Telephone mouthpiece and method for minimising wind noise

0001This invention relates to a telephone mouthpiece and a method for minimising wind noise, particularly, but not exclusively, for use with a telephone.

0002A conventional telephone mouthpiece will be described below with reference to the accompanying drawings.

0003A first example of the prior art, which has been disclosed in Japanese Patent Laid-Open No. 214400 (1990), will be described with reference to Fig. 11 which is a block schematic diagram.

0004A microphone 51 in the arrangement of Fig. 11 converts a vocal sound to an electric signal and outputs a voice signal. This voice signal is supplied to a high-pass filter 58 and a low-pass filter 59 via a preamplifier circuit 52, respectively.

0005The high-pass filter 58 is designed so as to pass signals at a frequency of 150 Hz or higher. The low-pass filter 59 is designed so as to pass low frequency signals containing a main factor of wind noise at a frequency of 150 Hz or lower.

0006An output signal of the high-pass filter 58 is supplied to an input terminal of an adder circuit 61.

0007An output signal of the low-pass filter 59 is supplied to another input terminal of the adder circuit 61 via an automatic level control circuit 60. At this time, the automatic level control circuit 60 supplies the output signal of the low-pass filter 59 to the input side of a variable gain amplifier circuit 60a, and results in the variable gain amplifier circuit 60a supplying its output signal to another input terminal of the adder circuit 61. Simultaneously, the automatic level control circuit 60 supplies the output signal of the variable gain amplifier circuit 60a to a level detection circuit 60b, and reduces the gain of the variable gain amplifier circuit 60a in accordance with a detection level determined by the level detection circuit 60b. Namely, the automatic level control circuit 60 is designed so as to have no attenuation in the absence of wind noise and an increase in the level of attenuation as the wind noises level increases.

0008However, the first example of the prior art has some problems such as a complex electric circuit construction and an unnatural feeling whilst talking caused by the alteration of the speaker's voice quality due to an alteration in the frequency response when the wind noise prevention function is ON.

0009Next, a second example of the prior art will be explained. The second example of the prior art has been disclosed in Japanese Utility Model Application Laid-Open No. 139649 (1989).

0010The second example is shown in Figs. 12 and 13 of the drawings.

0011Fig. 12 shows a side view of a handset partly broken away. Fig. 13 represents a fragmentary view taken in the direction of an arrow in Fig. 12.

0012Referring to Fig. 12 and Fig. 13, a protuberant telephone mouthpiece surface 76 has a dome-like shape with its top positioned at the centre of a telephone mouth-piece unit 73 connected to a base 72. An acoustic perforation 721 is formed slightly apart from the top of the dome-like shaped telephone mouthpiece surface 786 in the direction opposite to a receiver unit 74. The acoustic perforation 721 is so formed as to be apart from the receiver unit 74 as it goes from a front air chamber 78 towards the telephone mouthpiece surface 76.

0013When speaking using the above hand-set 71, the user usually has his/her ear tightly pressed to the receiver unit 74 for catching the other speaker's voice through an electrodynamic receiver 75. The cheek of the speaker is in close contact with the surface of the base 72. The user's lips are also located too close to the side of the protuberant domed-shaped telephone mouthpiece unit 73 in the direction of the receiver unit 74.

0014When the user speaks in this situation, his/her breathing is directed to the telephone mouthpiece unit 73, so that the breathing, which is a form of wind, is caused to flow directly to the section of the telephone mouth-piece surface 76 in the direction of the receiver unit 74, as an arrow A shows.

0015The opening section of the acoustic perforation 721 located apart from the top of the telephone mouthpiece surface 76 in the direction towards the receiver unit 74. The acoustic perforation 721 is so formed as to be apart from the receiver unit 74 as is the front air chamber 78 apart from the telephone mouthpiece surface 76.

0016This construction serves to minimise the possibility of wind entering into the acoustic perforation 721 directly, thus suppressing turbulence occurring therein and reducing noise. Since the space between the speaker's lips and the acoustic perforation 721 is remarkably small, the vocal sound itself becomes almost non-directional, resulting in little degradation in the voice signal. The first prior proposed arrangement requires no additional parts or major design modification, reducing the wind noise at a minimum cost.

