NASA CASE NO.

PRINT FIG. _1_

NOEICE

The invention disclosed in this document resulted from research in aeronautical and

space activities performed under programs of the National Aeronautics and Space

Administration. The invention is owned by NASA and is, therefore, available for

licensing in accordance with the NASA Patent Licensing Regulation (14 Code of

Federal Regulations 1245.2).

To encourage commercial utilization of NASA-owned inventions, it is NASA policy to

grant licenses to commercial concerns. Although NASA encourages nonexclusive

licensing to promote competition and achieve the widest possible utilization, NASA

will provide the necessary incentive to the licensee to achieve early practical

application of the invention.

Address inquiries and all applications for license for this invention to NASA Patent

Counsel, Langley Research Center, Code 212, Hampton, Virginia 23681-0001.

Serial No." 08/243,665

05/11/94 LaRC

(NASA-Case-LAR-14977-1) ACOUSTIC

CALZBRATIQN APPARATUS FOR

CALIBRATING PLETHYSMOGRAPH[C

ACOUSTIC PRESSURE SENSORS Patent

Application (NASA. Langley

Research Center) lkp

G3/52

N95- 20 76 3

Unclas

0039286

J
LAR 14977-1 AWARDS ABSTRACT 

ACOUSTIC CALIBRATION APPARATUS FOR CALIBRATING 

PLETHYSMOGRAPHIC ACOUSTIC PRESSURE SENSORS 

The present invention relates to plethysmographic sensors and, 

more particularly, to an apparatus for calibrating plethysmographic 

acoustic pressure sensors. 

According to the present invention, an apparatus includes a 

transmission material having an acoustic impedance approximately 

matching the acoustic impedance of the actual acoustic medium. The 

transmission material is preferably water since water has an 

acoustic impedance approximately equal to the acoustic impedance of 

human tissue. An elastic balloon-type container holds the 

transmission material. A test sensor and a reference sensor are 

mounted on the container. A sound producing device, typically a 

hydrophone, is coupled to the container and transmits acoustic 

signals into the container so that acoustic signals pass through 

the transmission material and the container to the reference sensor 

and the test sensor. The test sensor can be calibrated based on a 

comparison of the signals received by the reference sensor and the 

test sensor. The container is made of a material having a low 

acoustic impedance and a high elasticity. 

The present invention is applicable for calibrating many types 

of plethysmographic sensors and is particularly useful for 

calibrating plethysmographic acoustic pressure sensors used in 

fetal heart monitors currently being developed. 

Inventors: Allan J. Zuckerwar David C. Davis 

Home Addresses: 128 Sandpiper St. 115 Lakeview Dr. 

Newport News, VA Grafton, VA 

23602 23692 

SSN : 

Employer: NASA Langley Research Center 

Initial Evaluator: David L. Gray 

Serial No.: 08/243,665 

Filed: 5/11/94 
LAR 14977-1 - 1 - PATENT APPLICATION

ACOUSTIC CALIBRATION APPARATUS FOR CALIBRATING

PLETHYSMOGRAPHIC ACOUSTIC PRESSURE SENSORS

5

10

Origin of the Invention

The invention described herein was made by employees of the U.S.

Government and may be manufactured and used by or for the

government for governmental purposes, without the payment of any

royalties thereon or therefor.

Background of the Invention

Field of the Invention

15 The present invention relates to plethysmographic sensors and,

more particularly, to a method and apparatus of calibrating

plethysmographic sensors.

20

Description of the Related Art

Many plethysmographic instruments utilize plethysmographic

sensors to measure blood flow, blood pressure, heart rate, breathing

sounds, or other such functions, on the surface of the human body.

Examples of plethysmographic sensors or devices which use these

25 type of sensors are disclosed in Baker, U.S. Patent No. 4,781,200 -

Ambulatory Non-lnvasive Automatic Fetal Monitoring System; Pfohl, U.S.

