WO2010015976A2

WO2010015976A2 - A loudspeaker arrangement

Info

Publication number: WO2010015976A2
Application number: PCT/IB2009/053337
Authority: WO
Inventors: Okke Ouweltjes; Gerben Kooijman; Joris A. M. Nieuwendijk; Ronaldus M. Aarts
Original assignee: Koninklijke Philips Electronics N.V.
Priority date: 2008-08-06
Filing date: 2009-07-31
Publication date: 2010-02-11
Also published as: WO2010015976A3

Abstract

A loudspeaker arrangement comprises an acoustic enclosure (101) which is divided into at least two subchambers (103, 105). An audio radiator (109) is mounted in a first subchamber (103) of the at least two subchambers (103, 105) such that it radiates outwardly of the acoustic enclosure (101). A port (107) is included for pneumatically connecting the first subchamber (103) and the second subchamber (105). The system is designed such that a Helmholtz frequency of the first subchamber (103) and the port (107) is at least five times higher than a resonance frequency of the audio radiator (109). The loudspeaker arrangement may provide an improved trade-off between sound pressure levels, audio quality and size and may in particular provide improved low frequency performance without introducing disadvantages typically associated with conventional ported speaker and passive radiator designs.

Description

A loudspeaker arrangement

FIELD OF THE INVENTION

The invention relates to a loudspeaker arrangement and in particular, but not exclusively, to loudspeakers for providing high audio quality in a wide audio bandwidth.

BACKGROUND OF THE INVENTION

In the field of loudspeaker design there is a desire for louder sound levels and higher audio quality from smaller physical loudspeaker dimensions. In particular, there is a desire for louder and better bass from smaller cabinets. However, in general these requirements are mutually exclusive and speakers tend to be designed to provide desirable trade-offs between conflicting preferences. For example, loudness is related to the amount of air that a loudspeaker displaces and in order to displace large amounts of air at low frequencies, large sound transducers (with large effective areas) are typically required.

In order to improve the trade-offs between the conflicting preferences, a number of different speaker designs have been developed. The simplest speaker design tends to be a closed speaker but unfortunately this design also tends to result in a restricted bass output. A more efficient way to reproduce low frequencies is to use a bass reflex design wherein a bass reflex port acts as a lumped mass and the volume of the cabinet acts as an additional spring. Such a bass reflex design may provide an additional low frequency resonance which can be used to augment the bass response of the driver and which may extend the frequency response of the driver/enclosure combination to below the range the driver could reproduce in a sealed box.

The Helmholtz frequency fth of the bass reflex system is determined by the port length L_ph, the cabinet volume Vb, and the port area A_ph, according to:

In practice, the values for Vb, and fth are chosen to give the desired frequency response. As indicated by the previous equation, it is necessary for small cabinets to make the port longer or the port area smaller in order to keep the same low, resonance frequency. However in practice, there are disadvantages associated with a long or narrow port. In particular, a long port tends to exhibit standing waves in the midrange frequency band which causes coloration in the sound of the loudspeaker system. Also a small port area is responsible for an increase of the air velocity in the port which eventually leads to unwanted turbulent noise.

These two physical restrictions have lead to a speaker design wherein a passive radiator replaces the bass reflex port. The passive radiator is typically a loudspeaker without a driver element/motor, i.e. it is simply a membrane with a mass M_mp suspended with a spring constant C_mp. The passive radiator thus acts in the same way as the bass reflex port namely as a lumped mass coupled with a spring effect. Typically, the spring constant of the passive radiator itself is set sufficiently low to have negligible effect and it can accordingly be ignored for the tuning of the resonance frequency of the passive radiator system.

The moving mass of the passive radiator is determined by:

where M_mp is the total moving mass of the passive radiator, M_ms is the moving mass of the loudspeaker driver, S_p is the passive radiator membrane area, Sd is the cone area of the driver, α is the system compliance ratio (WJVb where V_as is the equivalent volume of the driver and Vb is the volume of the cabinet), h the system tuning ratio (ft,/f_s where ft is the resonance frequency of the cabinet and f_s is the resonance frequency of the driver) and δ is the passive radiator compliance ratio (V_ap/Vb where V_ap is the equivalent volume of the passive radiator and Vb is the volume of the cabinet). The values for h, α and δ are then chosen to provide the desired frequency response.

