ear defenders: measurement methods and comparative results

Ear Defenders: Measurement Methods

and Comparative Results* I. C. WEBSTER

U. S. NAVY ELECTRONICS LABORATORY

Ear defenders offer positive protection against loud noise, provided

that they have sufficient attenuation and fit the wearer correctly.

Outlined are methods of measuring the capabilities of ear defenders

and the results of measurements taken in a number of laboratories.

S O far in this symposium )~ou have learned about the physical

properties of noise, how to measure noise, where to find it, its effects on man, and the roles that various men play in combatting it. Now we get down to the weapons at our disposal for combatt ing noise. The offensive weapons that attack the source or the transmission path will be discussed later. Our present concern is with the defensive weapons, namely, those that can be placed in our ear canals, over our ears, or over our heads. Our purpose is to discuss methods of evaluating these defenders and to present some general results of such evalua- tions.

Wi th in each of the three cate- gories of in, over, and surrounded by there is one other functional breakdown. The sole job of one group of defenders is to attenuate external sound, but another group of defenders must, in addition, admit wanted sound. Those in this second group contain receivers (earphones) of some type for pre-

senting wanted sounds, say radio signals, to the ear.

Those devices that fit in the ear canal are usually called plugs and consist of (I) cotton (with or without wax impregnation) which can

* An address before the First West Coast Noise Symposium in Los Angeles, December 3, 1954.

be formed by the individual to conform to his own ear canal, (2) pre-shaped plugs made of yielding materials in various sizes intended to fit all ear canals, and (3) individually molded plugs made to fit a particular ear canal. This latter type is often used in conjunction with a hearing-aid type receiver (earphone) to admit wanted signals

while excluding ambient external noises.

Muffs (or "cushions," "sockets," "pads," etc.) fit over the ears, not in them, and are held either in a headband or a helmet. Certain headbands and helmets also contain receivers (earphones), while others do not.

T h e use of shields that fit over the whole head is very limited to date. One such crude shield is shown in Fig. 1. This shield has been used as a portable audiometric test room when making hearing tests aboard ships at sea.

Measurement Methods

T h e m o s t u n i v e r s a l l y u s e d method of evaluating the attenuation characteristics of ear defenders is the absolute-threshold-shift method. Briefly, this method consists of finding a person's open-ear threshold of hearing and then re- determining his threshold when he is wearing defenders. A threshold

is the level of the faintest audible sound. The difference between the two thresholds is a measure of the at tenuation of the ear defender. I f the defender is a plug, this method can be accomplished by using a clinical audiometer with earphones. However, the more usual method is to use a loudspeaker in a sound treated room, so that any device, plug, muff, or shield can be tested. This is usually called a free field test.

FIG. 1. Picture of celotex-lined cylindri-

cal hehnet that can be fitted snugly a round the face and shoulders of a

listener. Th i s type of crude helmet has been used as a por table audiometric test

booth aboard ships.

34 NOISE Control

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There are many variations of the absolute-threshold-shift method. The test can be monaural or binaural. If the monaural method is used, the ear not being tested must be sealed of[ to give a greater attenuation than the device under evaluation in the other ear. In testing rooms where test tones gen- erate interfering standing waves, bands of noise or warble-tones are more reliable stimuli than pure tones. Several alternative psychophysical methods can be used. The question arises as to whether the usual procedures of clinical audi- ometry or the method of adjustment should be used for the threshold-shift method. A recent report from England 1 covering the evaluation of 17 different defenders re- ports the use of binaural, free-field (loudspeaker) testing with pure tones and the clinical audiometric technique. We have tried many methods 2 but prefer free-field testing with either pure tones or bands of noise and the method of adjustment. t This method is shown in Fig. 2.

There are a few precautions or limitations to the absolute-thresh-

-['The American Standards Association Committee Z24-W-26 is now writing a standard which should specify which variant of the method is the most ac- ceptable.

old-shift method which should be pointed out. First, it is very sensi- tive to the ambient noise in the testing room. Any such noise will cause the open-ear free-field threshold to be abnormally high (the threshold will be noise-masked). When the defender is worn, the effects of room noise are minimized. Therefore, the difference between the two thresholds (the measured at tenuation of the defender) becomes progressively smaller as the room noise increases. In general, if any audible noise is present in the room, it is too noisy. A second factor to take into account is the increase in physiological noise occasioned by put t ing some- thing in or over the ears. T o notice this effect, just place your hands over your ears and take a few deep breaths. + These noises tend to raise the threshoId when defenders are worn, increasing the apparent at tenuation of the defender at threshold levels. A third factor, which has as yet no satisfactory explanation, concerns the difference in sound-pressure level required

Using your hands exaggerates the effect because of muscle tremors, but the effect remains if passive defenders are used. This also points out a potential reason why an earphone held by hand and an earphone mounted in a headband give different results in threshold tests.

