Abstract:
An earphone has a casing, containing a speaker, the casing being adapted to fit within the outer ear of a user at the entrance to the ear canal of the user. The earphone can be used in a noise cancelling earphone system, with signal processing circuitry connected to the microphone and to the speaker. The signal processing circuitry is adapted to receive the ambient noise signal from the microphone, and to apply the ambient noise signal to a filter having a controllable amount of gain, for generating a noise cancellation signal for transmission to the speaker. The result is that, however the earphone is worn within the outer ear of a user, an amount of sound leakage lies within a predetermined range, such that the amount of gain to be applied by the signal processing circuitry falls within a relatively narrow range.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    This invention relates to an earphone, and in particular to an earphone for use in a noise cancellation system. 
         [0003]    2. Description of the Related Art 
         [0004]    It is known to provide a noise cancellation system, for use with a sound-reproducing device such as an earphone. The sound-reproducing device includes a speaker, for receiving electrical signals representing a wanted sound, such as music or speech, from a portable music player, telephone handset, or the like. The noise cancellation system includes a microphone provided on the sound-reproducing device, to generate an electrical signal representing ambient noise. This ambient noise signal is then applied to signal processing circuitry to generate a noise cancellation signal, and the noise cancellation signal is applied to the speaker. 
         [0005]    The purpose of the signal processing circuitry is to generate a noise cancellation signal that, when applied to the speaker, produces a sound that is equal in magnitude but opposite in phase to the ambient sounds reaching the user&#39;s ear. If this can be achieved, destructive interference will have the effect of reducing the noise that can be heard by the user. 
         [0006]    In order to achieve this, it is known, for example from GB-2445984A, that the signal processing circuitry needs to apply frequency-selective filtering to the ambient noise signal, and that this frequency-selective filtering needs to take account of the frequency-dependent amplitude and phase characteristics of: the response of the noise microphone; any electronic amplification in the signal processing circuitry; and the response of the speaker. These characteristics are generally relatively stable for any given individual earphone device and, subject to manufacturing tolerances, they can be determined for any model of earphone. 
         [0007]    In addition, however, the frequency-selective filtering needs to take account of two further factors, namely the frequency-dependent amplitude and phase characteristics of the acoustic path from the surroundings into the ear of the user, and the phase and frequency response of the acoustic path from the speaker to the ear of the user. These are both dependent on the leakage characteristics of the earphone, that is, the leakage in the coupling of the earphone to the ear of the wearer. 
         [0008]    It is known that the frequency-dependent characteristics of the leakage path can vary widely, depending on how the sound-reproducing device interacts with the ear of the user. More specifically, one important factor is the area of the leakage, which affects both the amplitude and phase of all signals perceived by the ear. For example, in the case of an earphone that is intended to be worn within the outer ear of the user, the frequency-dependent leakage characteristics will depend on the exact shape of the user&#39;s ear, and on how tightly the earphone is pushed into the ear. 
         [0009]    This has the effect that it is difficult to perform frequency-selective filtering that is sufficiently representative of the frequency-dependent amplitude and phase leakage characteristics. 
       SUMMARY OF THE INVENTION 
       [0010]    According to a first aspect of the present invention, there is provided an earphone, comprising: a casing, containing a speaker, wherein the casing is adapted to fit within the outer ear of a user at the entrance to the ear canal of the user; wherein the casing has a front surface intended to be located adjacent to the entrance to the ear canal of the user; wherein the casing has a guide, protruding from the front surface of the casing, and suitable for locating in the ear canal of the user; wherein the casing is adapted to allow sound to pass through a sound-permeable portion of the front surface; and wherein the casing has a plurality of sound channels, leading across the front surface of the casing from the sound-permeable portion to a periphery of the first surface of the casing. 
         [0011]    According to a second aspect of the present invention, there is provided a cushion, for use on a casing body of an earphone containing a speaker, wherein the casing is adapted to fit within the outer ear of a user at the entrance to the ear canal of the user; wherein the cushion has a front surface intended to be located adjacent to the entrance to the ear canal of the user; wherein the cushion has a guide, protruding from the front surface of the cushion, and suitable for locating in the ear canal of the user; wherein the cushion is adapted to allow sound to pass through a sound-permeable portion of the front surface; and wherein the cushion has a plurality of sound channels, leading across the front surface of the cushion from the sound-permeable portion to a periphery of the first surface of the cushion. 
