WO2002013574A2

WO2002013574A2 - Bending wave loudspeaker

Info

Publication number: WO2002013574A2
Application number: PCT/GB2001/003310
Authority: WO
Inventors: Paul Burton
Original assignee: New Transducers Ltd
Current assignee: NVF Tech Ltd
Priority date: 2000-08-03
Filing date: 2001-07-24
Publication date: 2002-02-14
Anticipated expiration: 2003-02-03
Also published as: GB0018997D0; AU2001275698A1; WO2002013574A3

Abstract

A loudspeaker comprising a panel capable of supporting bending waves, a low frequency transducer mounted to the panel for exciting bending waves in the panel at frequencies below a predetermined frequency, a high-frequency transducer mounted to the panel for exciting bending waves in the panel at frequencies above the predetermined frequency, and crossover circuitry for supplying a signal to the low-frequency transducer at frequencies below the predetermined frequency and to the high-frequency transducer for frequencies above the predetermined frequency, wherein the predetermined frequency is substantially equal to the coincidence frequency.

Description

TITLE: BENDING WAVE LOUDSPEAKER

DESCRIPTION

TECHNICAL FIELD

The invention relates to a bending wave loudspeaker, and in particular to a bending wave loudspeaker that operates both above and below the coincidence frequency.

BACKGROUND ART Bending waves are transmitted on a plate with a propagation velocity that varies with frequency; the waves are dispersive. Thus there will in general be a frequency at which the speed of propagation in the plate matches the speed of propagation in free air (about 343 m/s) . The actual radiation characteristic of bending waves and also the power response are different above and below the coincidence frequency, due to an increase in the coupling of the bending waves to air above coincidence. Thus, when bending waves are driven by a transducer to produce an acoustic output there will in general be an increase in the axial and overall power response of the loudspeaker above the coincidence frequency. Moreover, at the coincidence frequency itself a wave propagating in the panel will couple to the air adjacent to the panel to produce a non-axial narrow band peak in the overall power response of the transducer which becomes superimposed on the step function described above, which causes the step to have an asymmetric shape.

Such steps and peaks in the frequency or power response of loudspeakers are particularly difficult to deal with since simple electrical compensation methods are more effective at dealing with changes in slope. The stiffer the panel, the higher the vibrational propagation velocities and thus the lower the coincidence frequency. It is common for the coincidence frequency to lie within the audible frequency range, often in the critical mid-band frequency range where the human ear is most sensitive. Therefore, a means for dealing with the sonic effects caused by coincidence would be of real and practical benefit to designers of systems using bending wave panels as loudspeakers.

As well as disrupting an otherwise smooth power response, coincidence can cause colouration or reflections if the loudspeaker is being used in a conventional stereo or audiovisual system positioned in a typical domestic listening room. Thus the control of coincidence also has the potential to control such room colouration.

A number of prior methods have been used to control the effects of coincidence. One such method is that described in WO00/33612 to New Transducers Limited. Two transducers are placed at a distance apart that corresponds to half of the wavelength of sound in the panel at coincidence frequency. Therefore, at the coincidence frequency the output from the transducers will destructively interfere to reduce the peak in output at the coincidence frequency. Another approach described in the same patent application is to place two transducers less far apart but to delay the signal to one of the transducers in order that at the coincidence frequency the waves destructively interfere. However, neither of these methods deal with all of the effects of coincidence and in particular although they can reduce the peak at the coincidence frequency they do not deal with the step increase in sound output above coincidence .

Anisotropic panel materials have also been suggested for control, and in such materials, where the bending stiffness is not the same for different wave propagation directions, the coincidence frequency will differ. Thus, the coincidence frequency region may be 'smeared' which will reduce the effect of the coincidence peak. However, the anisotropy option is not always available and in any case the effect of coincidence cannot be fully controlled.

A further approach is to use a notch filter of LCR form to reduce the overall energy output at and just above coincidence. However, this would entail a compromise being struck between a flat axial and a flat power response. An LCR notch filter is a parallel circuit of a capacitor and inductor. Normally it is damped by a parallel resistor. The whole filter of 3 parallel components is then wired in series with the load, in this case, the exciter or transducer.

Accordingly, no existing system for controlling coincidence effect is fully satisfactory.

DISCLOSURE OF INVENTION According to the invention, there is provided a loudspeaker comprising a panel capable of supporting bending waves, a low frequency transducer for exciting bending waves in the panel at frequencies below a predetermined frequency, a high-frequency transducer for exciting bending waves in the panel at frequencies above the predetermined frequency, and crossover circuitry for supplying a signal to the low-frequency transducer at frequencies below the predetermined frequency and to the high-frequency transducer for frequencies above the predetermined frequency, wherein the predetermined frequency is substantially equal to the coincidence frequency. This approach is similar in one respect to that of WO97/09846 which describes the use of a low frequency transducer and a high frequency transducer. However, that earlier document does not disclose the use of a configuration for controlling coincidence in which the crossover frequency is substantially the coincidence frequency.

