CA2314862A1 - Arrangement for directing sound into a microphone with reduced noise, especially in handsets - Google Patents
Arrangement for directing sound into a microphone with reduced noise, especially in handsets Download PDFInfo
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- CA2314862A1 CA2314862A1 CA 2314862 CA2314862A CA2314862A1 CA 2314862 A1 CA2314862 A1 CA 2314862A1 CA 2314862 CA2314862 CA 2314862 CA 2314862 A CA2314862 A CA 2314862A CA 2314862 A1 CA2314862 A1 CA 2314862A1
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- H—ELECTRICITY
- H04—ELECTRIC COMMUNICATION TECHNIQUE
- H04M—TELEPHONIC COMMUNICATION
- H04M1/00—Substation equipment, e.g. for use by subscribers
- H04M1/02—Constructional features of telephone sets
- H04M1/19—Arrangements of transmitters, receivers, or complete sets to prevent eavesdropping, to attenuate local noise or to prevent undesired transmission; Mouthpieces or receivers specially adapted therefor
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Abstract
This invention is a device to reduce the generation of extraneous noise in a microphone, such as the mouthpiece of a telephone handset, due to airflow at the entrance to the microphone. The problem of airflow is particularly acute for the plosive sounds such as "p" and "t" for which the velocity of air flow is high.
The invention provides the mouthpiece with an array of small holes, a backing chamber, and a sound tube leading to the microphone transducer which is offset from all the holes. The holes are preferably circular and have a maximum diameter of 1 mm.
Significant reductions in broadband airflow noise are achieved.
The invention provides the mouthpiece with an array of small holes, a backing chamber, and a sound tube leading to the microphone transducer which is offset from all the holes. The holes are preferably circular and have a maximum diameter of 1 mm.
Significant reductions in broadband airflow noise are achieved.
Description
ARRANGEMENT FOR DIRECTING SOUND INTO A MICROPHONE
WITH REDUCED NOISE, ESPECIALLY IN HANDSETS
S BACKGROUND OF THE INVENTION
1.FIELD OF THE INVENTION
The generation of extraneous noise due to airflow at the entrance to a microphone is an inherent problem with telephone handsets, including cellular and PCS
handsets.
The problem is particularly acute for the plosive sounds such as "p" and "t"
for which the velocity of airflow is high. The airflow can generate noise when flowing over a sharp edge of a hole, and air turbulence generated within the hole will lead to pressure fluctuations, i.e., acoustical noise. This mechanism is, in fact, the most common source of airflow noise in the telephone handset mouthpiece.
The invention has immediate application in telephony handsets that have a mouthpiece held near the user's mouth. These handsets include those for desktop telephones, for cellular units, and for PCS units. Application to microphones for intercom and kiosk systems is also appropriate. There are other application areas for which the extraneous airflow noise is not due to the articulation of plosive sounds but is due to ambient wind noise. The intercom systems in use at outdoor drive-through windows have noise generated by gusts of wind. The invention is directly applicable to these application areas as well.
WITH REDUCED NOISE, ESPECIALLY IN HANDSETS
S BACKGROUND OF THE INVENTION
1.FIELD OF THE INVENTION
The generation of extraneous noise due to airflow at the entrance to a microphone is an inherent problem with telephone handsets, including cellular and PCS
handsets.
The problem is particularly acute for the plosive sounds such as "p" and "t"
for which the velocity of airflow is high. The airflow can generate noise when flowing over a sharp edge of a hole, and air turbulence generated within the hole will lead to pressure fluctuations, i.e., acoustical noise. This mechanism is, in fact, the most common source of airflow noise in the telephone handset mouthpiece.
The invention has immediate application in telephony handsets that have a mouthpiece held near the user's mouth. These handsets include those for desktop telephones, for cellular units, and for PCS units. Application to microphones for intercom and kiosk systems is also appropriate. There are other application areas for which the extraneous airflow noise is not due to the articulation of plosive sounds but is due to ambient wind noise. The intercom systems in use at outdoor drive-through windows have noise generated by gusts of wind. The invention is directly applicable to these application areas as well.
Reduction of airflow noise is typically addressed by modifying the entrance holes) of the cavity leading to the transducer portion of the microphone. Angling and bevelling of the holes, the use of louvers, and the shaping of the cavity between entrance and transducer are modifications that can reduce noise levels. However, significant airflow noise remains despite these efforts.
Another approach for dealing with wind noise has been the use of windscreens, often in the form of a porous foam cover or other cover having many small apertures.
Such windscreens are common in microphones used outdoors. However such covers are not acceptable for use in telephone handsets since they may harbor germs and are difficult to clean.
Also known are noise canceling microphones, sometimes referred to as pressure gradient microphones, particularly used to reduce background noise. These have a vibratable diaphragm which is actuated by speech or other sounds and is arranged such that sound paths are provided to both front and rear sides of the diaphragm to reduce the effect of unwanted background noise signals. Examples of such microphones are described in the following patents:
U.S. Pat. No.4,850,016, issued Ju1.18, 1989 to Groves et al.;
U.S. Pat. No.5,282,245, issued Jan.25, 1994 to Anderson;
U.S. Pat. No.5,329,593, issued Ju1.12, 1994 to Lazzeroni et al.;
U.S. Pat. No.6,009,184, issued Dec.28, 1999 to Tate et al.
Other patents showing noise canceling microphones are U.S. Patents Nos.5,343,523, issued Aug. 30, 1994 to Bartlett et al., and 5,239,578, issued Aug. 24, 1993 to Regen and Kingsley, which describe approaches for canceling ambient noise from speech signals; neither address airflow noise at the entrance holes.
Other approaches are shown in the following patents:
U.S. Patent No.5,701,354, which issued Dec.23, 1997 to Komoda and Murata, describes a telephone mouthpiece design that purports to reduce wind noise. In this invention the distance between the single entrance hole and the microphone is increased using a transverse duct; it is claimed that the wind noise levels will decrease more rapidly than speech signals as the duct length is increased. To avoid low-frequency duct resonances, the duct is partitioned into sections that communicate through slits.