( • POWER ATTENUATOR OSCILLATOR '~ AMPLIFIER OR NOISE BANDS

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Fro. 2. Block diagram O f equipment for absolute-threshold-shift method of measuring the acoustic attenuation of ear defender. The psycho-physical method of adjustment is used in conjunction with loudspeaker presentation and binaural listening. Typical threshold traces are shown. The difference between averaging lines drawn through the traces yields the attenuation measurement.

for threshold, and also for equal loudness, between open-ear tests and tests where the ear is partially covered. Munson and Wiener ~ point out that when an earphone is worn over the ear, the pressure, as measured by probe tubes in the ear canal, for equal loudness (and presumably also at threshold) is some 4 to 6 decibels greater than when the earphone is removed. This difference exists even though the diaphragm of the earphone is removed and holes drilled through it, allowing the free entry of sound. Apparently, if the ear is closed at the outer end (even if closed with an acoustic window), the sound pressure required for threshold increases above the free-field sound pressure for threshold. This again would tend to inflate the attenuation values assigned to a defender, at least if the defender be worn over the ear as a muff. Whether this applies to plugs cannot be stated at the present time.

High-Level Testing The first two of these three lim-

itations can be avoided by using methods that do not involve absolute thresholds. Tha t is, room noises and physiological noises can be minimized by working at sound- pressure levels well above threshold. Two commonly used methods for doing this are shown in Fig. 3. The masked-threshold method ne- cessitates an active earphone under the defender and is, therefore, limited to the evaluation of muffs and shieIds. The procedure is to fill the room with noise and to determine a person's noise-masked threshold with and without the defenders. The masked threshold is the level of the faintest audible sound that can be heard in the presence of noise. The difference in threshold

�9 is a measure of the amount of noise exclusion. For the condition where the listener is wearing defenders, as long as the noise is loud enough to cause the masked threshold to be at least 20 db higher than the absolute threshold (masking equal to or greater than 20 db), this method is valid.

The loudness-balance method in its simplest form consists of listening to a tone (or band of noise)

September 1955 35


over a loudspeaker, first without the defender and then with the defender, and adjusting an attenuator in the loudspeaker channel for equal loudness under the two conditions. The difference in attenuator seuings is the measure of the amount of noise exclusion. This is very time-consuming, since the defender must be taken off and re- placed for each judgement, so usually a modified procedure is used. This involves the placing of a reference earphone on one ear. The tones from this earphone are equated in loudness to the tones of a loudspeaker, heard by the other ear, once protected by defender and once not. (Care must be taken that the sounds from the loudspeaker do not leak through to the ear with the reference earphone.) The difference in at tenuation set- tings for equal loudness on the reference earphone gives a measure of the sound attenuation of the defender under test.

All of the above methods are psycholog ica l or psychophys i ca l methods. Tha t is, the listener has to participate by making threshold or loudness determinations. For all devices except plugs, a miniature microphone or a probe microphone can be placed under the defender as shown schematically in Fig. 4. In this phys ica l m e t h o d a series of meter readings are taken with and without the defender and the difference measures the attenuation. The person acts only as a vehicle for mount ing the defender and the microphone. An alternative and less satisfactory physical method utilizes a microphone in a dummy head. This method does not yield valid results in general due to the inability to design a dummy head with the same acoustic and mechanical im- pedance characteristics as a real head. We might distinguish between these two alternative physical methods by calling the first a real ear measurement and the second an artif icial ear measurement.

