EP2719194A1

EP2719194A1 - An audio speaker arrangement

Info

Publication number: EP2719194A1
Application number: EP12729735.6A
Authority: EP
Inventors: Julien Laurent Bergere
Original assignee: Koninklijke Philips NV
Current assignee: Gibson Innovations Belgium NV
Priority date: 2011-06-09
Filing date: 2012-06-04
Publication date: 2014-04-16
Also published as: JP2014519293A; RU2013158172A; WO2012168849A1; CN103583053A; BR112013031157A2

Abstract

In a speaker arrangement, a first sound transducer (101) reproduces sound in a lower frequency range and has a first on-axis direction (109). A second sound transducer (103) reproduces sound in the lower frequency range and has a second on-axis direction (111). A third sound transducer (103) reproduces sound in a higher frequency range and has a third on-axis direction (113). The third sound transducer (105) is positioned between the first sound transducer (101) and the second sound transducer (103). Furthermore, a first angle between the first on-axis direction (109) and the second on-axis direction (111) is between 20° and 120°; and a second angle between the first on-axis direction (109) and the third on- axis direction (111) is less than the first angle. The arrangement may render sound with a wide sound stage and may provide a good approximation to a point source.

Description

An audio speaker arrangement

FIELD OF THE INVENTION

The invention relates to a loudspeaker arrangement and in particular, but not exclusively, to loudspeakers for approximating point source audio reproduction. BACKGROUND OF THE INVENTION

Audio reproduction from electrical signals is ubiquitous in today's society. A critical operation for generating the rendered sound is the conversion from electrical to acoustic signals performed by the sound transducer.

For many audio applications, the ideal sound radiator may be characterized as a dimensionless full bandwidth omni-directional pulsating sphere also referred to as a 'point source'. However, it is in practice impossible to provide such sound radiation characteristics and attempts to approach such an ideal sound generation has proved difficult and challenging as the requirements tend to be conflicting. For example, it is difficult for a very small speaker (i.e. approaching a dimensionless speaker) to move large amounts of air which is required for reproduction of bass frequencies at significant sound levels.

Traditional loudspeaker boxes typically contain two or more transducers that are aligned vertically and which partly share the reproduction of the same frequency range around the cross-over region. This tends to result in highly directional speakers which exhibit strong interference patterns in the vertical plane.

EP09756820 discloses a loudspeaker arrangement which seeks to address some of the disadvantages of conventional speakers and to specifically provide a point source audio rendering. The speaker arrangement of EP09756820 provides a very wide and regular dispersion of the sound, resulting in wide sound stage and wide sweet spot. However, the sound reproduction provided by the speaker arrangement is not perfect in all scenarios, and therefore further improvement would be desirable.

Hence, an improved speaker arrangement would be advantageous, and in particular a speaker arrangement allowing reduced speaker size, reduced cost, facilitated manufacturing, increased design flexibility, improved audio quality, facilitated deployment, increased point source approximation 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 speaker arrangement comprising: a first sound transducer for reproducing sound in a lower frequency range and having a first on-axis direction and a first acoustic center; a second sound transducer for reproducing sound in the lower frequency range and having a second on-axis direction and a second acoustic center; a third sound transducer for reproducing sound in a higher frequency range and having a third on-axis direction and a third acoustic center, wherein the third sound transducer is positioned between the first sound transducer and the second sound transducer; a first angle between the first on-axis direction and the second on- axis direction is between 20° and 120°; a second angle between the first on-axis direction and the third on-axis direction is less than the first angle.

The invention may provide an improved speaker arrangement. Specifically an improved audio quality may be achieved in many embodiments. The speaker arrangement may allow lower frequencies to reach a listener via both reflected (e.g. of a ceiling) and direct paths resulting in improved focus of the rendered sound and an increased perceived impact of the rendered sound. Furthermore, the sensitivity to the specific characteristics of the listening environment can be reduced compared to many speaker arrangements, such as e.g. speaker arrangements using lower frequency speakers in an upfiring configuration. At the same time, the speaker arrangement may in comparison to traditional loudspeaker arrangements provide a wide and regular dispersion of the sound, resulting in wide sound stage and wide sweet spot. A low degree of vertical interference may be achieved in many embodiments.

In many embodiments, a reduced size speaker arrangement may be achieved which for example may be suitable for bookshelf loudspeaker implementations. An improved sound quality may be achieved in many scenarios. In particular, increased point source characteristics may be achieved. An improved trade-off between directional and non- directional sound can be achieved resulting in a focused sound image being generated at the same time as a point source approximation is perceived.