Patent No. 4,981,139 - Vital Signs Monitoring and Communication

System; Dickson, U.S. Patent No. 4,458,687 - Trans-Telephonic

Acoustical and Electrical Heart Valve Monitor System; Kroll, U.S. Patent

30 No. 4,672,976 - Heart Sound Sensor; Kroll, et al., U.S. Patent No.
LAR 14977-1 -2- PATENT APPLICATION

4,763,660 - Flexible and Disposable Electrode Belt Device; Flowers, U.S.

Patent No. 4,258,720 - Strain Gauge Plethysmograph; and Shirley, et al.,

U.S. Patent No. 4,784,154- Interference Resistant Biomedical

Transducer.

5 Plethysmographic sensors include acoustical pressure sensors.

Various problems occur in providing calibration of plethysmographic

acoustical pressure sensors so that the calibration proves to be accurate

and reliable when the sensors are applied in actual in-service conditions.

Typically, the calibration of a plethysmographic acoustical pressure sensor

10 loses its validity when the sensor is removed from the calibration facility

and installed into actual in-service conditions. This problem generally

occurs if the specific acoustic impedance of the plethysmographic

acoustic pressure sensor is not substantially higher than the specific

acoustic impedance of the calibration transmission medium and the

15 specific acoustic impedance of the actual in-service transmission medium.

The transmission medium of most plethysmographic acoustic

pressure sensors is human tissue, which may be assumed to have a

specific acoustic impedance near that of water. If the specific acoustic

impedance of the plethysmographic acoustic pressure sensor is not

20 substantially higher than that of water, then the calibration must take

place in a medium having a specific acoustic impedance comparable to

that of water in order to match the environmental conditions of calibration

to the environmental conditions of actual in-service use. This matching of

environmental conditions poses a difficult problem for conventional

25 calibration techniques.

Also, actual in-service conditions require that the plethysmographic

sensors are mounted by a strap which is wrapped around the torso or a

limb, or are applied independently. Therefore, the mounting condition of

the plethysmographic acoustical pressure sensor in calibration should

30 match the mounting conditions of actual service.
LAR 14977-1 -3- PATENT APPLICATION

Conventional methods of calibrating plethysmographic acoustical

pressure sensors include the acoustical coupler, the reverberation

chamber, and the free field.

The acoustical coupler consists of a small rigid chamber with

5 sealed ports to seat a sound source, a reference sensor, and the test

sensor. The internal volume contains either a gas (air) or a liquid (water)

and has dimensions which are much smaller than an acoustical

wavelength, thus assuring uniform acoustical pressure throughout the

volume. Acoustic pressures generated by the sound source are measured

10 by the test and reference sensors and a comparison of the measurements

provides the basis for the calibration. A calibration in a conventional

acoustic coupler does not meet the mounting requirements of actual inservice conditions.

The reverberation chamber contains the same components as the

15 acoustical coupler, except that the volume dimensions of a reverberation

chamber are much larger than an acoustical wavelength and generally

large enough to contain the test and reference sensors. Irregular

reflections from the chamber walls generate a uniform sound energy

density and insure that the test and reference sensors are subjected to

20 the same sound pressure levels.

A free field calibration takes place in a very large volume (for

example, the atmosphere, a lake, or an ocean) or in an anechoic chamber,

such that the sound field is undisturbed by reflections. In this situation, a

calibrated sound source generates a known sound pressure at a distance

25 where the test sensor is located.

Accurate and reliable calibration in a reverberation chamber or free

field in water is possible with the test sensor mounted similarly to inservice conditions. However, this procedure requires that the test sensor

be submerged in water. This submersion may prove destructive since

30 many plethysmographic acoustic pressure sensors are not hermetically
LAR 14977-1 -4- PATENT APPLICATION

sealed. As an example, Schanz and Schilling (Acustica, vol. 65, pp. 267-

298 (1988)) describe a free field reciprocity method of calibrating PVDFfoil sensors, but the method requires the immersion of the sensor in a

tank of water.

5 Further, an acoustical wavelength in water at, for example, 20 Hz

is approximately 74 m and the required expanse of the free field must be

several times an acoustic wavelength. Therefore, this method may prove

impractical.

