WO2007109828A1

WO2007109828A1 - Loudspeaker system with acoustic equalisation

Info

Publication number: WO2007109828A1
Application number: PCT/AU2007/000185
Authority: WO
Inventors: Zeljko Velican; Graeme John Huon
Original assignee: Immersion Technology Property Limited
Priority date: 2006-03-28
Filing date: 2007-02-21
Publication date: 2007-10-04
Also published as: WO2007109832A1; AU2007231525A1; AU2007231529A1

Abstract

A loudspeaker system is disclosed including a vented enclosure and at least one electroacoustic transducer mounted in the enclosure. The loudspeaker system includes at least one resonance controlling structure associated with the vented enclosure for controlling and/or cancelling a resonance peak in the enclosure to provide a flatter frequency response for the system and/or to reduce distortion in said system. The resonance controlling structure includes an open end and a closed end and is arranged such that sound waves entering the open end are reflected and arrive out of phase at the open end. The resonance controlling structure may include at least one resonance controlling duct or waveguide placed with its open end near an end of a venting duct associated with the vented enclosure.

Description

LOUDSPEAKER SYSTEM WITH ACOUSTIC EQUALISATION

Field of invention

The present invention relates to the field of loudspeakers and in particular to a resonance controlling structure for controlling and/or cancelling a resonance peak in a loudspeaker system which uses a vented enclosure.

Background art

A primary goal in the design of a loudspeaker system is to establish a maximum acoustic output across a selected band of frequencies known as a pass band with faithful reproduction and the best efficiency that the designer can achieve in the smallest enclosure. Typically the loudspeaker system includes several frequency bands integrated by a crossover network to form the pass band. An important aspect of faithful reproduction is to maintain a constant output level across the pass band for a given input level. This is referred to as flatness of frequency response. A desirable consequence of a flat frequency response is linearity of the associated phase response.

There are many approaches in the prior art in an attempt to achieve flatness of response along with other aspects of faithful reproduction as well as high acoustic output, high efficiency and a small physical size.

A common problem with prior art designs is that resonance peaks and dips, usually caused by unwanted resonances, occur in the frequency response. Prior art solutions include use of damping materials, baffles and a range of other additions to the system. Disadvantages of prior art solutions include loss of efficiency, increased enclosure size, increased cost and unwanted secondary effects in other parts of the pass band of the loudspeaker system.

Fig. 1 shows a prior art loudspeaker system 10 that may benefit from correction of an unwanted resonant peak (refer Fig. 2). The loudspeaker system 10 in Fig. 1 includes an electroacoustic transducer 1 1 mounted in vented enclosure 12. Enclosure 12 includes a venting duct 13 for venting the enclosure. The front of the cone of transducer 1 1 radiates sound directly to the listening environment. The venting duct 13 is configured to conduct sound from the rear of the cone of transducer 1 1 to the listening environment.

The frequency response of loudspeaker system 10 is extended to the lowest frequencies of the pass band by utilising sound radiated from venting duct 13, such sound providing an extension to the response centred on a desired extension frequency near the low end of the pass band. The frequency extension is achieved by tuning the combination of the venting duct 13 and enclosure 12.

The venting duct used in tuning a loudspeaker system is frequently a cause of an unwanted resonance peak or peaks in the response of the system because it typically exhibits a pipe resonance and overtone peaks that fall within the pass band of the system. In addition resonance peaks may occur as a result of reflections between parallel surfaces, reflections caused by impedance mismatches, panel resonances and the like. Sometimes in a multi- band loudspeaker system where the pass band is made up of say a subwoofer, mid-band and tweeter, a resonance created by the subwoofer may create unwanted resonance peaks in the mid-band section of the pass-band and so on. These may be of sufficient magnitude to be a problem. Often a resonance peak is of substantial magnitude and spread over a narrow range of frequencies. Such a resonant peak is difficult to control with damping materials because damping materials affect all frequencies and do not provide loss limited to a relatively narrow band of frequencies.

The present invention may provide a structure for controlling and/or cancelling the effects of an unwanted narrow band resonance peak or peaks in a loudspeaker system. This may have the effect of not only providing a flatter response but also reducing distortion by removing harmonics. The resonance controlling structure may be arranged such that it may not require additional space and may operate without lowering efficiency of the loudspeaker system.