The lower frequency range and the higher frequency range may be divided by a cross-over frequency with the lower frequency range comprising frequencies below the cross-over frequency and the higher frequency range comprising frequencies above the crossover frequency. The cross-over frequency may specifically be the frequency at which the first and second sound transducers produce the same sound pressure level as the third sound transducer with the sound pressure level being measured at a distance of 1 meter of the second sound transducer when measured in anechoic conditions.

The acoustic center for a sound transducer may specifically be a point from which the rendered audio is perceived to originate.

The first sound transducer may specifically be a high efficiency tweeter. The speaker arrangement may further comprise a drive circuit for providing a lower frequency drive signal to the first transducer and a higher frequency drive signal to the second transducer from an input audio signal.

The on-axis direction of a sound transducer may specifically be a symmetric radiation-axis. For example, a sound transducer may be rotationally invariant or symmetric around the on-axis direction. The on-axis direction may be the direction of highest sound output of the sound transducer. Thus, the on-axis direction may correspond to the direction in which the maximum sound energy is radiated. The on-axis direction may specifically be defined by an axis through a center of the sound transducer.

The first and second sound transducers may be arranged to receive the same drive signal. In particular, the first and second sound transducers may be coupled in a parallel configuration.

In many embodiments a compact loudspeaker arrangement may be achieved while still providing good low frequency reproduction. Indeed, in many embodiments, the sound transducers may be mounted in an enclosure having a volume of less than 40 liters.

In accordance with an optional feature of the invention, the second angle is no less than 25% of the first angle.

This may provide improved performance in many scenarios. In particular, it may provide sound radiation with advantageous interference, dispersion, and/or directionality characteristics leading to an improved sound stage being perceived.

In accordance with an optional feature of the invention, the second angle is no more than 75% of the second angle.

In many embodiments, the second angle is advantageously no less than 25% and no more than 75% of the first angle. In accordance with an optional feature of the invention, a cross-over frequency between the lower frequency range and the higher frequency range is no higher than twice the speed of sound divided by a distance between the first acoustic center and the second acoustic center.

Equivalently, in many embodiments a distance between the first acoustic center and the second acoustic center is no more than twice a wavelength at a cross-over frequency between the lower frequency range and the higher frequency range.

This may provide improved performance in many embodiments and may in particular allow the first and second sound transducers to substantially be perceived as a single sound source. An improved interference characteristic for the first and second sound transducers may be achieved. In particular, destructive interference between the radiated sound waves may be prevented or reduced in many scenarios.

In accordance with an optional feature of the invention, the speaker arrangement further comprises a drive circuit for driving the first, second and third sound transducers. The drive circuit may be arranged to provide a delay for the third sound transducer relative to the first sound transducer.

This may provide improved performance in many embodiments and may in particular provide a speaker arrangement capable of providing high quality sound reproduction and of providing an improved approximation to a point source sound radiation. In particular, the approach may provide an (approximately) time coherent sound radiation resulting in the speaker arrangement being perceived as a single point sound source.

In accordance with an optional feature of the invention, the delay is no more than twice a nominal delay corresponding to a length between the third acoustic center and a midpoint between the first acoustic center and the second acoustic center divided by a speed of sound.

This may provide improved performance in many embodiments and may in particular provide an improved time coherency and closer approximation to a point sound source.

In accordance with an optional feature of the invention, the delay is no less than 70% of the nominal delay and no more than 130% of the nominal delay.

This may provide improved performance in many embodiments and may in particular provide an improved time coherency and closer approximation to a point sound source. In accordance with an optional feature of the invention, the drive circuit is arranged to drive the first sound transducer and the second sound transducer substantially identically.

This may allow improved performance and may in particular provide a rendered sound which is perceived to be of high quality and which provides a wide and dispersed sound stage yet with high impact and without being perceived as diffuse. The approach may provide improved interference characteristics and a close approximation to a point sound source.

In accordance with an optional feature of the invention, speaker arrangement further comprises an enclosure, the enclosure comprising: a first baffle in which the first transducer is mounted; a second baffle in which the second transducer is mounted, the second baffle being angled relative to the first baffle; a third baffle in which the third transducer is mounted, wherein the third baffle provides a transition between the first baffle and the second baffle.

This may provide facilitated implementation and/or improved audio performance.

In accordance with an optional feature of the invention, a dimension of the third baffle orthogonally to the plane comprising the third on-axis direction and an axis between the first acoustic center and the second acoustic center is smaller than a dimension of the first baffle orthogonally to a plane comprising the first on-axis direction and the axis between the first acoustic center and the second acoustic center.

