SOUND

TECHNICAL FIELD

The present invention relates generally to the field of sound reproduction devices, more specifically to headphones and still more specifically to headphones configured to be used both as traditional headphones, where the left and right headphones are placed on the respective outer ear of the user and as sound reproduction portable devices that can be placed on the shoulder portion of the user, such that the outer ears of the user are not occluded by the respective left and right telephone housings and associated earcubs.

BACKGROUND OF THE INVENTION

Acoustic reproduction devices configured somewhat similar to traditional headphones but being able to provide sound energy directly to the outer ear of a user via an earcub as well as from the headphone through air to the unoccluded outer ear of the user is known in the art.

Thus, prior art document EP 3035699 A1 discloses an acoustic device comprising two loudspeakers, one of which emits sound energy in a direction towards one ear of the user and the other of which emits sound energy towards the other ear of the user. The two loudspeakers are provided in respective slider blocks that are configured to be slidably attached to the respective end portions of a headband that connects the loudspeaker housings. This document does not provide any information about which frequency band the two loudspeakers are designed to emit.

It is a problem with this prior art device that when it is placed on the shoulder portion of the user and hence not with the sound outlet from the transducers coupled directly to the outer ears of the user, sound will not only be transmitted to the outer ears of the user but also into the surroundings, which may not be desired by the user for several reasons. For instance, the user may fear that using the device may annoy people in the surroundings and he may also feel a lack of privacy as he may not want other people to be aware of what he is listening to.

There is consequently a need for a device of the kind known from the prior art that overcomes or at least reduces these problems. DISCLOSURE OF THE INVENTION

The present invention relates to a hybrid acoustic reproduction device comprising at least two operational modes:

Mode A: Headphones mode in which the device functions as a traditional headphone that provides sound energy directly to the respective ears of the user via an earcub that may surround the pinna of the user’s outer ear completely.

Mode B: Neckphones mode in which the device is placed on the shoulder portion of the user and hence provides acoustic energy to the outer ears of the user via a certain distance and at the same reduces the undesirable radiation of sound into the user's surroundings. The provision of sound to the respective outer ears of the user is the desired effect of the invention, whereas the provision of sound to the user’s surroundings is undesirable as mentioned above. It is a prime object of the invention to avoid or reduce this undesired side effect of driving the device in the neckphone mode and to avoid undesired sound radiation to the user’s surroundings.

In a preferred embodiment of the invention, the device is capable of automatically switching between the two modes when the device detects that the geometrical configuration of the device changes as a consequence of the position of the device being changed from the normal headphone position (mode A) to the neckphone position (mode B) or vice versa.

In Mode A the device is configured similar to a regular headphone that is worn on the head of the user and isolates the ears of the user by means of the left and right earcups. When the device is hung on the neck of the user and the earcups are rotated at an angle a such that the earcups are radiating sound outwardly from the chest of the user, the device in a preferred embodiment automatically starts working in the Mode B (Neckphone).

As each of these two functional modes are acoustically very different, the electro-acoustic requirements of the device relating to high-quality sound reproduction in both modes cannot be achieved by a traditional headphone.

Requirement for Mode A (headphone): The device should preferably be provided with a full bandwidth sound emitting transducer. As the sound emitting transducer of the device in this mode is acoustically sealed from the environment by the earcups it can be driven with quite limited electrical power and as the sound emitting transducer is substantially aligned with the ear canal, a good frequency response is obtainable also at high frequencies.

Requirement for Mode B (neckphone): The device will require a sound emitting transducer with a high power rating capable of creating a similar loudness level in the ears of the user as in Mode A.

Additionally, the axis of the sound emitting transducer will not be aligned with the ears of the user, with the consequence that the directional audio radiated by the sound emitting transducer at high frequencies will be greatly attenuated at the entrance to the user’s ear canal. The radiation of high frequencies not solely in the direction of the user’s ear canal but in other directions in space further causes some serious drawback when the device is used in the openacoustics mode (Mode B). The problem is that the acoustic waves are radiated into the open space rather than into a closed and sealed cavity (Mode A). Due to this fact, simply using the sound emitting transducer that is used in mode A as the only transducer in mode B would lead to at least three different problems:

Problem 1: Privacy: It is not possible for the user to prevent others from knowing what the user is hearing or to listen to the user’s conversations while having voice calls in public.

