GB2373126A

GB2373126A - Loudspeaker driver with adapted natural resonance frequency

Info

Publication number: GB2373126A
Application number: GB0124292A
Authority: GB
Inventors: Julian Fordham; Martin Colloms; Graham Bank
Original assignee: New Transducers Ltd
Current assignee: NVF Tech Ltd
Priority date: 2000-10-13
Filing date: 2001-10-10
Publication date: 2002-09-11
Also published as: GB0025125D0; GB0124292D0

Abstract

A loudspeaker 10 comprises an exciter system 12 mounted to acoustic radiator 14, the natural resonance frequency of the exciter system 12 being adapted in order to improve the frequency response of the acoustic output of the acoustic radiator. Embodiments show the driver system exciting bending wave vibrations in a distributed mode loudspeaker. The natural frequency response of the exciter system is shifted away from frequencies that might interfere with the acoustic output of the radiator by adding a member 20 to the loudspeaker to alter the mass/stiffness of the exciter system (a foam block is shown). Also disclosed is a method for determining a natural resonance frequency of the exciter system and shifting it to a frequency which complements the acoustic output of the radiator.

Description

TITLE : LOUDSPEAKER DRIVER DESCRIPTION TECHNICAL FIELD The invention relates to loudspeakers and more particularly to loudspeaker drivers. The invention particularly relates to the class of loudspeakers known as bending wave panel-form loudspeakers, e. g. resonant panelform speakers of the kind described in International application W097/09842.

BACKGROUND ART The technology described in W097/09842 has come to be known as distributed mode or DM technology. Such loudspeakers comprise a resonant panel-form member and a vibration transducer or exciter mounted to the panel-form member to excite resonant bending-wave vibration in the panel-form member.

Such vibration exciters generally possess a unique or natural resonant frequency which may have a significant Q value. If the natural resonant frequency of the exciter occurs within the operating bandwidth of the loudspeaker, the natural resonant frequency of the exciter may interfere with the acoustic response of the loudspeaker. This interference is more common at low frequency and may lead to magnification of low frequency modes inherent in the panelform member. As a result the acoustic response of the speaker may have a very'boomy'sound at these particular modes.

DISCLOSURE OF INVENTION According to the present invention, there is provided a method for improving a frequency response of a loudspeaker comprising an acoustic radiator and an exciter system mounted to the acoustic radiator for exciting bending-wave vibration in the acoustic radiator to produce an acoustic output, the method comprising determining a natural resonance frequency of the exciter system and shifting in frequency the natural resonance frequency of the exciter system to a frequency which complements the acoustic output of the acoustic radiator.

The natural resonance frequency of the exciter system may be shifted to a frequency which is outside the frequency range of the loudspeaker. In particular, the natural

resonance frequency may be shifted to a frequency which is below a lowest frequency of the loudspeaker.

The loudspeaker may be a panel-form loudspeaker and the acoustic radiator may be in the form of a panel. The panel may have a distribution of resonant modes. The panel may be flat and may be lightweight. The material of the panel may be anisotropic or isotropic.

The panel may be capable of supporting bending wave vibration, particularly resonant bending wave mode vibration. The properties of the panel may be chosen to distribute the resonant bending wave modes substantially evenly in frequency, i. e. to smooth peaks in the frequency response caused by"bunching"or clustering of the modes.

In particular, the properties of the panel may be chosen to distribute the lower frequency resonant bending wave modes substantially evenly in frequency. The lower frequency resonant bending wave modes are preferably the ten to twenty lowest frequency resonant bending wave modes of the panel.

The location of the exciter system and in particular of each exciter in the exciter system may be chosen to couple substantially evenly to the resonant bending wave modes in the panel, in particular to lower frequency resonant bending wave modes. In other words, the exciter may be mounted at a location where the number of vibrationally active resonance anti-nodes in the acoustic radiator is relatively high and

conversely the number of resonance nodes is relatively low. Any such location may be used, but the most convenient locations are the near-central locations between 38% to 62% along each of the length and width axes of the acoustic radiator, but off-centre. Specific or preferential locations are at 3/7,4/9 or 5/13 of the distance along the axes; a different ratio for the length axis and the width axis is preferred. Preferred is 4/9 length, 3/7 width of an isotropic panel having an aspect ratio of 1: 1. 13 or 1: 1. 41.

The natural resonance frequency of the exciter system may be shifted to a frequency which does not interfere with the resonant modes which are excited in the panel. The natural resonance frequency of the exciter system may be shifted to a region of the frequency response of the loudspeaker where there are no resonant modes or the distribution of resonant modes is sparse.

The natural resonance frequency of the exciter system may be shifted by modifying the mass or stiffness of the exciter system. The mass, mechanical stiffness or damping of the exciter system may be altered by the addition of members which have moderate values of compliance and mechanical resistance. The members may be made from soft, compliant foams which lower the amplitude of specific modes.