This may provide improved performance in many embodiments and may in particular reduce the effect of radiated rearwards of the third sound transducer being reflected and focused in a forward direction. It may thus attenuate undesired reflections.

In accordance with an optional feature of the invention, at least one of the first, the second and the third baffle has a curved profile.

In accordance with an optional feature of the invention, the third baffle and the third sound transducer are symmetric with respect to the first sound transducer and the second sound transducer.

This may provide facilitated implementation and/or improved audio performance. In accordance with an optional feature of the invention, a distance between the first acoustic center and the second acoustic center is less than three times a largest dimension of the first acoustic transducer.

This may provide improved performance in many embodiments, and may in particular provide a speaker arrangement providing a time coherent sound radiation and an improved approximation to a point source sound radiation. In particular, the feature may reduce interference between the transducers thereby providing an improved sound image perception.

In accordance with an optional feature of the invention, a smallest distance between the first and second sound transducers is less than three times a largest dimension of the third sound transducer.

This may provide improved performance in many embodiments and may in particular provide a speaker arrangement providing a time coherent sound radiation and an improved approximation to a point source sound radiation. In particular, the feature may reduce interference between the transducers thereby providing an improved sound image perception.

According to an aspect of the invention there is provided a method of providing a speaker arrangement, the method comprising: providing a first sound transducer for reproducing sound in a lower frequency range and having a first on-axis direction and a first acoustic center; providing a second sound transducer for reproducing sound in the lower frequency range and having a second on-axis direction and a second acoustic center; and providing a third sound transducer for reproducing sound in a higher frequency range and having a third on-axis direction and a third acoustic center; wherein the third sound transducer is positioned between the first sound transducer and the second sound transducer; a first angle between the first on-axis direction and the second on-axis direction is between 20° and 120°; a second angle between the first on-axis direction and the third on-axis direction is less than the first angle.

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 is an illustration of an example of a speaker arrangement in accordance with some embodiments of the invention;

Fig. 2 is an illustration of an example of a speaker arrangement in accordance with some embodiments of the invention;

Fig. 3 is an illustration of an example of a speaker arrangement in accordance with some embodiments of the invention;

Fig. 4 is an illustration of an example of a speaker arrangement in accordance with some embodiments of the invention; and

Fig. 5 is an illustration of an example of a speaker arrangement in accordance with some embodiments of the invention;

Fig. 6 is an illustration of an example of a speaker arrangement in accordance with some embodiments of the invention;

Fig. 7 is an illustration of an example of a speaker arrangement in accordance with some embodiments of the invention;

Fig. 8 is an illustration of a cross-section of an example of a speaker arrangement in accordance with some embodiments of the invention;

Fig. 9 is an illustration of a cross-section of an example of a speaker arrangement in accordance with some embodiments of the invention; and

Figs. 10-13 illustrate examples of measurements of directivity for different speaker arrangements.

DETAILED DESCRIPTION OF SOME EMBODIMENTS OF THE INVENTION

Fig. 1 and 2 illustrates an example of a speaker arrangement in accordance with some embodiments of the invention. Fig. 1 illustrates a cross-sectional view whereas Fig. 2 illustrates a frontal view.

The speaker arrangement comprises a first low frequency (sound) transducer 101 which in the specific example is a low frequency loudspeaker. The speaker arrangement further comprises a second low frequency (sound) transducer 103 which in the specific example is also a low frequency loudspeaker. In addition, the speaker arrangement comprises a high frequency (sound) transducer 105 which in the specific example is a high frequency and high efficiency tweeter. In the system, the sound transducers 101, 103, 105 are mounted in an enclosure 107.

In the example the first low-frequency transducer 101 and the second low- frequency transducer 103 are substantially identical transducers. In particular, they may be the same type of transducer and may only differ in terms of manufacturing tolerances etc. However, it will be appreciated that in other embodiments the first low-frequency transducer 101 and the second low-frequency transducer 103 may not be identical units.

The two low frequency sound transducers 101, 103 are arranged to render sound in the same frequency range and specifically may be driven by the same drive signal. The high frequency sound transducer 105 is arranged to render sound in a frequency range higher than the two low frequency sound transducers 101, 103. For example, the speaker arrangement may, as is illustrated in Fig. 3, comprise a cross-over filter 301 that filters an incoming signal such that lower frequencies are fed to the two low frequency sound transducers 101, 103 whereas higher frequencies are fed to the high frequency sound transducer 105. The cross-over filter 301 may be designed as a high pass filter in the path to the high frequency sound transducer 105 and a low pass filter in the path to the low frequency sound transducers 101, 103.

