Active noise control stethoscope - Patent Review 5539831

I claim:

1. In an electronic stethoscope, including:

first sensor means for detecting sound exclusively from a predetermined source and converting said sound to a first electrical signal;

second sensor means for primarily sensing background sounds and not said sound from a predetermined source and converting said background sounds to a second electrical signal;

digital signal processor means for correlating said first and second signals generated by the respective sensor means in order to obtain a filtered electrical signal from which said background sounds have been cancelled; and

speaker means responsive to said filtered electrical signal for reproducing said sound from said predetermined source,

the improvement wherein said first and second sensor means are affixed to a common housing arranged such that the second sensor means senses external noise in a vicinity of the main sensor but not sounds made by the predetermined source, and wherein said first sensor means has an impedance which is substantially different from the impedance of air.

2. A stethoscope as claimed in claim 1, wherein said first sensor means is a piezoceramic transflexural actuator.

3. A stethoscope as claimed in claim 2, wherein said piezoelectric transflexural actuator comprises a thin layer of piezoelectric material bonded to a thin conductive metal disc.

4. A stethoscope as claimed in claim 3, wherein said conductive metal disc is bonded to a conductive metal cylinder which closes any direct path by which airborne noise can reach the piezoelectric element.

5. A stethoscope as claimed in claim 4, wherein said first sensor means further includes a waterproof insulating coating on an active side of the actuator for broadening a resonance frequency of the actuator.

6. A stethoscope as claimed in claim 4, wherein said second sensor means is positioned adjacent the first sensor means, and wherein said first sensor means faces the predetermined source and is between the predetermined source and the second sensor means.

7. A stethoscope as claimed in claim 4, further comprising a second layer of piezoelectric material bonded to a thin conductive metal disc which in turn is bonded to an end of said cylinder which is opposite to an end of said cylinder to which said first metal disc is bonded, the second layer and disc forming a second sensor to serve as a backup for the sensor means or as a reference source for active noise control.

8. A stethoscope as claimed in claim 4, wherein said cylinder is made of brass.

9. A stethoscope as claimed in claim 1, further comprising a third sensor means arranged to be positioned on the predetermined source but away from the first sensor means to thereby form a means for sensing background sounds which have coupled to the predetermined source.

10. A stethoscope as claimed in claim 9, wherein said third sensor means is identical to said first sensor means.

11. A stethoscope as claimed in claim 9, wherein said third sensor means is a microphone.

12. A stethoscope as claimed in claim 9, further comprising a third sensor means positioned in the vicinity of the first sensor means but but facing in a direction different from the direction faced by the predetermined source so as to sense background sounds in the vicinity of the first sensor means.

13. A stethoscope as claimed in claim 1, wherein said processing means is a digital signal processor and said digital signal processor includes means for implementing an LMS algorithm.

14. A stethoscope as claimed in claim 1, further comprising means for inverting a signal sent to the speaker means in order to exploit a binaural masking level difference effect.

15. In an active noise control system, including:

first sensor means for detecting sound from a predetermined source and converting said sound to a first electrical signal;

second sensor means for sensing background sound and converting said background sound to a second electrical signal;

digital signal processor means for correlating said first and second signals generated by the respective sensor means in order to obtain a filtered electrical signal from which said background sounds have been cancelled; and

speaker means responsive to said filtered electrical signal for reproducing said sound from said predetermined source,

the improvement comprising means for generating an antinoise signal to which the speaker is also responsive in order to minimize noise perceived by a user of a headset in which said speaker is situated, wherein the antinoise cancelling signal is the sum of two components: 1.) a filtered-X LMS generated signal that uses an outer earmuff microphone for an input microphone and an inner earmuff microphone as an error microphone, in order to cancel periodic acoustical noise, and 2.) a random noise cancelling signal for cancelling random acoustical noise based on input solely from the inner microphone.

16. A system as claimed in claim 15, further comprising means for inverting a signal sent to the speaker means in order to exploit a binaural masking level effect.

17. A system as claimed in claim 15, wherein said processor means includes a digital filter having an input x.sub.n and an output y.sub.n,

wherein ##EQU4## and wherein the coefficients ##STR5## of the antinoise digital filter are chosen so as to minimize a quantity representative of an ear sensitivity weighted sound pressure level.