Comparisons among Methods How well do the results from

the different methods compare with each other? In summarizing the work done at Harvard Uni-

. . . . i , I I

,Ll . l I --I m

Fro, $. Blogk diagrams of ma~ked-threshold-ahi/t (top) and loudness-balance (bottom) methods for measuring the a c o . _ ~ a ~ o | eat degenders. See text for procedures.

versity during World W a r I I on developing the V-51R earplug, Wiener and Miller 4 show a comparison of the results of the absolute-threshold-shift and the loudness-balance methods. The results are very similar, never differing by more than 3 db. Neely? on the other hand, found differences on the order of 10 db below 1000 cycles per second with good agreement above 2000 cps. Differ- ences of this type could be caused by a testing room that was not sufficiently quiet. In evaluating the shield shown in Fig. 1, we2 obtained a comparison among the masked-threshold-shift, loudness-balance, and real ear physical methods as shown in Fig. 5. With a shield large enough to get a microphone and/or earphone un- derneath, the methods agree within 5 db.

Dickson et al 1 obtained comparable results between the real ear physical and absolute-threshold-shift methods. In evaluating muffs, a miniature microphone w a s placed in the ear canal, causing an appreciable change in the volume and configuration of the canal. The microphone was not acting merely as a passive, non-interfering measuring device. In spite of this, the results of the two methods agreed very well. They also found that the variability of the measures from person to person was as great or greater when using the real ear physical measurements as when using the psychophysical results. However, they found that their determination of attenuation by the real ear physical (miniature microphone) method did not compare with a similar set of real ear physical measures made by ~u

MIVJE ~ ~ AMFUFIER NOISE BANDS

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EIo. 4. Block diagram of equipment used fog physical measuremenl of the acou~lit attenuation of ear defenders. Such measurement can be made in ~ases where a probe o~ miniature microphone can be placed under the defender. The defender is either worn by a person (rea/ e a r testing) or mounted on a dummy head ( a r t i f i c ~ l r

testing). Frequency response curves are m ~ in the ear canal with and withou~ the defender, and the difference between them is the amount of acoustic attenuation.

36 NOISE Control


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Fta. 5. Comparison of attenuation measurements on the cylindrical celotex helmet, using three different measuring methods.

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FIG. 6. Variability in attenuation measurements on the V-Sll~ ear plug as measured by Dickson et al.1 The open circles are the mean values, based on the measurements of 20 listeners, The central unshaded area is the interquartile range of the results on these 20 listeners. The extreme upper and lower boundaries of the shaded area are ---+ 2 times the standard deviations of the results on 20 listeners. If another group of listeners were tested on the same defender under the same testing conditions, the results of half the listeners would be expected to fall in the unshaded central area. The results of 95% of the listeners would be expected to fall between the extreme upper and lower boundaries of the shaded area.

The crosses are the mean attenuation values as determined by making 20 measures on one listener. The vertical line is the interquartile range of these 20 trials.

kie G on the same muffs. I t appea r s tha t Dickson e t al 1 a n d W i l k i e 6 used d i f fe ren t types o f m i n i a t u r e in i c rophones which m a y have been d i f ferent in physical size. T h i s m i g h t h a v e c o n t r i b u t e d to the ir d i f ferences . P r o b e t u b e s m i g h t m i n - i m i z e such d i f ferences .

V a r i a b i l i t y of A t t e n u a t i o n M e a s - u r e m e n t s

W e now know in a g e n e r a l sort of way what methods are available for e v a l u a t i n g the a t t e m t a t i o n ca- pab i l i t i e s of ear dc lenders . Before buy ing any defenders , we shou ld l ike to know wha t is the consensus of the experts . s to which gives ~i~e grea tes t a t t e n u a t i o n at the par t i cu- la r f requency or b a n d of f requen- cies we are in te res ted in. We also want to know which d e f e n d e r will p r o v i d e /be greatest amonr~t of at- t e n u a t i o n to the grea tes t m t m b c r o[ peop le a~xl w h e t h e r i t will pro- vide such attctmatioH each a n d every time. Sect ion A below deals with the v a r i a b i l i t y of a t t c m . ~ t i o n measurements , first lov the same d e f e n d e r when worn b~ d i l t e ren t people, then when r epea l ed meas- u rements are m a d e on the same perso~L Sectbm B gives the al tenuat iol l restdls as d e t e r m i n e d by many d i l l e r e m labora to r i es on t[/e same d e f e n d e r (V-51R plug) . l 'yl~- ical a~ tenua t ions will be shown for plugs, mutt,s, he lmets , a~M colnbi- na t ions the reof in a l a te r section.

