apps.dtic.milproduced by Lewis Generator No. 688 (Table VII) peaks at 16 kHz due to ... sufficiently...

July, 1977 DCIEM Technical Report No. 77X45

0

EXPOSURE TO ULTRASONIC CLEANER NOISE A )IN THE CANADIAN FORCES

R.B. CrabtreeS.E. Forshaw

~~ L ~

1Behavioural DivisionDefence and Civil Institute of Environmental Medicine1133 Shepphard Avenue West, P.O. Box 2000Downsview, Ontario M3M 3B9

DEPARTMENT OF NATIONAL DEFENCE - CANADA

DISRTUTI.NSTATEMENT A

Apprevod for public release;Distribution Unlimited

TABLE OF CONTENTS

Page

ABSTRACT .................................................... v

INTRODUCTION ................................................ 1

PROCEDURE ................................................... 2

RESULTS AND DISCUSSION ........................................ 2

CONCLUSIONS ................................................... .... 4

RECOMMENDATIONS ............................................. 5......5

ACKNOWLEDGEMENTS ................................................. 6

REFERENCES .......... ........................................ 7

TABLES ............................................................ 9

FIGURES ..................................................... ..... 19

Nils

BY

oI Iil' nISP.CIAL

ABSTRACT

The high-frequency noise produced by ultrasonic cleaning de-vices at CFB North Bay and CFB Trenton is sufficiently intense toproduce effects such as nausea, headaches, tinnitus and fatigueamong exposed personnel. Although the 20-kHz one-third octave-band sound pressure levels observed close to these units are wellunder 140 dB (the level below which damage to the human ear isthought not to occur), they nevertheless exceed the levels re-commended for hearing conservation (105 dB at an operator's posi-tion, 95 dB within 15 feet of an operator). The most effectivemeans of reducing the noise radiated from these cleaners is tocontain each unit in an appropriately ventilated enclosure or room.Personnel operating or working close to units not enclosed shouldwear hearing protection.

v

INTRODUCTION

Ultrasound (the sound produced by an ultrasonic source) isdefined as sound occuring at frequencies above the audible range ofman (typically above 16 or 17 kHz). Human exposure to intense levelsof ultrasound has become relatively common since the introduction ofjet engines into military and civilian aircraft operations. Theultrasonic energy in close proximity to these engines can be asintense as the audible-frequency components of the engine's noise(Macpherson and Thrasher, 1959; Parrack, 1966). The effects thatresult from exposure to such levels of ultrasonic energy were firstobserved on a large scale in personnel working around military jetaircraft. Termed 'ultrasonic sickness', these effects includeheadaches, vertigo, nausea and excessive fatigue. Acton and Carson(1967) have reported that these subjective effects do not occur unlessan individual's hearing extends to at least 17 kHz and the soundpressure level in the 17-kHz region exceeds 78 dB. They have notedthat women experience adverse symptoms more often than men, and youngmen more often than old, presumably due to differences in high-frequency hearing acuity rather than sex or age.

Ultrasonic devices have now found wide application in industrialprocesses such as drilling, cleaning and welding. The Canadian Forcesemploy ultrasonic cleaning systems (e.g., Lewis Ultrasonic CleanerModel L/C 136H manufactured by the Lewis Corporation (Figures 1 and2); Hyper-Intense Proximinal Scanning Ultrasonic Cleaner (HIPS) ModelAC 2858-IX, manufactured by Cavitron Ultrasonics Inc. (Figures 3 and4)) for aircraft maintenance purposes, and personnel working nearthese cleaners have reported symptoms of 'ultrasonic sickness'.

Because the units operating these devices (the Aircraft Main-tenance Development Unit (AMDU), CFB Trenton, and the AircraftMaintenance Control and Records Office (AMCRO), CFB North Bay) did nothave the equipment required to measure ultrasound, the Sonics Sectionof DCIEM was requested to take the following action:

1. Determine the levels of ultrasound being produced by thecleaning units.

2. Provide information on the hazards associated with ex-posure to ultrasound.

3. Measure the effectiveness of the enclosure fabricated atCFB North Bay and the Cleaning Rooms at CFB Trenton inreducing the amount of ultrasound being radiated.

* I.