In practice, the system may be tuned by adding mass to the cone of the passive radiator in order to get the desired performance and in particular to reduce the resonance frequency sufficiently. As indicated by the previous equation, the moving mass of the passive radiator depends on both the enclosure volume (via a) and its cone area. Accordingly, a small enclosure requires a high moving mass. For example, a small 2 liter cabinet, containing a 4" subwoofer with a 6" passive radiator would need approximately 8Og of moving mass for the radiator. However, this is highly undesirable in practice as it tends to lead to mechanical (vibrational) problems. For example, the mechanical vibrations may become audible or the radiator's cone suspension may even collapse under the heavy mass. Although, the moving mass can be reduced by decreasing the radiator's membrane area, this will prevent to radiator from reproducing high sound pressure volumes.

Another problem with many speaker designs is that when a high signal level is applied, the excursion of the sound transducers (loudspeaker element's) cone, coil or magnet can exceed the normal working range resulting in damage to the cone suspension and/or coil/magnet assembly. This risk is especially significant at low frequency sound reproduction where the cone excursion is relatively large. This problem is often solved by electronically limiting the signal level applied to the driver element. However, this approach is not always suitable and indeed loudspeakers may often be used with electronic circuitry and amplifiers that do not comprise functionality for limiting the cone excursion of the speaker elements being driven.

Hence, an improved speaker arrangement would be advantageous and in particular an arrangement allowing increased flexibility, improved audio quality, reduced size, increased sound levels, improved mechanical operation, increased cone excursion protection/limitation and/or improved performance would be advantageous.

SUMMARY OF THE INVENTION

Accordingly, the Invention seeks to preferably mitigate, alleviate or eliminate one or more of the above mentioned disadvantages singly or in any combination.

According to an aspect of the invention there is provided a loudspeaker arrangement comprising: an acoustic enclosure divided into at least two subchambers; an audio radiator mounted in a first subchamber of the at least two subchambers, the audio radiator being mounted to radiate outwardly of the acoustic enclosure; a port for pneumatically connecting the first subchamber and the second subchamber; wherein a Helmholtz frequency of the first subchamber and the port is at least five times higher than a resonance frequency of the audio radiator.

The invention may provide an improved loudspeaker arrangement.

Specifically, an improved trade-off between audio quality, sound reproduction levels and size can be achieved in many embodiments. Alternatively or additionally, the invention may provide improved speaker protection and may specifically reduce or limit cone excursions in many embodiments. Specifically, by providing a loudspeaker arrangement design wherein the Helmholtz frequency is maintained at least five times higher than the resonance frequency of the audio radiator, improved performance can be achieved. In particular, the combination of the first subchamber and the port may be arranged to provide an effective mass of the radiator and port arrangement which exceeds the effective mass of the audio radiator itself (e.g. when measured in a large chamber) and the effective mass of air in the port. Thus, a high effective mass may be provided thereby resulting in improved low frequency performance. Furthermore, this may be achieved without requiring a high mass of the audio radiator itself thereby mitigating mechanical problems. Furthermore, in comparison to a bass reflex system, the port may be shorter and/or have an increased cross sectional area thereby reducing the problems associated with standing waves. Furthermore, the air flow through the port is substantially reduced thereby reducing degradations associated with air turbulence.

In many embodiments, an improved low frequency sound quality may be achieved from a smaller loudspeaker enclosure.

The audio radiator is mounted to radiate outwardly of the acoustic enclosure. Specifically, the audio radiator can radiate directly into the acoustic environment external to the loudspeaker arrangement enclosure. The audio radiator may specifically be mounted in a hole in an external wall of the speaker enclosure. Thus, the audio radiator does not (mainly) radiate into another subchamber or internal enclosure of the loudspeaker arrangement. Specifically, the audio radiator may be mounted to radiate freely into a room in which the loudspeaker arrangement is located. The main or maximum radiation direction of the audio radiator is specifically in a direction out of the enclosure.

The port may have any suitable dimensions and may for example in some embodiments be short and narrow (e.g. it may even be a small hole in a barrier wall between the subchambers. Also, the shape and/or dimensions may be varied along the port. For example, a long port may be folded or the port may for example have a conical shape with an increasing (or decreasing) cross sectional area. The port may have any cross section and may for example have rounded edges in order to reduce turbulences.

The effective mass of the audio radiator may in the speaker arrangement correspond to a combination of a mass of the audio radiator and a mass of an air component in the port.

In accordance with an optional feature of the invention, a resonance frequency of the second chamber is below 500 Hz. This may allow improved performance and may in particular allow improved low frequency performance for the speaker arrangement.

In accordance with an optional feature of the invention, the audio radiator is an electro acoustic transducer driver.

