Since the earliest testing using tuningforks, assessment of bone-conductedhearing sensitivity has been an essen-

tial component of audiometric differentialdiagnosis. It was Bekesy, in 1932, whodemonstrated that it was possible to cancela bone-conducted tone by introducing anair-conducted tone of the same frequency,but with different phase. This supported thenotion that the two signals had the samemechanical displacement patterns at thebasilar membrane—a topic of some debateat the time. In the 1960s Tonndorf added toBekesy’s work by providing a detaileddescription of three different modes of bone-conduction transmission.

From a clinical standpoint, bone-conduction testing has pretty much alwaysbeen a routine part of an audiologic evalua-tion (at least since these evaluations havebeen called “audiologic”). Today, the testingis so routine that it sometimes is conductedin a rather casual manner. When immittancefindings are normal, maybe it’s not conductedat all. But like all things that we do over andover, it’s useful to step back and take a criticallook at what we are actually doing. We’vefound an author to provide us that per-spective on bone-conduction testing for thismonth’s Page Ten.

Robert H. Margolis did his early audio-logic training at Kent State University beforeearning his PhD from the University of Iowa.He worked at the UCLA Department ofOtolaryngology and the Syracuse UniversityDepartment of Communication Sciences andDisorders before joining the University ofMinnesota Department of Otolaryngology asprofessor of audiology in 1988. His researchhas focused on developing improved methodsfor hearing assessment, including acousticimmittance, electrocochleography, and pure-tone and speech audiometry.

In 2000 he established Audiology Incor-porated to develop and commercialize auto-mated hearing tests. His work on automatedaudiometry has been supported by small busi-ness technology transfer grants from theNational Institutes of Health. He is currently acollaborator on the NIH Toolbox project(www.nihtoolbox.org) to provide a standardhearing assessment tool for large epidemio-logic and clinical outcome studies.

When Bob isn’t thinking about bone-conduction secrets or automated testing,you can find him on the tennis court (wherehe prefers to serve wide to the ad court) orcarefully pouring a dark Belgian beer (whichhe prefers to serve in the middle of the back-court). For now, Dr. Margolis is serving upsome useful information about that old testof ours, bone conduction. He’s provided afew things to think about that might alter yourclinical practices.

GUS MUELLERPage Ten Editor

1Aren’t you the guy who published anarticle on bone conduction called

“Audiology’s Dirty Little Secret”? Why areyou airing audiology’s dirty laundry inpublic?That was me, but I was not referring to bone-conduction in general, just one or two specific lit-tle secrets. Frequently during my short 35-year careerin this field, I have heard complaints about air-bonegaps and bone-air gaps (bone-conduction thresh-old worse than air-conduction threshold) that don’tfit the patient’s audiometric picture and thereforemust be wrong. That is, you see a significant air-

bone gap when you’re pretty convinced it’s a completely sensorineural hearingloss. Or even more puzzling, bone scores that are 10-15 dB worse than air scores.

In the development of AMTAS®, an automated pure-tone test (see www.audiologyincorporated.com), we have noticed these “errors” even more frequently thanwe see them in manual audiometry. So I’ve been trying to get to the bottom of it.

2But doesn’t that simply mean that audiologists get moreaccurate thresholds than automated procedures?

Maybe–but maybe not. Let’s refresh our memory on the variability associated withair-bone gaps. This is not a new topic. Studebaker addressed this very issue in anattempt to clarify that when there is variability associated with a measurement,the measured value doesn’t always land on the mean.1 He modeled the air-bonegap as a normally distributed variable with a standard deviation of 5 dB. His modelpredicts that in the absence of a conductive hearing loss the air-bone gap is 0 dBonly 38% of the time. If four frequencies are tested, air-bone gaps at all frequen-cies would be zero only 2% of the time and gaps of 10, 15, and 20 dB are expectedto occur much more often than you might think.

3Interesting. Do you have anything to contribute to theexplanation?