18. A system as claimed in claim 17, wherein the ear sensitivity weighted sound pressure is given by the formula: ##EQU5## wherein the weighting factor W(f) is defined by the following equation:

wherein M(f) is a factor used to remove the frequency dependent sensitivity of the microphone, E(f) describes the frequency dependent sensitivity of the human ear, the unwanted source noise measured by the first microphone is given by p.sub.s (t), and the truncated Fourier transform of p.sub.s (t) over a time window of 2T is denoted by p.sub.s (T,f), which in turn is defined by the following equations: ##EQU6##

19. A system as claimed in claim 18, further comprising means for calculating a trapezoidal approximation for the functional value, gradient, and hessian of the ear sensitivity sound pressure level , using a modified Newton's method to find a minimum, and applying current values for P.sub.s (f) and the response ##STR6##

20. A system as claimed in claim 19, wherein the hessian is determined by the following procedure: ##EQU7##

21. A system as claimed in claim 15, further comprising means for checking for insufficiently damped modes.

22. In an electronic stethoscope, including:

first sensor means for detecting Sound exclusively from a predetermined source and converting said sound to a first electrical signal;

second sensor means for primarily sensing background sounds and not said sound from a predetermined source and converting said background sounds to a second electrical signal;

digital signal processor means for correlating said first and second signals generated by the respective sensor means in order to obtain a filtered electrical signal for reproducing said sound from said predetermined source,

the improvement wherein said first sensor means has an impedance which is substantially different from the impedance of air,

wherein said first sensor means is a piezoceramic transflexural actuator comprising a thin layer of piezoelectric material bonded to a thin conductive metal disc, said disc being in turn bonded to a conductive metal cylinder which closes any direct path by which airborne noise can reach the piezoelectric element,

wherein said second sensor means is positioned adjacent the first sensor means and said first sensor means faces the predetermined source and is between the predetermined source and the second sensor means, and

wherein said second sensor means is identical in construction to said first sensor means.

23. In an electronic stethoscope, including:

first sensor means for detecting sound exclusively from a predetermined source and converting said sound to a first electrical signal;

second sensor means for primarily sensing background sounds and not said sound from a predetermined source and converting said background sounds to a second electrical signal:

digital signal processor means for correlating said first and second signals generated by the respective sensor means in order to obtain a filtered electrical signa for reproducing said sound from said predetermined source,

the improvement wherein said first sensor means has an impedance which is substantially different from the impedance of air,

wherein said first sensor means is a piezoceramic transflexural actuator comprising a thin layer of piezoelectric material bonded to a thin conductive metal disc, said disc being in turn bonded to a conductive metal cylinder which closes any direct path by which airborne noise can reach the piezoelectric element,

wherein said second sensor means is positioned adjacent the first sensor means and said first sensor means faces the predetermined source and is between the predetermined source and the second sensor means, and

wherein said second sensor means is a microphone.

24. In an active noise control system, including:

sensor means for detecting sound from a predetermined source and converting said sound to an electrical signal;

speaker means responsive to said electrical signal for reproducing said sounds from said predetermined source; and

means for generating antinoise in order to minimize noise perceived by a user of a headset in which said speaker is situated, the improvement wherein said antinoise generating means includes at least one microphone positioned inside the earcup containing said speaker, processor means including means for generating an antinoise signal for driving said speaker in order to minimize noise detected by said at least one microphone,

wherein said means for generating an antinoise signal comprises means for driving said speaker in response to random noise detected by said at least one microphone, and

wherein said processor means includes a digital filter having an input x.sub.n and an output y.sub.n,

wherein ##EQU8## and wherein the coefficients ##STR7## of the antinoise digital filter are chosen so as to minimize a quantity representative of an ear sensitivity weighted sound pressure level.