The crossover circuitry may comprise a low pass filter connected to the low-frequency transducer and a high pass filter connected to the high-frequency transducer.

The high pass filter may include additional attenuation to reduce the response above coincidence.

The low frequency transducer can be adapted for low frequency use, for example by including a heavier transducer capable of inputting more power into the panel . Conversely, the high-frequency transducer may be optimised for high frequency operation, for example by having a lower mass voice coil. By dividing the frequency response at or near the coincidence frequency the drivers and the crossover circuitry may provide a large number of adjustable parameters to enable to interchange of electrical power between the low and high frequency transducers to control the overall transfer function in the manner of a loudspeaker crossover network to control the overall frequency and power response. Conventional loudspeakers drive multiple drivers using crossover networks. In the present application, a single panel radiator is driven using two separate transducers to enhance control of the output .

The placement of the high frequency transducer is less critical than that of the low-frequency transducer. Thus the low-frequency transducer can be located at a preferential location or site as taught in prior patent applications to New Transducers Limited, for example WO97/09842. The high-frequency transducers may be placed at another location; the larger density of resonant bending wave modes at higher frequencies may allow reasonable coupling to resonant bending wave modes at a variety of transducer locations.

The high-frequency transducer may in particular be placed at or close to nodal lines of low frequency modes to minimise the coupling of high-frequency transducer to those modes and also to reduce the effect of the high-frequency transducer on the lower resonant modes. Since the high- frequency transducer will often be the smaller transducer its location at a quieter position in terms of the lower resonant bending wave mode can improve its performance and reliability. Intermodulation effect will be ameliorated.

BRIEF DESCRIPTION OF DRAWINGS For a better understanding of the invention, and purely by way of example, specific embodiments of the invention will now be described, with reference to the accompanying drawings in which

Figure 1 shows a loudspeaker arrangement according to the invention,

Figure 2 shows the output of a bending wave panel with no control at the coincidence frequency,

Figure 3 shows the output of the panel of Figure 1 in which the coincidence effect is controlled,

Figure 4 shows the frequency response of a crossover circuit ,

Figure 5 shows a crossover circuit that produces the response of Figure 4,

Figure 6 shows an alternative crossover response, Figure 7 shows a crossover response of an alternative crossover circuit including attenuation,

Figure 8 shows a crossover circuit for producing the response of Figure 7,

Figure 9 shows a yet further crossover circuit showing an asymmetric response, and

Figure 10 shows a crossover circuit for producing the crossover characteristics of Figure 9.

BEST MODE FOR CARRYING OUT THE INVENTION

Referring to Figure 1, a panel (1) capable of supporting resonant bending wave modes has a low-frequency exciter (5) mounted on the panel at a preferential location or site for coupling to lower frequency resonant bending wave modes, and a further transducer (3) coupled to the panel for exciting higher frequency resonant bending wave modes. Crossover circuitry (7) is connected to both the lower and higher frequency transducers (3,5) and a signal input at the signal terminals (9) is split by the crossover circuitry so that the frequencies below the crossover frequency of the crossover circuitry are directed to the lower frequency transducer (5) and frequency above the characteristic frequency of the crossover circuitry are connected to the high-frequency transducer (3) . The crossover circuitry accordingly includes a low-pass filter (11) connected to the low-frequency transducer. The low pass filter includes an inductor (17) in series with the signal and a capacitor (15) in parallel across the signal.

Similarly, the crossover circuitry includes a high- pass filter (7) connected to the high-frequency transducer. The high-pass filter includes a capacitor (21) in series with the signal and an inductor (19) across the signal path.

The acoustic output of the panel driven without any crossover circuitry is shown in Fig.2. As can be seen, the sound output has a plateau (31) at lower frequency, a peak (33) at the coincidence frequency and a further plateau (35) at a higher sound level than the low frequency plateau (31) at frequencies above the coincidence frequency.

In order to control this response, the crossover frequency of the crossover circuitry (7) is arranged to be at the coincidence frequency. The crossover circuitry can be arranged to produce the sound output (36) shown in Fig.3.

A number of examples of crossover circuitry will now be described. Fig.4 shows one particular crossover response at which at the crossover frequency each of the low-pass and high-pass filter are down 3 dB from their plateau values. Such a frequency response can be obtained with low and high-pass filters as shown in Fig.5. The low- pass filter includes an inductor (17) in series with the signal and a capacitor (15) across the signal. The high- pass filter includes a capacitor (21) in series with the signal and an inductor (19) in parallel with the signal. A further crossover response is shown in Fig.6 which differs from Fig.3 only in that the power output is down 6 dB at the crossover frequency. This can be achieved by using second order low and high pass filters as is known.