Problem 2: Power consumption: In mode B, the sound emitting transducer must provide a higher acoustic output to provide the same sound pressure level at the entrance to the user’s ear canal than in mode A, as the entrance to the ear canal is further away from the sound emitting transducer than in the mode A and the axis of the sound emitting transducer is misaligned with the entrance to the ear canal. This means that directional audio waves (frequencies over 3kHz) will suffer a huge attenuation in the path to our ears and a lot of acoustic power will be wasted.

Problem 3: Acoustic contamination: All of the sound that is not focused towards our ears will be radiated into our environment. That will be annoying for the people in the surroundings especially when the device is being used in urban public spaces.

The above problems are solved by the claimed invention according to which there is provided a sound reproduction device with extended acoustic bandwidth that in one mode of the device operates as a traditional headphone and in another mode of the device operates as a neckphone, in both states with a high sound quality.

According to the invention, there is provided an acoustic reproduction device comprising left and right sound radiating units connected to each other by a headband, where the left and right sound radiating units where each is provided with a first sound emitting transducer (9) and one or more second sound emitting transducers, where the first sound emitting transducer is configured to emit broadband sound signals and where the one or more second sound emitting transduces are configured to emit sound signals in a mid and high frequency band wherein the left and right sound radiating units each comprises a speaker housing and a corresponding earcup attached to the speaker housing, where the speaker housing is defined by a longitudinal axis, and where the sound radiating unit has an outer circumferential surface portion in which the sound outlet(s) of the one or more second transducers is/are provided and where the speaker housing has an end face that faces the earcup, in which end face the sound outlet of the first sound emitting transducer is provided.

In an embodiment of the invention, the one or more second sound emitting transducers is/are provided in an outer circumferential surface portion of the speaker housing.

In an embodiment of the invention the one or more second sound emitting transducers is/are provided in and outer circumferential surface portion of the earcub.

In an embodiment of the invention the device is configured to comprise at least two operational modes (A, B), such that in the first mode (A) sound is only emitted by the first sound emitting transducer and in the second mode (B) sound is emitted by both the first and the one or more second sound emitting transducer.

In an embodiment of the invention the sound outlet(s) of the one or more second sound emitting transducers are located in the outer circumferential wall of the tubular body such that when the device is in the second mode (B), the longitudinal axes (RL, RR) of the respective loudspeaker housings extend substantially perpendicularly to the plane of the headband.

In an embodiment of the invention the one or more second sound emitting transducers is/are configured to have a directional sound radiation pattern with one main lobe of sound radiation extending about a main radiation axis R, which main radiation axis R in the second mode (B) of the device is pointing substantially in the direction towards the respective left and right ear of the user when the device is placed on the shoulder portion of the user. In an embodiment of the invention the mid and high frequency range is the frequency range above approximately 1500 Hz. It is however noted that the lower limiting frequency of the mid 30 and high frequency band depends on the configuration and dimensions of the second sound emitting transducers) as well as of the configuration and exact positioning of the sound outlets on the circumferential wall of the respective loudspeaker housings.

In an embodiment of the invention the device is provided with a microcontroller and detection means, where the detection means are configured to sense the orientation of the respective loudspeaker housings relative to the headband and based on this orientation to provide a control signal to the microcontroller in the device, where the microcontroller is configured to shift between the two operational modes (A, B) based on the orientation of the loudspeaker housings.

In an embodiment of the invention the device is provided with a microcontroller and detection means, where the detection means are configured to sense the orientation of the respective loudspeaker housings relative to each other and based on this orientation to provide a control signal to the microcontroller in the device, where the microcontroller is configured to shift between the two operational modes (A, B) based on the orientation of the loudspeaker housings.

In an embodiment of the invention the detection means is a rotation sensor configured to sense the rotation of the respective loudspeaker housing relative to the headband.

In an embodiment of the invention, the device comprises:

- a signal processing block configured to receive a left and a right input signal, such as the left and right channel signals of a stereophonic signal, and to provide processed or unprocessed versions of the left and right input signal as left and right output signals from the signal processing block;

- a microcontroller unit (MCU) configured to provide mode control signals indicating the mode (A, B) of the device, where the mode (A, B) is sensed by a mode sensor or indicator; - left and right first sound emitting transducers placed respectively in the left and right loudspeaker housings and having a broadband frequency response, such as a frequency response covering substantially the entire human audible frequency range;

- left and right second sound emitting transducers placed respectively in the left and right loudspeaker housings and configured to emit sound in a mid and high frequency region, such as the frequencies above approximately 1500Hz;

- a high-pass filter configured to receive the output signals from the signal processing block and providing high-pass filtered output signal versions of the output signals from the signal processing block;

- a low-pass filter configured to receive the output signals from the signal processing block via a switch and providing lowpass filtered output signal versions of the output signals from the signal processing block;

- a controllable switch configured to receive the output signals from the signal processing block and under control of a control signal from the MCU to provide the output signals from the signal processing block to the respective left and right first sound emitting transducers without filtration or to provide the output signals from the signal processing block to the respective left and right first sound emitting transducers filtered through the lowpass filter.

