Shelter System Multiple Noise Analysis and Design in order to meet

MIL-STD-1474 Noise Limits Spec

Sockkyu LEE1; Ingi BAEK2; Sanghyun KIM3; Jeung LEE4; Jiho CHOI5

Mechanical Engineering R&D Lab, LIGNex1 Co., Ltd. , Korea

ABSTRACT

Shelter System is a control center for military purposes and is used for controlling reconnaissance system,

missile system, etc. The outside shape of the Shelter System is similar to a container carried by a cargo ship.

In the interior of the Shelter System there are several pieces of noise-producing electronic equipment such as

workstations, fans and air conditioners. As the internal operators of the Shelter System conduct several

specific missions over a long period of time in this noisy environment, one of the most important

specifications for the Shelter System is the internal noise level in the operator’s ear. In this paper, as

MIL-STD-1474 Noise Limits required by MIL-STD-1472 Human Engineering, it requires under 65 dB (a) or

75dB (a) noise level in the operator’s ear. So, to predict the at-ear position, we first measured the component

noise level at various locations - workstations, fans, air conditioners, etc. We then analyzed Multiple Noise

with measured data and related numerical formula considering position and modified factors. Also we

analyzed Multiple Noise with Actran, acoustic analysis software. So we suggested the design to meet Noise

Limits Spec. After the manufacture, we tested the noise level and compared it with the prediction.

Keywords: Multiple noise, Shelter System, Equipment noise, MIL-STD-1474 Noise Limits

1. INTRODUCTION

Shelter System is a control center for military purposes and is used for controlling reconnaissance

systems, missile systems, etc. The outside shape of the Shelter System is similar to a container carried

by a cargo ship. In the interior of the Shelter System there are several pieces of noise-producing

electronic equipment such as workstations, fans and air conditioners. The shape is shown in Fig. 1.

Figure 1 Shelter System

As the internal operators of the Shelter System conduct several specific missions over a long period

of time in this noisy environment, one of the most important specifications for the Shelter System is

the internal noise level in the operator’s ear. In this paper, it is assumed that the duty is 8 hours

1 sockkyu.lee@lignex1.com 2 ingi.baek@lignex1.com 3 kimsh1207@lignex1.com 4 jeunglee@lignex1.com 5 jiho.choi@lignex1.com

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without hearing protection devices. So, the goal of noise limits is the range of Zone E, below Fig. 2.

Figure 2 Permissible noise exposure limits in MIL-STD-1472G(1)

Zone E, Maximum exposure without protection: For exposures of less than 8 hours, personnel

without hearing protection shall not be exposed to noise levels exceeding 84 dB(A). When personnel

remain for more than 8 hours in spaces with a high noise level, an Leq(24) of 80 dB(A) shall not to be

exceeded. Consequently, for at least a third of each 24 hours, the personnel shall be subject to an

environment with a noise level not exceeding 75 dB(A).(1)

As MIL-STD-1474D Noise Limits required by MIL-STD-1472G Human Engineering, it requires

under 65 dB (A) or 75dB (A) noise level in the operator’s ear because of category E or F as the

frequency of conversation. In the study, it is assumed that the goal noise limits is category E, 75dBA.

The noise limits in MIL-STD-1472D is shown in Tab.1 and 2.

Table 1 Steady-state noise categories in MIL-STD-1474D(2)

Category System Requirements

A No direct person-to-person voice communication required. Maximum design limit. Hearing

protection required.

B Electrically-aided communication via attenuating helmet or headset required. Noise levels are

hazardous to unprotected ears.

C No frequent direct person-to-person voice communication required. Occasional shouted

communication may be possible at a distance of 30 cm. Hearing protection required.

D

No frequent person-to-person voice communication required. Occasional shouted

communication may be possible at a distance of 60 cm. Levels in excess of Category D require

hearing protection.

E Occasional telephone or radio use or occasional communication at distances up to 1.50 m

required. (Equivalent to NC-70)

F Frequent telephone or radio use or frequent communication at distances up to 1.50 m required.

(Equivalent to NC-60).

Table 2 Steady-state noise limits (dBA) for personnel-occupied areas in MIL-STD-1474D(2)

Limit Category A B C D E F

A-Weighted Limit (dBA) 108 100 90 85 75 65

SIL-4 Limit 67 57

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2. NOISE SOURCE MEASUREMENT AND NOISE LEVEL ESTIMATION

2.1 Noise Source Measurement(10)

In order to estimate the noise at the ear position, we first measured the component noise level at

various locations - workstations, ventilator fans, air conditioners, etc.

Shelter system, since a plurality of internal noise source generates noise at the same time,

becomes audible to the production of a composite noise form. Therefore, the noise of the noise

source measured, in consideration of the value and quantity, and calculates the equivalent noise level.