I have a friendly amendment to the Studebaker model, one that was suggested tome by my friend and colleague Aaron Thornton. Aaron pointed out that the air-bone gap is a normally distributed variable, as Studebaker told us, but it is the dif-ference between two normally distributed variables (the air- and bone-conductionthresholds) that have different variances. That produces a normally distributedvariable with variability that is greater than either air conduction or bone con-duction alone. If we assume standard deviations of 5 dB for air-conduction thresh-olds and 7 dB for bone-conduction thresholds, the standard deviation of theair-bone gap is 8.6 dB. The new model predicts air-bone gaps of 0 dB for only21% of thresholds and gaps of 10 dB or more (air-bone gaps and bone-air gaps)almost half the time (48% of thresholds).

A few secrets about bone-conduction testingBy Robert H. Margolis

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Robert H. Margolis

Let’s look at the example in Figure 1. This audiogram wasobtained from a patient who is an experienced listener, hav-ing participated in many research studies. The audiometerwas calibrated to the ANSI standard (S3.6-2004)2 and boneconduction was tested with the vibrator on the forehead,using the appropriate mastoid-forehead corrections. Ignor-ing 4000-Hz for a moment, the air-bone gaps are well withinthe expected variability for patients with sensorineural hear-ing loss. But it looks a little sloppy, right? Note thatAMCLASS®, our validated audiogram classification system,called the hearing loss in the right ear a mixed loss becauseof the air-bone gap at 4000 Hz.3-5 If this was a manual audio-gram and I had any doubt about the bone-conduction thresh-olds, I would be tempted to nudge them toward smallerair-bone gaps, especially if I had already obtained normalimmittance results.

4I agree. It doesn’t look quite right.

But wait, I wasn’t finished telling the story. Now let’s look atanother example in Figure 2. What’s wrong with this audio-gram? Nothing, right? I know from previous audiograms,

patient history, tympanometry,and otoscopy that the patienthas a sensorineural hearing loss.But the likelihood, based onvariability of air-conduction andbone-conduction testing thatthe patient has 0-dB air-bonegaps at four frequencies in eachear is 1 in 250,000. Let me re-peat that: 1 in 250,000!

Most of us should not seean audiogram like this in ourprofessional careers. The audio-gram in Figure 1 is plausible.The one in Figure 2 is almostcertainly fudged. But, if every-thing matches nicely, the ENTdoctor wouldn’t walk back tothe booth and question it. (It’sour dirty little secret.) I know,you’re thinking that you’ve seenmany audiograms conductedby competent audiologistswhere everything matches upnicely. What I’m saying is thataudiologists don’t always reportair-bone gaps that occur inpatients with sensorineuralhearing loss.

5So you’re saying thataudiologists are inher-

ently dishonest?

I would never say that. I amsaying that bone conduction is

a biased experiment. When we are testing bone conductionwe almost always have an idea of what the result is going tobe. We get these premonitions from previous audiograms,thresholds at other frequencies, other test results like immit-tance and otoscopy, and patient history. If all these sources ofinformation point toward sensorineural hearing loss, the audi-ologist is biased toward recording bone-conduction values thatare equal to air-conduction thresholds.

6But if audiologists are honest people, whywould they report inaccurate bone-conduction

thresholds?

Two reasons. First, it is the nature of bias that we don’t alwaysrecognize that our behavior is affected by biasing factors. Thereis literature on effects of bias on human behavior that showsthat performance is affected even when the person is aware ofthe potential for bias. See, for example, Messick and Sentis.6

Second, as an audiologist, I want to communicate thecorrect status of the patient’s hearing. If I know in my heartthat the patient has a sensorineural hearing loss, I am lesslikely to report air-bone gaps that are expected to occur as aresult of the inherent variability of the measurements. I might

12 THE HEARING JOURNAL Page Ten FEBRUARY 2010 • VOL. 63 • NO. 2

AMTAS� Audiometric ReportSession : X00000-20071126-3Test Date : Mon Nov 26 16:48:07 2007

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Figure 1. Audiogram with air-bone gaps that are expected from variability of air-conductionand bone-conduction thresholds (except at 4000 Hz).

be concerned that an otolaryngologist would order furthertesting and/or follow-up appointments because of this “appar-ent” conductive loss. And, of course, we all have been taughtthat bone-air gaps are theoretically impossible, so I am biasedtoward under-reporting those when they occur even thoughthey are the expected result of the variability of the mea-surements.