10 Summary of the Invention

Accordingly, it is an object of the present invention to provide

reliable and accurate calibration of pressure sensitivity and frequency

response of surface mounted plethysmographic sensors.

15 It is a further object to provide a calibration apparatus in which (1)

the transmission medium has an acoustical impedance matching that of

human tissue (water), (2) the sensor installation conforms to that in

actual service conditions, and (3) immersion of the sensor in water or

other liquid is avoided.

20 Additional objects and advantages of the invention will be set forth

in part in the description which follows and, in part, will be obvious from

the description, or may be learned by practice of the invention.

The foregoing objects of the invention are achieved by providing an

apparatus for calibrating an acoustic sensor which is coupled to an actual

25 acoustic medium when used in actual in-service conditions, comprising a

transmission material having an acoustic impedance approximately

matching the acoustic impedance of the actual acoustic medium, an

elastic container which holds the transmission material, a first sensor

which is coupled to the container at a first location on the container, a

30 second sensor which is coupled to the container at a second location on
LAR 14977-1 -5- PATENT APPLICATION

the container, the second location being different from the first location,

and a sound producing device which is coupled to the container and

transmits acoustic signals inside the container.

5 Brief Description of the Drawings

10

These and other objects and advantages of the invention will

become apparent and more readily appreciated from the following

description of the preferred embodiments, taken in conjunction with the

accompanying FIG. 1, which is a diagram of an acoustic calibration

apparatus according to an embodiment of the present invention.

Detailed Description of the Preferred Embodiments

15 Reference will now be made in detail to the present preferred

embodiments of the present invention, examples of which are illustrated

in the accompanying drawing, wherein like reference numerals refer to

like elements throughout.

FIG. 1 illustrates an acoustic calibration apparatus according to an

20 embodiment of the present invention. The acoustic calibration apparatus

comprises a balloon 10, a sound source 12, a test sensor 14, a reference

sensor 16 and a suspension device 18. Balloon 10 acts as a container

and is filled with water 20. Sound generated by sound source 12,

located at the center of balloon 10, propagates through water 20 and

25 impinges upon both test sensor 14 and reference sensor 16. The

sensitivity of reference sensor 16 is known. Therefore, the sensitivity of

the test sensor 14 can be derived by comparison between measurements

of test sensor 14 and reference sensor 16.

The water-filled balloon 10 typically is made of 150-micron thick

30 vinyl and has a diameter of 0.3-0.4 m. A neck 22 of balloon 10 is
LAR 14977-1 -6- PATENT APPLICATION

opened to accommodate a tube 24 (preferably made of aluminum), which

seats sound source 12 in the center of balloon 10. An O-ring 26 presses

neck 22 against tube 24, thus providing a seal. A tight seal is not

necessary for successful operation of the acoustic calibration apparatus,

5 however, a tight seal operates to retain the position of sound source 12

relative to the center of balloon 10.

In the present embodiment of the present invention, test sensor 14

is mounted on a cummerbund 28 which is preferably wrapped around the

equator of balloon 10 so that test sensor 14 is in direct contact with the

10 vinyl surface of balloon 10. While FIG. 1 illustrates test sensor 14

mounted on cummerbund 28, it is appreciated by persons skilled in the

art that there are various other methods to mount test sensor 14.

However, test sensor 14 should be mounted in a manner which conforms

to mounting used in actual in-service conditions. For example, test

15 sensor 14 could be mounted to balloon 10 with a piece of adhesive tape.

Ambient vibrations generate pressure waves in water 20.

Therefore, balloon 10 is supported on a suspension device 18 to reduce

the contaminating effects of vibrations on input signals to test sensor 14

and reference sensor 16. Suspension device 18 is, for example, a foam

20 pad and resides on a base 19.

Sound source 12 is typically a hydrophone serving as an acoustic

projector. An example of a suitable hydrophone is an LC-32

manufactured by Atlantic Research Corp. A barrel 30 of sound source 12

is clamped to tube 24 by a clamp 34 in order to fix the position of sound

25 source 12. Clamp 34 is, for example, a conventional hose clamp. Tube

24 itself is supported by a stable fixture (not illustrated).