Summary of the invention

According to the present invention there is provided a loudspeaker system including: a vented enclosure; at least one electroacoustic transducer mounted in said enclosure; and at least one resonance controlling structure associated with said vented enclosure for controlling and/or cancelling a resonance peak in said enclosure to provide a flatter frequency response for said system and/or to reduce distortion in said system, wherein said resonance controlling structure includes an open end and a closed end and is arranged such that sound waves entering said open end are reflected and arrive out of phase at said open end.

The resonance peak may include at least one resonance peak 16 (refer Fig. 2) within the pass band of the loudspeaker system. In one form the resonance controlling structure may be adapted to generate at least one resonance notch 17 (refer Fig. 2) that may substantially cancel the at least one resonance peak 16.

The resonance controlling structure may include at least one resonance controlling duct or waveguide. The or each resonance controlling duct or waveguide may have a defined length and cross sectional area. The defined length may be substantially one quarter wavelength associated with the frequency of the resonance peak. The vented enclosure may be vented by a venting duct. The open end of the or each resonance controlling duct or waveguide may be placed near one end of the venting duct. The or each resonance controlling duct or waveguide may be of uniform or non-uniform cross-section. The or each resonance controlling duct or waveguide may be mounted wholly or at least partly inside the vented enclosure or wholly or at least partly outside the vented enclosure.

The dimensions of the or each resonance controlling structure may be determined by a process of modelling the loudspeaker system including the or each resonance controlling structure. Such a modelling process is known, for example, from US Patent 6,223,853 (Huon et al) assigned to the present applicant. In the modelling process waveguides may be represented as distributed elements. The modelling process may include simulating the system by means of an equivalent electrical circuit in which acoustical and mechanical elements in the system including the ducts or waveguides are represented in the circuit as equivalent electrical components.

The model may be optimized to achieve flatness of response by adopting initial values for the equivalent electrical components, analysing the circuit to produce a simulated response and examining for flatness of response. If the simulated response is not substantially flat, the values of said equivalent electrical components may be modified and the optimising step may be repeated with the modified values of the equivalent electrical components replacing the initial values.

If the simulated response is substantially flat, the modified values of said equivalent electrical circuit may be converted into acoustical and mechanical elements and the acoustical and mechanical elements may be incorporated into the system. Alternatively or additionally the dimensions and form of the resonance controlling structure may be determined by trial and error. A trial and error method may include the following general design rules.

The length (in meters) of the resonance controlling structure may be set to be approximately one quarter of the speed of sound in air (in meters/second) multiplied by the reciprocal of the frequency to be cancelled and/or controlled (in Hertz), which is a quarter wavelength of the frequency to be cancelled and/or controlled.

The cross sectional area of the resonance controlling structure may be set in proportion to the size of the resonance peak. The cross sectional area of the resonance controlling structure may be varied along its length to assist in broad-banding the resonance controlling structure.

One or more impedance discontinuities may be provided along the length of the resonance controlling structure to assist in broad-banding the resonance controlling structure.

The resonance controlling structure may be filled with a damping material to assist in broad-banding the resonance controlling structure.

Description of Preferred Embodiments Preferred embodiments of the present invention will now be described with reference to the accompanying drawings wherein :-

Fig. 1 shows a cross sectional view of a prior art loudspeaker system;

Fig. 2 shows a graphical representation of a representative frequency response associated with the loudspeaker system of Fig. 1 ; Fig. 3 shows a first embodiment of a resonance controlling structure according to the present invention;

Fig. 4 shows a second embodiment of a resonance controlling structure according to the present invention; Fig. 5 shows a third embodiment of a resonance controlling structure according to the present invention;

Fig. 6 shows a fourth embodiment of a resonance controlling structure according to the present invention;

Fig. 7 shows a fifth embodiment of a resonance controlling structure according to the present invention;

Fig. 8 shows a sixth embodiment of a resonance controlling structure according to the present invention;

Fig. 9 shows a seventh embodiment of a resonance controlling structure according to the present invention; Fig. 10 shows an eighth embodiment of a resonance controlling structure according to the present invention;

Fig. 11 shows a ninth embodiment of a resonance controlling structure according to the present invention; and

Fig. 12 shows a tenth embodiment of a resonance controlling structure according to the present invention; and

Fig. 13 shows an eleventh embodiment of a resonance controlling structure according to the present invention.