There is a good reason thatclinical trials are designed asdouble-blind experiments. Whenpeople have prior knowledge ofthe expected results, the outcomesare different from when they haveno prior knowledge. And this istrue with the most honest, ethi-cal humans on earth–all of whomare audiologists!

7You started out sayingthat air-bone gaps in

patients with sensorineuralhearing loss occur more fre-quently with automatedtests. How do you accountfor that?

It’s very simple. AMTAS andother automated tests are notbiased. They don’t care if thereis an air-bone gap or a bone-airgap. They report the results fromthe patients’ behavior uninflu-enced by any expectations. Theyhaven’t read Studebaker’s articleand they don’t have any dirty lit-tle secrets.

8Okay, I’ll buy that. I’veheard about erroneous

air-bone gaps at 4000 Hz.

Is this just something related to the variabilityyou’ve been talking about?Well, not entirely. There’s something else going on at 4000Hz. Look at the audiogram in Figure 3. This patient has a sen-sorineural hearing loss. The air-bone gaps at 4000 Hz areunlikely to be related to variability. You can prove that by test-ing the patient repeatedly. If you get the same air-bone gap allthe time it’s not the result of variability. Note that AMCLASSwants to call the hearing loss mixed in both ears. AMCLASS wasvalidated against the judgments of expert audiologists. Unlesswe ignore the air-bone gaps at 4000 Hz, the hearing loss ismixed in both ears. But it’s not.

9So where did that big air-bone gap come from?

My guess is that our bone-conduction, reference-equivalentthreshold force level at 4000 Hz is wrong. But in the studythat was the source of the standard bone-conduction thresholds,subjects with sensorineural hearing loss were tested at threelocations to derive bone-conduction, reference-equivalentthreshold force levels (RETFL) that would produce 0-dB air-bone gaps (on average).7

Then to verify that the values were correct, a new group ofsubjects with sensorineural hearing loss was tested and, sure

AMTAS� Audiometric ReportSession : X00000-20071205-1Test Date : Wed Dec 5 09:34:13 2007

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Figure 2. Audiogram with all 0-dB air-bone gaps.

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enough, their air-bone gaps averaged 0 dB at all test frequen-cies, including 4000 Hz. The threshold levels were incorpo-rated into the audiometer standard and we began calibratingbone conduction to those levels. But soon audiologists begannoticing air-bone gaps at 4000 Hz in patients with sensorineuralhearing loss.

10Is it possible that the patient can hear air-conduction radiation from the bone vibrator

at 4000 Hz and that causes bone conduction to bebetter than it should be?

That explanation continues to be kicked around, but it wasdebunked early on by Frank and Holmes (1981).8 They testedbone conduction in subjects with ears open and ears pluggedand got no difference at 4000 Hz.

In an AMTAS validation study we found the same unex-plained air-bone gap at 4000 Hz with the ears covered by cir-cumaural earphones and the bone vibrator on the forehead.8

If you can block the ear with an earplug as Frank and Holmesdid or with a sound-attenuating muff as we did and the air-bone gap remains, it is not due to acoustic radiation.

11But a lot of my friends don’t get air-bonegaps at 4000 Hz except when the patient

has middle ear disease.Why isn’t it always there?

Here’s another dirty little secret. Rumor has it that some cali-bration services have become tired of hearing complaints about4000-Hz air-bone gaps and calibrate bone levels at that fre-quency off standard. And they don’t always tell us they aredoing that.