This invention uses only a single entrance hole and does not attempt to prevent the formation of the wind noise, as compared to the present invention which also reduces wind noise. This aforementioned patent refers to a prior art, Japanese Utility Model Application Laid-Open No. 189649 (1989), which uses a protruding dome-shaped surface on a telephone mouthpiece to direct airflow away from the single entrance leading to the microphone.
In U.S. Patent No.5,905,803, issued May 18, 1999 to Dou et al., airflow noise is reduced by placing the microphone transducer within a recessed area of the mouthpiece and covering the recess with a thin film. This approach may not be appropriate for telephone handsets because of the need for electrostatic discharge protection and fabrication challenges.
In U.S. Patent No.4,887,693, issued Dec. 19, 1989 to Plice, a construction is described that makes a small microphone have a larger effective size, so that fluctuations in air pressure, due to airflow noise, are spatially averaged. A
tapered duct is used to couple incident sound between the large opening, preferentially covered with a windscreen, and the microphone transducer, inside the housing. A
single, relatively-large entrance hole is thus being used.
In U.S. Patent No.5,890,072, issued March 30, 1999 to Rabe, a shaped wave guide couples sound from entrance to microphone, acting as a low-pass acoustical filter but not necessarily reducing wind noise.
SUMMARY OF THE INVENTION
In accordance with this invention, the airflow noise is reduced to substantially lower levels through an innovative design of the microphone mouthpiece that makes use of several effects. First, the size of entrance hole has been found to be a critical factor in the generation of airflow noise. The use of holes of 1.0 mm diameter or less, and preferably 0.7 mm or less, or even less than 0.5 mm, leads to significant reductions in noise levels. For diameters of less than 0.1 mm, viscous and thermal boundary layer effects may limit acoustical performance by introducing large resistive loads. A
range of 0.1 mm to 0.7 mm may be the best for most applications.
Another approach for dealing with wind noise has been the use of windscreens, often in the form of a porous foam cover or other cover having many small apertures.
Such windscreens are common in microphones used outdoors. However such covers are not acceptable for use in telephone handsets since they may harbor germs and are difficult to clean.
Also known are noise canceling microphones, sometimes referred to as pressure gradient microphones, particularly used to reduce background noise. These have a vibratable diaphragm which is actuated by speech or other sounds and is arranged such that sound paths are provided to both front and rear sides of the diaphragm to reduce the effect of unwanted background noise signals. Examples of such microphones are described in the following patents:
U.S. Pat. No.4,850,016, issued Ju1.18, 1989 to Groves et al.;
U.S. Pat. No.5,282,245, issued Jan.25, 1994 to Anderson;
U.S. Pat. No.5,329,593, issued Ju1.12, 1994 to Lazzeroni et al.;
U.S. Pat. No.6,009,184, issued Dec.28, 1999 to Tate et al.
Other patents showing noise canceling microphones are U.S. Patents Nos.5,343,523, issued Aug. 30, 1994 to Bartlett et al., and 5,239,578, issued Aug. 24, 1993 to Regen and Kingsley, which describe approaches for canceling ambient noise from speech signals; neither address airflow noise at the entrance holes.
Other approaches are shown in the following patents:
U.S. Patent No.5,701,354, which issued Dec.23, 1997 to Komoda and Murata, describes a telephone mouthpiece design that purports to reduce wind noise. In this invention the distance between the single entrance hole and the microphone is increased using a transverse duct; it is claimed that the wind noise levels will decrease more rapidly than speech signals as the duct length is increased. To avoid low-frequency duct resonances, the duct is partitioned into sections that communicate through slits.
This invention uses only a single entrance hole and does not attempt to prevent the formation of the wind noise, as compared to the present invention which also reduces wind noise. This aforementioned patent refers to a prior art, Japanese Utility Model Application Laid-Open No. 189649 (1989), which uses a protruding dome-shaped surface on a telephone mouthpiece to direct airflow away from the single entrance leading to the microphone.
In U.S. Patent No.5,905,803, issued May 18, 1999 to Dou et al., airflow noise is reduced by placing the microphone transducer within a recessed area of the mouthpiece and covering the recess with a thin film. This approach may not be appropriate for telephone handsets because of the need for electrostatic discharge protection and fabrication challenges.
In U.S. Patent No.4,887,693, issued Dec. 19, 1989 to Plice, a construction is described that makes a small microphone have a larger effective size, so that fluctuations in air pressure, due to airflow noise, are spatially averaged. A
tapered duct is used to couple incident sound between the large opening, preferentially covered with a windscreen, and the microphone transducer, inside the housing. A
single, relatively-large entrance hole is thus being used.
In U.S. Patent No.5,890,072, issued March 30, 1999 to Rabe, a shaped wave guide couples sound from entrance to microphone, acting as a low-pass acoustical filter but not necessarily reducing wind noise.
SUMMARY OF THE INVENTION
In accordance with this invention, the airflow noise is reduced to substantially lower levels through an innovative design of the microphone mouthpiece that makes use of several effects. First, the size of entrance hole has been found to be a critical factor in the generation of airflow noise. The use of holes of 1.0 mm diameter or less, and preferably 0.7 mm or less, or even less than 0.5 mm, leads to significant reductions in noise levels. For diameters of less than 0.1 mm, viscous and thermal boundary layer effects may limit acoustical performance by introducing large resistive loads. A
range of 0.1 mm to 0.7 mm may be the best for most applications.
Where non-circular apertures are used, the maximum dimension of the apertures should be 1.0 mm, and preferably 0.7 mm, and the minimum dimension should be 0.1 mm.
Secondly, to maintain transmission of speech signals to the microphone, the number of these entrance holes needs to be increased as their diameters or cross-sectional areas are reduced. The optimum number will depend on the electroacoustic target of the transmit part of the handset and to what extent the impedance presented by the entrance holes is incorporated into the design. Single and double entrance holes have been identified as leading to airflow noise. Thus, a minimum of three entrance holes is necessary.
Thirdly, the sound tube leading from the backing cavity to the microphone transducer must not be in direct line with any of the small entrance holes. It is therefore preferred that a number of very small entrance holes lead into a backing cavity, with a sound tube having indirect alignment with entrance holes leading from the cavity to the microphone transducer. By using an array of entrance holes, there will be an azimuthal symmetry for both the acoustical signal and the residual formation of airflow noise, so the noise reductions will be achieved for a wider range of orientations of handset with respect to the user. It may be noted that although the prior art device of Lazzeroni et al. appears to have small holes, of unspecified size, some of these are in direct line with sound tubes leading to the microphone transducer.