The three sound transducers 101, 103, 105 thus provide a two-way speaker arrangement with the low frequency transducers 101, 103 predominantly generating sound in a lower frequency range and the high frequency transducer 105 predominantly generating sound in a higher frequency range The speaker arrangement has a cross-over frequency which may be defined as the frequency at which the two transducers 101, 103 and the high frequency sound transducer 105 contribute equally to the generated sound. Specifically, the cross-over frequency may be defined as the frequency at which the low frequency transducers 101, 103 and the high frequency transducer 105 produce the same sound pressure level at a distance of 1 meter of the high frequency transducer 101 when measured in anechoic conditions.

It will be appreciated that the cross-over and two way driving may be implemented by the explicit introduction of a cross-over filter. However, the frequency selective driving by the low frequency sound transducers 101, 103 and the high frequency sound transducer 105 may also fully or partially be determined by the different frequency responses of the sound transducers 101, 103, 105 themselves. Indeed, in some embodiments, the drive signal may be fed directly to all sound transducers 101, 103, 105 in parallel and the cross-over frequency may be determined as the frequency at which the sensitivity of the transducers is identical such that the generated sound pressure level is the same.

Specifically, the cross-over frequency may be determined as the cross-over frequency for the signal path(s) from the input to a sound measuring position, such as 1 meter in front of the first low-frequency transducer 101. The frequency at which the low frequency sound transducers 101, 103 provide the same sound pressure level as the high frequency sound transducer 105 for a signal being provided at the input to the drive circuit of the loudspeaker arrangement may be determined as the cross-over frequency. In the lower frequency range which comprises frequencies below the cross-over frequency, the low frequency sound transducers 101, 103 will provide a stronger sound pressure level and will dominate the sound rendering. In the higher frequency range which comprises frequencies above the cross-over frequency, the low frequency sound transducers 101, 103 will provide a stronger sound pressure level and will dominate the sound rendering.

It will be appreciated that the cross-over frequency may in some cases be determined to include the characteristics of an active drive circuit for the transducers 101,

103, 105. Thus, the drive circuit may in some embodiments or scenarios be considered part of the speaker arrangement and the impact of the cross-over filter may be included when determining a cross-over frequency for the system.

Thus, in the specific example the speaker arrangement comprises or is driven via a filter 301 which receives an audio signal for reproduction and which generates individual drive signals for the two low frequency transducers 101, 103 and for the high frequency transducer 105. The cross-over filter 301 performs a low pass filtering of the input signal to generate the drive signal for the low frequency transducers 101, 103 and a high pass filtering of the input signal to generate the drive signal for the high frequency transducer 105.

It will be appreciated that although the following description will focus on embodiments wherein the speaker system is a two-way system, other embodiments may use a three-way or higher system. For example, the frequency range covered by the low frequency transducers 101, 103 of Fig. 1 may be covered by a plurality of speakers in other

embodiments, such as for example a midrange speaker and a sub woofer.

Each of the transducers has an on-axis direction. The on-axis direction of a sound transducer may specifically be a symmetric radiation-axis. For example, a sound transducer may be rotationally invariant or symmetric around the on-axis direction. The on- axis direction may be the direction of highest sound output of the sound transducer. Thus, the on-axis direction may correspond to the direction in which the maximum sound energy is radiated. The on-axis direction may specifically be defined by an axis through a center of the sound transducer.

In the speaker arrangement, the first low frequency transducer 101, the second low frequency transducer 103, the and the high frequency transducer 105 are arranged such that their on-axis directions are at an angle to each other. Furthermore, the high frequency sound transducer 105 is positioned between the first low frequency transducer 101 and the second low frequency transducer 103.

The transducers 101, 103, 105 are specifically arranged such that the angle c w between the on-axis direction 109 of the first low frequency transducer 105 and the on-axis direction 111 of the second low frequency transducer is between 20° and 120°. Furthermore, the angle φ_τ between the on-axis direction 109 of the first low frequency transducer 105 and the on-axis direction 113 of the high frequency sound transducer 105 is less than the angle between the on-axis directions of the two low frequency sound transducers 101, 103.

The transducers are thus arranged to radiate in different directions with the low frequency sound being directed in two directions which are on either side of the radiation from the high frequency sound transducer 105.

Furthermore, the three transducers are positioned very close to each other and may provide a close approximation to a point source.

In typical use, the on-axis direction 109 of the first low-frequency transducer 101 may correspond to a horizontal direction and typically with a direction approximately towards the user. Therefore, the first low-frequency transducer 101 provides a fairly direct sound radiation to the listener. The high frequency sound transducer 105 typically provides sound radiation which is angled partially upwards and thus not directly towards the user. However, the arrangement is typically such that the angle of the high frequency sound transducer 105 is relatively modest such that both reflected and direct sound radiation takes place. The second first low-frequency transducer 103 is arranged in more of an upfiring configuration and therefore tends to provide a less direct sound radiation but instead results in increased reflections from e.g. ceilings, walls etc.