The mechanical stiffness of the member depends upon the physical dimensions and the mechanical properties of the

member, in particular the Young's modulus in compression or shear. The member may be in the form of a simple block of foam in compression and the effective stiffness (k) may be as defined in equation 1 :

EA k =-equation 1 t

where k = effective stiffness (N/m) E = compressive Young's modulus (N/m2) A = cross-sectional area (m2) T = thickness (m) The stiffness or rigidity may also be expressed in terms of the compliance (m/N) which is the reciprocal of the expression in equation 1. The member may be designed to work in a different manner than pure compression, e. g. in shear or tension and thus the expression for the effective stiffness (k) may be different.

The level of damping or energy absorption in the foam may be determined by the damping properties of the foam material itself. Alternatively or additionally, the level of damping or energy absorption in the foam may be determined by the type of the foam. For example, the foam may have an open cell structure and energy may be absorbed by the passage of air into and out of the foam structure.

Alternatively, the foam may have a closed cell structure and

gas may be enclosed in the cells whereby there is a contribution to energy absorption caused by the resistance of the cells to compression.

Alternatively or additionally, the energy absorption characteristics of the foam may vary with frequency, for example, at low frequencies e. g. less than 500 Hz, the level of damping may be different e. g. lower or higher, to that than at higher frequencies. Thus, the damping of the foam may be determined by altering the type and/or material of the foam and/or the frequency of operation.

Materials which show variation in mechanical properties (stiffness and damping) with frequency can be used for this application. However in most cases, the frequency region of interest is over a small range e. g. 50-150 Hz and therefore, variations in mechanical properties with frequency are unlikely to have a significant effect upon the resonance amplitude.

The members may be mounted into the exciter or on the panel. The advantage of mounting the members on the panel is that such members may be added after the exciter has been mounted on the panel. Thus, the performance of a preproduced loudspeaker may be improved. The relatively high mechanical impedance of the panel facilitates the placement of such members.

In some loudspeakers, the exciter system may comprise two or more exciters and may further comprise coupling means for coupling the exciters. As a result of the coupling, the exciter system may have greater stability and accuracy of exciter motion. Furthermore, if the coupling means is thermally conductive and capable of radiating heat it may act as an additional heatsink for the exciters. The or each exciter may be grounded or may be partially grounded.

Such multiple exciter systems are more commonly used in loudspeakers having large panels. Such multiple exciter systems generally have a high mechanical resonance Q and a low natural resonance frequency. Accordingly, since such loudspeakers generally have extended low frequency performance there is interference between the natural resonance frequency of the exciter system and the low resonant modes of the panel which impairs the acoustic response of the loudspeakers.

A member in the form of a generally compliant block may be mounted between the or each exciter and the panel.

Alternatively, the block may be mounted between the panel and the coupling means.

The mechanical stiffness, mass or damping of the block may be selected to change the natural resonance frequency of the exciter system to a more advantageous frequency, namely a frequency which complements the panel behaviour.

Alternatively, the frequency may be chosen to control and damp the degree of resonance required. Furthermore, the block may be selected such that peak excursion of the or each exciter is reduced, thus extending the life of the exciters and reducing harmonic distortion.

According to another aspect of the invention, there is provided a loudspeaker comprising an acoustic radiator and an exciter system mounted to the acoustic radiator for exciting the acoustic radiator to produce an acoustic output, the exciter system being adapted to have a natural resonance frequency which improves the frequency response of the acoustic output of the acoustic radiator.

References herein, both explicit and implicit, to acoustics or sound include references to infrasound and ultrasound.

The present invention is not limited to application in loudspeakers but can also be applied to other acoustic transducers such as microphones, couplers and the like.

BRIEF DESCRIPTION OF THE DRAWINGS The invention is diagrammatically illustrated, by way of example, in the accompanying drawings, in which : Figure 1 is a loudspeaker of known configuration; Figure 2 is a frequency response for the loudspeaker of Figure 1; Figure 3 is an embodiment of the present invention;

Figure 4 is a frequency response for the embodiment of Figure 2.

BEST MODES FOR CARRYING OUT THE INVENTION Figure 1 shows a prior art loudspeaker (10) comprising a resonant panel (14) and a transducer or exciter system mounted on the panel to apply bending wave energy thereto to cause the panel to resonate, e. g. as described in W097/09842. The exciter system comprises two electrodynamic inertial exciters (12) which are both mounted on the panel and a rigid, low mass beam (16) attached to both exciters (12). The beam (16) effectively couples together the mass of the two exciters (12) to shift the natural resonance frequency of the exciter system to a lower frequency. The natural resonance frequency of the exciter system interferes with a resonant mode of the panel and thus, the exciter system of Figure 1 has a magnified low frequency mode (18) as shown in the frequency response in Figure 2.

