Comparison of sensors. Measurement of respiratory ...

Measurement of respiratory acoustical signals. Comparison of sensors. H Pasterkamp, S S Kraman, P D DeFrain and G R Wodicka Chest 1993;104;1518-1525 The online version of this article, along with updated information and services can be found online on the World Wide Web at: http://chestjournal.chestpubs.org/content/104/5/1518

Chest is the official journal of the American College of Chest Physicians. It has been published monthly since 1935. Copyright1993by the American College of Chest Physicians, 3300 Dundee Road, Northbrook, IL 60062. All rights reserved. No part of this article or PDF may be reproduced or distributed without the prior written permission of the copyright holder. (http://chestjournal.chestpubs.org/site/misc/reprints.xhtml) ISSN:0012-3692

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Measurement Signals* Comparison

of Respiratory

Acoustical

of Sensors

,

M. D.; Steve S. Kraman, M.D. F C. C. F; M.S.E.E.; and George R. Wodicka, Ph.D.

Hans Pasterkamp, Paul D. DeFrain,

We assessed the performance of three air-coupled and four contact sensors under standardized conditions oflung sound recording. Recordings were obtained from three of the investigators at the best site on the posterior lower chest as determined by auscultation. Lung sounds were band-pass filtered between 100 and 2,000 Hz and sampled simultaneously with calibrated airflow at a rate of 10 kHz. Fourier techniques were used for power spectral analysis. Average spectra for inspiratory sounds at flows of 2 ± 0.5 Ifs were referenced against background noise at zero flow. Aircoupled and contact sensors had comparable maximum signal-to-noise ratios and gave similar values for most coustical

signals

from

are

assessed

ditionally

During

recent

number

of publications

tion

analysis

and

years,

Ofl

techniques

auditory

have

increasing

page

shown

proc-

1320

the

limits

frequency sounds

content

and

indicate

a potential

During sounds

bronchial are said

details

distribution value

provocation to show

application is in

where

a close

airway

patency also

upper

respiratory

and

correlation has

observed

Advances

for

central between

been

described

an

Department

meas-

airway

obstruction

sound

spectra

in physical

of

Pediatrics,

University

of

analysis

Unfortunately,

use

various

in-

methods

that

in

this

area

recently

has

been

.

and other in any recording

Advantages

and

sound sensors of respiratory

disadvantages

are fundaacoustical

of air-coupled-

versus contact-type sensors previously have been described but not formally compared in actual recording of respiratory sounds.6 We, therefore, decided to evaluate

the

selection techniques

ofsensors in situ. Also, the effect for processing of the recorded

sounds

relative

was

methods

performance

assessed,

for

and

of a representative

suggestions

presentation

of the

of different respiratory

of informative

acoustical

data

were

forth.

and

accelManitoba,

Canada (Dr. Pasterkamp); the VA Medical Center, Ky (Dr. Kraman); and the School of Electrical EngiPurdue University, West Lafayette, Ind (Mr. DeFrain and

Dr. Wodicka). This study was supported in part by a grant from the Whitaker Foundation and a National Science Foundation Young Investigator Award BCS-9257488 to Dr. Wodicka. Dr. Pasterkamp is supported by the Children’s Hospital ofWinnipeg Research Foundation. This study was presented in part at the 17th International Conference on Lung Sounds, August, 1992, Helsinki, Finland. Manuscript received December 3, 1992; revision accepted February 26, 1993. Reprint requests: Dr. Pasterkamp, CN503-840 Sherbrook Street, Winnipeg, Manitoba, Canada R3A 151

1518

the world

reaches

sound

laboratories.

standardization

SUBJECTS

models3 have

signal

differ from the choice of sound sensors, through the sampling and processing of sound signals, to the measurement and presentation of results. The need

put

clinically. technology

the lung sound

of respiratory

clinical

around

We used

in microprocessor

the

acoustical

introduction

into

emphasized Microphones mental parts

of human

at which

frequency noise level.

the

signals.

the

application. inspiratory

urements

erated

for

to provide

surpass

highest

=

background

vestigators

acquisisignal

F

systems

in median frequency that correlates with the of airflow obstruction even in the absence of both over the lung’ and at the trachea.2

Another

Winnipeg, Lexington, neering,

an

Digital

see

been that

respiratory

for clinical for example,

*Fmm

been

signals.

comment

sounds

Ofl

of normal

and

has

tra-

perception.