         [0012]    According to a third aspect of the present invention, there is provided a noise cancelling earphone system, comprising: an earphone, having a microphone for detecting ambient noise and generating an ambient noise signal, and a speaker, and signal processing circuitry, connected to the microphone and to the speaker, wherein the signal processing circuitry is adapted to receive the ambient noise signal from the microphone, and to generate a noise cancellation signal for transmission to the speaker, wherein the earphone comprises: a casing, containing the speaker, wherein the casing is adapted to fit within the outer ear of a user at the entrance to the ear canal of the user, and wherein the casing has a front surface through which sound from the speaker can pass; and a cushion, extending around the front surface of the casing, wherein the cushion extends discontinuously around a periphery of the front surface of the casing. 
         [0013]    This has the advantage that the amount of ambient noise that leaks past the earphone cannot be less than a certain minimum value, regardless of how tightly the earphone is pushed into the ear. Hence, the range of possible amplitudes in the characteristic of the leakage path is reduced, meaning that it is possible to perform frequency-selective filtering that is more likely to be representative of the frequency-dependent amplitude and phase leakage characteristics. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0014]    For a better understanding of the present invention, and to show how it may be put into effect, reference will now be made, by way of example, to the accompanying drawings, in which: 
           [0015]      FIG. 1  illustrates the use of an earphone in accordance with an aspect of the present invention; 
           [0016]      FIG. 2  shows a first noise cancellation system for use with the earphone of the present invention; 
           [0017]      FIG. 3  is a perspective view, showing the form of the earphone in accordance with an aspect of the present invention; 
           [0018]      FIG. 4  is a cutaway view, showing the earphone of  FIG. 3 ; 
           [0019]      FIG. 5  is a plan view of a cushion of the earphone of  FIG. 3 ; 
           [0020]      FIG. 6  is a perspective view of the cushion of  FIG. 5 ; and 
           [0021]      FIG. 7  is a side view of the cushion of  FIG. 5 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0022]      FIG. 1  shows a sound reproduction system  10 , including a signal source  12  and an earphone system  14 . The signal source  12  might be a playback device such as an MP3 player, or a device for receiving sound signals such a mobile phone handset, or the like. 
         [0023]    The earphone system  14  may include a jack  16  that plugs into the signal source  12 , and a signal processing unit  18 . Although a separate signal processing unit  18  is shown in  FIG. 1 , the invention is equally applicable to systems in which the signal processing takes place within the signal source, or even within the earphones themselves. 
         [0024]    In this example, the sound reproduction system  10  is a stereo system, and so the signal processing unit  18  includes respective leads  20 ,  22  connected to two earphones, of which only one earphone  24  is shown in  FIG. 1 , it being understood that the other earphone of the pair is simply a mirror image of the first. The leads  20 ,  22  may each be made up of several wires, allowing separate signals to be passed along them, as described in more detail below. 
         [0025]    The earphone  24  is of a size and shape that allows it to fit within the concha  26  at the entrance to the ear canal  28  in the outer ear  30  of a user  32 . 
         [0026]      FIG. 2  shows the general form of the noise cancellation system within the sound reproduction system  10 . Specifically, the signal processing unit  18  receives a wanted signal from the signal source  12  on an input  40 . This might for example be the signal representing the speech or music that the user wishes to hear. 
         [0027]    The wanted signal is applied to a first input of an adder  42 , and the output from the adder  42  is output over a first wire  44  in the lead  20  to a speaker  46  in the earphone  24 . 
         [0028]    The earphone  24  also includes at least one microphone  48 , for detecting ambient noise in the vicinity of the earphone. Ambient noise signals from the microphone  48  may be passed along a second wire  50  in the lead  20  to the signal processing unit  18 . 
         [0029]    The ambient noise signals are passed to a filter  52 , and to a gain unit  54  to generate a noise cancellation signal, which is applied to a second input of the adder  42 , so that it is added to the wanted signal as the latter is supplied to the speaker  46 . 
         [0030]    If the signal processing performed by the filter  52  and gain unit  54  in the signal processing unit  18  can be controlled appropriately, then the effect of applying the noise cancellation signal to the speaker  46  is to generate a sound that will cancel out the ambient noise to at least some extent, thereby making the wanted sounds more clearly audible. 
         [0031]    As is well known, effective noise cancellation requires that the filter characteristics of the filter  52  and the gain unit  54  should be well matched to the other characteristics of the system. Thus, the filter  52  can have a frequency response characteristic that compensates for any frequency dependent variations in the responses of the ambient noise microphone  48  or the loudspeaker  46 . Also, the filter  52  can have a frequency response characteristic that compensates for any frequency dependent variations in the ambient noise that reaches the user&#39;s ear around the earphone as it is worn. These characteristics of the filter  52  can be preset, based on knowledge of the earphone  24  with which the signal processing unit  18  is to be used. 