In Figure 3, the loudspeaker (10) of Figure 1 is altered by mounting a stiff, low density foam block (20) on the panel (14) underneath the beam (16) connecting the two exciters (12) and coupled to the beam. Attaching the foam block (20) between the panel (14) and the beam (16) alters the natural resonance frequency of the exciter system. The addition of the foam block (20) increases the effective compliance of the exciter system and shifts the resonance of the exciter system away from the resonant mode in the panel. As a result, the dominant low frequency mode (18) present in the prior art speaker of Figure 1 and shown in Figure 2 is smoothed and reduced to become a less dominant low frequency mode (22) as shown in Figure 4.

In this way the acoustic response of a bending wave panel speaker may be improved and the'boomy'resonant low frequency behaviour may be controlled.

Claims

CLAIMS 1. A method for improving a frequency response of a loudspeaker comprising an acoustic radiator and an exciter system mounted to the acoustic radiator for exciting bending-wave vibration in the acoustic radiator to produce an acoustic output, the method comprising determining a natural resonance frequency of the exciter system and shifting in frequency the natural resonance frequency of the exciter system to a frequency which complements the acoustic output of the acoustic radiator.
2. A method according to claim 1, wherein the natural resonance frequency of the exciter system is shifted to a frequency which is outside the frequency range of the loudspeaker.
3. A method according to claim 1 or claim 2, wherein the natural resonance frequency is shifted to a frequency which is below a lowest frequency of the loudspeaker.
4. A method according to any one of the preceding claims, wherein the acoustic radiator has a distribution of resonant modes and the natural resonance frequency of the exciter system is shifted to a frequency which does not interfere with the resonant modes which are excited in the acoustic radiator.
5. A method according to claim 4, wherein the natural resonance frequency of the exciter system is shifted to a

region of the frequency response of the loudspeaker where the distribution of resonant modes is sparse.
6. A method according to any one of the preceding claims, wherein the natural resonance frequency of the exciter system is shifted by modifying the mass or stiffness of the exciter system.
7. A method according to claim 6, wherein the mass or stiffness of the exciter system is altered by the addition of at least one of member having a moderate value of compliance and mechanical resistance.
8. A method according to claim 7, wherein the member is made from soft, compliant foam.
9. A method according to claim 7 or claim 8, wherein the member is mounted into the exciter.
10. A method according to claim 7 or claim 8, wherein the member is mounted on the acoustic radiator.
11. A method according to any one of claims 7 to 10, wherein the exciter system comprises two or more exciters and coupling means for coupling the exciters and the member is mounted between the or each exciter and the acoustic radiator.
12. A method according to any one of claims 7 to 10, wherein the exciter system comprises two or more exciters and coupling means for coupling the exciters and the member is

mounted between the acoustic radiator and the coupling means.
13. A method according to any one of claims 7 to 12, wherein the member is in the form of a generally compliant block and the mechanical stiffness, mass or damping of the block is selected to change the natural resonance frequency of the exciter system to a frequency which complements the acoustic radiator behaviour.
14. A method according to claim 13, wherein the block is selected such that peak excursion of the or each exciter is reduced.
15. A loudspeaker comprising an acoustic radiator and an exciter system mounted to the acoustic radiator for exciting the acoustic radiator to produce an acoustic output, the exciter system being adapted to have a natural resonance frequency which improves the frequency response of the acoustic output of the acoustic radiator.
16. A loudspeaker according to claim 15, wherein the natural resonance frequency of the exciter system is shifted to a frequency which is outside the frequency range of the loudspeaker.
17. A loudspeaker according to claim 15 or claim 16, wherein the acoustic radiator has a distribution of resonant modes and the natural resonance frequency of the exciter system is shifted to a frequency which does not interfere

with the resonant modes which are excited in the acoustic radiator.
18. A loudspeaker according to any one of claims 15 to 17, wherein the exciter system comprises two or more exciters and coupling means for coupling the exciters.
19. A loudspeaker according to any one of claims 15 to 18, whether the exciter system comprises at least one additional member having a moderate value of compliance and mechanical resistance to shift the natural resonance frequency of the exciter system.
20. A loudspeaker according to claim 19, wherein the member is made from soft, compliant foam.
21. A loudspeaker according to claim 19 or claim 20, wherein the member is mounted on the acoustic radiator.
22. A loudspeaker according to any one of claims 19 to 21, wherein the member is mounted between the or each exciter and the acoustic radiator.
23. A loudspeaker according to any one of claims 19 to 21, when dependent on claim 18, wherein the member is mounted between the acoustic radiator and the coupling means.
24. A loudspeaker according to any one of claims 19 to 22, wherein the member is mounted into the or each exciter.
25. A loudspeaker according to any one of claims 19 to 24, wherein the member is in the form of a generally compliant block and the mechanical stiffness, mass or damping of the block is selected to change the natural resonance frequency of the exciter system to a frequency which complements the acoustic radiator behaviour.