A. A m o n g P e o p l e a n d T r i a l s

l tow large arc the dif terences in a t t e n u a t i o n m e a s u r e m e n t s be tween di t levent peop l e w e a r ing the same d e f e n d e r arm be tween d i f ferent runs on the same person? Diekson et al ~ tested 20 d i f fe ren t p e o p l e on each o[ 17 defenders . Fo r certain defenders , one person ou t of the 20 was tested 20 d i f fe ren t t imes. l ,ct us see how lllttch v a r i a b i l i t y exis ted on a typical p lug , the V-51R, a n d a typica l muff, the R A F Acoust ics i , ab M a r k VI headpiece. F igures 6 a n d 7 show these resul ts as c o m p i l e d f rom re fe rence 1.

I n Figs. 6 a n d 7 we see two large s h a d e d areas s e p a r a t e d by a la rge u n s h a d e d cen t ra l a rea c o n t a i n i n g po in t s a n d ver t ica l lines. T h e large sol id circles r ep re sen t the average, or mean , a t t e n u a t i o ~ of the de-

September 1955 37


fender under test when the results as determined by all twenty men are averaged together. The "x's" represent the mean at tenuation of the defender as determined by averaging the 20 trials of a single person. The area between the two shaded areas represents the spread in at tenuation values for the 10 men whose results are the least extreme (interquartile range). Twenty-five percent of the attenuation values lie above this unshaded area (showing less attenuation) and 25% lie below. If similar tests were given to another group of men, 95% of the results would be expected to lie within the extreme boundaries of the shaded areas, including the unshaded area in the center. Only 5 men out of 100 would be expected to yield values lying above the top shaded area or below the lower one. When the V-51R atten- uat ion measurements are averaged from all seven frequencies, half of the measurements lie within a 13.7-db range, and 95% of the measurements would be expected within a 38-db range. T h a t is, the average height of the unshaded center area is 13.7 db, and the average distance between the extreme boundaries of the shaded area is "~8 db. T h e lines through the "x's" show the interquartile range of the 20 trials on one person. T h a t is, half the trials resulted in values lying somewhere on these lines. W h e n averaged over all seven frequencies, this turns out to be 9.9 db.

The same measurements on the Mark VI muff show that half of the values of the 20 people, averaged over seven frequencies, lie within a 6-db range, and 95% of the values would be expected in an 18-db range. Half of the trials on one person resulted in values in an average range of 7.3 db.

We can conclude from the above that the variability in results is much greater for the V-51K plug than for the Mark VI muff. Simi- lar measures of variability on other plugs and muffs show the same trends. This merely emphasizes the fact that plugs, even when sup- plied in various sizes, do not fit as well as muffs. This is not unex-

pected. The configurations of ear canals are more complex and variable than the relatively flat bony structures of the head around the ear. The conclusions to be drawn from this are that if you are go: ing to use pre-shaped defenders, a more universal fit is obtained by using muffs. Conversely, if plugs are used, great care must be taken to insure a proper fit, one that provides a maximum seal between the plug and the ear canal.

The experimental evidence of Dickson et al z has now shown us that a given muff or plug does not give the same at tenuation at any given frequency for different people or for different trials on the same person. They have shown the magnitude of the discrepancies that can be expected and have indicated that the primary cause of these discrepancies is the difference in fit or seal from one trial or person to another .

B. A m o n g Experimenters

Another way to show that attenuation is a statistic that fluctuates like earning power or productivity is to compare the results obtained at different laboratories.t, 4,5, g-9 Figure 8 shows such attenuation results on the V-51R plug. Also shown on Fig. 8 is the maximum extent of at tenuation as calculated from theoretical considerations by von Gierke and Warren. 1~ Thei r analysis shows attenuation of plugs to be limited at the low frequencies by skin compliance and at higher frequencies by the bone- conduction threshold. These theoretical limitations are discussed more fully in the preceding pa- per. 11 The shading on Fig. 8 out- lines the area into which 95~o of the at tenuation results, as determined by the various laboratories, would be expected to fall. Figure 6 outl ined a very large area (38 db wide, on the average) into which 95% of the results as determined

o n one of any number of listeners should fall. However, if an average value is plotted based on the results of many listeners, the variation is much smaller, and the area into which the results should fall is much smaller. I t is this smaller area, into which the averaged re-

sults of many listeners are expected to fall, that is plotted on Fig. 8. Statistically speaking, this is the spread of twice the standard error of the mean of the Flying Person- nel Research Committee results, plotted symmetrically around the average of the FPRC results. The FPRC data (Dickson et al z) are used because they are based on a large sample and are complete enough to allow the necessary calculations to be made.