The invention may allow an improved protection of the electro acoustic transducer driver. Specifically, the first subchamber may be designed to be small relative to the second subchamber (e.g. ten times smaller) thereby resulting in the air compressibility of the system being predominantly performed in the second subchamber. Thus, the air flow/ pressure transmission through the port is significant for the operation of the system and at high volume levels the air flow resistance increases substantially (e.g. due to non-linear turbulence effects). This increasing air flow resistance will provide an improved resistance to the movement of the cone of the electro acoustic transducer driver thereby reducing the excursion of this.

In accordance with an optional feature of the invention, the second subchamber further comprises a bass reflex port.

This may provide improved performance and may in particular provide improved low frequency audio quality.

In accordance with an optional feature of the invention, the audio radiator is a passive radiator.

The invention may provide an improved loudspeaker arrangement.

Specifically, an improved trade-off between audio quality, sound reproduction levels and size can be achieved in many embodiments. In particular, a specific passive radiator construction may provide improved low frequency performance (audio quality and/or sound pressure level) for a given loudspeaker enclosure size.

In accordance with an optional feature of the invention, the loudspeaker arrangement further comprises an electro acoustic transducer driver mounted in the second subchamber. The arrangement of an electro acoustic transducer driver and a passive radiator in two subchambers coupled by a port, provides a highly effective and high performance speaker design wherein many disadvantages associated with e.g. bass reflex ports or passive radiator designs can be mitigated or eliminated.

The electro acoustic transducer driver may specifically be mounted to radiate outwardly of the acoustic enclosure. Specifically, the electro acoustic transducer driver can radiate directly into the acoustic environment external to the loudspeaker arrangement enclosure. The electro acoustic transducer driver may specifically be mounted in a hole in an external wall of the speaker enclosure. Thus, the electro acoustic transducer driver may not radiate into another subchamber or internal enclosure of the loudspeaker arrangement. Specifically, the electro acoustic transducer driver may be mounted to radiate freely into a room in which the loudspeaker arrangement is located. The main or maximum radiation direction of the audio radiator is specifically in a direction out of the enclosure.

In accordance with an optional feature of the invention, the audio radiator and the electro acoustic transducer driver are mounted such that an on-axis direction of the audio radiator is substantially parallel to an on-axis direction of the electro acoustic transducer driver.

This may provide improved performance and/or an improved implementation.

In accordance with an optional feature of the invention, the loudspeaker arrangement comprises no other audio radiators than the audio radiator and the electro acoustic transducer driver.

This may provide improved performance and/or an improved implementation. In particular, it may reduce cost while ensuring high quality.

In accordance with an optional feature of the invention, a volume of the second subchamber is at least five times a volume of the first subchamber.

This may provide improved performance and in particular may provide an effective interaction between the audio impact of the second chamber and the combination of the first chamber, the audio radiator and the port.

In accordance with an optional feature of the invention, the port is a tube having a length at least twice a square root of a cross section area of the tube.

This may provide improved performance and may in particular allow the acoustic interaction of the audio radiator, the first chamber and the port to provide an effective low frequency extension of the speaker arrangement. In particular, it may in many scenarios allow the tube to provide an additional effective acoustic mass complementing the acoustic mass of the audio radiator.

The tube may be the only pneumatic coupling between the first and second subchamber. The tube may have any suitable cross-section such as e.g. a circular or oval cross section.

In accordance with an optional feature of the invention, the cross section area is less than a tenth of a cone area of the audio radiator.

This may provide improved performance and may in particular allow a preferential acoustic interaction of the audio radiator, the first chamber and the port. In accordance with an optional feature of the invention, the cross section area is less than 10 cm³.

This may provide improved performance and may in particular allow a preferential acoustic interaction of the audio radiator, the first chamber and the port.

In accordance with an optional feature of the invention, the Helmholtz frequency is below 500 Hz.

This may provide improved performance and may in particular provide an improved low frequency response for the speaker arrangement.

In accordance with an optional feature of the invention, a volume of the first subchamber is less than 2000 cm³.

This may provide improved performance and may in particular allow a preferential acoustic interaction of the audio radiator, the first subchamber and the port. In particular, the first subchamber may be a coupling chamber providing an air elasticity which is predominantly determined by the characteristics of the port.

In some embodiments, the volume of the first subchamber may be less than five times a volume taken up by the audio radiator within the first subchamber.

In accordance with an optional feature of the invention, the first subchamber and the port is arranged to reduce a cone excursion of the audio radiator at sound levels above a threshold.

This may provide improved performance and may in particular provide improved protection for the audio radiator.