Referring to Fig. 3, a first embodiment of a resonance controlling structure according to the present invention includes a duct or waveguide 20 of a given length and uniform cross-sectional area. The resonance controlling structure includes an open end 21 and a closed end 22. The resonance controlling structure is mounted inside vented enclosure 12 and is positioned with its open end 21 at or near the entrance to interior end 15 of venting duct 13 of vented enclosure 12. Referring to Fig. 4, a second embodiment of a resonance controlling structure according to the present invention includes two resonance controlling ducts or waveguides 30, 31. Each resonance controlling duct 30, 31 is positioned with a respective open end 32, 33 at or near interior end 15 of venting duct 13 of vented enclosure 12. Resonance controlling ducts 31 , 32 combine cancelling effects and may be used to cancel multiple resonance peaks having different frequencies or a single broad resonance peak.

Fig. 5 shows an alternative embodiment comprising two resonance controlling ducts or waveguides 40, 41 positioned with respective open ends 42, 43 at or near interior end 15 of venting duct 13 of vented enclosure 12.

Figs. 6-10 show further embodiments of resonance controlling structures 50, 60, 70, 80 and 90 that are similar to the embodiment in Fig. 2 except that the resonance controlling structures comprise ducts or waveguides of variable cross section or other non-uniform ducts or waveguides. Ducts or waveguides of non-uniform cross-section may be used to tailor the response of the ducts or waveguides to more effectively cancel and/or control a resonance peak.

Fig. 1 1 shows an embodiment of a resonance controlling structure comprising resonance controlling duct 100, wherein duct 100 is located wholly outside of vented enclosure 12. Duct 100 is positioned such that its open end

101 is at or near the entrance to interior end 15 of venting duct 13 of vented chamber 12.

Fig. 12 shows an embodiment of a resonance controlling structure comprising resonance controlling duct 1 10, wherein the open end 1 1 1 of resonance controlling duct 1 10 is placed near the exit end 14 of venting duct 13 of vented enclosure 12.

Multiple resonance controlling vent structures, structures including non-uniform cross-section and other features disclosed herein may be applied to the exit end 14 of venting duct 13. In some embodiments resonance controlling structures may be applied to both ends 14, 15 of venting duct 13.

Fig. 13 shows an embodiment of a resonance controlling structure comprising resonance controlling duct 120, wherein the open end 121 of resonance controlling duct 120 joins into a modified version of venting duct 13 (designated 13A) of vented enclosure 12 forming a junction with it. Finally it is to be understood that various alterations, modifications and/or additions may be introduced into the constructions and arrangements of parts previously described without departing from the spirit or ambit of the invention.

Claims

1 . A loudspeaker system including: a vented enclosure; at least one electroacoustic transducer mounted in said enclosure; and at least one resonance controlling structure associated with said vented enclosure for controlling and/or cancelling a resonance peak in said enclosure to provide a flatter frequency response for said system and/or to reduce distortion in said system, wherein said resonance controlling structure includes an open end and a closed end and is arranged such that sound waves entering said open end are reflected and arrive out of phase at said open end.

2. A loudspeaker system according to claim 1 wherein said enclosure is vented by a venting duct.

3. A loudspeaker system according to claim 1 or 2 wherein said resonance controlling structure includes at least one resonance controlling duct or waveguide.

4. A loudspeaker system according to claim 3 wherein the or each resonance controlling duct or waveguide has a defined length and cross- sectional area.

5. A loudspeaker system according to claim 4 wherein said defined length is substantially one quarter wavelength associated with said resonance peak.

6. A loudspeaker system according to claim 4 or 5 wherein said cross sectional area is substantially uniform over said length.

7. A loudspeaker system according to claim 4 or 5 wherein said cross sectional area is not uniform over said length.

8. A loudspeaker system according to any one of the preceding claims as appended to claim 2 wherein the open end of said resonance controlling structure is acoustically coupled to said venting duct.

9. A loudspeaker system according to claim 8 wherein the open end of said resonance controlling structure is acoustically coupled to one end of said venting duct that is substantially inside said vented enclosure.

10. A loudspeaker system according to claim 8 wherein the open end of said resonance controlling structure is acoustically coupled to one end of said venting duct that is substantially outside said vented enclosure.

1 1. A loudspeaker system according to any of the preceding claims wherein said resonance controlling structure includes damping material.

12. A loudspeaker system according to any of the preceding claims wherein said resonance controlling structure includes an impedance discontinuity.

13. A loudspeaker system substantially as herein described with reference to Figs. 3 to 13 of the accompanying drawings.