In an embodiment of the invention:

- the high-pass filtered output signal versions are provided to the respective second left and right sound emitting transducers via a first power amplifier;

- the low-pass filtered output signals are provided to the respective first left and right sound emitting transducers via a third power amplifier;

- the broadband signals are provided to the respective first left and right sound emitting transducers via a second power amplifier.

In an embodiment of the invention the device shifts between the two modes (A, B) in the following manner

- in mode A, the signal paths to the left and right second sound emitting transducers and the signal paths to the left and right first sound emitting transducer through the lowpass filter and the third power amplifier are interrupted by control signals from the MCU, while the signal path to the left and right first sound emitting transducers through the second power amplifier is established; and

- in mode B, the signal paths to the left and right second sound emitting transducers and the signal paths to the left and right first sound emitting transducer through the lowpass filter and the third power amplifier are established by control signals from the MCU, while the signal path to the left and right first sound emitting transducers through the second power amplifier is interrupted.

In an embodiment of the invention, the interruption of the signal paths takes place by turning off the power amplifier in the respective signal paths.

In an embodiment of the intervention, the interruption of the signal paths takes place by interrupting the input signals to the respective amplifiers (26, 30, 31) or filters (25, 29).

Interrupting the respective signal paths by turning off the amplifier in this signal path, could be a preferable solution as the tumed-off amplifiers do not consume any power and hence reduces the overall power consumption of the device. Alteratively, but less preferably, the respective amplifiers could as mentioned above be left turned on, and hence consume power, but either the input signals or the output signals of the amplifiers - or corresponding filters - could be interrupted. The acoustic performance would be the same in these two alternatives, but alternative one would be preferable if a reduction of power consumption is desired.

In the present context a mode sensor is any means configured to automatically detect whether the device is in mode A or in mode B. An example is a rotation sensor configured to sense the rotation of the respective loudspeaker housing relative to the headband. Another example would be a rotation sensor configured to sense the rotation of the left and right loudspeaker housings relative to each other. Contrary to this, a mode indicator is in the present context any means that enables the user manually to shift between the two modes of the device. An example would simply be a press-button provided on the device.

The hybrid acoustic sound reproduction device can additionally be used occasionally as a Bluetooth portable speaker. BRIEF DESCRIPTION OF THE DRAWINGS

Further benefits and advantages of the present invention will become apparent after reading the detailed description of non-limiting exemplary embodiments of the invention in conjunction with the accompanying drawings, wherein figure 1 shows an embodiment of the device according to the invention in the two operational modes: (a) in the headphone mode, i.e. placed in the traditional manner on the outer ears of the user and (b) in the neckphone mode, i.e. placed on the shoulder/chest portion of the user with the headband around the rear portion of the neck and with the left and right loudspeaker housings pointing upwards from the chest of the user; figure 2 shows a schematic representation of the device according to the invention with indications that define the geometry of the device in the headphone mode; figure 3 shows a schematic representation of the device according to the invention with indications that define the geometry of the device in the neckphone mode; figure 4 shows a prototype of an embodiment of one of the loudspeaker housings (the left) of the device according to the invention in which the sound outlet from the second sound emitting transducer can be seen and with the sound outlet pointing in four different directions (a through d) relative to the outer ear of the user. The figure also shown a plot of the frequency response of the second sound emitting transducer corresponding to the four different orientations of the second sound emitting transducer; figure 5 shows a perspective schematic drawing of an embodiment of a loudspeaker housing according to the invention comprising sound outlets from the first and second sound emitting transducers; figure 6 shows the effect of changing the orientation of the first sound emitting transducer in the headphone mode; figure 7(a) shows an embodiment of the invention in which the second sound emitting transducer is provided in the outer circumferential wall of the speaker housing; figure 7(b) shows an embodiment of the invention in which the second sound emitting transducer is provided in the outer circumferential wall of the earcub; and figure 8 shows an illustrative electric block diagram of an embodiment of the device according to the invention.