In accordance with the present invention therefore, a casing for directing sound into a microphone transducer defines an acoustical passage which includes:
at least three entrance apertures communicating with the exterior of the casing, each said entrance aperture having a maximum dimension not greater than 1 mm;
a chamber communicating with the entrance apertures; and at least one sound conduit connecting the chamber with the microphone transducer, the entrance apertures being arranged so that none are aligned with the sound conduit.
Preferably, the entrance apertures are between 0.1 and 0.7 mm in maximum dimension. Most preferably, the entrance apertures are circular holes which have a diameter between 0.1 and 0.7 mm diameter, although holes of less than 0.5 mm diameter may be used in some cases.
Between 10 and 40 entrance holes may be used; the larger number being needed where the holes are small. Preferably the holes surround a central area of the chamber, and the sound conduit communicates with the central area.
BRIEF DESCRIPTION OF THE DRAWINGS:
A preferred embodiment of the invention will now be described with reference to the accompanying drawings, in which:
Fig. 1 shows a sectional view through a mouthpiece of a prior art telephone handset;
Fig. 2 shows an enlarged view of the microphone portion of the prior art mouthpiece, showing air flow;
Figs. 3 and 4 illustrate an experimental set-up used for testing prior art mouthpieces and that of the present invention;
Fig. 5 is a graph showing the noise spectra obtained with tests on the prior art mouthpiece as shown in Fig. 1;
Fig. 6 is a plan view of another prior art mouthpiece used in the tests;
Figs. 7, 8 and 9 are graphs showing the noise spectra of the mouthpiece of Fig.
6 compared to the mouthpiece of Fig. 1, at different azimuth angles;
Figs. 10a, lOb, and lOc are diagrammatic drawings showing air flows in the mouthpiece of Fig.6;
Fig. 11 is a graph showing noise spectra obtained with the mouthpiece of Fig.
6, when an air deflector is used;
Fig. 12 shows a perspective view of a preferred embodiment of the present invention;
Fig, 13 shows a sectional view through the mouthpiece of Fig. 12;
Fig.14 shows an experimental version of the present invention used to test the its performance characteristics;
Secondly, to maintain transmission of speech signals to the microphone, the number of these entrance holes needs to be increased as their diameters or cross-sectional areas are reduced. The optimum number will depend on the electroacoustic target of the transmit part of the handset and to what extent the impedance presented by the entrance holes is incorporated into the design. Single and double entrance holes have been identified as leading to airflow noise. Thus, a minimum of three entrance holes is necessary.
Thirdly, the sound tube leading from the backing cavity to the microphone transducer must not be in direct line with any of the small entrance holes. It is therefore preferred that a number of very small entrance holes lead into a backing cavity, with a sound tube having indirect alignment with entrance holes leading from the cavity to the microphone transducer. By using an array of entrance holes, there will be an azimuthal symmetry for both the acoustical signal and the residual formation of airflow noise, so the noise reductions will be achieved for a wider range of orientations of handset with respect to the user. It may be noted that although the prior art device of Lazzeroni et al. appears to have small holes, of unspecified size, some of these are in direct line with sound tubes leading to the microphone transducer.
In accordance with the present invention therefore, a casing for directing sound into a microphone transducer defines an acoustical passage which includes:
at least three entrance apertures communicating with the exterior of the casing, each said entrance aperture having a maximum dimension not greater than 1 mm;
a chamber communicating with the entrance apertures; and at least one sound conduit connecting the chamber with the microphone transducer, the entrance apertures being arranged so that none are aligned with the sound conduit.
Preferably, the entrance apertures are between 0.1 and 0.7 mm in maximum dimension. Most preferably, the entrance apertures are circular holes which have a diameter between 0.1 and 0.7 mm diameter, although holes of less than 0.5 mm diameter may be used in some cases.
Between 10 and 40 entrance holes may be used; the larger number being needed where the holes are small. Preferably the holes surround a central area of the chamber, and the sound conduit communicates with the central area.
BRIEF DESCRIPTION OF THE DRAWINGS:
A preferred embodiment of the invention will now be described with reference to the accompanying drawings, in which:
Fig. 1 shows a sectional view through a mouthpiece of a prior art telephone handset;
Fig. 2 shows an enlarged view of the microphone portion of the prior art mouthpiece, showing air flow;
Figs. 3 and 4 illustrate an experimental set-up used for testing prior art mouthpieces and that of the present invention;
Fig. 5 is a graph showing the noise spectra obtained with tests on the prior art mouthpiece as shown in Fig. 1;
Fig. 6 is a plan view of another prior art mouthpiece used in the tests;
Figs. 7, 8 and 9 are graphs showing the noise spectra of the mouthpiece of Fig.
6 compared to the mouthpiece of Fig. 1, at different azimuth angles;
Figs. 10a, lOb, and lOc are diagrammatic drawings showing air flows in the mouthpiece of Fig.6;
Fig. 11 is a graph showing noise spectra obtained with the mouthpiece of Fig.
6, when an air deflector is used;
Fig. 12 shows a perspective view of a preferred embodiment of the present invention;
Fig, 13 shows a sectional view through the mouthpiece of Fig. 12;
Fig.14 shows an experimental version of the present invention used to test the its performance characteristics;
Fig.lS shows noise spectra obtained with the experimental version shown in Fig. 14; and Fig. l6 shows the noise spectra obtained with the preferred embodiment of Figs.l2 and 13.
DETAILED DESCRIPTION
Fig. 1 shows a prior art handset whose mouthpiece has a casing 10 with a single entrance hole 12, of 2.3 mm diameter, leading via a sound conduit 14 to a micro-phone transducer 16. The location of the transducer is 18 mm below the surface of the mouthpiece, which is typical for compliance with electrostatic discharge require-ments. Measurements of the airflow noise generated in a test handset have been made; these serve as a baseline to quantify noise mitigation approaches. Fig.
shows an enlarged view of the hole 12 and the sound conduit, and shows the mechanism of turbulence generated with a single hole mouthpiece of this kind.