         [0032]    The system shown in  FIG. 2  is a pure feedforward system, in which the ambient noise signals are passed through a fixed filter  52  and gain unit  54 . In other embodiments, the noise cancellation system can be an adaptive system, in which the earphone  24  also includes an error microphone, positioned close to the speaker  46 , and error signals generated by the error microphone are used to adjust the characteristics of the filter  52  and/or the gain unit  54  in use, in order to minimise the error signals. 
         [0033]    Whether the system is a pure feedforward system or an adaptive system, the level of gain applied by the gain unit  54  should be well matched to the characteristics of the system. One particularly relevant aspect of these characteristics can be described as the leakiness of the earphone. 
         [0034]    When the earphone  24  is held loosely in the concha  26  of the ear of the user, there is a relatively high leakage. That is, the earphone  24  provides a low acoustic resistance to ambient sounds reaching the ear canal  28  of the user, and a low acoustic resistance to sounds from the speaker  46  reaching the exterior. In such circumstances, a relatively high degree of noise cancellation is required, and so the gain value applied in the gain unit  54  to the ambient noise signals received from the noise microphone  48  must be relatively high, if effective noise cancellation is to be achieved. 
         [0035]    When the earphone  24  is held tightly over the entrance to the ear canal  28  of the user, it provides a high acoustic resistance to ambient sounds reaching the ear canal, and similarly a high acoustic resistance to sounds from the speaker  46  reaching the ambient environment, and there is said to be a relatively low leakage. In such circumstances, there is less noise reaching the ear requiring cancellation, and so the gain value applied in the gain unit  54  to the ambient noise signals received from the noise microphone  48  must be relatively low, if acceptable noise cancellation is to be achieved. 
         [0036]    In the illustrated embodiment, the gain value applied by the gain unit  54  is fixed, and so it is necessary to select a gain value that provides an acceptable degree of noise cancellation, however the earphone is used by the user. 
         [0037]      FIGS. 3 and 4  show a form of earphone  24 , in which the range of leakage values is restricted, despite differences in how the earphone might be worn in the ear of the user. 
         [0038]    Specifically,  FIGS. 3 and 4  show an earphone  24 , having a casing  60 . In this embodiment, the casing  60  includes a casing body  62 , which has a first end region  64  that is of a size and shape that allows it to be placed in the outer ear of the user, adjacent to the entrance to the user&#39;s ear canal. A second opposite end region  66  of the casing body  62  receives the lead  20  (not shown in  FIGS. 3 and 4 ). The casing body  62  may be made of a rigid plastic material, or any other suitable material that is rigid enough for the intended use. 
         [0039]    In this embodiment, the casing  60  also includes a cushion  68  mounted around the periphery of the first end region  64  of the casing body  62 . The cushion  68  may be made of a plastic material or any other material that is suitable for the intended use. The cushion may be made of a material, such as plastic or rubber, that is less rigid, i.e. softer, than the casing body  62 , and may be designed to be removable from the casing body  62  by slight stretching, so that it can be replaced if necessary. In this case, the cushion  68  acts as a gasket, providing a partial seal between the casing body  62  and the outer ear of the user. 
         [0040]    In other embodiments, the casing can have a unitary structure. That is, the casing body and the cushion can be formed as a single body. 
         [0041]    The casing body  62  also has one or more holes  70 , allowing ambient sound to enter the casing. 
         [0042]    The casing  60  defines an internal space  72 , into which can be fitted the speaker  46  and the microphone  48 . The speaker  46  (not shown in  FIG. 4 ) is positioned and oriented so that it directs sound out of the casing  60 , that is, upwards in the orientation shown in  FIG. 4 . A suitable speaker will typically direct sound out through a surface that is covered by a sound-permeable but water-resistant material, such as a mesh. 
         [0043]    The microphone  48  (not shown in  FIG. 4 ) is positioned so that it can detect ambient sound entering through the hole  70 . 
         [0044]      FIGS. 5 ,  6  and  7  show the cushion  68  removed from the casing body  62 . Specifically,  FIG. 5  is a plan view of the cushion  68 ,  FIG. 6  is a perspective view from above, and  FIG. 7  is a side view. 
         [0045]    The cushion  68  has a guide  74  protruding from its upper surface. The guide  74  is designed to be located in the entrance to the ear canal of the user, so that it assists in correct positioning of the earphone  24  in the outer ear of the user. Thus, the cross-sectional area of the guide  74  is smaller than the area of the entrance to the ear canal of the user so that it does not significantly prevent sound from entering the ear canal. 