These and other aspects, features and advantages of the invention will be apparent from and elucidated with reference to the embodiment(s) described hereinafter.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will be described, by way of example only, with reference to the drawings, in which

Fig. 1 illustrates an example of a speaker arrangement in accordance with some embodiments of the invention;

Fig. 2 illustrates an example of a speaker impedance as a function of frequency for different speaker designs;

Fig. 3 illustrates an example of cone excursion as a function of an applied signal level for different speaker designs; Fig. 4 illustrates an example of a speaker arrangement in accordance with some embodiments of the invention;

Fig. 5 illustrates an example of a sound pressure level as a function of frequency for different speaker designs;

Fig. 6 illustrates an example of a sound pressure level as a function of frequency for different speaker designs;

Fig. 7 illustrates an example of a cone excursion as a function of an applied signal level for different speaker designs; and

Fig. 8 illustrates an example of a speaker arrangement in accordance with some embodiments of the invention.

DETAILED DESCRIPTION OF THE EMBODIMENTS

Fig. 1 illustrates an example of a speaker arrangement in accordance with some embodiments of the invention.

The loudspeaker of Fig. 1 comprises an acoustic enclosure 101 which forms the external enclosure of the loudspeaker. The acoustic enclosure 101 is divided into a first subchamber 103 and a second subchamber 105 by internal barrier walls. The barrier walls do not allow air movement and pressure transmission between the subchambers 103, 105 through the barrier walls and thus provides an acoustic decoupling of these subchambers 103, 105.

The loudspeaker of Fig. 1 furthermore comprises a port 107 which provides a pneumatic connection between the first subchamber 103 and the second subchamber 105. Thus, in the example, the only air flow and between the first subchamber 103 and the second subchamber 105 is via the port 107. Furthermore, the barrier walls are in the specific example sufficiently stiff to ensure that the air pressure transmission between the subchambers 103, 105 is substantially restricted to that which takes part via the port (i.e. the impact of any movement or vibration of the barrier walls can be ignored).

The loudspeaker comprises two audio radiators 109, 111 which are mounted in the two subchambers 103, 105. Thus, one audio radiator 109 is located in the first subchamber 103 and the other audio radiator 111 is located in the second subchamber 105. Accordingly, apart from the coupling provided by the port 107, the two audio radiators 109, 111 are within the enclosure 101 substantially acoustically isolated from each other. Thus, within the enclosure 101, the acoustic interworking between the two audio radiators 109, 111 is substantially determined by the port 109. The two audio radiators 109, 111 are mounted on an external wall of the enclosure 101. Specifically, they are mounted such that the main radiation direction of the audio radiators 109, 111 is directly out into the acoustic environment in which the loudspeaker is located. Thus, the main radiation direction of the audio radiators 109, 111 is outwardly from the enclosure 101.

In the specific example, the first audio radiator 109 is a passive audio radiator. Thus, the first audio radiator 109 does not actively add acoustic energy to the system but rather moves as a consequence of the pneumatic conditions and the mechanical characteristics of the first audio radiator 109. The first audio radiator 109 may specifically be a loudspeaker without a driver element/motor, i.e. it is simply a membrane with a mass M_mp suspended with a spring constant C_mp. Thus, in the specific example, the first audio radiator 109 does not comprise any magnet or coil (or at least no electrical energy is provided to any coil).

Furthermore, in the specific example, the second audio radiator 111 is an electro acoustic transducer driver. Thus, the second audio radiator 111 is arranged to introduce acoustic energy to the system from an electrical signal being provided to the second audio radiator 111. Specifically, the second audio radiator 111 can be a conventional speaker element.

It will be appreciated that the first audio radiator 109 and the second audio radiator 111 may have similar or identical characteristics (e.g. size, membrane area etc). Specifically, the first audio radiator 109 and the second audio radiator 111 may be identical speaker elements with only the second audio radiator 111 being fed an electrical signal. In other embodiments, the first audio radiator 109 and the second audio radiator 111 may have substantially different characteristics. For example, in many embodiments, it may be an advantage to have a passive radiator (the first audio radiator 109) that has a much larger cone area than the active radiator (the second audio radiator 111) because the corresponding excursion is reduced given a certain sound pressure.

The design of the loudspeaker of Fig. 1 is furthermore such that a Helmholtz frequency of the first subchamber 103 and the port 107 is at least five times higher than a resonance frequency of the first audio radiator 109.

As will be well known to the person skilled in the art, the Helmholtz frequency is the frequency of a Helmholtz resonance which represents the phenomenon of air resonance in a cavity. Thus, the Helmholtz frequency corresponds to the Helmholtz resonance of the cavity formed by the port 107 and the first subchamber 103 (which is closed by the first audio radiator 109).