1. Tests on prior art devices: General Figs. 3 and 4 illustrate the airflow simulator which was constructed to simulate the gusts of air that project from the mouth while talking, particularly the plosive sounds.
The source of air was a compressed air line with a pressure reduction valve.
The exit nozzle 20 was stainless steel tubing (3 mm OD / 2 mm ID). The apparatus produced a collimated stream of air that made no perceivable noise. At a distance of 20 mm from the nozzle, the air stream has a width of 2 mm. Measurements were obtained using a TSI hot wire anemometer to measure flow velocities. Additional measurements showed that the stream was maintained for a distance of approximately 50 mm.
By having such a narrow stream of air, specific features on the mouthpiece can be selected for acoustical excitation.
The anemometer was also used to survey the velocity of air flowing from the mouth when a talker produced "p" sounds. A range of values was obtained that depended on the strength of the plosive sound and how long it was sustained. At a distance of 20 mm from the mouth, air speeds of 6 - 18 m/s were found. These speeds are comparable to those produced by the airflow simulator.
The airflow simulator was used as indicated in Fig. 4. For all measurements, the nozzle 20 was 20 mm away from the mouthpiece 10. Both handset and airflow simulator were positioned on tripods located some distance from other objects that might affect the local airflow. Both azimuthal angle cp and elevation angle 0, which are illustrated in Fig. 3, were controlled. There is a wide range of positions in which a handset might be held during typical usage. It is anticipated that the elevation angle would be between 30° and 90° and that the azimuthal angle would be in the range of -135° to + 135° for normal usage; often, though, a handset may be seen used with an angle of 180° when part of an intercom system.
2. Measurement of noise spectra for prior art devices The flow of air over the holes caused broadband noise, clearly audible. To quantify these observations, noise spectra were obtained under various conditions. The measurement system is sketched in Fig. 4 . The signal generated at the microphone was fed to a HP 35670A spectrum analyzer 22 via a bias 23 and an amplifier 24.
There was ample signal-to-noise and no gain from the measuring amplifier 24 (B&K
2610) was required. Spectra were obtained for a frequency range of 0 - 12.8 kHz, with a resolution of 32 Hz. Each spectrum is the rms average of 16 consecutive spectra.
The equivalent noise level can be obtained from a measured spectrum. The different frequency components need to be weighted according to their importance to the listener. An A-weighting has been accepted as the best compromise; this is a recognized standard in audio testing and provides a standard compensation for the fact that the average person's ears have differing sensitivity to different frequencies of sound.
For a measured spectrum V" and an A-weighting function Wn, with n being the frequency index, the total power is:
DETAILED DESCRIPTION
Fig. 1 shows a prior art handset whose mouthpiece has a casing 10 with a single entrance hole 12, of 2.3 mm diameter, leading via a sound conduit 14 to a micro-phone transducer 16. The location of the transducer is 18 mm below the surface of the mouthpiece, which is typical for compliance with electrostatic discharge require-ments. Measurements of the airflow noise generated in a test handset have been made; these serve as a baseline to quantify noise mitigation approaches. Fig.
shows an enlarged view of the hole 12 and the sound conduit, and shows the mechanism of turbulence generated with a single hole mouthpiece of this kind.
1. Tests on prior art devices: General Figs. 3 and 4 illustrate the airflow simulator which was constructed to simulate the gusts of air that project from the mouth while talking, particularly the plosive sounds.
The source of air was a compressed air line with a pressure reduction valve.
The exit nozzle 20 was stainless steel tubing (3 mm OD / 2 mm ID). The apparatus produced a collimated stream of air that made no perceivable noise. At a distance of 20 mm from the nozzle, the air stream has a width of 2 mm. Measurements were obtained using a TSI hot wire anemometer to measure flow velocities. Additional measurements showed that the stream was maintained for a distance of approximately 50 mm.
By having such a narrow stream of air, specific features on the mouthpiece can be selected for acoustical excitation.
The anemometer was also used to survey the velocity of air flowing from the mouth when a talker produced "p" sounds. A range of values was obtained that depended on the strength of the plosive sound and how long it was sustained. At a distance of 20 mm from the mouth, air speeds of 6 - 18 m/s were found. These speeds are comparable to those produced by the airflow simulator.
The airflow simulator was used as indicated in Fig. 4. For all measurements, the nozzle 20 was 20 mm away from the mouthpiece 10. Both handset and airflow simulator were positioned on tripods located some distance from other objects that might affect the local airflow. Both azimuthal angle cp and elevation angle 0, which are illustrated in Fig. 3, were controlled. There is a wide range of positions in which a handset might be held during typical usage. It is anticipated that the elevation angle would be between 30° and 90° and that the azimuthal angle would be in the range of -135° to + 135° for normal usage; often, though, a handset may be seen used with an angle of 180° when part of an intercom system.
2. Measurement of noise spectra for prior art devices The flow of air over the holes caused broadband noise, clearly audible. To quantify these observations, noise spectra were obtained under various conditions. The measurement system is sketched in Fig. 4 . The signal generated at the microphone was fed to a HP 35670A spectrum analyzer 22 via a bias 23 and an amplifier 24.
There was ample signal-to-noise and no gain from the measuring amplifier 24 (B&K
2610) was required. Spectra were obtained for a frequency range of 0 - 12.8 kHz, with a resolution of 32 Hz. Each spectrum is the rms average of 16 consecutive spectra.
The equivalent noise level can be obtained from a measured spectrum. The different frequency components need to be weighted according to their importance to the listener. An A-weighting has been accepted as the best compromise; this is a recognized standard in audio testing and provides a standard compensation for the fact that the average person's ears have differing sensitivity to different frequencies of sound.
For a measured spectrum V" and an A-weighting function Wn, with n being the frequency index, the total power is:
P = ~ 1~ (vn-Wn)l10 (1) Conversion to equivalent sound pressure levels requires specification of the microphone sensitivity s. Subsidiary measurements on the various microphone assemblies used in this study gave a typical sensitivity of s = 18 mV/Pa. The equivalent noise level (in dB) is then N = 10 log [ P l ( po s)2 ] (2) where po is the reference sound pressure level of 2 x 10'5 Pa.