In the system, the sound transducers are mounted in an enclosure 107 which in the example forms a closed chamber. The enclosure 107 may thus be a closed volume or may e.g. include a bass reflex port. Indeed, the enclosure may in some embodiments contain more than one chamber and may be designed to meet the specific preferences and requirements of the individual embodiment.

In the example of Fig. 1, the first low frequency transducer 103 is arranged in a front firing configuration. Thus, when the speaker arrangement is in an operational configuration, e.g. the enclosure 107 is placed on a substantially horizontal plane, such as a floor or shelf, the on-axis direction 109 of the first low frequency transducer 101 is at an angle of substantially 90° relative to vertical, i.e. it is substantially horizontal. The second low frequency transducer 103 is arranged in a full or partial upfiring configuration. Thus, when the speaker arrangement is in an operational

configuration, e.g. the enclosure 107 is placed on a substantially horizontal plane such as a floor or shelf, the on-axis direction of the low frequency transducer 101 is at an angle of 20° to 120° relative to horizontal.

Furthermore, the high frequency transducer 105 is arranged in a configuration which is in between the front firing of the first low-frequency transducer 101 and the upfiring of the second low-frequency transducer 103 . Thus, when the speaker arrangement is in an operational configuration, e.g. the enclosure 107 is placed on a substantially horizontal plane, such as a floor or shelf, the on-axis direction of the high frequency transducer 105 is at an angle of 20° to 120° relative to horizontal, but at a lower angle than the angle for the second low-frequency transducer 103.

Thus, the speaker arrangement is such that the lower frequency range is simultaneously radiated in an upwards and forwards direction, and typically reaches the listener via various reflections and indirect paths as well as via direct paths. The higher frequency range is from a single source radiated partially towards the listening position and partially upwards to provide reflected and more reverberant sound.

Thus, the arrangement provides simultaneous sound radiation of lower frequency sound to create both reflected and direct sound radiation, with the sound radiation of the higher frequency being in a direction in between these. The combined sound radiation interacts closely to provide a sound rendering which is perceived to be of high quality and which has advantageous characteristics.

In particular, the arrangement typically provides a wide sound stage with a wide sweet-spot. The arrangement may provide a wide and very regular distribution of sound with low inter-transducer interference, and in particular with low vertical interference. At the same time, the arrangement may have reduced sensitivity to variations in the acoustic environment. The speaker arrangement provides a particularly good approximation to a point sound source and may provide substantially time coherent sound radiation.

In the system particularly advantageous performance is found for the angle ( _w between the on-axis direction 109 of the first low frequency transducer 105 and the on-axis direction 111 of the second low frequency transducer being in the range from 20° to 100°, and in particular for being in the range from 60° to 90°. Also, particularly advantageous performance has been found when the high frequency sound transducer 105 is angled differently from both the first low-frequency transducer 101 and the second low-frequency transducer 103. In particular, advantageous performance is found for the angle φ_τ between the on-axis direction 109 of the first low frequency transducer 105 and the on-axis direction 113 of the high frequency sound transducer 105 being in the range from 25% to 75% of the angle cpw between the on-axis direction 109 of the first low frequency transducer 105 and the on-axis direction 111 of the second low frequency transducer, and in particular for being in the range from 40% to 60% of cpw-

These design parameters provide sound rendering which has a particularly advantageous trade-off between different characteristics and which provide a particularly advantageous audio perception.

In many embodiments, a particularly advantageous arrangement is provided by the high frequency sound transducer 105 being angled symmetrically with respect to the first low-frequency transducer 101 and the second low-frequency transducer 103. Thus, in many embodiments the angle φ_τ may be substantially half the angle (p_w.

In the example, the loudspeaker comprises three baffles in which the three transducers 101, 103, 105 are mounted. Each sound transducer is thus mounted in a support structure or surface (baffle). In the example, the enclosure is box-shaped except for the baffles for the second low-frequency transducer 103 and the high frequency sound transducer 105 being angled relative to the baffle for the first low-frequency transducer 101. The baffle for the high frequency sound transducer 105 is located between the baffle for the first low- frequency transducer 101 and the baffle for the second low-frequency transducer 103. Thus, the high frequency sound transducer baffle provides the transition between the baffles of the first low-frequency transducer 101 and the second low-frequency transducer 103. The baffle of the high frequency sound transducer 105 can indeed be considered to be the edge formed by the two baffles of the respective low frequency sound transducers 101, 103. For example, a rounded or smooth transition between the baffles of the low frequency sound transducers 101, 103 may be used as the baffle for the high frequency sound transducer 105. The positioning of the high frequency sound transducer 105 on the transition between the low frequency sound transducers 101, 103 is particularly favorable for low-directivity.