0017The second prior example, reduces the effect of the breathing noises of the talker. When using it outdoors, however, its ability to suppress noise due to turbulence from outside is hardly improved, because it is arranged to cope with the air flow in a certain direction only.

0018Features of arrangements to be described below, as examples, are that wind noise may be reduced, even out of doors, and that resonance due to wind noise in the acoustic duct section of a telephone mouthpiece may be reduced.

0019A telephone mouthpiece to be described below, as an example, includes an acoustic duct box provided with an acoustic perforation, a microphone unit provided on a surface of the acoustic duct box facing to another surface of the acoustic duct box having the acoustic perforation, a plurality of partitions provided within the acoustic duct box, and a plurality of slits respectively provided in the partitions.

0020The principle of the present invention will now be explained. In Fig. 1 and Fig. 2, P_A is an acoustic pressure caused by a speaker, P_B is an acoustic pressure caused by wind noises at an acoustic perforation 1. L is a distance where a voice of the speaker passes until reaching to the acoustic perforation 1, R is an acoustic resistance. 1 and 1' are acoustic duct distances between the acoustic perforation 1 and a microphone unit 2, respectively, r and r' are resistances of distance 1 and distance 1', respectively. In a general hand held portable phone, L is in centimetres and 1 is in millimetres, so the relation between R and r is R>r. Here, supposing that the length of the acoustic duct is 1' that is longer than 1 as shown in the figures, the relation between them is r'>r. As a result, the relation between the acoustic resistances is R>r'>r. While, generally, L>1' due to limitations when mounting the microphone in a hand-held-portable phone.

0021Next, attenuation ratios of the generated acoustic pressure P_A and the wind noise pressure P_B are compared in the cases in which the acoustic duct distances are 1 and 1'. To simplify the explanation, a condition that r=1, r'=2, R=3 and each acoustic resistance represents its sound source is supposed.

0022In this case, the attenuation ratio A of the generated acoustic pressure P_A is:<maths id="math0001" num=""><math display="block"><mrow><mtext>A=(r+R)/(r'+R)=(1+3)/(2+3)=4/5.</mtext></mrow></math><img file="EP0707403A2_D0001.tif" /></maths>

0023Like this, in the case in which the acoustic duct distance is 1', the generated acoustic pressure P_A is reduced 20% than the case that the acoustic duct distance is 1.

0024On the other hand, the attenuation ratio B of the wind noise acoustic pressure P_B is:<maths id="math0002" num=""><math display="block"><mrow><mtext>B=r/r'=1/2.</mtext></mrow></math><img file="EP0707403A2_D0002.tif" /></maths>

0025Like this, in the case in which the acoustic duct distance is 1', the wind noise acoustic pressure P_B is reduced 50% compared to the case in which the acoustic duct distance is 1.

0026This means that the longer the acoustic duct distance is set, the more the wind noise acoustic pressure P_B is reduced than the generated acoustic pressure P_A. Namely, setting the acoustic duct distance longer reduces wind noises more effectively.

0027However, setting the acoustic duct distance long only causes a resonance phenomenon due to the frequency response of wind noises.

0028To prevent this resonance phenomenon of wind noises, the acoustic duct is partitioned by slit partitions.

0029More, air chambers formed by the partitions serve to increase the attenuation of the wind noise acoustic pressure to the microphone greater than that of the generated acoustic pressure of a speaker.

0030As a result, the present invention enables the effect of wind noise to be reduced without causing a deterioration in the sound quality of a speaker.

0031The following description and drawings disclose, by means of an illustrative example, the invention which is defined in the appended claims, whose scope determines the extent of the protection sought.

0032In the drawings:- <ul id="ul0001" list-style="none" compact="compact"><li>Fig. 1 is a diagram for use in explaining the principle of the present invention,</li><li>Fig. 2 is also a diagram for use in explaining the principle of the present invention,</li><li>Fig. 3 is an exploded perspective view showing diagrammatically a part of a hand held portable phone mounted to employ one embodiment of the present invention,</li><li>Fig. 4 is a cross sectional view on the line A-A in Fig. 3,</li><li>Fig. 5 is a view showing the upper enclosure of the hand held portable phone of Fig. 3 reversed,</li><li>Fig. 6 are graphs enabling data obtained using an embodiment of the invention and of the prior art to be compared,</li><li>Fig. 7 illustrates diagrammatically an acoustic duct model in which primary resonance occurs,</li><li>Fig. 8 shows an equivalent circuit of the acoustic duct model of Fig. 7,</li><li>Fig. 9 shows the frequency response of the acoustic duct model of Fig. 7, and</li><li>Fig. 10 is a diagram showing an equivalent circuit of the presently preferred embodiment.</li></ul>

0033An arrangement illustrating an embodiment of the present invention will now be described.