3. Tests on mouthpiece with single entrance hole Tests were done on a handset whose mouthpiece has a single entrance hole leading to the microphone transducer, as sketched in Figs. l and 3. Measurements of the airflow noise generated in a test handset have been made; these serve as a baseline to quantify noise mitigation approaches.
Measured power spectra are shown in Fig. 5 for three angles of elevation of incident airflow, all for an azimuthal angle of 0° . Noise levels computed via Eq. (2) from the measured power spectra are 110 dB, 115 dB and 94 dB, for elevation angles of 45°, 60° and 90°, respectively. Variations with azimuth are found to be relatively small, as might be expected for a single entrance hole. Several mod ifications were examined, including bevelling of the entrance hole and locating the entrance hole in a shallow slot. These were found to give insignificant reductions in airflow noise.
Averaging the measurements over azimuth angles and for the various modifications, noise levels of 110 dB, 116 dB and 98 dB are obtained for the elevation angles of 45 °, 60° and 90°, respectively. These are the baseline values that will be used.
The problem with the use of a single microphone hole is that airflow catching the entranceway quickly creates an over pressure inside the hole. Incoming air is then redirected back out. The outgoing flow crosses the ingoing flow, as sketched in Fig.2, leading to the generation of turbulence and, hence, acoustical noise, in this region.
4. Tests on mouthpiece with two entrance holes The use of two holes leading to the microphone can reduce some of the noise generated by this pressure buildup mechanism. Airflow that catches one entrance hole flows down this hole, across the microphone, and out the other hole; pressure buildup is avoided and the entranceway interaction is avoided. This approach was found to work quite well for some azimuthal angles of incidence and very badly for others.
The test handset used for this investigation is shown in Fig. 6. Of the three shallow slots 26, 27 and 28, each approximately 1.6 mm x 9 mm x 1.5 mm, the outer two and 28 are largely decorative. From the top slot 26, two holes 26a and 26b, each of 2 mm diameter, proceed inward. The two holes join inside near the microphone transducer position. For the tests, the airflow was directed at the hole labeled 26a.
The microphone transducer inside is the same as that used in the single hole handset.
The airflow noise spectrum with this two-hole mouthpiece, for an azimuth of 0° and an elevation of 45°, is shown in Fig.7. The spectrum is 15-20 dB less than that of the single-hole handset. The computed noise level, averaging per Eq. (2), is 93 dB. This represents a 17 dB reduction in the noise level. In Fig. 8, the spectrum for an azimuth of 90° and an elevation of 45 ° is shown. The noise level of 102 dB is 14 dB below the level of the single-hole handset. In Fig. 9, the spectrum for an azimuth of 270° and an elevation of 45° is shown. Here, however, the noise levels are not lower. They are actually higher by 10 dB.
The huge difference between the noise levels for azimuths of 90° and 270° can be explained through examination of the airflow patterns in the handset, as illustrated in Fig. 10. At 90° azimuth, as shown in Fig. 10a, the air entering hole 26a can flow into the handset, past the microphone, and out the other hole, thus avoiding a pressure buildup and subsequent turbulent flow and noise generation. For the 270° azimuth though, as shown in Fig.lOb, the air coming out of the exit hole 26b interferes with the incoming flow into hole 26a, causing significant turbulence and, hence, noise generation. To test this hypothesis, a deflector 28 was installed, as shown in Fig. lOc, to prevent the interaction of the two air streams. The results are shown in Fig. 11.
The solid curve is the noise spectrum for an azimuth of 90°. The dashed and dotted curves are for an azimuth of 270°, without and with a deflector, respectively. Without the deflector, the noise levels are much higher. With the deflector, the noise spectra is comparable or lower than the spectra for 90°. The use of a deflector is not practical, but it does point out the need to avoid air flows that interfere with each other.
5. The present invention: Mouthpiece with multiple entrance holes The previous results lead into the current invention. To avoid the pressure buildup problem, more than one entrance hole is necessary. With two entrance holes, significant reductions were obtained for some angles of incidence but increases were obtained for others, because of interacting flow streams. It was realized that the degree of flow interaction could be reduced considerably by using a large number of entrance holes. With a large number of holes, the outgoing flow is distributed between many separate holes. The flow rate out any one hole is much less, so even if an outgoing flow stream collides with the ingoing airflow, the degree of interaction is much reduced compared to the two-hole approach.
The degree of flow interaction will be related to the cross-sectional area of the flow streams. Therefore, the smaller the entrance hole, the lower the airflow noise. It is necessary, of course, to ensure that the desired acoustical signals are transmitted to the microphone; the number of entrance holes will need to be increased as the hole diameter is decreased.
Figs. 12 and 13 show a preferred form of the invention. The mouthpiece casing has an array comprising two circular series of circular holes 112 arranged around a central, unapertured space, giving a total of 16 holes. All holes are less than 1.0 mm in diameter. As shown in Fig. l3, the holes lead into a backing chamber 113 which is circular and coaxially underlies the holes. A sound tube 114 leads from the bottom of the center of the chamber 113 to the microphone transducer 16. None of the holes 112 is aligned with the sound tube 114.
Tests on multiple hole mouthpiece Before the preferred embodiment of Figs. l2 and 13 was made, three handsets were fabricated to test the effect of multiple holes leading to a space, with all holes being non-aligned with the sound tube. Both handsets have 15 holes, 3 mm long, leading into a cylindrical chamber approximately 2 mm deep and 10 mm in diameter. Figure 14 shows a sketch of the arrangement. For one configuration, the holes are each 0.97 mm in diameter; for the second, the holes are 0.57 mm in diameter, and for the third they are 0.34 mm diameter. The holes are evenly distributed over a roughly circular area of the mouthpiece. The microphone hole leading from the chamber to the microphone is slanted because of the modular construction of this test handset and does not need to be duplicated in a practical implementation. All measurements reported here use a 270° azimuth (with many holes, though, no azimuthal dependence is anticipated and, in fact, no such dependence was observed). The airflow was directed at different parts of the mouthpiece. The "bottom" region is closest to the 180° azimuth end of the handset, the "top" region is closest to the 0°.
Some of the measured noise spectra for these test handsets are shown in Fig.
15.