A further crossover response is shown in Fig.7 which shows an electrical attenuation at higher frequencies. This can be achieved by adding resistors (23,25) to the high-pass filter, as shown in Fig.8.

A yet further crossover response includes asymmetry in the crossover, as illustrated in Fig.9. This may be achieved as shown in Fig.10 by adding a further inductor (27) to the low pass filter.

The crossover frequency (29) is illustrated in each of Figs.2, 3, 4, 6, 7 and 9. This crossover frequency can be arranged at or slightly above the coincidence frequency of the panel (1) .

The crossover approach allows a number of advantages to be achieved. Firstly, it allows control of variations in the panel's overall axial output levels around coincidence. Secondly, it allows the increased output levels above coincidence to be attenuated if required in order to maintain a smooth power response using well known resistors, passive or active attenuation techniques.

Since the crossover circuitry may have independent low and high frequency filters they can be used to equalise an asymmetrical axial frequency or power response or a non- symmetrical peak, for example by varying the shape or order of one or both of the filters - see Fig.10. Each of the low and high frequency exciters can be selected to optimally perform in their range. The low frequency exciter can be large with a higher force factor (product of voice coil winding length and magnetic field) and high inductance, while the high frequency exciter can be smaller and lighter. The small voice coil diameter and low mass of the high-frequency exciter will push the drumskin panel resonance or aperture effect, which occurs in the panel material inside the voice coil parameter, to higher and less critical frequencies. Furthermore, a typically observed lift in the power response above coincidence can be cancelled by using a small and lower sensitivity exciter with the more powerful low frequency exciter.

In a distributed mode loudspeaker with a single panel driven by two exciters covering different frequency ranges separated by an electrical crossover, the low frequency exciter works in a range which is less modally dense. Its location on the panel is therefore critical to maximise the number of panel modes excited in that panel range. Its position is accordingly to be optimised to effectively drive the lowest modes for good low frequency performance .

On the other hand, the panel may have a high density of bending wave modes in the higher frequency region, so the placement of the high frequency exciter allows more freedom.

The high frequency exciter may be usefully located in a low order nodal position or low frequency quiet spot, to avoid being disturbed by low frequency anti-nodal bending. This may reduce inter-modulation distortion.

Alternatively, it may be possible to locate the high frequency exciter at a nodal point at the coincidence frequency, particularly if the panel is very stiff and the coincidence frequency low. This will avoid modally driving the coincidence frequency. In this case the crossover point may be set below the coincidence frequency so that only the high frequency exciter is active at coincidence.

The techniques described assume that crossover frequency is set by the dominant coincidence frequency of the panel. This leaves exciter spacing as the main available variable to control the effects of a crossover between any drivers separated in space. Related effects are known as lobing and comb filtering. At least three approaches are possible to account for these.

Firstly, the exciters can be located less than half a wavelength apart in their overlap range.

Secondly, the exciters can be separated by several wavelengths at the crossover frequency. This will tend to de-correlate the outputs, which in conjunction with the complex modal distribution in the panel at the crossover frequency may result in good directivity and freedom from audible directionality and lobing interference notches. Thirdly, as taught above, if the high frequency exciter is located in a null position at the coincidence/crossover frequency, it will then drive the panel less effectively at that frequency range. Then off- axis frequency response lobes and comb filtering effects are reduced in proportion to the reduced exciter coupling in this range.

INDUSTRIAL APPLICABILITY The invention thus provides a simple mechanism for controlling coincidence effects in a bending wave panel speaker.

Claims

CLAIMS 1. A loudspeaker comprising a panel capable of supporting bending waves, a low frequency transducer mounted to the panel for exciting bending waves in the panel at frequencies below a predetermined frequency, a high- frequency transducer mounted to the panel for exciting bending waves in the panel at frequencies above the predetermined frequency, and crossover circuitry for supplying a signal to the low-frequency transducer at frequencies below the predetermined frequency and to the high-frequency transducer for frequencies above the predetermined frequency, wherein the predetermined frequency is substantially equal to the coincidence frequency.

2. A loudspeaker according to claim 1, wherein the crossover circuitry comprises a low pass filter connected to the low-frequency transducer and a high pass filter connected to the high-frequency transducer.

3. A loudspeaker according to claim 2, wherein the high pass filter includes additional attenuation to reduce the response above coincidence.

4. A loudspeaker according to any one of claims 1 to 3 , wherein the high frequency transducer is a moving coil device adapted for high frequency operating by a small diameter voice coil.

5. A loudspeaker according to any preceding claim, wherein the high-frequency transducer is adapted for high frequency operation by a low mass voice coil.

6. A loudspeaker according to any preceding claim wherein, the low-frequency transducer is located at a preferential location or site.

7. A loudspeaker according to any preceding claim, wherein the high-frequency transducer is placed at or close to nodal lines of low frequency modes to minimise the coupling of the high-frequency transducer to those modes and also to reduce the effect of the high-frequency transducer on the lower resonant modes .