Baby incubator noise : causes and some reduction methods

BABY INCUBATOR NOISE: CAUSES AND

Carl A. Wales

Submitted in Partial Fulfillment of the Requirements for the Degree of

Bachelor of Science at the

Massachusetts Institute of Technology

Signature redacted

Signature of Author..

Department of Mechanical Engineering, Aoril 15. 1975

Signature redacted

Signature

redacted

Chairman, Departmental Comlittee on Graduate Students

Archives

(MAR

4A P

BABY INCUBATOR NOISE: CAUSES AND SOME REDUCTION METHODS

by

Carl A. Wales

Submitted to the Department of Mechanical Engineering on April 15, 1975 in partial fulfillment of the requirements

for the Degree of Bachelor of Science.

ABSTRACT

An investigation into the internal acoustic noise of an Isolette Model C-86 infant (baby) incubator is discussed. The practical lower limit of internal noise is derived.

The internal noise is discussed with regard to its

characteristics, its sources and its paths. Methods to reduce internal noise are set forth and recommendations made to reduce internal noise to the practical lower limit. The major question of whether the incubator noise is harmful or helpful is not discussed by this author.

Thesis Advisor: D. Graham Holmes

Title: Asst, Prof., Dept. of Mechanical Engineering - 2

-ACKNOWLEDGEMENTS

The author would like to acknowledge the following:

. Air Shields, Inc., including Mr. Ridley and Mr. Weaver for providing the incubator on which this study was performed.

. Dr. Graham Holmes, for his supervision.

. The personnel of the MIT Acoustics and Vibration Laboratory for their assistance and the use of the lab's equipment.

. Prof. Leehey for his advice and guidance.

. Dr. Cochrane and the personnel of the nursery of the Boston Hospital for Women.

. Miss Brenda Raby, R.N., for assistance at the hospital and for answering many medically-oriented questions.

. Robert Gipe and George Foote for their assistance and advice.

-TABLE OF CONTENTS Page ABSTRACT... ₂ ACKNOWLEDGEMENTS... ₃ LIST OF FIGURES... 5 LIST OF TABLES... I. INTRODUCTION... 6

II. EQUIPMENT: DETERMINATION OF THE PRACTICAL LOWER LIMIT ON NOISE... 9

III. EXPERIMENTAL PROCEDURE... 12

IV. RESULTS AND CONCLUSIONS: DETERMINATION OF LOWER PRACTICAL LIMIT TO THE INTERNAL NOISE... 13

V. EQUIPMENT: NOISE ANALYSIS... 17

VI. EXPERIMENTAL PROCEDURE: INTERNAL NOISE... 19

VII. RESULTS AND CONCLUSIONS OF INTERNAL NOISE ANALYSIS... 21

VIII. EQUIPMENT: INTERNAL NOISE REDUCTION... 29

IX. EXPERIMENTAL PROCEDURE: NOISE REDUCTION METHODS... 30

X. RESULTS OF NOISE REDUCTION WORK... 31

XI. OVERALL CONCLUSIONS AND RECOMMENDATIONS... 34

REFERENCES... 37

APPENDIX 1... 38

BIBLIOGRAPHY... 41

-LIST OF FIGURES

Page 1(a) Equipment and Interconnections for

Measurements in Hospital Nursery... 10

l(b) Laboratory Analysis of Recorded Noise... 10

2 Equipment and Interconnections for Generating the Sound field in the

Transmission Loss Determination...

3 Hospital Nursery Room Levels... 14 4 Sound Isolation... ₁₅

5 Equipment and Interconnections for

Laboratory Work... 18

6 Incubator as Delivered... 22 6(a) Noise with Incubator Off... 23

7 Internal Noise Level Contours on the

Mattress...