Several observations can be made. It is evident that, in all cases, the spectra are well below the baseline spectrum for the baseline handset with single entrance hole. The best spectra show noise reductions of over 30 dB. Second, spectra for airflow directed at the "bottom" give higher noise levels. The air getting through the holes flows directly into the large hole leading to the microphone, causing a return to the one-hole problem of over pressure and turbulence generation in the microphone hole.
When the airflow is directed at the "center" or "top" positions, the spectra are lower and much lower, respectively. Third, the size of the holes are very important. The spectra for the handset with the smaller 0.57 mm holes are consistently lower than the spectra for the handset with 0.97 mm holes, and the spectra for the handset with 0.34 mm holes are even less.
Tests on Preferred Embodiment The preferred form of the invention shown in Figs.12 and 13 was also made and tested, in this case with circular holes of 0.57 mm. diameter. The results, shown in Fig. l6, indicate an even greater improvement in noise reduction.
CONCLUSIONS
The comparison of the different approaches for reducing airflow noise can be made easier by reducing the spectra to single number noise levels according to Eq.
(2). The A-weighting corresponds reasonably well to the noisiness that would be perceived by a listener. A summary of the equivalent noise levels measured for the various approaches used to reduce wind noise is shown in Table 1. It is clear from this summary that the use of a single entrance hole leads to relatively high airflow noise levels. The use of two entrance holes can achieve very low noise levels, for certain angles of incidence. But for other angles of incidence, those for which outgoing and ingoing air flows collide, noise levels are very high. The multiple entrance hole approach gives dramatically reduced noise levels for all angles of incident air flow.
It is clear from Table 1 that the best results are obtained by using smaller entrance holes (c.f., using 0.57 mm or 0.34 mm. diameter holes rather than 0.97 mm holes).
For diameters of less than 0.1 mm, though, viscous and thermal boundary layer effects may limit acoustical performance by introducing larger resistive acoustical loads. We propose that a range of 0.1 mm to 1 mm will give good results for most applications, but a range of 0.1 to 0.7 mm is preferable.
When the entrance hole diameters are decreased, the number of holes needs to be increased to maintain the acoustical performance for transmitting speech signals. The optimal number of entrance holes will depend on the electroacoustic target of the transmit part of the handset and to what extent the impedance presented by the entrance holes is incorporated into the design. To avoid significant airflow noise due to flow interactions, an absolute minimum of three entrance holes is necessary. More typically, 10-40 entrance holes would be suggested.
The data in Table 1 also indicate that the lowest noise levels are achieved when the air flow streams through the entrance holes do not enter the sound tube leading from the backing cavity to the microphone (i.e., compare the results for "top", "center" and "bottom" positions of airflow). The mouthpiece design should avoid having any entrance holes in direct line with the sound tube leading to the microphone.
Table 1.
Comparison of equivalent A-weighted noise levels (in dB) for various test handsets.
The angles of azimuth and elevation define the direction of incident airflow.
The multiple hole approaches are independent of azimuthal angle so only azimuth of 270 was used in measurements. The "top", "center", and "bottom" indicate where in the array of entrance holes the air flow was directed; the "top"
position was farthest away from the inner sound tube leading to the microphone.
Azimuth 0 90 180 270 ---Elevation 45 60 45 60 45 60 45 60 90 One entrance 110 116 110 116 110 116 110 116 98 hole Two entrance 93 99 102 95 120 113 102 hole Multiple entrance holes 0.34 mm. (center) 46 48 46 0.57 mm (center) 81 63 51 0.57 mm (bottom) 87 99 98 0.97 mm (top) 79 85 78 0.97 mm (center) 84 90 91 0.97 mm (bottom) 103 106 105 0.57 mm(annulus) 76 The use of an array of entrance holes has another advantage. As well as having the same, reduced airflow noise levels for all azimuthal angles, the acoustical response to speech signals will be the same for all azimuthal directions.
In summary, the lowest airflow noise levels are achieved by using (a) large number of entrance holes into a backing chamber, (b) holes with a small a cross section, and (c) a sound tube leading to the microphone that is not aligned with any of the entrance holes; with these (d) the acoustical response of the microphone is independent of azi-muthal angle.
3. Tests on mouthpiece with single entrance hole Tests were done on a handset whose mouthpiece has a single entrance hole leading to the microphone transducer, as sketched in Figs. l and 3. Measurements of the airflow noise generated in a test handset have been made; these serve as a baseline to quantify noise mitigation approaches.
Measured power spectra are shown in Fig. 5 for three angles of elevation of incident airflow, all for an azimuthal angle of 0° . Noise levels computed via Eq. (2) from the measured power spectra are 110 dB, 115 dB and 94 dB, for elevation angles of 45°, 60° and 90°, respectively. Variations with azimuth are found to be relatively small, as might be expected for a single entrance hole. Several mod ifications were examined, including bevelling of the entrance hole and locating the entrance hole in a shallow slot. These were found to give insignificant reductions in airflow noise.
Averaging the measurements over azimuth angles and for the various modifications, noise levels of 110 dB, 116 dB and 98 dB are obtained for the elevation angles of 45 °, 60° and 90°, respectively. These are the baseline values that will be used.
The problem with the use of a single microphone hole is that airflow catching the entranceway quickly creates an over pressure inside the hole. Incoming air is then redirected back out. The outgoing flow crosses the ingoing flow, as sketched in Fig.2, leading to the generation of turbulence and, hence, acoustical noise, in this region.
4. Tests on mouthpiece with two entrance holes The use of two holes leading to the microphone can reduce some of the noise generated by this pressure buildup mechanism. Airflow that catches one entrance hole flows down this hole, across the microphone, and out the other hole; pressure buildup is avoided and the entranceway interaction is avoided. This approach was found to work quite well for some azimuthal angles of incidence and very badly for others.
The test handset used for this investigation is shown in Fig. 6. Of the three shallow slots 26, 27 and 28, each approximately 1.6 mm x 9 mm x 1.5 mm, the outer two and 28 are largely decorative. From the top slot 26, two holes 26a and 26b, each of 2 mm diameter, proceed inward. The two holes join inside near the microphone transducer position. For the tests, the airflow was directed at the hole labeled 26a.
The microphone transducer inside is the same as that used in the single hole handset.