At acoustical frequencies below 20 kHz, sound source 12 radiates

sound isotropically throughout balloon 10 so that the sound pressure

levels at test sensor 14 and reference sensor 16 are equal.

30 The acoustic field inside balloon 10 is reverberant and composed of
LAR 14977-1 -7- PATENT APPLICATION

multiple reflections. Therefore, the sound pressure at test sensor 14 and

reference sensor 16 cannot be reliably inferred from the characteristics of

sound source 12. For this reason, reference sensor 16, having known

calibration, is positioned on the surface of balloon 10. Reference sensor

5 16 should present a very high acoustic impedance to the acoustic field in

order to ensure that the sound pressure at the face of reference sensor

16 is approximately twice the incident pressure.

A high acoustic impedance can be realized if reference sensor 16

utilizes a high specific acoustic impedance material (for example, a

10 piezoelectric ceramic) and is mounted with sufficient rigidity to resist

displacement by the incident acoustic pressure. A sufficiently rigid mount

can be achieved, for example, by pressing a massive bar 36 against

reference sensor 16. As illustrated in FIG. 1, test sensor 14 is mounted

on cummerbund 28. Therefore, it is preferable that reference sensor 16

15 is placed between cummerbund 28 and balloon 10. Massive bar 36

presses against reference sensor 16 through cummerbund 28.

Supporting instrumentation is used for driving sound source 12,

detecting the level of excitation detected by reference sensor 16 and test

sensor 14, and comparing the levels of excitation of reference sensor 12

20 and test sensor 14. Supporting instrumentation includes a function

generator 40, a power amplifier 42, a voltmeter 44, a differential amplifier

46, a digital signal analyzer 48 and an amplifier 50, although many other

combinations of supporting equipment are easily recognized and used by

persons skilled in the art.

25 Sound source 12 is excited by function generator 40 and power

amplifier 42, preferably in a continuous-wave, single-frequency mode.

Background signals due to ambient vibrations are generally so large that

the use of a gated pulse technique is precluded. The output of function

generator 40 is connected to power amplifier 42 via line 60 and power

30 amplifier 42 is connected to sound source 12 via line 62. Voltmeter 44 is
LAR 14977-1 -8- PATENT APPLICATION

connected to the output of power amplifier 42 via line 64 and indicates

the level of excitation.

Test sensor 14 may be single-ended or differential. If test sensor

14 is differential, then the test sensor signal is amplified in differential

5 amplifier 46 and measured on a voltage-reading instrument, such as

digital signal analyzer 48. If test sensor 14 is single-ended, then a single

ended amplifier will be used in place of differential amplifier 46.

Reference sensor 16 is generally single-ended and its output signal is

amplified by single-ended amplifier 50 and measured on a voltage-reading

10 instrument (not illustrated). The output of reference sensor 16 can be

amplified by amplifier 50 and read by digital signal analyzer 48 if digital

signal analyzer 48 is dual-channeled.

Test sensor 14 is connected to differential amplifier 46 via lines 66

and 68. Differential amplifier 46 is connected to digital signal analyzer 48

15 via line 70.

Reference sensor 16 is connected to amplifier 50 via line 72 and

amplifier 50 connects to digital signal analyzer 48 via line 74.

The procedure for obtaining a frequency response calibration of

test sensor 14 is as follows. The frequency of function generator 40 is

20 selected for a continuous wave (CW) signal. The respective gains of

differential amplifier 46 and amplifier 50 are adjusted to facilitate the

measurement of test sensor 14 and reference sensor 16 on digital signal

analyzer 48. The sensitivity of test sensor 14 is the test sensor output

voltage (divided by the gain of differential amplifier 46) divided by the

25 measured sound pressure derived from the calibrated response of

reference sensor 16. A series of such measurements at selected

frequencies will yield the frequency response of test sensor 14.