2

4. Recommend procedures and/or exposure limits in order tominimize the effects of ultrasound upon personnel.

PROCEDURE

The sound fields produced by one HIPS and three Lewis ultrasoniccleaners (in the AMCRO section (CFB North Bay), the AMDU section andNo. 3 Hangar (CFB Trenton)) (see Figures 4, 5 and 6) were measured(overall and octave-band sound pressure levels) using a Bruel andKjaer type 2209 Sound Level Meter. For certain conditions, the soundwas also recorded on a Nagra type IV-SJ Tape Recorder for subsequentone-third octave-band analysis.

RESULTS AND DISCUSSION

The results of the overall and octave-band noise measurements aregiven in Table I to V for various conditions and locations around thecleaners. It can be seen that the most intense noise produced by thecleaners occurs in the 16-kHz octave band. A narrow-band analysis ofthis noise shows, in fact, that its peak occurs in the 20-kHz one-third octave-band, the operating frequency of the HIPS and LewisUltrasonic cleaners (see Tables VI to IX). It is noted that the noiseproduced by Lewis Generator No. 688 (Table VII) peaks at 16 kHz due tothe inadvertent misadjustment of the machine's operating frequencyduring maintenance. Note also that considerable noise is generatedbelow 20 kHz due to cavitation in the cleaning solutions. Minutebubbles are formed in the liquid and grow until they reach a resonantsize, at which time they oscillate with increasing amplitudes untilimplosion occurs (Hughes, 1965).

The first question to be answered is whether these levels aresufficiently intense to cause tinnitus and the feelings of nausea andfatigue reported by personnel working in the vicinity of the cleaners.It is noted that the noise levels (in the 1.25- plus 16-kHz one-thirdoctave bands) produced by the ultrasonic cleaners (when not enclosed)range from 82 to 97 dB (see Tables VI to IX, last line), thus exceedingthe 78-dB criterion of Acton and Carson (1967), and are thereforeintense enough to produce the reported symptoms. One of the authors(RBC) himself experienced extraordinary fatigue and an 'unnaturalsensation' in his ears after a two-hour exposure (without hearingprotection) in the ultrasonic room at CFB Trenton.

A second question is whether the noise levels reported aboveare hazardous to hearing. Parrack (1966) has concluded that ultra-sonic fields should not be harmful to the human ear until theoctave-band or one-third octave-band sound pressure levels approach

3

140 dB.

At the same time, it is recognized that a hazard may exist dueto subharmonic energy accidentally generated by ultrasonic equip-ment. As a result Parrack has recommended that the 20-kHz one-third octave-band sound pressure level, measured at the ear of anoperator of ultrasonic generating equipment, should not exceed 105dB. Likewise, the 25-, 31.5- and 40-kHz one-third octave-bandsound pressure levels should not exceed 110, 115 and 115 dB re-spectively. Further, the sound pressure level in the 20-kHz one-

third octave band should not exceed 95 dB for general advetitiousexposures of people within 15 feet of the operator's position(Guignard, 1973). Although the 20-kHz one-third octave-band soundpressure levels observed around the unenclosed ultrasonic cleanersat CFB North Bay and CFB Trenton are well below the 140-dB limitthought to be non-injurious to hearing, the levels do exceed the105-dB criterion suggested by Parrack.

The enclosure fabricated at CFB North Bay, constructed from3/4 inch plywood, lined with one-inch styrofoam, and fitted witha top lid and front panel which are hinged with piano-type hinges,(Figures 8, 9 and 10) is effective in attenuating the noise pro-duced by their ultrasonic cleaner. At the operator's position(with the cleaner lid closed), the enclosure reduces the cleanernoise from 94 to 65 dBA, and in the 12.5- plus 16-kHz one-thirdoctave bands, from 85 to 55 dB.

It is observed that a vertical force of approximately 30pounds is required to lift the top lid on this enclosure. Theaddition of a mechanical assist and a small access panel to pro-vide access to the cleaner controls would reduce much of the in-convenience that has resulted from the use of the enclosure.

The rooms constructed in No. 3 Hanger (Figure 6) and in theA}4DU (Figure 7) at CFB Trenton I effectively reduce the sound pro-

duced by the ultrasonic cleaners in other areas of these buildings.In No. 3 Hanger, the sound pressure level at the operator's posi-

tion is 94 dBA; outside the room at the Silting Index Bench and

I The room in the AMDU is constructed using 1/2" gypsum board on

both sides of 2" x 4" studs. The room in No. 3 Hanger has wallswith 1/4" plywood on one side and 1/4" plywood and 1/2" tenteston the other side of 2" x 4" studs, with the space between filledwith fibreglass. No attempt has been tiade to seal the doors ineither room, and in fact, there are at least 1/2" air spaces underthe doors.