The high frequency sound transducer baffle is thus between the other baffles resulting in the high frequency sound transducer 105 being between the two low frequency sound transducers 101, 103. In the example, the three sound transducers 101, 103, 105 are aligned such that their center point, and specifically their acoustic centers fall substantially on a line when viewed from the a point on the on-axis direction of the first low-frequency transducer 101. Furthermore, the line is substantially vertical when the speaker arrangement is in its nominal operational position (i.e. when in use). However, it will be appreciated that in some embodiments, one or more of the transducers may be offset, e.g. sideways in the baffle. However, when in use, the vertical position of the acoustic center of the high frequency sound transducer 105 will be between the vertical position of the first low- frequency transducer 101 and the vertical position of the second low-frequency transducer 103.

In the arrangement, the distance between the transducers 101, 103, 105 is kept very close, and indeed in many embodiments the distance between the speakers is minimized as much as possible. Specifically, the distance between the acoustic centers of the sound transducers are kept as small as practicable. Indeed, in many embodiments the distance between the acoustic centers of the two low frequency sound transducers 101, 103 is less than three times a largest dimension of the first acoustic transducer 101 (and/or the second acoustic transducer 103). Also, the smallest distance between the first and second sound transducers 101, 103 is less than three times the largest dimension (typically the diameter) of the third sound transducer 105.

The speaker arrangement is thus constructed such that the acoustic centers of the sound transducers are maintained very close together. This may allow a very compact speaker unit to be provided, but more importantly it provides an improved interworking between the sound rendered by each sound transducer. In particular, it may allow the rendered sound to have a closer phase and time alignment.

The cross-over frequency for the speaker arrangement may be selected such that the individual transducers are used in the frequency ranges in which they perform best. Thus, the cross-over frequency may be selected to result in a good rendering of the audio band. In many embodiments, the cross-over frequency may advantageously be in the interval from 1.5 kHz to 3 kHz.

However, in addition, the cross-over frequency is also selected such that the rendering of the low frequency range from two sound transducers 101, 103 rather than from a single sound transducer does not introduce unacceptable and unintended degradations.

In particular, the cross-over frequency between the lower frequency range and the higher frequency range is designed to be no higher than twice the speed of sound divided by a distance between the first acoustic center and the second acoustic center. Equivalently, the distance between the first acoustic center and the second acoustic center may be limited to be no higher than twice a wavelength corresponding to the cross-over frequency.

Thus, the speaker arrangement is designed to meet the requirement: 2 - c

f jcc ≤ _D where f_c is the cross-over frequency, c is the speed of sound (e.g. measured at 20°) and D is the distance between the acoustic centers of the low frequency sound transducers 101, 103.

In many embodiments, the speaker arrangement advantageously meets the stricter requirement of:

Maintaining the cross-over frequency sufficiently low relative to the distance between the acoustic centers ensures that the interference between the radiated sound from the two low frequency sound transducers 101, 103 is maintained sufficiently low. In particular, it allows the sound radiation to be sufficiently coherent in all relevant directions and ensures that destructive interference between the two sound sources is kept sufficiently low.

In the example above, the speaker arrangement is a passive speaker arrangement comprising no amplification. Indeed, the driving of the sound transducers 101, 103, 105 is via a passive cross-over filter 301. However, it will be appreciated that in other embodiments, the speaker arrangement may be an active speaker arrangement comprising active drive circuits for the sound transducers 101, 103, 105. Such driving may for example be provided by a power amplifier prior to the cross-over filter 301 of Fig. 3. In other embodiments, each of the sound transducers may be provided with individual amplification. An example of this is illustrated in Fig. 4 wherein a power amplifier 401, 403, 405 is provided for each of the sound transducers 101, 103, 105 after the cross-over filter. An advantage of such a configuration is that that cross-over filter 301 operates at low power, and indeed that various signal processing can be performed at low power for each individual sound transducer 101, 103, 105.

In some embodiments, the signal for the high frequency sound transducer 105 may be delayed relative to the signals for the low frequency sound transducers 101, 103.