In the design, the Helmholtz frequency of the first subchamber 103 and the port 107 is thus set to be much higher than the resonance frequency of the first audio radiator 109. Accordingly, the first subchamber 103 and the port 107 are designed to provide a substantial modification of the effective acoustic mass of the first audio radiator 109. Specifically, it may ensure that the first audio radiator 109 operates in an acoustic environment wherein the air pressure of the first subchamber 103 provides an increased spring resistance to the movement of the membrane of the first audio radiator 109. This is specifically achieved by creating a more stiff air environment for the first audio radiator 109 to operate in. This can be achieved by ensuring that the volume of the first subchamber 103 and the cross sectional area of the port 107 are sufficiently small and that the length of the port 107 is sufficiently high.

Specifically, the coupling volume behind the first audio radiator 109 provided by the first subchamber 103 can be considered small, or in other words stiff, when the Helmholtz frequency of the port and the coupling volume is high in comparison to the original, i.e. the resonance frequency f_s of the first audio radiator 109.

The specific volume of the first subchamber 103 may depend on the specific requirements of the specific embodiment and may in particular depend on the size of the first audio radiator 109. In particular, in many embodiments a volume of the first subchamber 103 below around 2000 cm³ may provide a volume sufficiently large to allow easy mounting of sufficiently large radiators while ensuring that the volume is sufficiently low to provide sufficient stiffness. However, it will be appreciated that the approach may be used for any size arrangement and that the volume of the first subchamber 103 may be reduced to very small values for some embodiments (e.g. to a volume of, say, 0.1 liter).

The loudspeaker of Fig. 1 thus combines a port with the use of a passive radiator 109. The passive radiator 109 is coupled via a small volume to a port 107 which specifically is a small tube that leads to the larger cabinet volume (the second subchamber 105). Since the coupling volume provided by the first subchamber 103 is kept small and thus stiff, the air-mass inside the port 107 adds to the moving mass of the radiator 109. Additionally, the area ratio between the radiator membrane/cone and the port effectively acts as an acoustical lever. In this way, the effective air-mass of the port 107 is enlarged by the lever mechanism. The total moving mass of the passive radiator 109 consists of the sum of the its original moving mass and the amount of mass added in order to get the desired tuning frequency in a certain loudspeaker system:

^Mmp = ^Mm_Por_g + ΔMmp

where M _mporg is the mass of the first audio radiator 109 in air with normal atmospheric pressure and ΔM_mp is the effective mass provided by the port 107.

Thus, the effective mass of the first audio radiator 109 corresponds to the combination of the mass of the first audio radiator 109 itself and a mass of the air component in the tube.

The dimensions of the port can specifically be determined by:

A

L = AM_^ ps_P

where the L_p is the port length, A_p the port cross section area, p is the density of the gas in the enclosure (in this case air p= 1.21 kg/m³) and S_p is the cone area of the passive radiator.

The loudspeaker of Fig. 1 may eliminate or mitigate disadvantages typically associated with both the reflex port and passive radiator speaker design approaches for small cabinets. Specifically, the system does not have a high moving mass causing excessive vibration. Also, both higher order port resonances and unwanted turbulent noise of the port 107 are effectively attenuated by the passive radiator. Furthermore, the resonance provided by the port and the passive radiator 109 allows a high sound level and quality at lower frequencies for smaller enclosures.

In the example of Fig. 1, the first audio radiator 109 and the second audio radiator 111 are substantially parallel. Specifically, the on-axis direction of the passive radiator (the first audio radiator 109) is substantially parallel to the on-axis direction of the electro acoustic transducer driver (the second audio radiator 111). The two on-axis directions may specifically be parallel within an accuracy of, say 5-10°. This may in many embodiments provide a suitable speaker design with improved quality as both radiators may radiate directly towards a desired listening position. For example, in many scenarios, it may allow fewer constraints on the placement of the loudspeaker. Furthermore, in the example of Fig. 1, the loudspeaker comprises no other audio radiators than the passive audio radiator (the first audio radiator 109) and the electro acoustic transducer driver (the second audio radiator 111). This may provide a low cost loudspeaker with advantageous performance and in particular with advantageous low frequency performance. However, it will be appreciated that in other embodiments, other passive or active audio radiators may be included. It will also be appreciated that in some embodiments, the speaker enclosure may comprise additional subchambers. For example, the loudspeaker may in addition comprise a high frequency speaker element, such as a high frequency tweeter, located in a separate (and possibly acoustically isolated) subchamber.