It may be a reasonable solution because the source of theproblem appears to be an incorrect RETFL at 4000 Hz. Butif this really happens, they shouldn’t do it without telling us.

12Is that legal?

I’m glad you asked that question. Some state licensure lawsrequire that testing be performed with a calibrated audiometer.

That implies that levels are calibrated to the audiometer stan-dard. If we have a good reason for doing it and we have datato support it, we are probably on safe ground if we use a dif-ferent reference level. But we should do it with our eyes open.

13Do we actually have the data?

We have some, but they are conflicting. The Dirks et al. datashow no air-bone gap at 4000 Hz6 and our data show a 12-dBair-bone gap for manual testing and 22-dB air-bone gap forautomated testing.9 I suspect the difference is due to bias inmanual testing. We are testing a new group of sensorineuralhearing loss subjects now to shed more light on it. We hopeto report the results early this year.

14If 4000-Hz bone conduction is such aproblem, why don’t we just skip it?

I don’t think that would be a good idea. High-frequency air-bone gaps can be clinically important. Look at the audiogramin Figure 4. This patient came in with a complaint of auralfullness in the left ear. Her 226-Hz tympanogram was normal.High-frequency hearing losses like this are usually senso-rineural and it would be easy to send this person away with-out further evaluation.

But her 1000-Hz tympanogram was flat and an otomi-croscopic examination revealed middle ear effusion. The hear-ing loss and abnormal high-frequency tympanogram areconsistent with mass loading of the middle ear. She was treatedfor otitis media and the hearing loss resolved. Middle ear effu-sion in adults can be a sign of serious conditions such asnasopharyngeal carcinoma. I wouldn’t want to miss this case.I think we should do more high-frequency bone-conductiontesting, not less.

15How do you know that the 4000-Hz air-bonegap in this case is real rather than the same

erroneous finding you’ve been talking about?

Good question, and it’s not always an easy distinction. In thiscase, the 3000-Hz bone threshold, the high-frequency tym-panogram, and the careful ear examination confirmed thatthere was a real high-frequency conductive hearing loss. Thecase illustrates the importance of getting our 4000-Hz bonethresholds right. We don’t have a definitive answer yet, butwe should all know how our audiometers are calibrated anduse our diagnostic skills to interpret these cases appropriately.

16If tympanograms and acoustic reflexes arenormal, do we really have to test bone

conduction at all?

Yes. Let’s look at the case in Figure 5. This patient had fluctu-ating hearing loss and vertigo and, based on her symptoms,could easily be diagnosed with Ménière’s disease. In spite ofnormal immittance findings, there is an air-bone gap.

My colleague Lisa Hunter followed a group of patients likethis for several years before Rosowski and his colleagues atHarvard-MIT explained that patients with dehiscent supe-rior semicircular canals behave just like this.10,11 The enhanced

Figure 4. High-frequency conductive hearing loss.

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bone-conduction sensitivity is explained by a “third window”effect in which the opening of the bony labyrinth into thesubdural space results in greater cochlear stimulation by boneconduction.

In some cases with normal immittance findings and a doc-umented history of sensorineural hearing loss, bone-conductiontesting may be unnecessary. But more information is alwaysbetter if you want to understand your patient’s hearing. If youdon’t do it you won’t see the surprising cases that may teach ussomething. And you may make the wrong diagnosis.

17Let’s back up a moment. Earlier youmentioned forehead placement of the

bone-conduction vibrator. Is that a new way totest bone conduction? How does it compare tomastoid placement?

It was recognized very early in the development of hearing test-ing that it doesn’t really matter where you place the bone vibra-tor. Forehead placement has been around for decades and thereference equivalent threshold force levels are in the standard.It takes roughly 10 dB more force to reach threshold with fore-head placement than with mastoid placement. For many yearsthat was a problem because it restricted the maximum hearinglevels that could be produced. Current audiometers and bonevibrators can reach higher levels, so it’s no longer a problem.

