US 5,539,831 A · Active noise control stethoscope

Abstract

An active noise control stethoscope enables a physician or paramedic to check vital signs in the presence of high background noise levels. A digital processing technique is used to remove noise from the output of a main detection sensor, the detector being impedance mismatched with air and therefore less sensitive to external airborne noise. Instead of a microphone, the detector uses a piezoceramic transflexural actuator mounted in a cylindrical piece of brass, with a polyurethane coating placed over the active side of the sensor to keep the sensor waterproof and broaden the response of the sensor. An identical sensor is placed above the device to detect background noise adjacent the device, the signals being combined to obtain a signal free of background noise. A third sensor is also used to electronically remove noise detected by the main sensor, the third sensor being positioned to pick-up noise coupled through the patient's body. The time varying voltages from signals output by these sensors are digitized and processed by the digital signal processor, and the output used to drive the speakers in the headset. The digital signal processor uses a least mean squared algorithm to digitally subtract out the part of the detector signal that is correlated to the signals from the second and third sensors. In addition, noise penetrating the earcups of the headset is reduced by using the speakers to generate antinoise. The antinoise is generated by a filtered X-adaptive digital algorithm, and also by a random noise cancellation system which sets up an infinite impulse response filter in which the coefficients are periodically updated for minimizing an ear sensitivity weighted sound pressure level detected by a microphone inside the headset.

Claims29

Dependent claims

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.

3 more elements

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.

4 more elements

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.

4 more elements

Dependencies are inferred from the claim text.

Active noise control stethoscope

Life of this patent

Active noise control stethoscope

Life of this patent

Abstract

Claims29