It will be appreciated that the specific dimensions and characteristics may be different in different embodiments and implementations.

However, in many embodiments, the resonance frequency of the second subchamber 105 is below 500 Hz. This may specifically provide efficient performance and may allow the operating conditions for the second audio radiator 111 to be determined by the characteristics of the second subchamber 105 and to be substantially independent of the port, the first subchamber 103 and the first audio radiator 109. It may furthermore provide efficient low frequency operation which may be further enhanced by the resonance provided by the port 107, first audio radiator 109 and first subchamber 103 arrangement.

Also, in many embodiments, the Helmholtz frequency may preferably be below 500 Hz thereby providing an improved low frequency operation.

The approach may in particular provide improved low frequency performance for small speaker cabinets/ enclosures. Specifically, a high quality bass response can typically be provided for very small cabinets.

It will also be appreciated that the volume of the second subchamber 105 will typically be selected to be substantially larger than the volume of the first subchamber 103. In most embodiments, the volume of the second subchamber 105 is designed to be at least five times the volume of the first subchamber 103. This may allow an efficient low frequency response to be provided by the second audio radiator 111 operating in the second subchamber 105 while allowing the first subchamber 103 to provide sufficient stiffness to further enhance this low frequency response by the presence of the additional Helmholtz frequency.

The port 107 may specifically be provided by a tube which has a cross section that is much lower than the length of the tube. It will be appreciated that the tube may have any suitable cross section shape including for example a circular, rectangular, square or oval cross section shape. Specifically, the tube may have a length which is at least twice the square root of the cross section area of the tube. Typically, the length may be two or more times longer than the maximum cross section width of the tube. This may provide a suitable operation of the tube and may in particular ensure that the tube provides an efficient mass of air that can resonate with the desired properties.

Furthermore, the cross section area of the tube is maintained relatively low in order to provide the appropriate acoustic resonance effect that enhances the operation of the first audio radiator 109. Specifically, the cross section area of the tube may be less than a tenth of the cone (membrane) area of the audio radiator. Specifically, for most embodiments, an advantageous acoustic operation of the tube is achieved for a cross section area of less than 10 cm².

It will be appreciated that although the example of Fig. 1 includes a port formed by a single tube, the port may in other embodiments be formed by a plurality of pneumatic couplings between the two subchambers 103, 105. For example, the two subchambers 103, 105 may be connected via two (long and narrow) tubes.

In order to illustrate the improved performance of a loudspeaker in accordance with Fig. 1, two loudspeakers have been tested. For both of these, a 4" driver has been used in an enclosure with a 2 liter volume. A comparison speaker has been designed with a heavy passive radiator (M_mp = 80 g) whereas a test speaker uses an approach as described with reference to Fig. 1. Specifically, the test speaker includes a port between the two subchambers 103, 105 which has a circular cross section with a diameter of 14mm and with a length of 60mm. Fig. 2 illustrates the measured impedances of these loudspeakers with curve 201 providing the impedance of the comparison speaker and curve 203 providing the impedance of the test speaker. As can be seen, both speakers have a resonance frequency of 52 Hz and provide very similar performance. However, it was found that when using these loudspeakers for music reproduction, the heavy passive radiator of the comparison speaker exhibits excessive vibration even at modest sound pressure levels whereas the test speaker only exhibits minor vibration at even very high sound pressure levels.

The Helmholtz frequency fh can be calculated by:

where V_c is the coupling volume of the first subchamber 103, L_p is the port length, A_p is the port cross section area and c is the speed of sound. Typical values may for example be:

F_pr=30 Hz (resonance frequency of the first audio radiator 109) V_c=70 cm³ L_p=6 cm

resulting in a Helmholtz frequency of Fh=320 Hz and thus a modification of the resonance frequency of F_ph/F_pr ~ 10.

Another specific example may e.g. use the following parameters: F_pr=30 Hz (resonance frequency of the first audio radiator 109) V_c=220 cm³ L_p=6 cm

resulting in a Helmholtz frequency of Fh=I 85 Hz and thus a modification of the resonance frequency of F_ph/F_pr ~ 6.

The described construction may not only be used to improve the low frequency operation of the loudspeaker but may alternatively or additionally be used to limit the excursion of the cone of the first audio radiator 109. Specifically, the design may exploit that at high sound levels the nonlinear flow resistance R_nι in the port 107 becomes significant due to the high acoustic flow velocity. This may reduce air flow and thus result in a higher spring elasticity being provided by the air in the first subchamber 103. Accordingly, the excursions of the cone of the first audio radiator 109 may be attenuated at these higher sound levels.