25. A system as claimed in claim 24, wherein the ear sensitivity weighted sound pressure is given by the formula: ##EQU9## wherein the weighting factor W(f) is defined by the following equation:

wherein M(f) is a factor used to remove the frequency dependent sensitivity of the microphone, E(f) describes the frequency dependent sensitivity of the human ear, the unwanted source noise measured by the first microphone is given by p.sub.s (t), and the truncated Fourier transform of p.sub.s (t) over a time window of 2T is denoted by p.sub.s (T,f), which in turn is defined by the following equations: ##EQU10##

26. A system as claimed in claim 25, further comprising means for calculating a trapezoidal approximation for the functional value, gradient, and hessian of the ear sensitivity sound pressure level , using a modified Newton's method to find a minimum, and applying current values for P.sub.s (f) and the response ##STR8##

27. A system as claimed in claim 26, wherein the hessian is determined by the following procedure: ##EQU11##

28. A system as claimed in claim 24, further comprising means for checking for insufficiently damped modes.

29. A stethoscope as claimed in claim 1, wherein said actuator has an impedance which is approximately that of human flesh.

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BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to the field of active noise control, and in particular to an electronic stethoscope usable in a noisy environment.

2. Description of Related Art

Use of a stethoscope for auscultation, for example to detect lung noises or monitor heart beats, is often impossible in ambulances, medivac helicopters, and other emergency medical environments due to detection of extraneous noise and vibrations by the sensing device of the stethoscope, and because the signal output by the sensing device must compete with additional airborne noise that penetrates past the earpiece of the stethoscope and into the user's ear. In order to solve the problem of auscultation in a noisy environment, the stethoscope must limit both the direct detection of extraneous sounds by the sensing device of the stethoscope and the effect of noise which penetrates past the earpiece of the stethoscope. No such system currently exists, although solutions have been achieved for specialized situations. Such solutions have generally involved substituting an electrical transducer such as a piezoelectric microphone element for the vibration detector of a conventional stethoscope, and then applying electronic signal processing techniques to the resulting electrical signal.

There are several advantages to having a sensor with electronic output. First, the electronic output is amenable to filtering in order to receive the frequency band of interest. Thus, noise outside the frequency band of interest can be removed. Second, heart and lung sounds often get garbled by reverberation in the rubber tubes of conventional stethoscopes, but the electronic signal produced by the electronic sensor is not susceptible to such reverberations. Third, a sensor that generates an electronic signal is advantageous in that the electronic signal can be amplified and filtered to compensate for hearing loss specific to an individual physician. Finally, the electronic signal generated by the sensor can be used in conjunction with adaptive noise control techniques to further reduce unwanted noise.

The earliest forms of electronic signal processing in this context involved adaptive noise cancellation techniques based on subtraction of reference signals related to specific noise sources. For example, a DSP implementing a least means squared (LMS) algorithm was successfully used to remove unwanted 60 Hz noise which interfered with the recording of electrocardiograms (ECGs). For this application, the primary input signal was the ECG, which was correlated with a secondary input reference signal taken from a nearby electrical power outlet in order to obtain the part of the primary signal uncorrelated with the 60 Hz source of electrical interference.

In another application of the LMS filtering technique, a fetal ECG device was developed which cancelled out maternal heartbeat signal from a fetal heartbeat monitor. For this application, the primary signal came from a stethoscope placed near the infant, and the secondary reference signal was obtained from a stethoscope near the mother's heart. After removing the part of the primary signal correlated with the reference signal, the infant's heartbeat could be heard much more clearly.

More recently, rather than relying only on passive attenuation of external noise, research has focused on active noise control techniques. Here, active noise attenuation refers to the reduction of noise due to interference with a controlled secondary source of sound or "antinoise". No matter how noise-free the speaker's output, external airborne noise which penetrates to the listener's eardrum will still be a problem. In the case of a stethoscope, for example, external airborne noise that penetrates to the listener's eardrum and masks the relevant stethoscope signal can significantly interfere with the listener's interpretation of the signal. Since the source of noise does not come from the speaker driving signal, it cannot be controlled by simply passively filtering the speaker driving signal, but rather must be actively controlled.

The active control of sound in antinoise headsets is currently being investigated by many researchers. This headset research is divided between work using analog devices and digital (DSP) devices. Analog headsets have been under development for some time (see U.S. Pat. No. 4,445,675, and also U.S. Pat. Nos. 4,494,075, 4,644,581 and 4,856,118) and currently more than ten companies, including The Bose Corp., have made such headsets commercially available. Basic research on the design of DSP anti-noise headset systems is currently on-going. While the analog systems are less expensive, their cancellation performance is limited.