I t is quite obvious that the results (averages based on the testing of many listeners) from the various laboratories do not lie within the shaded area, so some explanation is required. Shaw et al ~ have shown that there is no large systematic difference between the free-field and earphone variations of the absolute-threshold-shift method of measuring the attenuation of ear plugs, so the earphone data from the School of Aviation Medicine need not be discarded on that count. Dickson et al z say that there is a systematic difference between the monaural and binaural methods that can be explained by the equation

A,~ ---- 1.lAb -- 4 db

where AN, is the monaural attenuation and A b is the binaural attenuation. This difference is due primarily to the fact that in binaural listening, the defender that fits the least well is the one that determines the results (the best- hearing ear takes over). As can be seen from the equation, between 6 and 7.5 db should be subtracted from the monaural readings (NDRC and SAM) to make them comparable to the binaural results. When this is done, the SAM results fall into the shaded area, leaving only the N D R C and ZW results outside.

Zwislocki ~ does not state whether his tests were binaural or monaural. If the monaural correction is added to his results, they fall within the shaded area. If no correction is made, the corrected N D R C results agree very well with the ZW results, and both show more low-frequency attenuation than any other investigation. This difference can probably be ac-

38 NOISE Control


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Fz6. 7. Variability in attenuation measurements on the RAF Acoustics Laborator Mark VI headpiece (muff) as measured by Dickson et al.Z The explanation of symbols, lines, shaded areas, etc. is the same as for Fig. 6.

| I IIII |

FI6. 8. Variability in attenuation measurements on the V-5IR ear plug, as measured at six different laboratories. All of the values shown here were obtained by the absolute- threshold-shift method. The NDRC results were obtained by using free-field stimulation of the right ears of 8 people at the Psycho-Acoustic Laboratory (Harvard University). The SAM (School of Aviation Medicine, Randolph Field, Texas) results were obtained by using earphone stimulation of each ear of 27 people. All other results were obtained hy binaural free-field stimulation. For subjects, NEL used 10 sailors, NSAM (Naval School of Aviation Medicine, Pensacola, Florida) used 5 hospital corpsmen, and the British (Flying Personnel Research Committee) used 20 persons working at the RAF Central Medical Establishments. The shaded area represents the spread of twice the standard error of the mean for the FPRC results.

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counted for by the care with which the plugs are fitted into the subjects' ears. When developing new defenders and using laboratory personnel for listeners, a good fit can be expected due to the past experience, training, and lack of fear among the subjects. Notice that the results that show the least a t tenuat ion are based on l0 USN sailors f rom N EL and 20 RAF men. At NEL, at least, the subjects had no previous experience with ear defenders. Th e i r instruc- tion was limited to an explanat ion that the purpose of the tests was to determine how well the defenders would at tenuate noise and that they had a choice of three sizes for obtaining the best fit. T h e experi- menter handed the subject the me- dium-size defenders, except in one case where obviously large ear canals suggested the large size, and showed him the recommended technique of insertion. Once the defenders were inserted, the exper imenter asked if they fit tightly and checked by touch to catch any obvious looseness. I t is quite pos- sible that, in some case or cases, the subject could have obtained a better fit, but only he would be able to judge that, through greater in- struction and experience.

T h e exper imenter felt that the difference in size between the me- d ium and large V-51R defenders was so great that another size between the two would do much to eliminate the at tenuat ion loss and variation apparently due to inade- quate fit.

The re is no question but that the seal of the plug is of vital importance in obtaining good low- frequency attenuation. Reference 9 shows data on the at tenuat ion of the V-51R modified by punching a hole through i t . T h e low-frequency at tenuat ion is gone. T h e same thing has been shown to be true of the Lee Sonic Ear-Valv when used in continuous types of noise. It is also known that imme- diately after a well-fitting plug has been inserted, there is a pressure developed on the ear d rum which causes increased low-frequency attenuation. This effect can be observed by any airplane passenger during climbing and descending.

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FIe. 9. V a r i a b i l i t y in a t t e n u a t i o n m e a s u r e m e n t s on ea r muffs d u e to a i r l e aks (da t a

f r o m y o n G i e r k e a n d W a r r e n , r e f e r e n c e 10). T h e cu rves l a b e l l e d "0.2 m m l e a k , " "0.5 m m l eak , " a n d "B.C. t h r e s h o l d " a r e t h e o r e t i c a l c a l c u l a t i o n s . T h e ones l a b e l e d " U s u a l

F i t " a n d " S e a l e d " are m e a s u r e m e n t s b a s e d on t h e t e s t i n g of l i s t eners .