An example of such an embodiment is illustrated in Fig. 5 which corresponds to the example of Fig. 4 but with the addition of a delay in the signal path for the high frequency sound transducer 105. The drive signal for the high frequency sound transducer 105 is in the example delayed with respect to the drive signal for the low frequency sound transducers 101, 103. The delay 501 may be implemented in any suitable way and may for example be implemented as an analog or digital delay line.

In the example, the delay may compensate for differences in the distance from the acoustic center of the different sound transducers to the listener. For example, Fig. 6 illustrates the speaker arrangement of Fig. 1. Fig. 6 illustrates that each of the sound transducers 101, 103, 105 has an acoustic center 601, 603, 605. Since the distance between the two low frequency sound transducers 101, 103 is relatively short, the two low frequency sound transducers 101, 103 may appear to radiate (direct) sound as if it was from a single transducer with an acoustic center 607 midway between the acoustic centers 601, 603 of the two low frequency sound transducers 101, 103.

The acoustic center 605 of the high frequency sound transducer 105 will generally be further forward of the acoustic center 607 of the combined effect of the two low frequency sound transducers 101, 103. Therefore, the direct sound wave from the high frequency sound transducer 105 will be in advance of the direct sound wave from the two low frequency sound transducers 101, 103. Accordingly, the system may delay the signal to the high frequency sound transducer 105 to compensate for the time difference between the radiated sound waves. The delay may thus result in an improved time coherence of the radiated sound thereby providing an improved audio quality.

The actual delay between the sound waves may depend on the exact position of the listener and specifically on the angle to the listener (and thus on the height of the listener). However, a nominal delay may be determined to correspond to the length d between the acoustic center 605 of the high frequency sound transducer 105 and the midpoint 607 between the acoustic centers 601, 603 of the low frequency sound transducers 101, 103. The corresponding delay may be determined as the A=d/c where d is the distance between the acoustic centers and c is the speed of sound. The nominal delay specifically corresponds to the delay for direct waves for a listener positioned along the on-axis direction 113 of the high frequency sound transducer 105. Although, the delay may vary for other positions, the nominal delay will typically be a good approximation.

Accordingly, the delay for the high frequency sound transducer 105 is typically advantageously no more than twice the nominal delay Δ, and indeed typically advantageously is no less than 70% and no more than 130% of the nominal delay Δ.

This may result in a sound rendering which is perceived to be substantially time coherent. In the example of Fig. 1 and 2, the speaker arrangement is implemented in a relatively simple enclosure. However, in some embodiments a more complex shaping of the enclosure may be used to improve the quality of the rendered audio.

In some embodiments, the baffle of the high frequency sound transducer 105 may be narrowed with respect to the baffles of the low frequency sound transducers 101, 103. Specifically the dimension of the baffle measured orthogonally to the plane comprising the on-axis direction 113 of the high frequency sound transducer 105 and an axis 609 between the acoustic centers of the low frequency sound transducers 101, 103 may be smaller than a dimension of the low frequency sound transducers baffles measured in the same direction, i.e. also measured orthogonally to this plane. Fig. 7 illustrates an example of such an

embodiment. The example corresponds to the front view of Fig. 2 but with a narrowing of the baffle for the high frequency sound transducer 105. Typically the narrowing is at the front of the baffle with the baffle receding backwards. The profile of the baffle may be curved and specifically may curve away as illustrated in Fig. 8 which shows a top-cross sectional view at the height of the center point of the high frequency sound transducer 105.

In some embodiments, one or both of the low frequency sound transducer baffles may also have a curved profile. For example, as illustrated in Fig. 9, the cross section may have a profile corresponding to the baffle curving backwards from the low frequency.

The described speaker arrangements may provide advantageous sound rendering in many scenarios. In particular, the approach may provide a better focusing of sound towards the listening position.

For example, compared to the speaker arrangement of EP09756820, the described speaker arrangement may provide an improved focusing of sound resulting in an improved sound stage perception and reduced sensitivity to variations in the acoustic environment.

For example, Fig. 10 illustrates a measurement of a horizontal directivity of a single upfiring low frequency transducer (woofer) in the speaker arrangement of

EP09756820. The low pass transducer behaves close to an omni-directional sound source, with quite a substantial amount of sound energy being sent to the back of the speaker box (180degrees). This results in undesirable coloration after reflection off the room back wall.

Fig. 11 illustrates a measurement of a directivity for a double low frequency transducer arrangement as described previously. As can be seen, much less sound energy is sent towards the back of the speaker in the important frequency interval between 500Hz and 2kHz.This results in improved sound quality. Fig. 12 illustrates an example of a measured front (0 degrees) to back (180 degrees) vertical directivity of an upfiring low frequency transducer (woofer) in the speaker arrangement of EP09756820. The arrangement exhibits strong energy being sent to the ceiling (90 degrees) between 700Hz and 2kHz, with a maximum of energy in the on-axis direction of the low frequency transducer (60 degrees in this example), and with significant sound energy being sent to the rear of the speaker box.