0034Fig. 3 is an exploded view showing a part of a hand held portable phone mounted in conjunction with an embodiment of the present invention, Fig. 4 is a cross section view on A-A in Fig. 3 and Fig. 5 is a view showing a reversed upper enclosure of the hand held portable phone of Fig. 3.

0035In the embodiment of Figs. 3, 4 and 5, the telephone mouthpiece includes a unit housing section 1, an acoustic duct section 2 which is tightly fitted thereto and which has been integrally formed with an upper enclosure 31 of the hand held portable phone and a microphone unit 3 connected to an electronic circuit base 33 within the hand held portable phone.

0036The unit housing section 1 formed of a rubber member has a cylindrical housing 12 for housing the microphone unit 3 at one end of a flat plate 11. A protrusion 13 is provided at the other end of the flat plate 11 to be positioned and fixed into a hole 34 formed in the electronic circuit base 33. The reason for forming the unit housing section 1 of a rubber member is to increase air tightness with the acoustic duct section 2 and to prevent a leak of the sound.

0037The acoustic duct section 2 includes an acoustic duct box 21 in the form of a box in close contact with the corresponding flat plate 11, and a cylindrical box 22 in close contact with the cylindrical housing 12 at an end of the acoustic duct box 21. The acoustic duct box 21 includes an acoustic perforation 25 at the other end of the acoustic duct box 21 and on a surface opposite to the surface facing the flat plate 11, and partitions 23 and 24 for partitioning the inside of the acoustic duct box 21 into two air chambers 26 and 27. The partitions 23 and 24 are provided with slots or slits 29 and 30, respectively.

0038The unit housing section 1 and the acoustic duct section 2 forming the telephone mouthpiece of the present embodiment are disposed between the upper enclosure 31 and a lower enclosure 32.

0039In the present arrangement, as shown in Fig. 4, it is supposed that the length of the acoustic duct is 15mm, the distance between the partitions 23 and 24 is 10mm, the length of a side of a slot or slit 29 is 1.0 mm, and the length of one side of the slot or slit 30 is 0.25 mm.

0040Fig. 6 shows comparison data of wind noises of the embodiment constructed as above and illustrating the invention and a conventional telephone mouthpiece which does not have a substantial acoustic duct, but which has a microphone closely touching an acoustic perforation.

0041As Fig. 6 shows, the telephone mouthpiece of the present embodiment acts to reduce wind noise more than the conventional telephone mouthpiece.

0042Next, an equivalent circuit of the illustrative embodiment will be explained.

0043To simplify the explanation, an experimental result of a primary resonance circuit will be explained first.

0044Fig. 7 shows an acoustic duct model employing primary resonance, and Fig. 8 shows an equivalent circuit of the acoustic duct model of Fig. 7.

0045In Fig. 7, D is the diameter of an acoustic perforation, t is the thickness of a wall of an acoustically perforated section and V is the volume of an acoustic duct. In the particular example, D=0.8 mm, t=0.8 mm and V=130 mm³.

0046When substituting an acoustic model by an equivalent circuit, an acoustic perforation (slit or slot) can be substituted by a coil L and a resistor R₁. Also, a volume, a length of an acoustic duct and a microphone can be substituted by a capacitor C, a resistor R₀ and a resistor R₂, respectively.