8 Accelerations of the Motor... 27

9 0 to 1000 Hz Spectrum with Microphone

Suspended in Mid-Air in the Baby

Com-partment... 35 10 0 to 1000 Hz Spectrum with the Microphone

Positioned at the Baby's Head position... 36

Al With Water in Tank... 39 A2 Without Water in Tank... 40

-INTRODUCTION

The noise problem in the baby incubator was discussed

by Falk and Woods [1]. Their data (measured at a hospital)

gives the level inside an incubator which was not operating as 61 dB and 35 dB(A). They list the levels with the in-cubator operating as 74.5 1.8 dB and 57.7 3.3 dB(A).

Isolette, a major manufacturer of incubators, was interested in determining if incubators are, indeed, this noisy,

what the characteristics of the noise are, the sources of the noise, the paths for noise transmission, and possible solutions to reduce the internal noise.

In addressing himself to this problem, this author divided the project into three parts:

1. Determine the practical lower noise limit. 2. Measure and analyze the internal noise.

3. Decide on methods of internal noise reduction.

The determination of the practical lower noise limit gave a reference upon which to base any efforts in practical understanding and reducing the noise. In this regard, the background level was measured in the Special Care Nursery at a Boston Hospital. Using this level and the trans-mission loss from outside to inside the incubator, which was measured in the lab, the lowest obtainable level

-side the incubator could be determined. Noise reduction efforts to reduce self noise below this level would be im-practical.

With regard to the internal noise level and characteristics, the internal amplitude levels were

measured with different configurations and conditions in-side the incubator. Narrow-band spectrum analysis was con-ducted to determine sources of noise as well as

propa-gation paths.

Noise reduction methods consisted of alterations to the motor mounting and the fan, and interfering with the propagation paths. Theoretical methods were considered but not tried, for the changes necessary to the incubator were

too majcr.

For this project, an Isolette Model C-86 Infant In-cubator (See Pictorial 1) equipped with the intensive care hood (and not equipped with iris access ports) was used. Both the Infant Servo Control and the standard power pack were used. The incubator was mounted on the standard cart which is supplied with the incubator.

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8-PICTORIAL 1

II. EQUIPMENT: DETERMINATION OF THE PRACTICAL LOWER LIMIT ON NOISE

In the Special Care Nursery of the Boston Hospital for Women data was taken in real time using a sound level meter. Approximately twenty minutes of noise was tape

recorded also for further data reduction in the laboratory. Figure l(a) shows the specific equipments and their inter-connections used in the work at the hospital.

Analysis in the lab of the noise which was tape re-corded was done with a narrow band spectrum analyzer and spectrum averager. The specifics of the set-up in the lab are shown in Figure l(b). Transmission loss was determined with the equipment shown in Figure 2.

A piston phone with an output of 124 decibels (dB)

(re 2 x 10-5 N/M 2) at 250 Hz was used in all cases for calibration. Corrections for changes in barometric pres-sure were ignored for being well below the accuracy

( l dB) for this project. The reference of 2 x 10-5 N/M is used for all decibel measurements in this project.

-FIGURE 1-A

EQUIPMENT ANT INTERCONNECTIONS FOR MEASUREMENTS IN HOSPITAL NURSERY

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FIGURE 1-B 03

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EQUIPMENT AND INTERCONNECTIONS FOR GENERATING THE SOUND FIELD IN THE TRANSMISSION LOSS DETERMINATION

B & K 1402 bwMCINTOS' 240 ATLAS 60W

RANDOM NOISE GENERATOR - POWER AMP HORN SPAKER

EQUIPMENT AND INTERCONNECTIONS FOR MEASURING THE SOUND INSIDE THE INCUBATOR

B & K 4220 PISTON PHONE

ISOLETTE B & K 4133 B & K 2619 B & K 2607 MODEL C-86 MICROPHONE PREAMPLIFIER MEASURING AMP

HEWLETT-PACKARD 7015

FEDERAL SCIENTIFIC UA-15A FEDERAL SCIENTIFIC 1015 X-Y PLOTTER

SPECTRUM ANALYZER SPECTRUM AVERAGER

OSCILLOSCOPE

III. EXPERIMENTAL PROCEDURE

At the Boston Hospital for Women, twenty minutes of noise was recorded in various rooms in the Special Care

Nursery. A calibration tone was recorded on the tape at the

beginning and the end of the noise. Measurements were then taken using the sound level meter which was calibrated before the measurements were taken and checked for calibration after-wards. Readings were taken at various locations in each of

the rooms in the nursery, in one octave bandwidths, linear full range and A-weighted full range. (All measurements

were taken to the nearest whole dB.)