Unlike analog systems, a digital antinoise system can adaptively redefine its operating parameters in order to seek out the optimal way to cancel a particular noise problem. However, in practice, most DSP algorithms only remove periodic noise in ANC headsets. Periodic noise is much easier to cancel than broadband random noise. In emergency medical environments where most of the noise is periodic, the conventional algorithms are satisfactory. Nevertheless, there are many situations in which random noise cancellation is required.

Even where the external noise source is periodic, active noise control techniques by themselves may be inadequate to completely eliminate the background noise. Problems include inadequate reference sources for attenuation of both electronic noise in the primary signal and acoustical noise near the ear of the stethoscope's user, a primary signal which is too weak in relation to the noise sources, and differences in sound between the electronically filtered speaker output and the sound to which the user is accustomed. The present invention seeks to provide complete solution to these and other problems by using a variety of electronic processing techniques and by combining these signal processing improvements with improvements in the hardware by which the electrical signals to be processed are obtained.

SUMMARY OF THE INVENTION

It is a first objective of the invention to provide an electronic stethoscope that enables emergency medical personnel to auscultate in the presence of a high background noise level, by providing an improved primary signal sensor and an optimized combination of both passive and active noise cancellation technology in order to increase the signal-to-noise ratio of the signals output to drive a headset speaker, and reduce the effect of external airborne noise which can mask sounds output by the headset speaker.

It is a second objective of the invention to provide an active noise cancellation (ANC) or active noise reduction (ANR) system capable of cancelling random as well as periodic noise.

These objectives are accomplished in part by a unique detection sensor for picking up sound from the patient directly, in which the transducer has an impedance which is matched to that of human flesh rather than with the air, and therefore is much less sensitive to external airborne noise than commercial electronic stethoscopes with conventional microphones having very light diaphragms designed to minimize the impedance mismatch with air.

In an especially preferred embodiment of this aspect of the invention, the detection sensor is a piezoelectric transflexural actuator, designed to be excited by lung and heart sounds when in contact with the patient and produce an electric signal representative of the lung and heart sounds with no external power.

The objectives of the invention are further accomplished by providing additional sensors for electronically removing noise that the first sensor detects, all three sensors outputting time varying electronic signals, which are subsequently digitized and processed by a digital signal processor used to drive speakers in the headset.

In an especially preferred embodiment of the invention, the second sensor is used for measuring airborne noise in the vicinity of the main detection sensor and the third sensor is placed away from the lungs and heart on the patient's body so that these sounds will not get subtracted out from the first sensor, the third sensor measuring sound and vibrations, e.g., from a stretcher, that have already coupled into the patient's body.

According to another aspect of the invention, the digital signal processor uses an LMS algorithm, capable of cancelling random noise, to digitally subtract out the part of a first sensor signal that is correlated to the signals from second and third sensors.

According to yet another aspect of the invention, the digital signal processor is also used to calculate a correct manner to drive the speaker to make antinoise, in order to cancel sound penetrating the earcup so that only sound corresponding to the digitally processed first sensor signal is heard in the earcup, the antinoise reference source including, if necessary, two microphones to better account for movement of the user's head and varying noise levels.

Finally, the preferred embodiments of the invention also provide for processing of the filtered sensor output to make the effects of electronic processing transparent to the user so that the sounds generated thereby mimic those produced by the conventional stethoscope to which most medical personnel are accustomed.

It will be appreciated by those skilled in the art that many features of the invention, including the novel random noise cancelling algorithm, may be used in an active control headset for purposes other than auscultation. For example, such a headset can feed through electronically transmitted communications from a radio, while cancelling unwanted noise. This might be used by a person operating a lawnmower or other noisy equipment to hear radio broadcasts, or by a person driving a military tank in a noisy battle to hear electronically transmitted commands.

Another possible application is to configure the preferred stethoscopes for use by a plumber to detect leaks in long underground pipes. Digging up long pipes is expensive, time consuming, and scars the land, so detection of leaking water using a stethoscope can be highly advantageous. However, because pipes often are laid alongside busy roads, noise can be a significant problem. Similarly, the preferred stethoscopes could be used to hear termites in wood and insects inside fruit and grain, even in environments which are otherwise prohibitively noisy, on a larger scale than is possible with random sampling.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of the principal components of an active noise control stethoscope arranged according to principles of a preferred embodiment of the invention.

FIG. 2 is a perspective view of a piezoelectric sensor for use in the stethoscope of FIG. 1.