As an example, Fig. 3 illustrates an example of the first audio radiator's 109 (the passive radiator's) cone excursion at the resonance frequency of f = 100 Hz as a function of the applied signal voltage for a loudspeaker having a port 107 in the form of a tube with a length L of 6 cm and a circular cross section with a diameter of d=16 mm as well as for a tube with a length L of 1.5 cm and a diameter of d=8mm. As can be seen, the excursions of the loudspeakers are identical for low and moderate signal levels. However, at high signal levels there is a clearly reduced excursion of the passive radiator's cone for the configuration with the smallest tube.

It will be appreciated that this approach is not necessarily applied to a passive radiator but may also be applied to an electro acoustic transducer driver. An example of such a system is illustrated in Fig. 4.

The loudspeaker of Fig. 4 specifically comprises an acoustic enclosure 401 which forms the external enclosure of the loudspeaker. The acoustic enclosure 401 is divided into a first subchamber 403 and a second subchamber 405 by internal barrier walls. The barrier walls prevent air movement and pressure transmission between the subchambers 403, 405 and thus provide an acoustic decoupling of the subchambers 403, 405.

The loudspeaker of Fig. 4 furthermore comprises a port 407 which provides a pneumatic connection between the first subchamber 403 and the second subchamber 405. The loudspeaker furthermore comprises a single audio radiator 409 which is mounted in the first subchamber 403. The audio radiator 409 is specifically mounted on an external wall of the enclosure 401 and is mounted such that the main radiation direction of the audio radiator 409 is directly out into the acoustic environment in which the loudspeaker is located. Thus, the main radiation direction of the audio radiators 409 is outwardly from the enclosure 401.

In the loudspeaker of Fig. 4, the volume Vi of the first subchamber 403 is small compared to the volume V₂ of the second subchamber 405 such that the compressibility of the air in the enclosure 401 is mainly in the volume V₂ of the second subchamber 405. Furthermore, the design of the loudspeaker of Fig. 1 is such that the Helmholtz frequency of the first subchamber 403 and the port 407 is at least five times higher than the resonance frequency of the first audio radiator 409.

In the example, the first audio radiator 409 is an electro acoustic transducer driver which is fed the electrical signal that is to be rendered as a sound signal. Thus, in this example, the first audio radiator 409 provides the acoustic energy to the system. Furthermore, the construction with the first subchamber 403 and the port 407 allows the effective mass of the first audio radiator 409 to be increased. Furthermore, the low frequency performance of the system may be controlled by the larger volume of the second subchamber 405.

However, the system may furthermore be designed such that nonlinear flow effects in the port 407 limit the cone excursion of the first audio radiator 409 at high signal levels. Specifically, at high signal levels, the nonlinear flow resistance R_ni in the port 407 becomes significant, due to the high acoustic flow velocity as given by: p^ I v I

K₁ =- IS

where, p is the mass density of the medium (air), v is the acoustic velocity in the port 407, S is the cross sectional area of the port 407, and K is a constant known as the minor loss parameter (this depends on e.g. the exact geometry of the tube, and usually is close to one).

This effect is in the example of Fig. 4 used to reduce the cone excursion of the first audio radiator 409 at high signal levels when compared to a conventional loudspeaker cabinet design wherein the driver is mounted in a relatively large volume. For example, for Vi=O.05 liter, V₂=I liter, and a length and diameter of the port of L=I mm and d=7 mm respectively, Fig. 5 illustrates the sound pressure level (SPL) as a function of frequency for a 1 mV applied signal voltage compared to the SPL for a conventional closed box loudspeaker design with a volume equal to Vi+V₂. Fig. 6 illustrates the corresponding SPLs for an applied signal of 30 V. Fig. 7 illustrates the cone excursion of the first audio radiator as a function of the applied signal voltage at 100 Hz (i.e. the resonance frequency of the system). It can be seen that at low input voltages, the frequency responses of both speaker designs are virtually the same (except for the dip at about 1 kHz, which is above the intended working range). At high input voltages, the SPL around the resonance frequency is much lower for the loudspeaker cabinet with the port than for a conventional closed box loudspeaker. The associated reduced loudspeaker cone excursion can be observed in Fig. 7 which demonstrates a reduction up to a factor of around three in the specific example.

It will be appreciated that, the amount of cone excursion reduction can be modified by modifying the specific characteristics and physical dimensions of the configuration including the length and cross section area of the port 407.