0047It is known that each value of a circular acoustic perforation is given by the following equation.<maths id="math0003" num=""><math display="block"><mrow><msup><mrow><mtext>L = 2(t + 0.75 × D)/344000 × D</mtext></mrow><mrow><mtext>2</mtext></mrow></msup><mtext> × π [H].</mtext></mrow></math><img file="EP0707403A2_D0003.tif" /></maths><maths id="math0004" num=""><math display="block"><mrow><mtext>C = 2V/344000 [F]</mtext></mrow></math><img file="EP0707403A2_D0004.tif" /></maths><maths id="math0005" num=""><math display="block"><mrow><msub><mrow><mtext>R = 1/ω</mtext></mrow><mrow><mtext>0</mtext></mrow></msub><mtext> × Q × C [Ω]</mtext></mrow></math><img file="EP0707403A2_D0005.tif" /></maths>

0048Where, the units are in mm, 344000 is the speed of sound, ω₀ is the angular speed at the point of resonance, and Q represents the sharpness of the resonance.

0049The frequency response of the acoustic duct model is shown in Fig. 9.

0050From the frequency response of Fig. 9, the following relations are derived.<maths id="math0006" num=""><math display="block"><mrow><msub><mrow><mtext>Q = f</mtext></mrow><mrow><mtext>0</mtext></mrow></msub><mtext>/if = 2300/450 = 5.1</mtext></mrow></math><img file="EP0707403A2_D0006.tif" /></maths><maths id="math0007" num=""><math display="block"><mrow><mtext>C = 2V/344000 = (2 × 130)/344000 ≅ 756µF</mtext></mrow></math><img file="EP0707403A2_D0007.tif" /></maths><maths id="math0008" num=""><math display="block"><mrow><msub><mrow><mtext>R</mtext></mrow><mrow><mtext>1</mtext></mrow></msub><msub><mrow><mtext> + R</mtext></mrow><mrow><mtext>0</mtext></mrow></msub><msub><mrow><mtext> = 1/(ω</mtext></mrow><mrow><mtext>0</mtext></mrow></msub><msup><mrow><mtext> × Q × C) =1/(2300 × 2π × 5.1 × 756 × 10</mtext></mrow><mrow><mtext>8</mtext></mrow></msup><mtext>) ≅ 0.018[Ω]</mtext></mrow></math><img file="EP0707403A2_D0008.tif" /></maths>

0051From a real measurement, R₀=0.4Ω and R₂=0.006Ω.

0052Therefore,<maths id="math0009" num=""><math display="block"><mrow><msub><mrow><mtext>R</mtext></mrow><mrow><mtext>1</mtext></mrow></msub><mtext> = 0.1Ω,</mtext></mrow></math><img file="EP0707403A2_D0009.tif" /></maths><maths id="math0010" num=""><math display="block"><mrow><msup><mrow><mtext>L = 2(0.8 + 0.15 × 0.8)/344000 × 0.8</mtext></mrow><mrow><mtext>2</mtext></mrow></msup><mtext> × 3.14 ≅ 4.0[H].</mtext></mrow></math><img file="EP0707403A2_D0010.tif" /></maths>

0053Next, each value of the equivalent circuit of the present invention is calculated.

0054When the slot or slit is a square, as in the arrangement being described, D is calculated by substituting the area of a circular slot or slit by an equivalent area. As a result, D of the slot or slit 29 of the present arrangement is 1.1 mm approximately, and D for the slot or slit 30 is 0.28 mm approximately.

0055In addition, R∝t/s is known. Where, s is an area of a cross section of the acoustic perforation (slot or slit).

0056Similarly to the above, an equivalent circuit of the present arrangement can be calculated, resulting in the circuit of Fig. 10. In Fig. 10, the acoustic perforation 25 is replaced by a series circuit of a coil L₁ and a resistor R₁. A slit 29 is replaced by a series circuit of a coil L₂ and a resistor R₂. A slit 30 is replaced by a series circuit of a coil L₃ and a resistor R₃. Air chambers 26, 27 and 28 are replaced by capacitors C₁, C₂ and C₃, respectively. The microphone unit 3 is replaced by a resistor R₄. The length of the acoustic duct is substituted by R₀. The acoustic duct section 2 as a whole is equivalent to a multi-staged low pass filter.

0057Simulation based on the equivalent circuit shown in Fig. 7 enables the values to be varied of the distance between the partition 23 and the partition 24 (the positions of air chambers 26, 27 and 28), and of the sizes of the slots or slits 29 and 30 to values other than those of the particular arrangement described.

0058It will be understood that, although embodiments of the invention have been described, by way of example, with reference to the accompanying drawings, variations and modifications thereof, as well as other embodiments, may be made within the scope of the appended claims.