Before equalizing your ear-drum pressure by swallowing or yawn- ing, listen to the background noise, then notice the increase in low- frequency energy after the equali- zation of air pressure.

Another incident at N E L shows 0 the importance of ear-plug fit. For one of the experiments in a training course in acoustic measurements, the students were asked to ,o compare the a t tenuat ing properties of cotton and the V-51R plug. Minimal instructions were given ~o them concerning how to fit the plugs, and they operated in pairs _z in the absence of detailed super- z O

vision. T h e results of several pairs ~ ~o of students showed cotton to be ,z the superior sound at tenuator. Be- ~< fore plugs can be used to good ad- 4o vantage, aid in fitting and instructions should be given the user.

Typical Values of Attenuation ~o

Plugs A point of interest regarding

Fig. 8 is that the Dickson et al 1 results (solid circles) which are based on the largest sample of men (N - -20) agree at all frequencies to within 2 db with some results obtained by D. E. Wheeler? 2 Wheel-

er's results, not shown on Fig. 8, were average at tenuat ions of nine different plugs. Figure 8 can, therefore. be considered to give typical

average results oil plugs in general and also to show the m a x i m u m at- tenuat ion that can be expected if a near-perfect seal is made by the plug. This m a x i m u m is shown by the calculations of yon Gierke and War ren lo and by the N D R C exper imenta l results. Other investiga- tions 18 have found results as good as the N D R C results when evalu- at ing plugs that were very similar, namely, the V-29, and when using similar test procedures. Reference 9 also shows similar excellent results for the Austral ian equivalent of the V-29 and V-51, calIed the A T L (Acoustic Tes t ing Labora- tory, Sydney, Australia).

Muffs and Cloth H e l m e t s

T h e seal of a muff to the head is as impor tan t in obta in ing maxi- m u m at tenuat ion as is the fit of ear plugs. Figure 9 shows the results of some calculations on an equivalent circuit and some experimental results obta ined by yon Gierke? ~ His theoretical calculations show that as the d iameter of the air leak undernea th a muff increases f rom 0.2 m m to 0.5 m s , the a t tenuat ion decreases about 35 db at frequencies below 500 cps.

CENTER FREQUENCY OF OCTAVE BAND

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FIG. 10. T y p i c a l a t t e n u a t i o n m e a s u r e m e n t s on ea r muffs . T h e " S e a l e d L i m i t " a n d

"B.C. t h r e s h o l d " cu rves a r e t he s a m e as those o n Fig. 9. T h e r e s u l t s o n (1) t h e " R A F

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t he " D E W A v g " a re f r o m r e f e r e n c e 11.

40 NOISE Control


Exper imental ly , he ob ta ined an average difference of about 14-db at- tenuat ion between the frequencies of 250 and 2000 cps for muffs that were t ightly sealed as opposed to muffs with the usual seal.

The a t tenua t ion of some repre- sentative muffs are shown in Fig. 10. T h e curve showing max imum expected a t tenua t ion is based on the sealed results of yon Gierke ~o below 1000 cps and his bone-con- duct ion curve above 1000 cps. T h e curve based on the average results of five commercial ly avai lable muffs eva lua ted by Whee le r 12 lies about 15 db above the m a x i m u m expected results. T h e a t tenua t ion of the R A F Acoustics Lab Mark VI muff lies wi th in 5 db of the expected max imum. T h e Mark VI muffs were improved through five developmental models ~ and are probably near op t imum for the par t icu lar type of construction used. T h e other curve shown represents the a t tenua t ion of the s tandard U.S. Navy donut cushions, NAF-48490-1, used with headset PHA-6 and Permoflux earphones. T h e i r a t tenuat ion is comparable to that of the Mark VI up to 2000 cps.

