst A2-2 was carbon fibre skin on aluminium honeycomb.
  • Panel Type B (Nm) ⁇ (Kg/m 2 ) Zm (Ns/m) Size (mm) A2-1 Glass on PC Core 10.4 0.89 29.3 5 x 592 x 420 A2-2 Carbon on AI Core 57.6 1.00 60.0 7.2 x 592 x 420 A5-1 PC on PC core 1.39 0.64 7.5 2 x 210 x 149 A5-2 Carbon on Rohacell 3.33 0.65 11.8 2 x 210 x 149 A5-3 Rohacell core 0.33 0.32 2.7 3 x 210 x 149
  • Panels were mounted onto a back enclosure with adjustable depth using a soft polyurethane foam for suspension and acoustic seal.
  • the enclosure depth was made adjustable on 16,28,40 and 53mm for set 'A' and on 20,50,95 and 130mm for set 'B' panels.
  • Various measurements were carried out at different enclosure depths for every test case and result documented.
  • Panel velocity and displacement were measured using a Laser Vibrometer.
  • the frequency range of interest was covered with a linear frequency scale of 1600 points.
  • the set-up shown in Figure 16 was used to measure the panel mechanical impedance by calculating the ratio of the applied force to the panel velocity at the drive point.
  • Z m F V
  • a special jig was made to allow the measurement of the internal pressure of the enclosure at nine predetermined points as shown in Figure 21.
  • the microphone was inserted in the holes provided within the back-plate of an A5 enclosure jig at a predetermined depth, while the other eight position holes were tightly blocked with hard rubber grommets.
  • the microphone was mechanically isolated from the enclosure by an appropriate rubber grommet during the measurement.
  • Figure 25a shows the log-velocity spectrum of a free radiating, A5-1 panel clamped in a frame, radiating in free space equally from both sides.
  • the solid line represents the simulation curve and the dashed line is the measure velocity spectrum.
  • the panel goes in resonance with the exciter.
  • the discrepancy in the frequency range above 1000 Hz is due to the absence of the free field radiation impedance in the simulation model.
  • Figure 25b shows the same panel as in Figure 25a but this time loaded with two identical enclosures, one on each side of the panel, with the same cross-section as the panel and a depth of 24mm.
  • a double enclosure was designed and used in order to exclude the radiation impedance of free field on one side of the panel and make the experiment independent of the free field radiation impedance. It is important to note that this laboratory set-up was used for theory verification only.
  • a very small enclosure used with a DML will render it independent of its immediate environment and make the system predictable in its acoustical performance.
  • the mathematical model developed demonstrates the level of complexity for a DML in the coupled system. This throws a sharp contrast between the prediction and design of a DML and that of the conventional piston radiator. Whilst the mechanoacoustical properties of a cone-in-box may be found by relatively simply calculations (even by a hand calculator) those associated with a DML and its enclosure are subject to complex interactive relationships which render this system impossible to predict without the proper tools.
  • the directivity of the enclosed system changes substantially from a dipolar shape to a near cardioid behaviour as shown in Figure 17. It is envisaged that the directivity associated with a closed-back DML may find use in certain applications where stronger lateral coverage is desirable.
  • Power response measurements were found to be most useful when working with the enclosed DM system, in order to observe the excessive energy region that may need compensation. This is in line with other work done on DM loudspeakers, in which it has been found that the power response is the most representative acoustic measurement correlating well to the subjective performance of a DML. Using the power response, it was found that in practice a simple band-pass or a single pole high-pass filter is all that is needed to equalise the power response in this region.

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  • Engineering & Computer Science (AREA)
  • Signal Processing (AREA)
  • Physics & Mathematics (AREA)
  • Acoustics & Sound (AREA)
  • Otolaryngology (AREA)
  • Health & Medical Sciences (AREA)
  • Multimedia (AREA)
  • Details Of Audible-Bandwidth Transducers (AREA)
  • Diaphragms For Electromechanical Transducers (AREA)
  • Obtaining Desirable Characteristics In Audible-Bandwidth Transducers (AREA)
  • Measurement Of Mechanical Vibrations Or Ultrasonic Waves (AREA)
  • Soundproofing, Sound Blocking, And Sound Damping (AREA)
  • Piezo-Electric Transducers For Audible Bands (AREA)
  • Fittings On The Vehicle Exterior For Carrying Loads, And Devices For Holding Or Mounting Articles (AREA)

Claims (5)