For the determination of the transmission loss, an experiment was run in the lab (twice to insure valid data).

A noise field was generated in the room using a random noise

generator. With the field present, but without the incubator, measurements were taken to check for any significant normal modes of the room. The incubator was then placed in the

field and measurements were taken inside the incubator. In addition a plot was made of the frequency spectrum both

in-side and outin-side the incubator. (Note: for these tests, the incubator was not running.)

-IV. RESULTS AND CONCLUSIONS: DETERMINATION OF LOWER PRACTICAL LIMIT TO THE INTERNAL NOISE

The results of the noise study of the hospital nur-sery are shown in Figure 3. Spectrum analysis of the re-corded noise confirmed the data taken from the level measure-ments. The noise in the hospital for purposes of further determination efforts was 63 dB(A).

The sound field in the room for the two transmission loss experiments was 93 dB and 102 dB. Measured inside the incubator at the position of the babies head the level was measured at 76 and 87 dB. This gives a transmission loss

(TL) of approximately 15 dB. The spectra of inside and out-side the incubator as shown in Figure 4 concurs with this approximate value. This TL differs from what be

theoretical-ly determined for the plexiglass because of the various air

leaks in the hood. (Determining the TL using 1 octave band levels did not differ significantly from using the broad-band level as above).

With the field in the hospital of 63 dB(A) and the TL of the incubator this places the practical lower noise level inside the incubator at 48 dB(A). This figure is con-servative because the hood on the incubator used did not have

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nurses on duty). In addition the Chief of Pediatrics at that hospital uses monitoring equipment biased by personal preference towards quieter models.

-V. EQUIPMENT: NOISE ANALYSIS

A. A Strobotac (General Radio Model 1531) was used for

determining motor rotation rate. It was calibrated with its internal calibration which uses the 60 Hz power line voltage as a standard.

B. Laboratory noise work was conducted using the equipment and interconnections shown in Figure 5. The wind

screen was used only for the noise contour studies and was not necessary for other work. The level recorder was used for determining long term (one-half to one day continuous running) variation in noise level. Calibration was done using the piston phone (described earlier).

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MICROPHONES PREAMdPLIFIER MaSURING AMP

VI. EXPERIMENTAL PROCEDURE: INTERNAL NOISE

The first step in studying the internal noise was to determine the rotation rate and blade passage rate of the fan and its motor. This was done using a strobotac

calibrated against line frequency. The fan was marked and rotation rate checked. Then the fan was removed from the motor to check the unloaded rotation rate of the motor. The fan was equipped with twenty blades which multiplied by the rotation rate gives the blade passage rate.

The second step was to determine the airflow pattern inside the incubator. This was done by releasing some

smoke into the plenum chamber underneath the base plate of the incubator, letting the smoke flow with the circulating air.

Having taken care of the background aspects, the basic set-up for noise studies inside the incubator was

established. The narrowest frequency range which still con-tained the significant data was determined by continually narrowing the range of the analyzer until the full spectral window was filled with data. Also the number of spectra to be averaged was determined by trial and error until the minimum number was found which did not reflect meaningless

fluctuations.

-The standard condition for the incubator was without water and without a baby. Measurements were taken in a

quiet lab to remove as many external influences as possible. Using a wind screen the noise level contour was taken in-side the incubator to determine the variations which related to position of the microphone inside the baby compartment.

Modifications were made to the incubator after running initial tests. These modifications included running with a simulated baby (a baloon of water with 35 ppt salt by weight), running without a fan on the motor (but with the motor running), selection of wet or dry air for the baby compartment, use of the different power packs, and the

location of the make-up air tube in the inlet hole to the fan volute, additional modifications (and results) are listed in the Appendix.