Muffs are often used in helmets with and wi thout earphones. Fig-

I06 i

i06 O I

CENTER FREQUENCY O F O C T A V E BAND

212 4;~5 850 1700 3400 i i 1 i i

\ \

\ \

m

z

z

4o

68O0 I

13600 I

50

T Y P I C A L COMBINATIONS

\ \

\ \

\ \ R A F

M K I I~ S M R

B J a / I N S E R T

/

" ' " " " - - Ds

62.5 125 250 500 IO00 12000 4000 8000 " 16000

FREQUENCY IN CPS

FIG. 12. T y p i c a l a t t e n u a t i o n m e a s u r e m e n t s of c o m b i n a t i o n s of devices . T h e " R A F

Mk I and SMR" results are from reference 1. The "BJ and Insert" results are from NEL data. The "DEW Est. Max." results are from reference 10.

ure 11 shows some typical attenuation curves. T h e best of the re- cently developed British helmets is the R A F T y p e M he lmet? which has between 1 and 5 db less atten- uat ion than their Mark VI muff. T h e other curve shown is for do-

CENTER FREQUENCY OF OCTAVE BAND ~l Z 425 FrO0 6800 3600

i i i " i i i ~ i

T Y P I C A L ' ~.~. CLOTH " ~" " ~ H E L M E T S

\

z_ \ z \ \ / ~ STD

z / /

RAF 40 / TYPE M

50

t I l l 12000 14000 I I 62.5 125 250 500 IO00 8000 t6000

FREQUENCY IN CPS

Fw,. l 1. T y p i c a l a t t e n u a t i o n m e a s u r e m e n t s on c l o t h h e l m e t s . T i l e " U S N S T D " d a t a are from reference 2; the "RAF Type M" from reference 1.

nuts with Permoflux earphones in the cloth inner l iner of the USN s tandard flying helmet. W h e n the ha rd shell is worn over the inner liner, the a t tenua t ion is nearly equivalent to the Type M cloth helmet. None of these helmets are as good as their equivalent muffs in headbands in the crucial region a round lO00 cps. T h e difference must be in the relat ive pressure exer ted against the head, or the seal, and apparen t ly headbands ex- ert greater pressure than helmets. Some recent unofficial results on new cloth helmets made for launching crews on aircraft carriers show substant ia l improvements in a t tenua t ion over any depic ted in Fig. 11.

Com bi na t i ons of Defenders I t is well established that the at-

tenuat ion actual ly observed when two defenders, say a p lug and a muff, are worn s imul taneously is less than the sum of the separate at tenuat ions. However, in most cases the total a t t enua t ion exceeds the a t tenua t ion of e i ther defender used by itself. T h e a t tenua t ion of a few combinat ions of defenders is shown in Fig. 12. T h e best com- b ina t ion as regards a t tenuat ion ,

September 1955 41


not comfort, that Dickson et al a found was the Mark I muff over the SMR plug. Also plotted is a curve for the at tenuation provided by wearing the hard-shelled Sound Asorb helmet over Air- phones (individually molded in- serts with hearing-aid-type earphones attached). T h e same helmet over Permoflux earphones provides between 5 to 10 db less attenuation below 2000 cps, but this combination is at least 5 db better at all frequencies above 250 cps than the same earphone with donuts in standard headset PHA-6. These combinations approach the attenuation maximum (limited by bone conduction) estimated by D. E. Wheeler and contained in reference 10.

Effectiveness of Defenders as Evaluated by Other Means

One of the pr imary reasons for wearing ear defenders is to avoid noise-induced permanent hearing loss. Consequently, attempts have been made to evaluate defenders by their ability to minimize temporary hearing losses in high-level- noise fields. Everyone using this ex- pediency is quick to point out that the relationship between temporary and permanent hearing loss has not as yet been established. However, the common-sense relationship between temporary and permanent losses seems clear enough to make this type of eval- uat ion fairly common. Zwislocki 7 has reported some cases of protection against temporary hearing loss by factory workers wearing defenders, as opposed to those who did not. Davis et al a~ used various defenders and combinations in the presence of jet-aircraft noises on aircraft carriers and found that muffs alone prevent temporary hearing losses in levels up to 140 db, while the V-51R plug alone or in combination prevented losses in levels of 150 db. T h e y recom- mend that both plugs and muffs be used in the most noisy situa- tions. Dickson et al 1 ranked the devices they tested in their ability to prevent hearing loss for speech, as calculated from pure-tone losses, when worn by listeners subjected to 123 db of propeller and /or jet noises for controlled periods of

time. They rank five muffs ahead of the best plug and then four more muffs ahead of the remain- ing four plugs. These rankings cor- relate very highly with their rank- ing of the ability of the defenders to interfere with the reception of speech in a quiet background.