Studies

increase degree wheeze,

system auscultation.

on computer-assisted

of these

respiratory

respiratory

by subjective

there

For editorial essing

the

spectral parameters. Unexpectedly, less sensitivity (lower signal-to-noise ratio) at high frequencies was observed in the air-coupled devices. Sensor performance needs to be characterized in studies of lung sounds. We suggest that lung sound spectra should be averaged at known airfiows over several breaths and that all measurements should be reported relative to sounds recorded at zero flow. (Chest 1993; 104:1518-25)

used

seven

sensors

for respiration

acoustic

subjects

for

the

consent.

The study

Committee were

on

healthy

in height

from subjects

the study.

they

flow

range

intensity before

oflung

sounds

after All

commonly

of us served giving

by the Purdue

subjects.

was

sounds

the

flow

posterior

the experiment

three

Us.

as

informed

University participants

The chest

point was


month

of Electrical

pneumotachograph The

of maximum identified

for successive

Acoustical

the

sat in a soundproof

on an oscilloscope.

marked

of Respiratory

during

at the School

subjects

a calibrated

lower and

infection

place

The signal

at 2 ± 0.5

Measurement

took

through the

set

tract

University.

breathed

observed over

ofthose

1). Three

was approved

of human

a respiratory

Purdue and

while

lion

had

recording

chamber

of lung

protocol use

(Table

male nonsmokers, ranging in age from 24 to 47 years, 166 to 183 cm, and in weight from 62 to 83 kg. None

before The

METHODS

studies

recording the

of the

Engineering,

AND

that are representative

target sound

by auscultaplacement

Signals(Pastes*amp

of

eta!)

Table

ofSound

1-Dimensions

Sensors

before

spectral

calculated Height, Sensors

Diameter,

g

overlap

mm

the

155*

ECM77*

12.0

1.7

5.6

12.0

1.5

5.6

8.9

2.0

7.6

RadioShackNo.33-1052t HP 210501

EMT25C

PPG

No.

power

52.2

14.0

15.4

28.0

Digital

8.0

9.9

28.0

5.1

2.1

20.0

FYSPac2II *sony

Corp.

Montvale,

,

tRadio Shack, Hewlett-Packard, §Siemens,

Tandy

Iselin,

IIPPG

lung

of lung

sounds

sound

(below

which

band

Tex.

and

below

than

NJ.

(phonopneumography)

sensor,

University,

Technion

Belgium.

sensors

were

first placed

were

with

affixed

in coupling

double-sided

diameter,

pressure

20 mm.

by

placement

directly

while

chambers.

to

recording

chamber

bore

hole.

a closed-cell with

of the

taped

The

a lateral

microphone was mm in diameter,

The

the

chest was

cavity The

a central

was

coupler

foam

disk,

hole

air-coupled

plastic

where

this

sound

spectra

Lung

Radio

and

On

31

(range, contained number

in diameter

for

sensor

was

surface with standard masking tape. kept the same for each subject throughout

The the

amplified

four-pole

Butterworth-type

digitized

at 10 kHz

converter

(4801a,

ware

used

was

Calif).

provided

acquisition,

personal

computer

control

analysis

of the

sound

fast signals.

flow

using

signals

were

Customized and

(NB3,

signal Hz,

analog-to-digital

Mass).

playback

respiratory

2,000

a 12-bit

analysis,

and ensured

2,048-point

and

Woburn,

data

least four complete used

with

Digital-to-analog

quality

We

Corp.,

100 and

Sound

channel

ADAC

to the individual

between filters.