Vibration spectra were taken using an accelerometer (not calibrated because only relative measurements were needed) mounted on the motor and on the base plate of the baby chamber.

For all acoustic measurements the system was calibrated frequently using the pistonphone.

-VII. RESULTS AND CONCLUSIONS OF INTERNAL NOISE ANALYSIS

The results of studying the spectra and levels col-lected are:

1. As seen in Figure 6, the frequencies of all significant noise are below 1000 Hz. This low frequency content allowed most efforts to be devoted to study of the limited range 0 to

1000 Hz, considerably reducing the amount of

data that needed to be collected and analyzed.

2. Figure 7 shows the significance of microphone location. Because of the normal mode excitation around 185 Hz, the difference in noise level varies 7 dB(A) between a mode and an antimode.

3. The location of the make-up air tube within the supply hole to the fan volute can vary the noise level up to 4 dB(A).

4. Removal and reinsertion of the power pack may

affect levels up to 4 dB(A). This is a significant problem not only in making laboratory data

col-lection more time consuming by the requirement of taking the average of multiple readings, but also in making it difficult to predict levels in an incubator in use and being operated by hospital

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5. Even after relocating electrical components on the circuit board in the infant servo control

(ISC) power pack to insure isolated motor mounting, there is a difference between the standard power pack and the ISC power pack with the ISC being up to 5 dB(A) noisier.

6. The presence of the simulated baby had no effect. Consequently this author feels that all the work done in the project is applicable to in-use

situations.

7. The selection of wet or dry (or some combination) air had no effect. This points away from air-born sound paths through the plenum chambers.

Consequently, the sources of the noise were determined to be:

The spectra and level results are:

1. All significant noise was below 1000 Hz as seen

in Figure 6.

2. The location where the microphone was positioned had an effect up to 7 dB(A) as seen in Figure 7.

-3.

The location of the make-up tube has up to

4

dB(A) affect.

4.

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to 4 dB(A) affect.

5.

There was a difference between the power packs

with the infant servo control power pack being

up to 5 dB(A) noisier.

6.

The presence of the simulated baby had no effect.

7.

Selection of wet or dry (or some combination)

had no affect.

The sources of the noise were determined to be:

.

Improper fan volute design which includes a

non-concentric air intake into the volute.

.

Improper fan design.

.

The vibration of the motor (which is shown in

Figure 8).

The paths for sound propagation between source and

baby compartment are primarily structure born as various

alterations and tests could not prove the existence of

airborn path.

In conclusion, the noise level inside the incubator

is high and is caused by various oversights in design.

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-VIII. EQUIPMENT: INTERNAL NOISE REDUCTION

The laboratory equipment for work in noise

re-duction methods was the same as used (and described) above for noise analysis.

For practical reasons a sound level meter (described previously) was used at the factory to measure levels in-side the incubators.

-IX. EXPERIMENTAL PROCEDURE: NOISE REDUCTION METHODS

The procedure for determining noise reduction methods consisted of installing a possible method and running the same tests as were run for initial noise analysis. Re-sults were compared and additional modifications made and retested. To eliminate the differences induced by uncon-trollable factors such as removal and insertion of the power pack, the tests were run up to ten times with seemingly

identical set-ups. The results were then averaged.

After the shock mount (for the motor) testing and testing of the fan alterations the effectiveness of these methods was tested at the factory using five incubators which were chosen at random from the production line. The

internal noise was measured using the sound level meter. Levels were recorded before modification and after each modification. Power packs were rotated through each

in-cubator to show individual characteristics of each inin-cubator and each power pack, as well as to provide better analysis of each modification.

-X. RESULTS OF NOISE REDUCTION WORK

Shock mounts for the motor and minor fan modifications were tried with success. Two other methods were tried

without success, and some untried theoretical methods were examined.