It will also be appreciated that the operation of the loudspeaker of Fig. 4 is equivalent to that of the loudspeaker of Fig. 1 and that the comments and considerations provided with respect to the loudspeaker of Fig. 1 can also (as appropriate) be applied to the loudspeaker of Fig. 4.

In some such embodiments, the second subchamber 405 may comprise a bass reflex port. For example, Fig. 8 illustrates an example of the loudspeaker of Fig. 4 further enhanced by comprising a bass reflex port 801. This may in many embodiments provide an improved low frequency response of the loudspeaker.

In this example, the internal port 407 prevents too high acoustic velocities in the bass reflex port 801 which otherwise may result in unwanted turbulent noise. Furthermore, even if turbulent noise is generated in the internal port 407, the bass reflex port 801 may act as an acoustic filter preventing this noise from being transmitted to outside the cabinet.

It will be appreciated that in other embodiments, the second subchamber 405 may comprise a passive radiator rather than a bass reflex port.

Although the present invention has been described in connection with some embodiments, it is not intended to be limited to the specific form set forth herein. Rather, the scope of the present invention is limited only by the accompanying claims. Additionally, although a feature may appear to be described in connection with particular embodiments, one skilled in the art would recognize that various features of the described embodiments may be combined in accordance with the invention. In the claims, the term comprising does not exclude the presence of other elements or steps.

Furthermore, although individually listed, a plurality of means, elements or method steps may be implemented by e.g. a single unit or processor. Additionally, although individual features may be included in different claims, these may possibly be advantageously combined, and the inclusion in different claims does not imply that a combination of features is not feasible and/or advantageous. Also the inclusion of a feature in one category of claims does not imply a limitation to this category but rather indicates that the feature is equally applicable to other claim categories as appropriate. Furthermore, the order of features in the claims do not imply any specific order in which the features must be worked and in particular the order of individual steps in a method claim does not imply that the steps must be performed in this order. Rather, the steps may be performed in any suitable order. In addition, singular references do not exclude a plurality. Thus references to "a", "an", "first", "second" etc do not preclude a plurality. Reference signs in the claims are provided merely as a clarifying example shall not be construed as limiting the scope of the claims in any way.

Claims

CLAIMS:

1. A loudspeaker arrangement comprising: an acoustic enclosure (101) divided into at least two subchambers (103, 105); an audio radiator (109) mounted in a first subchamber (103) of the at least two subchambers (103, 105), the audio radiator (109) being mounted to radiate outwardly of the acoustic enclosure (101); a port (107) for pneumatically connecting the first subchamber (103) and the second subchamber (105); wherein a Helmholtz frequency of the first subchamber (103) and the port (107) is at least five times higher than a resonance frequency of the audio radiator (109).

2. The loudspeaker arrangement of Claim 1 wherein a resonance frequency of the second chamber (105) is below 500 Hz.

3. The loudspeaker arrangement of Claim 1 wherein the audio radiator (109) is an electro acoustic transducer driver.

4. The loudspeaker arrangement of Claim 3 wherein the second subchamber (105) further comprises a bass reflex port (801).

5. The loudspeaker arrangement of Claim 1 wherein the audio radiator (109) is a passive radiator.

6. The loudspeaker arrangement of Claim 5 further comprising an electro acoustic transducer driver (111) mounted in the second subchamber (105).

7. The loudspeaker arrangement of Claim 6 wherein the audio radiator (109) and the electro acoustic transducer driver (111) are mounted such that an on-axis direction of the audio radiator (109) is substantially parallel to an on-axis direction of the electro acoustic transducer driver (111).

8. The loudspeaker arrangement of Claim 6 wherein the loudspeaker arrangement is a loudspeaker comprising no other audio radiators than the audio radiator (109) and the electro acoustic transducer driver (111).

9. The loudspeaker arrangement of Claim 1 wherein a volume of the second subchamber (105) is at least five times higher than a volume of the first subchamber (103).

10. The loudspeaker arrangement of Claim 1 wherein the port (107) is a tube having a length at least twice a square root of a cross section area of the tube.

11. The loudspeaker arrangement of Claim 10 wherein the cross section area is less than a tenth of a cone area of the audio radiator (109).

12. The loudspeaker arrangement of Claim 10 wherein the cross section area is less than 10 cm².

13. The loudspeaker arrangement of Claim 1 wherein the Helmholtz frequency is below 500 Hz.

14. The loudspeaker arrangement of Claim 1 wherein a volume of the first subchamber (103) is less than 2000 cm³.

15. The loudspeaker arrangement of Claim 1 wherein the first subchamber (103) and the port (107) is arranged to reduce a cone excursion of the audio radiator (109) at sound levels above a threshold.