4

at the Filter Bench, the levels are 54 and 51 dRA respectively. Inthe 12.5- plus 16-kHz one-third octave bands, the sound pressurelevels inside the room (with the ultrasonic cleaner lid open) are 91and 60 dB respectively.

Likewise, the sound pressure levels inside the cleaner room inthe ANDU are 79 to 95 dBA (depending upon operating conditions (Table)): outside the room it is 50 dBA.

Placing plastic absorbers on the surface of the ultrasoniccleaner fluid has about the same effect on sound radiation as doesclosing the lid of the cleaner. Without absorbers on the surface, forexample, the sound radiated from the cleaner in the 12.5- plus 16-kHzone-third octave band drops ,rom 90 to 88 dB when the lid is closed.Leaving the lid open and placing absorbers on the surface of thecleaner fluid reduces the radiated sound from 90 to 87 dB. Closingthe cleaner lid and placing absorbers on the surface of the fluid doesnot result in additional noise reduction, due presumably to the factthat other modes of radiation become dominant.

CONCLUSIONS

The high-frequency noise produced by ultrasoni.- cleaning devicesat CFB North Bay and CFB Trenton is sufficiently intense to produceeffects such as nausea, headaches, tinnitus, fatigue etc., amongexposed personnel.

Although the 20-kHz one-third octave-band sound pressure levelsobserved close to these units are well under 140 dB (the level belowwhich damage to the human ear is thought not to occur), they never-theless exceed the levels recommended for hearing conservation (105 dBat an operator's postion, 95 dB within 15 feet of an operator).

The enclosure fabricated at CFB North Bay reduces the noiseproduced by the ultrasonic cleaner in the AMCRO below the level wherethe above effects begin to occur. The addition of a mechanical deviceto assist in lifting the enclosure lid, and a small access panel toprovide access to the cleaner controls, would make the cleaner moreconvenient to use.

The rooms constructed in No. 3 Hanger and in the AMDU at CFBTrenton effectively reduce the sound produced by the ultrasoniccleaners from propagating to other areas of these buildings. Ofcourse, personnel required to work inside these rooms receive noprotection from the generated noise.

The plastic absorbers on the surface of the cleaning fluid insidethe ultrasonic tanks have about the same effect on reducing radiatedsound (by 2 to 3 dB) as does closing the cleaner lid. It has beensuggested that a greater reduction might be achieved (perhaps 10 dB onthe A-weighted scale) by isolating the ultrasonic tank (Figure 2) fromthe remainder of the cleaner unit. It would appear, however, thatthis rather complex modification is not warranted since the resultingreduction in noise level would not be sufficient to completelyalleviate the above exposure effects.

The most effective means of reducing the noise radiated fromultrasonic cleaners is to contain each unit in an appropriatelyventilated room or enclosure. An enclosure should include an easy-to-operate lid and convenient access to the cleaner controls.

RECOMMENDATIONS

1. Ultrasonic cleans should be enclosed to minimize the effectsof ultrasonic exposure upon operators and personnel workingin proximity with the devices.

2. Personnel who operated ultrasonic cleaners that are not ef-fectively enclosed, or who work in environments where the noiseradiated from such cleaners produces effects such as nausea,

headaches, fatigue, tinnitus etc., should wear Canadian Forcesstandard issue ear plugs or earmuffs while being thus exposed.

6

ACKNOWLEDGEMENTS

The authors wish to thank Capt. J. Butterfield, AMDU, CFB Trenton,and Capt. R. Payne, AMCRO, CFB North Bay, for the cooperation and assist-ance provided by they and their staffs during this investigation.

7

REFERENCES

1. ACTON, W.I. & CARSON, M.B. (1967). Auditory and subjective effectsof airborne noise from industrial ultrasonic sources. Brit. Jour.Ind. Med., 24, p 297.

2. GUIGNARD, J.C. (1973). A basis for limiting noise exposure forhearing protection. Report No. AMRL-TR-73-90. Aerospace MedicalResearch Laboratory, Wright-Patterson Air Force Base, Ohio, U.S.A.