Then, reflecting the impact of the distance spaced apart to the point you want to know, and calculates

the final composite noise. So, at the first, noise source was measured as each condition and distance.

The Main noise source is shown in Tab. 3.

Table 3 Internal Main noise sources of Shelter

Units Cabinets

Ventilator Airduct #1 #2 #3

Workstation 5ea 2ea - - -

VME rack 2ea - - - -

UPS 1ea - - - -

Data recorder - 1ea - - -

Backbone S/W - 1ea - - -

Server unit - - 3ea - -

Data processor - - 4ea - -

Ventilator Fan - - - 3ea -

Air-conditioner - - - - 2ea

2.1.1 Workstation In order to measure the max noise level, we gave the simulated load to workstation. And also

we measured three cases-only a workstation, workstations built in cabinet opened door, workstations

built in cabinet closed door as below fig. 3. And the frequency domain noise level with NC line is

shown as below fig. 4 and fig. 5.(8) As fig. 5A, for high frequency noise, sound-absorbing material

will be applied on the door.(3,6)

Figure 3 Measurement results dB(A) (workstation)

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Figure 4 Measurement results with NC lines (workstation)

Figure 5 Measurement results with NC lines (workstation)

2.1.2 Ventilator It was measured to target the loaded ventilator in shelters basic backbone of similar size and

structure. In view of the adverse conditions, the noise in the Full RPM state of the ventilator was

measured. Measurements Case may operate only one Article ventilation fan, it was measured in a

state in which all doors were closed. The measurement results are shown as below fig.6. And the

frequency domain noise level with NC line is shown as below fig. 7.

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Figure 6 Measurement results dB(A) (ventilator)

Figure 7 Measurement results with NC lines (ventilator)

2.1.3 Air-conditioner The Air-conditioner is a 18,000 BTU/h-class military product. Similarly reflected in the

mechanical mounting conditions in the chamber wall, the noise was measured. The noise was

measured as air flow intensity "strong", "weak" mode. By applying the air duct outlet, the noise was

measured by a "strong" mode.

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Figure 8 Measurement results dB(A) (air-conditioner)

Figure 9 Measurement results with NC lines (air-conditioner)

2.2 Noise Level Estimation

The equivalent noise level will be calculated through related numerical formula considering

position with measure noise source data. Considering real effect, factors of formula will be modified

for estimation. Then, reflecting the impact of the distance spaced apart to the point you want to know,

and calculates the final composite noise.

2.2.1 Composite Noise Level Calculation Formula

Sound pressure level(SPL)

Sound pressure level(SPL) is a logarithmic measure of the effective pressure of a sound relative

to a reference value.(3) Sound pressure level, denoted Lp and measured in dB, is defined by (1)

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2

0

1010

P

PLogLp (1)

Multiple sources

The formula for the sum of the sound pressure levels of n incoherent radiating sources is

Inserting the formulas

In the formula for the sum of the sound pressure levels yields(3)

....log10...

log10

2

0

2

0

2

2

0

1102

0

222

21

10 dBp

p

p

p

p

pdB

p

pppL nn

(2)

,,...,2,1,10 102

0

nip

p dB

L

ii

(3)

.10...1010log10 10101010

21

dBL dB

L

dB

L

dB

L n

(4)

Distance

According to the inverse proportional law(5,9), when sound level Lp1 is measured at a distance

r1, the sound level Lp2 at the distance r2 is

2

12 1 10

2

10p p

rL L Log dB

r

(5)

2.2.2 Modified Factor for Noise Level Estimation Equations When comparing the noise measurement test results with the theoretical equation (eq.5) results

according to the distance, as Fig.10~Fig.11 below it shows a big difference in the slope. Thus, using

the coefficient of the slope difference between each measured value and the calculated value , it is

expressed as a modified theoretical calculation, as eq.6 and eq.12.

Workstation

Modified Equation for a workstation only (eq.6).

2

12 1 10

2

0.4 10p p

rL L X Log dB

r

(6)

Modified Equation for a workstation built in cabinet (eq.7)

2

12 1 10

2

0.4 10p p

rL L X Log dB

r

(7)

Modified Equation for two or three workstations built in cabinet (eq.8)

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2

12 1 10

2

0.5 10p p

rL L X Log dB

r

(8)

Figure 10 Noise Correction results (workstation)

Ventilator fan

2

12 1 10

2

0.2 10p p

rL L X Log dB

r

(9)

Figure 11 Noise Correction results(ventilator, air-conditioner)

Air-conditioner

Modified Equation for “strong” mode air-conditioner (eq.10)

2

12 1 10

2

0.4 10p p

rL L X Log dB

r

(10)

Modified Equation for “weak” mode air-conditioner (eq.11)

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2

12 1 10

2

0.35 10p p

rL L X Log dB

r

(11)

Modified Equation for “strong” mode air-conditioner applying the air duct outlet (eq.12)

2

12 1 10

2

0.35 10p p

rL L X Log dB

r

(12)

2.2.3 Consideration of Measured Results and Final Modified Factor As Fig.3, a workstation, a rack-type equipment, is measured at about 70 dBA at the front.