The airflow noise spectrum with this two-hole mouthpiece, for an azimuth of 0° and an elevation of 45°, is shown in Fig.7. The spectrum is 15-20 dB less than that of the single-hole handset. The computed noise level, averaging per Eq. (2), is 93 dB. This represents a 17 dB reduction in the noise level. In Fig. 8, the spectrum for an azimuth of 90° and an elevation of 45 ° is shown. The noise level of 102 dB is 14 dB below the level of the single-hole handset. In Fig. 9, the spectrum for an azimuth of 270° and an elevation of 45° is shown. Here, however, the noise levels are not lower. They are actually higher by 10 dB.
The huge difference between the noise levels for azimuths of 90° and 270° can be explained through examination of the airflow patterns in the handset, as illustrated in Fig. 10. At 90° azimuth, as shown in Fig. 10a, the air entering hole 26a can flow into the handset, past the microphone, and out the other hole, thus avoiding a pressure buildup and subsequent turbulent flow and noise generation. For the 270° azimuth though, as shown in Fig.lOb, the air coming out of the exit hole 26b interferes with the incoming flow into hole 26a, causing significant turbulence and, hence, noise generation. To test this hypothesis, a deflector 28 was installed, as shown in Fig. lOc, to prevent the interaction of the two air streams. The results are shown in Fig. 11.
The solid curve is the noise spectrum for an azimuth of 90°. The dashed and dotted curves are for an azimuth of 270°, without and with a deflector, respectively. Without the deflector, the noise levels are much higher. With the deflector, the noise spectra is comparable or lower than the spectra for 90°. The use of a deflector is not practical, but it does point out the need to avoid air flows that interfere with each other.
5. The present invention: Mouthpiece with multiple entrance holes The previous results lead into the current invention. To avoid the pressure buildup problem, more than one entrance hole is necessary. With two entrance holes, significant reductions were obtained for some angles of incidence but increases were obtained for others, because of interacting flow streams. It was realized that the degree of flow interaction could be reduced considerably by using a large number of entrance holes. With a large number of holes, the outgoing flow is distributed between many separate holes. The flow rate out any one hole is much less, so even if an outgoing flow stream collides with the ingoing airflow, the degree of interaction is much reduced compared to the two-hole approach.
The degree of flow interaction will be related to the cross-sectional area of the flow streams. Therefore, the smaller the entrance hole, the lower the airflow noise. It is necessary, of course, to ensure that the desired acoustical signals are transmitted to the microphone; the number of entrance holes will need to be increased as the hole diameter is decreased.
Figs. 12 and 13 show a preferred form of the invention. The mouthpiece casing has an array comprising two circular series of circular holes 112 arranged around a central, unapertured space, giving a total of 16 holes. All holes are less than 1.0 mm in diameter. As shown in Fig. l3, the holes lead into a backing chamber 113 which is circular and coaxially underlies the holes. A sound tube 114 leads from the bottom of the center of the chamber 113 to the microphone transducer 16. None of the holes 112 is aligned with the sound tube 114.
Tests on multiple hole mouthpiece Before the preferred embodiment of Figs. l2 and 13 was made, three handsets were fabricated to test the effect of multiple holes leading to a space, with all holes being non-aligned with the sound tube. Both handsets have 15 holes, 3 mm long, leading into a cylindrical chamber approximately 2 mm deep and 10 mm in diameter. Figure 14 shows a sketch of the arrangement. For one configuration, the holes are each 0.97 mm in diameter; for the second, the holes are 0.57 mm in diameter, and for the third they are 0.34 mm diameter. The holes are evenly distributed over a roughly circular area of the mouthpiece. The microphone hole leading from the chamber to the microphone is slanted because of the modular construction of this test handset and does not need to be duplicated in a practical implementation. All measurements reported here use a 270° azimuth (with many holes, though, no azimuthal dependence is anticipated and, in fact, no such dependence was observed). The airflow was directed at different parts of the mouthpiece. The "bottom" region is closest to the 180° azimuth end of the handset, the "top" region is closest to the 0°.
Some of the measured noise spectra for these test handsets are shown in Fig.
15.
Several observations can be made. It is evident that, in all cases, the spectra are well below the baseline spectrum for the baseline handset with single entrance hole. The best spectra show noise reductions of over 30 dB. Second, spectra for airflow directed at the "bottom" give higher noise levels. The air getting through the holes flows directly into the large hole leading to the microphone, causing a return to the one-hole problem of over pressure and turbulence generation in the microphone hole.
When the airflow is directed at the "center" or "top" positions, the spectra are lower and much lower, respectively. Third, the size of the holes are very important. The spectra for the handset with the smaller 0.57 mm holes are consistently lower than the spectra for the handset with 0.97 mm holes, and the spectra for the handset with 0.34 mm holes are even less.
Tests on Preferred Embodiment The preferred form of the invention shown in Figs.12 and 13 was also made and tested, in this case with circular holes of 0.57 mm. diameter. The results, shown in Fig. l6, indicate an even greater improvement in noise reduction.
CONCLUSIONS
The comparison of the different approaches for reducing airflow noise can be made easier by reducing the spectra to single number noise levels according to Eq.
(2). The A-weighting corresponds reasonably well to the noisiness that would be perceived by a listener. A summary of the equivalent noise levels measured for the various approaches used to reduce wind noise is shown in Table 1. It is clear from this summary that the use of a single entrance hole leads to relatively high airflow noise levels. The use of two entrance holes can achieve very low noise levels, for certain angles of incidence. But for other angles of incidence, those for which outgoing and ingoing air flows collide, noise levels are very high. The multiple entrance hole approach gives dramatically reduced noise levels for all angles of incident air flow.
It is clear from Table 1 that the best results are obtained by using smaller entrance holes (c.f., using 0.57 mm or 0.34 mm. diameter holes rather than 0.97 mm holes).
For diameters of less than 0.1 mm, though, viscous and thermal boundary layer effects may limit acoustical performance by introducing larger resistive acoustical loads. We propose that a range of 0.1 mm to 1 mm will give good results for most applications, but a range of 0.1 to 0.7 mm is preferable.
When the entrance hole diameters are decreased, the number of holes needs to be increased to maintain the acoustical performance for transmitting speech signals. The optimal number of entrance holes will depend on the electroacoustic target of the transmit part of the handset and to what extent the impedance presented by the entrance holes is incorporated into the design. To avoid significant airflow noise due to flow interactions, an absolute minimum of three entrance holes is necessary. More typically, 10-40 entrance holes would be suggested.