3. HUGHES, D.E. (1965). 'Biological effects of ultrasound. ScienceJour., 7, p 39.

4. MACPHERSON, P.A., & THRASHER, D.B. (1959). A "near field" study ofjet engine noise. DRML Technical Memorandum No. 27-11. Toronto.

5. PARRACK, H.O. (1966). Effect of air-borne ultrasound on humans.Int. Audiology, 5, p 294.

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TABLE VI

ONE-THIRD OCTAVE-BAND SOUND PRESSURE LEVELS INdB re 2xl0 - 5 N/m2 AT THE OPERATOR POSITION OF THE ANCRO ULTRASONIC CLEANER

CENTRE LID OPEN LID CLOSED LID CLOSED

FREQUENCY CABINET OPEN CABINET CLOSED CABINET CLOSED

6.3 kHz 78dB 74dB 53dB

8 kHz 81dB 79dB 55dB


12.5 kHz 77dB 75dB 52dB

16 klilz 86dB 84dB 52dB

20 kHz 105dB lOOdB 78dB


31.5 kHz 80dB 79dB 50dB


12.5ktiz + 16kHz 87dB 85dB 55dB

15

TABLE VII

ONE-THIRD OCTAVE-BAND SOUND PRESSURE LEVELS IN

dB re 2x10 -3 N/M2 AT THE OPERATOR POSITION OF THE AMDU ULTRASONIC CLEANERS

CENTRE LEWIS GEN. NO. 1063 LEWIS GEN. NO. 688 HIPS ULTRASONICFREQUENCY LID OPEN LID CLOSED LID OPEN LID CLOSED CLEANER

6.3 kHz 64dB 62dB 53dB 53dB 79dB

8 kHz 71dB 73dB 55dB 55dB 77dB


12.5 kHz 56dB 55dB 83dB 81dB 71dB


20 kHz 103dB 98dB 61dB 60dB 103dB


31.5 kHz 70dB 67dB 75dB 67dB 73dB


12.5kHz + 16kHz 97dB 95dB 90dB 90dB 82dB

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16

TABLE VIII

ONE-THIRD OCTAVE-BAND SOUND PRESSURE LEVELS IN dB re 2x10 5 N/m2 OF THEAMDU ULTRASONIC CLEANER, USING PLASTIC GEOMETRIC SHAPES AS SURFACE ABSORBERS

CENTRE LID OPEN LID CLOSED

FREQUENCY SHAPES IN SHAPES OUT SHAPES IN SHAPES OUT

6.3 k}Iz 73dB 73dB 74dB 69dB

8 kHz 89dB 76dB 77dB 73dB


12.5 kHz 77dB 75dB 77dB 73dB


20 kHz 106dB llOdB 107dB 107dB


31.5 kHz 80dB 77dB 76dB 73dB


12.5kHz + 16kHz 87dB 90dB 87dB 88dB

17

TABLE IX

ONE-THIRD OCTAVE-BAND SOUND PRESSURE LEVELS IN db re2x10- 5 N/M2 IN AND AROUND THE CFB TRENTON ULTRASONIC CLEANER (HANGAR NO. 3)

INSIDE CLEANING ROOM OUTSIDE CLEANING ROOMCENTRE SILTING BENCH FILTER BENCHFREQUENCY LID OPEN LID CLOSED LID OPEN LID CLOSED

6.3 kHz 69dB 73dB 37dB 47dB

8 kllz 72dB 72dB 37dB 37dB


12.5 kHz 73dB 74dB 35dB 37dB


20 kHz 1OldB 100dB 67dB * 65dB


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ROR,'T NO: ]Y21E:4 ';rchnical J><}ort 7,'C;,

TITLE: Exposure to Ll.rLonic C1c..1uir Noiz':O(2 inthe Canadian Forces.

DATED: Julif 77

AUTHORS: )R.B. /Crabt reel'& S.E./ Forshaw

SECURITY GR;.DING: UNCL-SSIFIED initial distribution Dec 77

2- DSIS / /Plus distribution I1- DSIS Rerort Collection . - '

1- Microfiche Section

i- DPMI BRITA TN1- CFYS Ministry of Defence1- GF School of ,,eromedical Training 5- DRICi- CISTI (microfiche only)1- NUW UNITED STK2...1- Dept. Veteran:- %ffairs C Chief Medical Adviser) 3- DDC/TIZ1- Air Canada ,Ui-:inistrttive Officer of Medical

ServicesAttn: Dr. Keith Eady

/ - /. K - //

//

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