Mission Statement Neonatal Acoustic Solution’s purpose, in cooperation with Children’s Hospital Central California (CHCC), is to find methods to reduce noise levels to 55 dB in the neonatal ICU through the application of existing technologies, creation of new products, and alteration of

patterns of behavior with a focus on efficiency and cost effectiveness, while minimizing the impact on hospital staff.

Noise Reduction in Neonatal ICU Paul Baker, Rogelio Grijalva, Elton Leung, Roberto Rios-Rios

University of California Merced Mechanical Engineering Capstone Design Spring 2012

Theory

Discussion

ANR Circuit

Introduction

Analysis

Wave Theory

Sound is the vibration of the air, and when air

vibrates it creates areas of high and low

pressure. This pressure wave can be

described by this equation:

When two or more waves coincide the

interaction between them and resulting

combined pressure wave can be described

as:

While this is helpful, for our purposes the

most important equation is this one:

This equation represents the resulting

amplitude when two waves interfere with

each other. Since we want to reduce noise

we need to either absorb sound or generate

a waveform which, when added to the

existing noise, causes the left side of this

equation to go toward zero. This concept is

referred to as destructive interference.

Active Noise Reduction (ANR)

Active noise cancellation works on the principle of destructive interference. When two

waves with the same phase but opposite amplitudes overlap, the peaks of one

interfere with the troughs of the other, which leads to a reduction in total amplitude.

In our research we found that if you are able to keep the canceling source within ¼ of

a wavelength of the noise source you can achieve global cancellation without all the

complicated sensors, microphones and logic circuits that 3D cancellation normally

requires. For our project the frequency we needed to cancel out was in the 1-1000 Hz

range, and at 1000 Hz, ¼ of the wavelength is approximately 3.5 inches, so as long as

unit remains close to the source it should affect the entire room.

Our ANR circuit is comprised of 3 main parts: the input, inverter, and amplifier. The

input is a unidirectional microphone connected to a unity gain operational amplifier

acting as a pre-amp. This boosts the signal level of the microphone such that the rest

of the circuit can use it (microphones usually have very low voltages). The second

stage is an inverter, which in our circuit is again an operational amplifier though in this

case it is utilizing its inverting input. This chip takes the incoming microphone signal

and flips its phase 180°. This means that anywhere there used to be a peak there is

now a trough, and vice versa. The last stage is a low noise audio amplifier with 20-

200dB gain. This section gives the signal enough power to operate a speaker. The

specific components are noted in the circuit diagram below.

Active Feedback Signage

We found that noise made by hospital

staff was not continuous, but did spike

up to unacceptable levels during shift

changes and procedures that involved

more than a single nurse. While the staff

is trained to keep their voices down, this

clearly is not effective when they are

focused on other things. The goal of the

sign is to alert the staff when their voices

are getting too loud.

The concept is identical to the highway

speed signs that tell you your current

speed as you pass. While you know that

the speed limit is 65, that does not mean

you always travel at that speed.

Sometimes you end up going faster

without noticing, and the sign reminds

you to slow down. Similarly, our signs

are designed to alert the staff when their

voices are nearing the 55 dB level. This

prevents undue noise spikes, while our

other two solutions actively reduce the

ambient noise level.

Our aim was to reduce noise levels to 55dB. With the ANR averaging 10dB and the new ceiling providing approximately

2dB, the overall noise reduction is expected to be around 12dB. Given a starting level of 67dB this means we achieve our

goal. That said, though we accomplished a great deal this semester, there is much that could be done to improve the design

of both our circuits.

The ANR circuitry works best against low frequency sound, but currently it is forced to process all noise. The addition of a

low pass filter following the pre-amp would allow us to exclude frequencies outside our ideal range. This would decrease

feedback potential, unintentional high frequency amplification, and could slightly decrease overall power consumption in the

circuit. Similarly, even though the sign is only intended to target human noise, it currently registers all surrounding noise,

which means alarms may cause it to generate false positives. An idea that we did not have time to implement was an

adjustable band pass filter. It would allow the user to select a range of frequencies in which the alarms operate and exclude

those noise from registering on the LED display. This is useful because if the sign is going into the red due to alarms the

staff may begin to ignore it, which defeats its purpose.