The data in Table 1 also indicate that the lowest noise levels are achieved when the air flow streams through the entrance holes do not enter the sound tube leading from the backing cavity to the microphone (i.e., compare the results for "top", "center" and "bottom" positions of airflow). The mouthpiece design should avoid having any entrance holes in direct line with the sound tube leading to the microphone.
Table 1.
Comparison of equivalent A-weighted noise levels (in dB) for various test handsets.
The angles of azimuth and elevation define the direction of incident airflow.
The multiple hole approaches are independent of azimuthal angle so only azimuth of 270 was used in measurements. The "top", "center", and "bottom" indicate where in the array of entrance holes the air flow was directed; the "top"
position was farthest away from the inner sound tube leading to the microphone.
Azimuth 0 90 180 270 ---Elevation 45 60 45 60 45 60 45 60 90 One entrance 110 116 110 116 110 116 110 116 98 hole Two entrance 93 99 102 95 120 113 102 hole Multiple entrance holes 0.34 mm. (center) 46 48 46 0.57 mm (center) 81 63 51 0.57 mm (bottom) 87 99 98 0.97 mm (top) 79 85 78 0.97 mm (center) 84 90 91 0.97 mm (bottom) 103 106 105 0.57 mm(annulus) 76 The use of an array of entrance holes has another advantage. As well as having the same, reduced airflow noise levels for all azimuthal angles, the acoustical response to speech signals will be the same for all azimuthal directions.
In summary, the lowest airflow noise levels are achieved by using (a) large number of entrance holes into a backing chamber, (b) holes with a small a cross section, and (c) a sound tube leading to the microphone that is not aligned with any of the entrance holes; with these (d) the acoustical response of the microphone is independent of azi-muthal angle.
Claims (10)
1. A casing for directing sound into a microphone, wherein the casing defines an acoustical passage including:
at least three entrance apertures communicating with the exterior of the casing, each said entrance aperture having a maximum dimension not greater than 1 a chamber communicating with the entrance apertures; and at least one sound conduit connecting the chamber with the microphone transducer, the entrance apertures being arranged so that none are aligned with the sound conduit.
at least three entrance apertures communicating with the exterior of the casing, each said entrance aperture having a maximum dimension not greater than 1 a chamber communicating with the entrance apertures; and at least one sound conduit connecting the chamber with the microphone transducer, the entrance apertures being arranged so that none are aligned with the sound conduit.
2. A casing according to claim 1, wherein said entrance apertures are less than 0.7 mm in maximum dimension.
3. A casing according to claim 1, wherein from 10 to 40 entrance apertures are provided.
4. A casing according to claim 1, wherein said entrance apertures each have a minimum dimension of at least 0.1 mm.
5. A casing according to claim 1, wherein said entrance apertures each have maximum and minimum dimensions between 0.5 mm and 0.1 mm.
6. A casing according to claim 1, wherein said entrance apertures are arranged to surround a central area of said chamber, and wherein said sound conduit communicates with said central area of the chamber.
7. A casing according to claim 1, wherein the apertures are circular holes.
8. A telephone handset incorporating a casing in accordance with claim 1.
9. A telephone handset having microphone transducer and a casing which defines an acoustical passage into the microphone transducer, said acoustical passage having from 10 to 40 circular entrance apertures arranged to surround a central chamber which communicates with the entrance apertures, each of said apertures having a diameter of less than 1.0 mm.
and a sound conduit connecting the chamber with the microphone transducer, the sound conduit communicating with said central area of the chamber and being out of alignment with all of the entrance apertures.
and a sound conduit connecting the chamber with the microphone transducer, the sound conduit communicating with said central area of the chamber and being out of alignment with all of the entrance apertures.
10. A telephone handset according to claim 9, wherein said apertures all have a diameter of less than 0.7 mm.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA 2314862 CA2314862A1 (en) | 1999-08-06 | 2000-08-02 | Arrangement for directing sound into a microphone with reduced noise, especially in handsets |
Applications Claiming Priority (3)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| CA2,279,719 | 1999-08-06 | ||
| CA 2279719 CA2279719A1 (en) | 1999-08-06 | 1999-08-06 | Device for airflow noise reduction for handsets |
| CA 2314862 CA2314862A1 (en) | 1999-08-06 | 2000-08-02 | Arrangement for directing sound into a microphone with reduced noise, especially in handsets |
Publications (1)
| Publication Number | Publication Date |
|---|---|
| CA2314862A1 true CA2314862A1 (en) | 2001-02-06 |
Family
ID=25681107
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| CA 2314862 Abandoned CA2314862A1 (en) | 1999-08-06 | 2000-08-02 | Arrangement for directing sound into a microphone with reduced noise, especially in handsets |
Country Status (1)
| Country | Link |
|---|---|
| CA (1) | CA2314862A1 (en) |
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US6741717B2 (en) | 2001-09-28 | 2004-05-25 | Mitel Knowledge Corporation | Device for reducing structural-acoustic coupling between the diaphragm vibration field and the enclosure acoustic modes |
| CN113132839A (en) * | 2020-11-24 | 2021-07-16 | 美律电子(深圳)有限公司 | Electronic device |
| WO2023273963A1 (en) * | 2021-06-29 | 2023-01-05 | 影石创新科技股份有限公司 | Microphone assembly and camera |
-
2000
- 2000-08-02 CA CA 2314862 patent/CA2314862A1/en not_active Abandoned
Cited By (4)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US6741717B2 (en) | 2001-09-28 | 2004-05-25 | Mitel Knowledge Corporation | Device for reducing structural-acoustic coupling between the diaphragm vibration field and the enclosure acoustic modes |
| CN113132839A (en) * | 2020-11-24 | 2021-07-16 | 美律电子(深圳)有限公司 | Electronic device |
| CN113132839B (en) * | 2020-11-24 | 2023-03-07 | 美律电子(深圳)有限公司 | Electronic device with a detachable cover |
| WO2023273963A1 (en) * | 2021-06-29 | 2023-01-05 | 影石创新科技股份有限公司 | Microphone assembly and camera |
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