A hydrophone used as sound source 12 radiates electromagnetic

signals as well as acoustical signals, as is well-known in the art. The

30 electromagnetic interference (EMI) signals will be detected by a test
LAR 14977-1 -9- PATENT APPLICATION

sensor having a high electrical impedance. A piezoelectric sensor is an

example of such a test sensor. The EMI signals are generally of far

greater amplitude than the acoustic pressure signals. Suppression of the

EMI signals requires that the test sensor or hydrophone be completely

5 surrounded by an electrical shield which is connected to earth ground.

This can be accomplished by any of three methods.

First, if the test sensor already has a built-in electrical shield, then

this shield must be connected to earth ground.

Second, in the absence of a built-in shield, a layer of copper-coated

10 kapton foil wrapped around the test sensor will provide effective shielding

if the copper is connected to earth ground. The presence of the foil will

not affect acoustical propagation to the test sensor.

Finally, the vinyl balloon 10 can be coated with a thin metallic layer

connected to earth ground. This will not affect the acoustic propagation

15 to the test sensor, but will suppress EMI signals propagating beyond the

confines of the balloon wall.

Sound source 12 can be any transducer capable of projecting

sound underwater, such as, for example, a magnetostrictive transducer.

Further, sound source 12 need not be located at the center of balloon 10,

20 but rather can be located at the distal end of tube 24 (outside balloon

10). In this case, tube 24 would serve as a waveguide.

The transmission medium (water) has an acoustic impedance

closely matching that of human tissue. However, test sensor 14 remains

outside the medium during the calibration procedure. Therefore, there is

25 no need to immerse test sensor 14. This is an important aspect of the

present invention, since many test sensors are not hermetically sealed.

The present invention has the advantage that test sensor 14 can be

installed on balloon 10 in a manner conforming to that in actual in-service

conditions. Thus, when test sensor 14 is removed from the calibration

30 apparatus, the calibration of test sensor 14 remains valid when test
LAR 14977-1 -10- PATENT APPLICATION

sensor 14 is reinstalled for use in actual in-service conditions.

The calibration method utilizes water as the propagation medium to

simulate human tissue; however, the test sensor remains outside the

propagation medium. This feature is important if the test sensor is not

5 hermetically sealed.

The calibration method permits a test sensor installation

corresponding to the actual in-service condition. As a result, the

calibration remains valid when the test sensor is removed from the

acoustic calibration apparatus and reinstalled for service.

10 The present embodiments of the present invention use balloon 10

as a container to hold water 20; however, a container other than a

balloon can be used in the present invention. The container should have

a low acoustic impedance and a high elasticity. Such a container should

preferably have an elastic modulus approximately equal to the elastic

15 modulus of soft rubber, which is approximately .0005 X 101° N/m 2. The

container could also be selected so as to have an elastic modulus not

exceeding approximately 10% of the bulk modulus of water (which is

approximately 2 X 108 N/m2). Therefore, the container should be selected

to have an elastic modulus not exceeding approximately 2 X 108 N/m 2.

20 Although a few preferred embodiments of the present invention

have been shown and described, it would be appreciated by those skilled

in the art that changes may be made in these embodiments without

departing from the principles and spirit of the invention, the scope of

which is defined in the claims and their equivalents.

25 What is claimed is:
LAR 14977-1

//

PATENT APPLICATION

ACOUSTIC CALIBRATION APPARATUS FOR CALIBRATING

PLETHYSMOGRAPHIC ACOUSTIC PRESSURE SENSORS

5

Abstract of the Disclosure

An apparatus for calibrating an acoustic sensor. The apparatus

includes a transmission material having an acoustic impedance

approximately matching the acoustic impedance of the actual acoustic

medium existing when the acoustic sensor is applied in actual in-service

10 conditions. An elastic container holds the transmission material. A first

sensor is coupled to the container at a first location on the container and

a second sensor coupled to the container at a second location on the

container, the second location being different from the first location. A

sound producing device is coupled to the container and transmits acoustic

15 signals inside the container.
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