Another way of making an over- all comparison among defenders is to determine how much the over- all level of a noise is a t tenuated by the defender. In the rank order determined by Dickson et al 1 for this criterion, the plugs rank ahead of the muffs. T h e reason is that at frequencies below 1500 cps, plugs at tenuate better than muffs, and most typical noises have greater energies below than above 1500 cps. Based on average results over many different plugs and muffs,a, a0 plugs at tenuate better than muffs by between 8 and 14 db at 250 cps, the difference grad- ually decreasing to 0 db at 1500 cps and reversing so that muffs attenuate bet ter by between 6 and 10 db above 4000 cps.

This dependence of at tenuat ion on frequencies makes it difficult to pick a best defender. If it is as- sumed that the at tenuat ion of defenders is adequately described by average results, then the attenuation of low frequencies is accomplished better with plugs and high- frequency at tenuat ion is better accomplished with muffs. Since the fit of plugs is more variable than muffs, it must be kept in mind that certain peopIe will get much better a t tenuat ion from plugs than the average, but, like- wise, many people will get less. T h e general rule is that if the plug is too comfortable, beware; it is probably not at tenuating well. An exception here might be the comfortable wax-impregnated cotton preparations that are readily available and at tenuate quite satisfac- torily. On the other hand, the variation of at tenuat ion with fit will be much less with muffs. I f the safety criterion is to protect every last person, then muffs are the best choice. If the criterion is to get the greatest over-all attenuation, then close fitting plugs are preferable. No single plug or muff is best for all jobs and, in fact, in

many cases the simultaneous use of both is preferable. One positive statement that can be made is that if hearing loss f rom working con- tinuously in high-level noise is to be prevented, use some defender. Whether it is a muff, plug, helmet, shield, or a combination thereof, any good defender (this excludes dry cotton) will help prevent deaf- ness due to noise. In fact, Davis et al 14 say that for present-day noise levels, what we now have available is sufficient if properly used.

References 1 E. D. D. Dickson, R. HincIlcliffe, and

L. J. Wheeler, "Ear defenders," Flying Personnel Research Committee Report 884, Air Ministry, London (June 1954).

2j . c . Webster, H. R. Beitscher, and J. A. Silkwood, "Acoustic attenuation of noise shielding devices," NEL Report 482 (March 1954).

W. A. Munson and F. M. Wiener, "In search of the missing six db," J. Aconst. Soc. Am. 24, 498-501 (1952).

4NDRC, Div. 17, Summary Technical Report, Vol. 3, Transmission and Recep- tion of Sounds Under Combat Conditions, Chap. 2, Fig. 57 (Washington, D. C., 1946).

K. K. Neely, "An assessment of the acoustical insulation properties of the Lee Sonic Ear-Valv," Canada Defense Re- search Med. Lab. Report 100-1 (July, 1952).

6 D. R. Wilkie, "The acoustic properties of flying helmets," Flying Personnel Research Committee Report 732, Air Ministry, London (May 1950).

7 j . Zwislocki, "New types of ear pro- tectors," J. Acoust. Soc. Am. 24, 762-764, Fig. 5 (1952).

s G. Tolhurst, "Noise attenuation characteristics of the B. F. McDonald ear muffs," USN School of Aviation Medi- cine Special Report 5~-22 of 18 Nov. 1953, Proj. No. TED-PEN-AE 1403.

9W. A. Shaw and P. S. Veneklasen, "The development of ear wardens type V-51R," Off. Sci. Res. Dev. Report 5122 (1 July 1945).

a0 H. E. von Gierke and D. R. Warren, "Protection of the ear from noise: limit- ing factors," in H. W. Ades, et al, BENOX report: An Exploratory Study of the Bio- logical Effects of Noise (Chicago Univ., 1 Dec. 1953).

11 p, S. Veneklasen, "Methods of noise control--personal protection," NOISE CONTROL 1, NO. 5, 29 (1955).

12 Personal Communication of Nov. 15, 1954.

18N. A. Watson and V. O. Knudson, "Ear defenders," J. Acoust. Soc. Am. 15, 153-159 (1944).

14 H. Davis, D. H. Eldredge, A. Glorig, and W. D. Halstead, "Bio-acoustics stud- ies of high intensity aircraft engine noise at the Naval Air Test Center and aboard USS Coral Sea," Contract N6ori-02044 between Office of Naval Research and Uni- versity of Chicago.

42 NOISE Control


ear defenders: measurement methods and comparative results

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