18So why do most people continue to usemastoid placement?

The common use of mastoid placement is related more to theearphone used to provide masking to the non-test ear than tothe bone vibrator. The supra-aural earphones that are mostcommonly used produce a large occlusion effect in the low fre-quencies that shifts the bone-conduction thresholds. That’swhy we uncover the test ear during masked bone-conductiontesting. Then we have to turn the whole arrangement aroundto test the other ear. When you use occluding earphones there

is no advantage to forehead bone because you still have torearrange the transducers when you switch ears to leave thenon-test ear uncovered.

But if we had an earphone that didn’t produce an occlu-sion effect, we could place the bone-conduction vibrator onthe forehead, earphones over both ears, and test air conduc-tion and bone conduction without moving the transducers.This would make manual testing more efficient and makeautomated testing possible. That’s how AMTAS works.

There is a tendency to think that when we place the vibratoron the right mastoid we are testing the right ear. Of course, welearn in basic audiology that there is no interaural attenuationfor bone conduction, but inexperienced testers may forget that.With forehead bone conduction it is clear that you only knowwhich ear heard the tone if you properly mask the non-test ear.

19I’m liking the idea of using foreheadplacement. Is there a good non-occluding

earphone that can be used for air-conductiontesting and for masked bone?

There certainly is. The Sennheiser HDA200 earphones thatmany clinics use for extended high-frequency testing can beused for the conventional frequency range as well. The refer-ence equivalent threshold sound pressure levels are in the stan-dard so it can be calibrated and used for air-conduction testing.It is also a thousand times more comfortable than supra-auralearphones and provides good ambient noise attenuation so youcan test in any reasonably quiet space–not just sound rooms.

20Mastoid bone or forehead bone. Automatedtesting or manual testing. Supra-aural

earphones or circumaural earphones. There aresome choices in pure-tone testing, aren’t there?

Yes there are, but there has been very little innovation, despitethe fact that audiologists spend more time performing pure-tone audiometry than any other single activity. We have text-books that have been teaching the same method for decadesand we have an audiometer standard that stifles innovation.We can do better. We need to think outside the box.

REFERENCES1. Studebaker GA: Intertest variability and the air-bone gap. J Sp Hear Dis 1967;

32:82-86.2. American National Standards Institute: ANSI S3.6-2004, American National Stan-

dard Specification for Audiometers. New York: Acoustical Society of America, 2004.3. Margolis RH, Saly GL: Toward a standard description of hearing loss. IJA 2007;

46:746-758.4. Margolis RH, Saly GL: Distribution of hearing loss characteristics in a clinical popu-

lation. Ear Hear 2008a;29:524-532.5. Margolis RH, Saly GL: Asymmetrical hearing loss: Definition, validation, prevalence.

Otol Neurotol 2008b;29:422-431.6. Messick DM, Sentis KP: Fairness, preference, and fairness biases. In Messick DM,

Cook KS, eds., Equity Theory: Psychological and Sociological Perspectives. New York:Praeger, 1983.

7. Dirks DD, Lybarger SF, Olsen WO, Billings BL: Bone conduction calibration: Cur-rent status. J Sp Hear Dis 1979;44:143-155.

8. Frank T, Holmes A: Acoustic radiation from bone vibrators. Ear Hear 1981;2:59-63.9. Margolis RH, Glasberg BR, Creeke S, Moore BCJ: AMTAS®-Automated Method

for Testing Auditory Sensitivity: Validation studies. IJA 2009, in press.10. Songer JE, Rosowski JJ: A mechano-acoustic model of the effect of superior canal

dehiscence on hearing in chinchilla. J Acoust Soc Am 2007;122:943-951.11. Merchant SN, Rosowski JJ: Conductive hearing loss caused by third-window lesions

of the inner ear. Otol Neurotol 2008;29:282-289.

Figure 5. Audiogram from a patient with an inner eardisorder (probably dehiscent superior semicircular canal)that produces air-bone gaps.

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