FIG. 3 is a schematic diagram illustrating the principles of antinoise generation.

FIG. 4 is a block diagram of the processor of FIG. 1.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 is a schematic diagram of the principal components of an active noise control stethoscope constructed in accordance with the principles of a preferred embodiment of the invention.

The two background noise sources which interfere with sound produced by a patient are indicated in FIG. 1 by the reference numerals 1 and 2. These sources are the background noise 1 in the vicinity of the patient and the background noise 2 in the vicinity of the headset.

In order to minimize detection of noise source 1, main sensor 3, which is placed against the patient's body for directly sensing sounds originating therein, has an impedance matched with that of human flesh and mismatched with that of air, so that it is less sensitive to external airborne noise. Instead of a conventional microphone, the preferred main sensor 3 uses a piezoceramic transflexural actuator consisting of a thin layer of piezoelectric material 4 bonded to a thin conductive metal disc 5, which in turn is bonded to a conductive metal cylinder or housing 6 as shown in FIG. 2. Such piezoelectric transflexural disks are manufactured, for example, by Murata Erie North America, Inc. (part No. 7NB-41-25DM-1).

A polyurethane coating or potting 7, for example CONATHANE.TM. manufactured by Conap, is placed over the active side of this circular flat sensor so that when placed in contact with the patient, the polyurethane coating serves to keep the sensor waterproof and broaden the response of the sensor. Without the polyurethane coating, the piezoceramic sensor would have a sharp resonance frequency, and the physician would only hear a narrow frequency band of the sound in the patient's body. The coating 7 and a gasket 7' conduct heat slowly, and prevent contact of the patient with the metal parts of the sensor, which might otherwise feel uncomfortably cold.

The cylinder or housing 6 is preferably made of brass and includes a first cylindrical cavity 60 to which is attached the perimeter of the circular piezoceramic device by applying a thin layer of conductive epoxy and radio service cement around the perimeter of the cavity ledge. Behind the piezoceramic transflexural element made up of piezoelectric material 4 and metal disc 5 is a second cylindrical cavity 61 which allows the sensor to be displaced back and fourth. As illustrated, the brass housing 6 is wider than the piezoceramic element and metal disc 5 so that when pressed against the patient, the housing closes any direct path for airborne noise to reach the sensor.

In order to connect a shielded cable 62 to the sensor, a cavity 67 is drilled into the side of the cylinder, with an extension 63 and 64 permitting a fine insulated wire 65 to be fed to the piezoceramic disc 4. A metal tube 66 is preferably provided to anchor cable 62 and is dimensioned to fit snugly within a cavity 67 in communication with cavity 63 in order to provide a path to ground via wire 68. A wire from the cable is soldered at 69 to another wire 65. The wire 65 is preferably very thin to avoid transmitting wire vibrations to the sensor which could cause voltage fluctuations.

An identical second sensor, sensor 8, is placed adjacent the first sensor 3. For example, in the illustrated embodiment, sensor 8 is integrated into housing 6 by providing symmetric cavities 80 and 81, metal disc 82, piezoceramic disc 83, polyurethane potting 84 and connecting wire 85 in extension 86 of cavity 63, although sensor 8 could also be provided separately. For purposes of providing a reference signal for noise cancellation, sensor 8 must be placed near the point where the stethoscope makes contact with the patient, but not in contact with the patient's body, so that it measures only the external noise 1 in the vicinity of the main sensor, rather than sounds made by the patient's body itself. A third identical sensor 9 is placed on the patient's body away from the source of the sounds of interest to the physician or paramedic using the stethoscope, generally the patient's heart and lungs, in order to detect background sounds which have coupled to the patient's body, for example through a stretcher.

In a variation of the above, sensor 8 may be in the form of a small conventional microphone rather than a piezoceramic transflexural actuator. Although such a microphone would be more susceptible to external airborne noise in the vicinity of the stethoscope sensor, the substitution alleviates the problem of loud noises being created when sensor 8 is accidentally tapped. The increased susceptibility to external noise can be alleviated by filtering the output of the microphone to detect only those frequency bands detected by the main sensor 3.

Sensors 8 and 9 serve as reference signal sources for digitally removing noise 1 that main sensor 3 detects. The time varying voltages output by sensors 3, 8, and 9 are digitized and processed by d