While COMSOL provides a decent representation of the acoustics of our room, there are more specialized pieces of

software that would give a more accurate picture. Odeon is one of these, and though we wanted to use it for this project it

did not fit in our budget this semester. Future work could include this analysis as well.

COMSOL Model

Overall performance will be analyzed by a

COMSOL model of a sample room. We will

place equivalent point noise sources and

accurate material properties into the model

and run it using the acoustics module, which

generates a representation of existing sound

level pressures in the room.

Our research indicated that while ANR on

average reduces noise levels by around 10dB

for low frequency noise (less than 1500 Hz).

Using this 10dB as a starting point we will

reduce the output of the point sources by that

amount and change the NRC value of the

ceiling. After running the analysis again this will

give us a new graph which shows the

approximate reduction in overall room noise as

a result of our designs.

Noise Reduction Calculation

To determine the improvement that could be expected from

swapping out the ceiling tiles, we performed a noise reduction

calculation. The concept behind this is a comparison between

the total absorptive area before and after the change. The

relationship is below:

NR = log Aa/Ab

The procedure is to calculate the total surface area in a given

room (we used a typical ward with 6 beds). For each surface

multiply the area by the noise reduction coefficient (NRC) of

the material. The sum of these products represent the effective

absorptive area Ab. The second step is to assume a change in

the room composition. In our case this was a change in the

NRC value of the ceiling tiles (0.5 to 0.95). Following the same

procedure with the new ceiling generates Aa, from which we

get the noise reduction in dB.

Neonatal Acoustic Solutions

Signal

Inverter Pre-amp

Audio

Amplifier

Active Signage Circuit

Pre-amp

Signal

Inverter

Low Gain

Amplifier

Display

Driver

Active Signage Model ANR Device Model COMSOL Room Model

Material NRC

Linoleum 0-0.05

Plywood 0.1-0.15

Drywall 0.05-0.2

Acoustic Tile 0.5-0.75

Ecophon 0.95

Glass .05-.10

Case NR

(dB)

Best 2.45

Average 1.45

Problem: Excessive noise in the CHCC Neonatal ICU – OSHA standard mandates 55dB Max.

Background: Medical research has demonstrated the importance of noise regulation in a hospital

setting. Noise can be harmful to the development of fragile neonates and has been

shown to increase recovery times. A previous study done by the hospital showed an

average noise level of 66-71dB. However, there were three main concerns in our

designs: they need to be safe for the babies, financially viable, and should not

prevent the hospital staff from performing their duties.

Investigation: Our initial task was to measure the sound levels for ourselves. Some data from that

experiment is to the right. We discovered that there were two main sources of noise:

equipment and staff. The equipment produced low frequency periodic noise

whereas the staff were responsible for intermittent wide band bursts. These

represent very different types of noise pollution and demanded separate solutions.

Course of

Action:

We decided upon a three pronged solution: two targeted and one general. To

combat the periodic machine noise generated by the ventilators, oscillators, and

pumps we developed an Active Noise Reduction device. In response to the people

noise we devised an Active Feedback Sign which gives the staff immediate

feedback about their volume. Lastly, to cover everything our targeted solutions

missed, we recommended changing out the ceiling for better Acoustic Panels, which

affect all frequencies and provide overall noise reduction in the room.

Room #2605 High (dBc)

Low (dBc)

Room Ambient: 67.1 62.4

Oscillator/Ventilator: 81 78.8

Diaphragm Valve: 87.4 84.1

Suction Regulator A: 67.1 62

Suction Regulator B: 74.8 64.8

Room #2604

Room Ambient: 63.9 57.4

Ventilator: 81.2 63.4

Room #2606

Ventilator 74.3 72.4

HVAC 68.7 64.6

Speaker

Source: ecalculator.com