DETAILED DESCRIPTION OF THE INVENTION

In the following a detailed description of an example embodiment of the invention is given. It is however understood that the principles of the invention could be embodied in other ways without departing from the scope of the invention as defined by the claims.

With reference to figure 1 there is shown the placement of the device according to the invention in the two operational modes:

(a) the headphone mode, i.e. placed in the traditional manner with the two loudspeaker housings 1 and 2 and associated earcubs, respectively placed over the left and right outer ears of the user; and

(b) the neckphone mode, i.e. placed on the shoulder/chest portion of the user with the headband around the rear portion of the neck and with the left and right loudspeaker housings pointing upwards from the chest of the user.

With reference to figure 2, the respective longitudinal axes indicated by the vectors Rtand RR are in the headphone mode substantially coincident (although in the representation of figure 2 with opposite directions as indicated by the vectors Rt and RR respectively). The headband 3 is lying in/about plane P in figure 2, which also means that in the headphone mode, the loudspeaker housings 1 and 2 have their respective longitudinal axis extending substantially in plane P, i.e. substantially in the same plane as the headband 3. According to the invention, which is also apparent from figure 1, the loudspeaker housings 1 and 2 can turn about their respective “vertical" axes xt and XR as indicated by the arrows n and m. The respective end faces L and R of the left and right loudspeaker houses comprises the sound outlet from the respective first sound emitting transducers as it will be described in connection with figure 4 below. The longitudinal axes Rtand Rnare substantially perpendicular to the end faces L and R of the loudspeaker housings. With reference to figure 3, the loudspeaker housings 1 and 2 have been rotated from the headphone mode shown in figure 2 to the second mode, i.e. the neckphone mode. In the neckphone mode, the left and right end faces L and R extends substantially in parallel with the plane P, i.e. the respective longitudinal axes RL and RR are pointing substantially perpendicularly to the plane P. However, on order to take account of the specific shape of the user’s chest and neck, and also optionally to be able to optimize the orientation of the longitudinal axes relative to the body of the user, specifically to the outer ears of the user, certain tolerances to the direction of the longitudinal axes are provided. This is indicated in figure 3 by the respective solid angles ARuand ARR.

Although not shown in figures 2 and 3 the device comprises means configured to detect the mode of the device. Such means could be implemented by rotation sensors for instance inserted between the respective loudspeaker housing and the headband 3. Other kind of mode detection means could also be conceived and would fall within the scope of the invention as defined by the claims.

With reference to figure 4 there is shown a prototype of an embodiment of one of the loudspeaker housings (the left) of the device according to the invention generally designated by reference numeral 6 both in figures 4 and 5, in which the sound outlet 10 from the second sound emitting transducer can be seen and with the sound outlet pointing in four different directions (a through d) relative to the pinna 5 of the user. The placement on the torso portion of the acoustic mannequin upon which the loudspeaker housing is provided in this figure corresponds to the position in mode B (the neckphone mode) of the device. The figure also shown a plot of the frequency response in dB of the second sound emitting transducer for a frequency-independent input signal corresponding to the four different orientations of the sound outlet of the second sound emitting transducer. As it appears from the plot, the second sound emitting transducer is highly directional and has its main lobe of sound radiation substantially in the directions indicated by the arrows P in figure 4. As it appears from the plot, a substantial increase of the gain of the high-frequency output from the second sound emitting transducer can be achieved by proper orientation of the main lobe of sound radiation from the second sound emiting transducer.

In mode B, the first sound emitting transducer is not couplet directly to the outer ear of the user as in mode A, which means that more power must be provided to the transducer, and the longitudinal axis (axis of the main radiation lobe) is perpendicular to the ear canal of the user, which means that the high frequencies are much attenuated at the entrance to the ear canal. Furthermore, high frequency sounds will be radiated not only to the entrance of the ear canal but also to the user's environment causing annoyance to people around the user and making it difficult for the user to keep private conversations. In mode B of the device, the audio signal is split into low frequencies (that are radiated omnidirectionally) and mid-high frequencies (that are radiated directionally). More details about the sound emitting transducers and other functional entities of the device will be given in connection with figure 7.