The motor vibration contributed large amounts of noise to the internal noise in the baby compartment. Through the use of Uniroyal type 301A Shock Mounts (the required parameters of the Shock Mounts were determined through the use of the Barry Mounts Catalogue (Ref. 2), the contribution from the motor was lowered to well below the practical lower noise limit. What the precise level was depended on which power pack was used, but both power

packs had levels below 40 dB(A) when operated without the fans.

Alterations were made to the fan itself in an effort to reduce the noise it generated. The motor end of the fan had stiffening ribs which were working as radial fan blades and causing a lower pressure stall area around the shaft. Relief of this pressure either by removal of the ribs or by drilling holes through the end of the fan reduced the noise from the fan to make the overall level inside the baby

compartment to below the practical lower limit.

-These two reduction methods were tested at the factory on other incubators with the results listed in Table 1. Note, however, that up to 3 dB(A) of the figures

in the table come from measuring the levels at the factory at the baby's head as opposed to in mid-air where most laboratory measurements were made.

Removal of the heater proved that the heater reduced the noise rather than contributed any.

Reducing the airflow reduced the noise significantly which the author concluded was an added indication that the

fan is stalling in normal operation. Selection of a new fan seems almost impervious. Reference 3 concludes that backward facing fan blades are the best type for quietest operation.

Additional theoretical methods of noise reduction include the use of non-parallel ends to the baby compartment which should reduce any multiple reflections of sound off of the ends of the compartment.

-TABLE 1

Power Pack A(ISC) Power Pack B(Standard)

w/o

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with

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0.84 0.40

Each X,a

based on

10 measurements

- 33 -Fan Shocks w/o w/o

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XI. OVERALL CONCLUSIONS AND RECOMMENDATIONS

The results of the factory trip proved two important things:

1. Results are variable from incubator to incubator, and even on the same incubator from one moment to the next.

2. There is the possibility that the incubator in the laboratory studies was in fact unique and therefore the noise reduction achieved on it will not achieve the same level of results as on other incubators-even those made by the same

factory.

The noise contours indicate notable contributions to the noise level measured where the microphone is located. Spectrum analysis and the noise contour plot indicated a normal mode associated with this problem. (See Figures 9 and 10). Possibly alterations to the ducting at the supply and exhaust ports could change the noise contour pattern,

by reducing the excitation at the normal mode frequency of

approximate 185 Hz.

Further efforts should be devoted to improving the design of the fan used, matching the fan volute to the fan, and better designed ports for the circulation system.

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REFERENCES

1. S.A. Falk, M.D., and N.F. Woods, R.N., M.N., "Hospital Noise Levels and Potential Hazards".

2. Catalogue from Barry Mounts.

3. J.B. Graham, "How to Estimate Fan Noise", Sound and Vibration, May 1972.

-APPENDIX 1

ADDITIONAL MODIFICATIONS TESTED TO DETERMINE THE SOURCES OF INTERNAL NOISE

A. Levels taken with and without the heater coil in place revealed the presence of the heater reduces the noise

by a slight amount (less than 2 dB(A).)

B. _{The presence of water in the wet air chamber reduced} the noise a slight amount in upper frequencies, but did not affect the major noise content. (See Figures

Al and A2).

C. _{Operation with the filter taken off the rear of the}

in-cubator increased the level considerably. This was not investigated further as normal operation will never be conducted without a filter on the incubator.

D. _{Isolation of the hood by removing the hinges did not}

alter the levels inside the incubator.

E. _{Inserting baffles in the ducting in the plenum chamber}

did not change the level leading to the conclusion that the noise was not airborne.

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----BIBLIOGRAPHY

ACOUSTIC MEASUREMENT, Bruel & Kjaer MECHANICAL VIBRATIONS, Bruel & Kjaer

FUNDAMENTALS OF ACOUSTICS, Kinsler & Frey NOISE REDUCTION, Beranek

NOISE AND VIBRATION CONTROL, Beranek

HANDBOOK OF NOISE MEASUREMENT, General Radio BRUEL & KJAER CATALOGUE