However, when attached to the cabinet assembly, it is measured at about 65.2 dBA due to the side

sound absorbing, when the front door applied, it is measured at about 51.4 dBA due to the front

sound absorbing material. As Fig.3B, when attached to the cabinet assembly, the workstations are

measured at similar dBA whether the number of workstation is two or three.

In Fig.3,Fig.6, Fig.8, it shows a large variation between measurements and numerical calculations

as the distance. Numerical calculations estimate the sound energy is assumed to be established in the

form of spherical wave from spreading to an open space with no reflective effect. But because of

states which the equipment is installed, the actual noise source is spread to go to unidirection from a

equipment location, it is estimated as a result, an acoustic reflection effect in the closed reflected

space. Also, the goal of the noise source measurement is to finally predict the noise in the enclosed

space of a shelter system. So, it was measured in three cases such as open area, closed area, anechoic

chamber. As a result, as shown in Fig.10, Fig.11, the result of measurement in a certain work area

(open area), such as workstations and the result of measurement in anechoic chamber, such as

air-conditioner was in a tendency to be corrected to 0.4~0.5 about distance attenuation slope factor,

in case of ventilator(enclosed space, at basic shelter), the result of measurement was in a tendency to

be corrected to 0.2 about distance attenuation slope factor. Because this noise measurement is the

purpose of calculating the composite noise within a certain closed space, for a conservative review, it

sets the attenuation slope factor as 2 for estimation. By utilizing Eq.4, Eq.9, the results of calculated

composite noise is shown in tab.4.(4,7)

Table 4 Multiple noise of cabinets (with door/airduct) [Unit ; dB(A)]

Units Cabinets

Ventilator Airduct #1 #2 #3

Workstation 51.8 51.8 - - -

VME rack 52.0 - - - -

UPS 45.0 - - - -

Data recorder - 45.0 - - -

Backbone S/W - 52.0 - - -

Server unit - - 52.0 - -

Data processor - - 45.0 - -

Ventilator Fan - - - 68.0 -

Air-conditioner - - - - 65.5

Multiple noise(Front) 60.4 56.9 57.8 72.8 68.5

Total multiple noise(1m) 70.5

Total multiple noise(2m) 69.3

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When illustrated reflecting properties of the noise in the Tab.4, it is shown in Fig.12.

Figure 12 Multiple noise Estimation (Table.4)

2.2.4 Actran Analysis and design modification For multiple noise analysis, an analysis model was constructed with Actran S/W as shown in

Fig13.

Considering only the reflection effect, in order to estimate the complex noise, noise sources were

assumed to be point source. And air damping was properly considered.

Figure 13 Actran Analysis Model

Figure 14 Actran Analysis Results

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As shown in Fig.14, between the cabinet and the walls, the ventilator fan noise is spread by the

gap. There was a gap which would spread ventilator fan noise through. So it is suggested

modifying the design, and block the noise.

3. COMPARISON OF ESTIMATION AND REAL RESULTS

As shown in Fig.15, Fig.16, measurements which were measured at pre-production and post

-production system in a similar position were compared. In the closed door case as Fig.15, it is

assumed that high frequency noise was decreased because of sound-absorbing material on the door.

Figure 15 Comparison of Before Measurement results with After (workstation)

Figure 16 Comparison of Before Measurement results with After (ventilator fan , air-conditioner)

Because of the noise characteristics change before and after manufacture, there are different

results. But for a conservative prediction method, it is meaningful to use the formula method and

Actran S/W.

As shown in Tab.5, The maximum value of analysis and experimental data have similar results,

within the range of 5%.

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Table 5 Comparison of analysis and test results

CASE POSITION(dBA)

#1 #2 #3 #4

Estimation results with formula 60.0 60.0 62.0 69.3

Estimation results with Actran S/W 65.5 67.0 66.8 67.7

Real results after manufacture 67.5 66.8 67.1 66.8

4. CONCLUSIONS

In this paper, we used the same or similar equipment to measure the noise source for estimation at

pre-manufacture, the actual multiple noise was predicted by noise-related formula applied modified

coefficient. The formula considered the reflection effect and was properly predicted.

Also we used the Actran S/W to analyze the multiple noise and we suggested the design to meet

Noise Limits Spec.

After manufacture, we measured real multiple noise at the ear’s position, and we compared the

analysis and test results. So, the maximum value of analysis and experimental data have similar

results, within the range of 5%.

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