         [0046]    When seen in plan view, as seen most clearly in  FIG. 5 , the cushion  68  is generally circular, and the guide  74  is located close to the outer periphery of the cushion  68 , at a position that is diametrically opposed to the direction in which the second end  66  of the casing body  62  extends. 
         [0047]    A sound aperture  76  is provided in the upper surface of the cushion  68 . As can be seen, the aperture  76  is of a generally elliptical shape, and it is formed in the half of the circular shape of the cushion  68  that is nearest to the guide  74 . This has the effect that the aperture  76  is positioned close to the entrance to the user&#39;s ear canal in use. The upper surface of the cushion  68  surrounding the aperture  76  is typically substantially impermeable to sound, so that all of the sound generated by the speaker  46  passes through the aperture  76 . Although an aperture is shown here, it would equally be possible to provide an area that is more permeable to sound than its surrounding area of the upper surface. 
         [0048]    In addition, the guide  74  has a generally concave cross-sectional shape, as seen most clearly in  FIGS. 5 and 6 , so that sound passing through the aperture  76  is guided into the ear canal of the user when the earphone is being worn as described above. 
         [0049]    The cushion also has three predetermined sound leakage channels  78 ,  80 ,  82 , which are formed in the upper surface of the cushion  68 , and extend from the aperture  76  towards the outer periphery of the cushion  68 . More specifically, the channel  80  leads from the aperture  76  in a direction directly away from the guide  74 , while the channels  78 ,  82  are opposite each other, and are each perpendicular to the channel  80 . Although three sound channels are shown here, any suitable number of channels (for example in the range from two to six, inclusive) can be provided. 
         [0050]    The result of forming the predetermined sound leakage channels  78 ,  80 ,  82  in the upper surface of the cushion  68  is that the upper surface is discontinuous where it contacts the surface of the user&#39;s concha  26 . 
         [0051]    The effect of this discontinuity is that the earphone  24  is unable to provide an acoustic seal for the entrance to the user&#39;s ear canal  28 , and hence that there will always be a significant amount of leakage of ambient noise past the earphone  24  into the user&#39;s ear, and of sounds from the speaker  46  to the environment. This has the result that, in use, the acoustic resistance to ambient sounds reaching the ear canal  28  of the user cannot reach a very high value, regardless of how the user chooses to wear the earphone, and in particular regardless of how tightly the user attempts to press the earphone into his concha. 
         [0052]    Although the acoustic impedance to ambient sounds reaching the ear canal  28  of the user will still vary, depending on how the user chooses to wear the earphone, the range of this possible variation will be less than would be the case if an acoustic seal could be formed. 
         [0053]    The amount of sound leakage of ambient noise past the earphone  24  into the user&#39;s ear can conveniently be discussed in terms of the area of the available leakage paths. For example, in the case of an earphone having a smooth upper surface, for one typical user this leakage area might be in the region of 5 mm 2  if the device is pressed against the surface of the concha, increasing to 10 mm 2  if the earphone is worn loosely in the ear. These leakage areas will also vary from one user to another. Thus, wearing the earphone more loosely can increase the leakage area by 100%. 
         [0054]    This means that it is necessary to attempt to select the characteristics of the filter  52  and/or the gain unit  54  in such a way that it provides acceptable noise cancellation across this range of leakage areas. However, the large percentage variation in the leakage area means that it is difficult to achieve this. 
         [0055]    By contrast, in the case of an earphone as described here, if the predetermined sound leakage channels  78 ,  80 ,  82  have a total cross-sectional area of 10 mm 2 , then the total available leakage area might be in the region of 15 mm 2  if the device is pressed against the surface of the concha, increasing to 20 mm 2  if the earphone is worn loosely in the ear. Thus, in this case, wearing the earphone more loosely can increase the leakage area by 33%. 
         [0056]      FIG. 7  shows the cross-sectional area A of the predetermined sound leakage channel  82 . 
         [0057]    As before, it is necessary to attempt to select the characteristics of the filter  52  and/or the gain unit  54  in such a way that it provides acceptable noise cancellation across this range of leakage areas. However, the smaller percentage variation in the leakage area means that it is easier to achieve this. Furthermore, in an adaptive system, i.e. where the filter characteristics and/or the gain are adaptive, there will be a smaller range for adaptation, which is advantageous. 
         [0058]    This means that the gain value applied in the gain unit  54  to the ambient noise signals received from the noise microphone  48  can be set to a relatively high value, and this will be suitable for providing effective noise cancellation across the range of leakage values that can be achieved. 
         [0059]    There is therefore provided an earphone that allows noise cancellation circuitry to provide signal processing that deals more effectively with the ambient noise that can reach the ear of the user.