  1. Verfahren zur Modifizierung des modalen Verhaltens einer akustischen Multimodenresonanzbiegewellenpaneeleinrichtung, gekennzeichnet durch den Schritt des Bringens des Resonanzpaneels (5) in enge Nachbarschaft mit einer Grenzoberfläche (2, 3, 14), um einen Hohlraum dazwischen festzulegen, wobei der Hohlraum (13) zumindest einen Abschnitt einer Fläche des Paneels (5) umschließt und so angeordnet ist, dass er akustische Abstrahlung von dem Abschnitt der Paneelfläche eingrenzt, und wobei die enge Nachbarschaft derart ist, dass die der einen Paneelfläche zugewandte Grenzoberfläche des Hohlraums eine Fluidkopplung an das Paneel (5) bewirkt und einen Resonanzhohlraum (13) festlegt, in dem X- und Y-Kreuzmoden dominant sind.
  2. Verfahren nach Anspruch 1, das den Schritt des Abdichtens des Hohlraums (13) umfasst.
  3. Verfahren nach Anspruch 1 oder Anspruch 2, bei dem das Verhältnis des Hohlraumvolumens zu dem eingeschlossenen Paneelflächenbereich, ausgedrückt in ml zu cm2 im Bereich von 10:1 bis 0,2:1 liegt.
  4. Verfahren nach einem der vorhergehenden Ansprüche, das den Schritt des Anbringens des Paneels (5) in und das Abdichten des Paneels (5) an der den Hohlraum festlegenden Einrichtung (2) durch eine Umfangsumrandung (6) umfasst.
  5. Verfahren nach Anspruch 4, bei dem die Umrandung (6) elastisch ist.
EP99914672A 1998-04-07 1999-04-06 Akustisches gerät Expired - Lifetime EP1070437B1 (de)

Applications Claiming Priority (3)

Application Number Priority Date Filing Date Title
GB9807316 1998-04-07
GBGB9807316.6A GB9807316D0 (en) 1998-04-07 1998-04-07 Loudspeaker
PCT/GB1999/001048 WO1999052322A1 (en) 1998-04-07 1999-04-06 Acoustic device

Publications (2)

Publication Number Publication Date
EP1070437A1 EP1070437A1 (de) 2001-01-24
EP1070437B1 true EP1070437B1 (de) 2006-07-26

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EP99914672A Expired - Lifetime EP1070437B1 (de) 1998-04-07 1999-04-06 Akustisches gerät

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EP (1) EP1070437B1 (de)
JP (1) JP2002511681A (de)
KR (1) KR20010042491A (de)
CN (1) CN100417304C (de)
AR (1) AR019019A1 (de)
AT (1) ATE334567T1 (de)
AU (1) AU3340099A (de)
BG (1) BG104811A (de)
BR (1) BR9909496A (de)
CA (1) CA2326193A1 (de)
DE (1) DE69932507T2 (de)
EA (1) EA200001038A1 (de)
GB (1) GB9807316D0 (de)
HK (1) HK1030327A1 (de)
HU (1) HUP0103916A3 (de)
ID (1) ID27518A (de)
IL (1) IL138310A0 (de)
NO (1) NO20005056L (de)
NZ (1) NZ506732A (de)
PL (1) PL343440A1 (de)
SK (1) SK14932000A3 (de)
TR (1) TR200002920T2 (de)
TW (1) TW462201B (de)
WO (1) WO1999052322A1 (de)
ZA (1) ZA200004675B (de)

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NO20005056L (no) 2000-12-06
JP2002511681A (ja) 2002-04-16
AR019019A1 (es) 2001-12-26
WO1999052322A1 (en) 1999-10-14
ZA200004675B (en) 2002-02-27
IL138310A0 (en) 2001-10-31
CA2326193A1 (en) 1999-10-14
BG104811A (en) 2001-07-31
CN1296719A (zh) 2001-05-23
DE69932507T2 (de) 2007-07-19
TR200002920T2 (tr) 2000-12-21
NO20005056D0 (no) 2000-10-06
EP1070437A1 (de) 2001-01-24
HUP0103916A2 (hu) 2002-03-28
GB9807316D0 (en) 1998-06-03
HK1030327A1 (en) 2001-04-27
NZ506732A (en) 2002-11-26
TW462201B (en) 2001-11-01
ID27518A (id) 2001-04-12
KR20010042491A (ko) 2001-05-25
EA200001038A1 (ru) 2001-04-23
ATE334567T1 (de) 2006-08-15
CN100417304C (zh) 2008-09-03
SK14932000A3 (sk) 2001-05-10
AU3340099A (en) 1999-10-25
PL343440A1 (en) 2001-08-13
HUP0103916A3 (en) 2002-12-28
DE69932507D1 (de) 2006-09-07

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