per

for

IBM-compatible Torrance,

according

band-pass-filtered

of the

artifact-free

soft-

display

Epson

on

America

an Inc.,

transforms

A Hanning

data

sound

rec)rded

sounds

recording

over

for power window

Table

at

spectral

was

applied

2-Perftwmance

Subject

above

We

to less

calculated

the

and the frequency

and

band

analysis

attenuation

significance

of the

dropped

regression

plot

to the of power

(decibel/octave).

were

assessed

by re-

Newman-Keuls was

multiple

accepted

when

prob-

0.05.

average, 16.0

the to 30.3

length

six inspirations of spectra within 18 to

spectra The

from an average slopes of the

sounds

recorded compared

sensors

(Table

trates

these

with

ofSound

1

the background

those

with

findings noise signal

20.8

sensor,

s and

(range, 4 to 9). The average the target flow range was 34

recorded

2). A significantly observed

background lung sound

was and

noise

of3l samples (range, 9 to 70). spectral curves of inspiratory air-coupled microphones were

with

steeper

was

subject

57). We computed

(range,

uation

of recording

s) for each

in

with

greater the

contact

sound

Sony

electret

attenuation (p2,000

25.0

-

17.5

955

>2,000

26.0

-

12.4

765

Sensors

signal

sensors

of variance

Statistical

than

limits

frequencies

noise.

linear

phones (p2,000 935

ratio.

plot from 300 to 700 Hz. reaches

background

noise

level.

CHEST

I

104

I 5 I NOVEMBER,


1993

1519

0.01

Subject

No. 3

0.001

0.0001

I E-5

N

I LC)

I E-6

I E-7

I I E-8

I E-9

IE-lO 0

500

1000

1500

Frequency FIGURE 1. Comparison in situ of two signals and noise F1,, lowest frequency second quartile frequency (median greatest.

the

1,000 were

spectral curves Hz indicate, close

to

or

and missing data points therefore, that the lung at

background

mean

I 000

-

noise

three contact sensors, plotting difference between the lung sound signal reaches background noise level; F,,.,,, frequency at which the ratio of signal to noise is

above sounds

level.

(Hz)

and

air.cotipled at which frequency);

Q2,

2000

difference

500 and

The

this

case

in spectral 1,000

Hz.

was

obtained

slopes The

is most

greatest with

evident

between

signal

band

width

PPG

sensor.

For

the

in the

± SD

_______________________

V

1st quartile

U

median

0

3rd quartile

A

spectral

(Q1)

(Q2)

(Q3)

edge (SEw)

N I >

.1

0

C

a)

#{149}1

#{149}

C. U.

I

.

:

FIGURE

of2±0.5

1520

I

I

I

Sony

Sony

RadiO

ECM155

ECM77

#33-1052

2. Measurements Us.

on average

lung

Shack

sound

spectra,

FYSPaC2

obtained

with

HP

PPG

Siemens

21050

#201

EMT25C

seven

Measurement

sensors

at inspiratory

of Respiratory

Acoustical


flows

Signals

(Pasterkemp

etaQ

30

25

-0-

EMT25C

(#2)

-,---

EMT25C

(#9)

-s-----

PPG

#201

20 #{149}0

15 U) 0

C 10

C C)

Cl)

No. 4

Subject

I 000

I 500

Frequency 3.

FIGURE

sensitivity EMT 25C).

FYSPac2 depicts

sensor, the

the

mechanical

Comparison (maximum

in situ of three contact signal-to-noise ratio) between

spectral

peak

resonance

near

of this

2,000

Hz

device.

sensors, two

(Hz) showing a significant difference in effective sensors of the same make and model (Siemens

significant used

a characteristic

The values

FYSPac2 transducer gave significantly higher for all spectral parameters compared with those

either

of the higher

other with

tation. Figure

sensors the Sony

(Fig and

2). The first Radio Shack

reduction

sensor

alternatively

quartile was microphones

of

sensitivity

in

for the measurements of

some

this

damage

4 illustrates

Siemens

2, reflecting

particular

sustained

our

the

in Table

sensor

during

observations

or

transpor-

in one

subject

compared with the HP, PPG, and Siemens sensors (p