With reference to figure 5 there is shown a perspective schematic drawing of an embodiment of a loudspeaker housing 6 according to the invention comprising sound outlets 9, 10 from the first and second sound emitting transducers, respectively. The speaker housing according to this embodiment is cylindrical comprising a circumferential side face 7 and opposing end faces, of which only the front face 8 is shown. In the front face 8, the sound outlet 9 of the first sound emitting transducer is located, in this embodiment radially displaced from the longitudinal axis RR of the loudspeaker housing. The relative size and positioning of the sound outlets can be chosen differently from what is shown in figure 5, taking into account the effect on the radiation patter of different placements, sizes and shapes of the sound outlets.

With reference to figure 6 there is shown the effect of changing the orientation of the first sound emitting transducer on the frequency response of an embodiment of the device according to the invention in mode A (the headphone mode). Specifically, the plot shows the frequency response of the device in mode A measured at the microphone in the acoustic manequin on which the device is provided.

The first sound emitting transducer is a full-range transducer covering substantially the whole audible frequency range. The transducer must have such power handling capacity that it is able to function in both mode A (where it is coupled directly into the confined cavity formed by the earcubs, pinna and the ear canal of the user) and in mode B, where it does not radiate sound into a confined cavity, as in mode A, but to the surroundings. This has at least the following three effects that increases the requirements of the first sound emitting transducer, when it works in mode B: (i) the transducer diaphragm is loaded by another radiation impedance, which will change (and likely reduce) its ability to radiate sound power from the diaphragm of the transducer, (ii) the sound pressure generated by the transducer will decrease because of the increased distance to the ear canal entrance compared to the mode A condition, and (iii) while the low frequencies will be radiated omnidirectionally, the mid and high frequencies will be radiated in a directional pattern with the main radiation lobe not directed towards the ear canal of the user, thereby causing attenuation of the sound pressure of the mid and high frequency sounds that are received at the entrance to the ear canal.

In the prototype embodiment of the invention, a first sound emitting transducer with a high sensitivity (i.e. the efficiency of the transducer driver to convert electric power into acoustic power) and a power handling capacity of up to 3WRMS with a very low fo has been used in order to ensure a good sensitivity in the 20-200Hz frequency band. A high sensitivity is both important in order to obtain the best posible sound quality in mode, but also in order to ensure the best possible ANC system performance.

As it appears from the plot shown in figure 6, in mode A the first sound emitting transducer must be aligned and tilted in the optimal manner to fully align the sound radiation with the ear canal of the user. For the specific sound emitting transducer used in the prototype of the invention it was found that the position indicated by reference numeral 16 and a tilt angle a of 6 degrees provided the best overall performance. Thus, the frequency response indicated by reference numeral 13 in figure 6 corresponds to the position indicated by 16 to the right of the figure and to a tilt angle a of 0 degrees. The frequency response indicated by reference numeral 14 in figure 6 corresponds to position 16 and a tilt angle of 6 degrees.

With the first transducer used in the prototype of the invention it was found that in mode A only about 10mW RMS was needed in order to obtain a sound pressure level of 90-96 dBSPL in the ear canal of the user. However, in order to have a certain headroom not to drive the amplifier that drives the first sound emitting transducer into saturation, an amplifier with an audio output power of 125mW RMS has been used in the prototype.

In mode A, the acoustic performance of the device according to the invention can (as an option) be optimized by means of the signal processing block 20 shown in figure 7, which can optionally be embedded for example into a QCC5124 chipset. Thus, the signal processing block 20 is configured to minimize non-linear distortion and to create a V-shaped frequency response that enhances the low frequencies. All or most of a required high-frequency extension or enhancement is in the prototype obtained by the transducer alignment and tilt angle as shown in figure 6 as well as by the transducer specifications (frequency response) itself. It would however also be possible, at least within certain limits, to shape the high- frequency response by means of the signal processing block 20. With reference to figure 7 there is shown two embodiments of the sound radiating unit according to the present invention comprising a speaker housing 1 and a corresponding earcupT (corresponding to the left sound radiating unit 1, T in figure 1. It is however understood that similar embodiments of sound radiating units could also be used as the left sound radiation unit 2, 2* shown in figure 1). In figure 7(a) one second sound emitting transducer 34 is provided in the outer circumferential wall portion of the speaker housing 1. In figure 7(b) one sound emitting transducer 36 is provided in the outer circumferential wall portion of the earcupT. It is understood that alternatively more than one second sound emitting transducers could be provided either in the outer circumferential wall portion of the speaker housing or in the earcup The provision of second sound emitting transducers in both the speaker housing and in the earcub would also be possible according to the present invention.