Fig. 13 illustrates the corresponding front (Odegrees) to back (180 degrees) vertical directivity of a double low frequency transducer arrangement as described previously. As can be seen, much less sound energy is sent towards the ceiling and towards the rear of the speaker box. The maximum of energy is at 30 degrees, the median direction between the on-axis directions of the two low frequency transducers. In the arrangement, the two low frequency transducers combine so as to provide a directivity pattern that resembles a point source from listening angles of +/-90degrees horizontally, and +/-30 degrees vertically (for the case with a 60 degree angle between low frequency transducers).

The invention can be implemented in any suitable form. The elements and components of an embodiment of the invention may be physically, functionally and logically implemented in any suitable way. Indeed the functionality may be implemented in a single unit, in a plurality of units or as part of other functional units.

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 speaker arrangement comprising:

a first sound transducer (101) for reproducing sound in a lower frequency range and having a first on-axis direction (109) and a first acoustic center (601);

a second sound transducer (103) for reproducing sound in the lower frequency range and having a second on-axis direction (111) and a second acoustic center (603);

a third sound transducer (103) for reproducing sound in a higher frequency range and having a third on-axis direction (113) and a third acoustic center (607),

wherein

the third sound transducer (105) is positioned between the first sound transducer (101) and the second sound transducer (103);

a first angle between the first on-axis direction (109) and the second on-axis direction (111) is between 20° and 120°;

a second angle between the first on-axis direction (109) and the third on-axis direction (111) is less than the first angle.

2. The speaker arrangement of claim 1 wherein the second angle is no less than 25% of the first angle.

3. The speaker arrangement of claim 1 wherein the second angle is no more than 75% of the first angle.

4. The speaker arrangement of claim 1 wherein a cross-over frequency between the lower frequency range and the higher frequency range is no higher than twice the speed of sound divided by a distance between the first acoustic center and the second acoustic center.

5. The speaker arrangement of claim 1 further comprising a drive circuit for driving the first, second and third sound transducers (101, 103, 105) , and wherein the drive circuit is arranged to provide a delay for the third sound transducer (105) relative to the first sound transducer (101).

6. The speaker arrangement of claim 5 wherein the delay is no more than twice a nominal delay corresponding to a length between the third acoustic center (607) and a midpoint between the first acoustic center (601) and the second acoustic center (603) divided by a speed of sound.

7. The speaker arrangement of claim 6 wherein the delay is no less than 70% of the nominal delay and no more than 130% of the nominal delay.

8. The speaker arrangement of claim 5 wherein the drive circuit is arranged to drive the first sound transducer (101) and the second sound transducer (103) substantially identically.

9. The speaker arrangement of claim 1 further comprising an enclosure, the enclosure comprising:

a first baffle in which the first sound transducer (101) is mounted;

a second baffle in which the second sound transducer (103) is mounted, the second baffle being angled relative to the first baffle;

a third baffle in which the third sound transducer (205) is mounted, wherein the third baffle provides a transition between the first baffle and the second baffle.

10. The speaker arrangement of claim 9 wherein a dimension of the third baffle orthogonal to the third on-axis direction (113) and an axis between the first acoustic center

(601) and the second acoustic center (603) is smaller than a dimension of the first baffle orthogonal to the first on-axis direction (109) and the axis between the first acoustic center (601) and the second acoustic center (603).

11. The speaker arrangement of claim 9 wherein at least one of the first baffle, the second baffle and the third baffle has a curved profile.

12. The speaker arrangement of claim 9 wherein the third baffle and the third sound transducer (105) are symmetric with respect to the first sound transducer (101) and the second sound transducer (103).

13. The speaker arrangement of claim 1 wherein a distance between the first acoustic center (601) and the second acoustic center (603) is less than three times a largest dimension of the first acoustic transducer (101).

14. The speaker arrangement of claim 1 wherein a smallest distance between the first sound transducer (101) and the second sound transducers (103) is less than three times a largest dimension of the third sound transducer (105).

15. A method of providing a speaker arrangement, the method comprising:

providing a first sound transducer (101) for reproducing sound in a lower frequency range and having a first on-axis direction (109);

providing a second sound transducer (103) for reproducing sound in the lower frequency range and having a second on-axis direction (111);

providing a third sound transducer (103) for reproducing sound in a higher frequency range and having a third on-axis direction (113),

wherein