An advantage of the embodiment shown in figure 7(b) would be that contrary to the embodiment shown in figure 7(a) in which the position of the sound outlet of the second sound emitting transducers) is/are fixed relative to a plane containing the two speaker housings and the headband, in the second embodiment the position and orientation of the one or more sound outlets from the second sound emitting transducers can be adjusted in order to improve the performance of the device.

With reference to figure 8 there is shown an electric block diagram of an embodiment of the device according to the invention. It is understood that the different signal processing and controlling functional blocks shown in the block diagram can be integrated into a single physical unit or that they can be distributed over the device according to the invention, if desired. The different functional blocks can be implemented digitally or for some of the blocks by analog means or by a combination of digital and analog implementation.

The block diagram shown in figure 8 will be described in detail in the following, wherein the necessary power supply is provided as indicated by reference numeral 18.

Stereophonic audio input signals to the device can be provided via a jack connection 21 (IL and IR respectively) or alternatively they can be provided wirelessly for instance using a Bluetooth connection as indicated in functional block 20 in figure 8.

In functional block 20 the left and right audio input signals (such as Land IR respectively) can be processed for instance to reduce non-linear distortion or the shape of the frequency response of the device or to provide active noise cancellation (ANC). For the latter purpose use is made of signals provided by microphones shown as functional block 19. One or more microphones may be comprised by block 19.

Based on the input signals Block 20 provides two output signals Si and S2 respectively that are processed or un-processed versions of the respective input signals. Si indicates thus a processed or un-processed version of the left input signal and S2 indicates a processed orall together un- processed version of the right input signal. Similarly S3, Ss, ST, Seand S11 indicate left signals and S4, Se, Ss, Swand S12 indicate right signals.

The device further comprises a control unit or microcontroller (MCU) 22 that provides control signals Ci, C2, Cs and C* to various functional blocks. Input signals to the control unit are provided by a mode sensor 23 and an ANC key 24. In preferred embodiments of the invention, the mode sensor functions automatically, for instance when the loudspeaker housings are rotated relative to the headband, but it would also be possible to use a manually operated mode selector, as for instance a push button on the device.

The output signals Si and S2from the signal processing block 20 are provided to a high-pass filter (HPF) 25 that determines the mid and high frequency signal components that are to be provided to the left and right second sound emitting transducers, respectively, that are symbolically indicated by block 27. The output signals S3 and S4from the HPF 25 are provided as input signals to a first power amplifier 26 that provides the output signals ST and Ss that drives the second sound emitting transducers 27. In figure 8, the power amplifier 26 has a maximum power of 1WRMS, but it is understood that the power requirements depend on the specific sound emitting transducers 27. The power amplifier 26 is controlled from the switch unit 22 via control signal Cs such that it turns off the output signals ST and Ss to the second sound emitting transducers when the device is in mode A, where it functions as a traditional headphone.

The output signals Si and S2 from the signal processing block 20 are also provided to the controllable switch unit 28 which can be controlled from the MCU 22 via control signal C2. In mode A, the switch unit 28 provides the full bandwidth signals Si and S2 to a second power amplifier 30 that provides output signals Se, S10 as left and right signals to the first sound emitting transducer 32. In mode B the switch unit provides the full bandwidth signals Si and S2 to the lowpass filter (LPF) 29 that provides lowpass filtered versions Ss and Se of the full bandwidth signals Si and Sato a third power amplifier 31, the respective output signals Sn and S12 of which are provided to the left and right first sound emitting transducer 32, respectively. In mode A, the second power amplifier 30 is turned on and the third power amplifier 31 is turned off by the respective control signals Cs and C*. Similarly, in mode B the second power amplifier 30 is turned off and the third power amplifier 31 is turned on.

The specific characteristics of the LPF 29 and the HPF 25 can be chosen as required. In the prototype embodiment of the invention both filters have a cutoff frequency of 2kHz.

With respect to the signal processing carried out in the signal processing block 20, it is noted that for mode B, different signal processing techniques are applied to the audio signal than for mode A, as in mode B a higher idle noise level and level of non-linear distortion can be tolerated as the distance from the transducer to the ear is larger and the interfering background noise is much more powerful, as the ears of the user are not occluded by the respective loudspeaker housing and earcub.