Contribution of theexternalauditory meatusto auditory sensitivity underwater Harry Hollien and Stephen Feinstein Communication SciencesLaboratory, University of Florida, Gainesville,Florida 32611 (Received 23 December 1974; revised 13 March 1975)

Support for, and a refinementof, the hypothesisthat divers hear primarily by bone conductionwas provided by Hollien and Brandt [J. Acoust. Soc. Am. 46, 384-387, (1969)]. To further test this hypothesis,the thresholdsof sevensubmergedlistenerswere obtained(at frequenciesof 0.25, 0.50, 1.0, 4.0, and 8.0 kHz) under three different conditions:(1) while they wore a full 3/16-in. wet suit

with no hood, (2) while wearinga full 3/16-in. wet suit with a 3/16-in. hood, and (3) while wearing a full 3/16-in. wet suit and hood with 1/4-in. rubber tubes passingthrough the hood to the external auditory meatuses.There were no significantdifferencesbetweenthe conditionsinvolving the use of a hood but thresholdswere significantlylower in the middle and high frequenciesfor the no-hood condition.These findingsprovide further supportfor the hypothesisthat underwatersoundenergy is transducedpredominantlyby bone conductionrather than by the normal middle ear linkage. Subject Classification:65.50;30.20; 65.22, 65.75.

INTRODUCTION

be remembered that, as early as 1966, Bauer and Torick argued that an earphoneis not a bone conductor, e.g.,

An earlier experiment (Hollien and Brandt, 1969) demonstrated that human underwater auditory sensitivity is not altered by either water or air bubbles impinging upon the tympanic membrane. Since the transmission characteristics of these two conditions are so disparate, we reasoned

that

if the middle

ear

is functional

under-

water, this change in impedance within the meatus should have been reflected in the subjects' performance. Since

then, however, Bauer (1970) has arguedthat this occluded bubble procedure does not constitute a valid test of the functioning middle ear underwater. His contention is based on his earlier work with underwater earphones

(Bauer and Torick, 1966), as well as an analysisof an equivalentcircuit of the tympanum(Zwislocki, 1957) andthe external ear (Bauer, Rosenbeck,andAbbagnaro, 1967). In his model, Bauer representsthe ear in air as a two-section inductive-capacitive with

an end correction

factor.

equivalent circuit

When the external

ear

is

partially filled with water and an air bubble is held between the water column and the tympanum, the model

becomes a three-section equivalent circuit, again with an end correction factor; the portion of the canal containing the air is represented, as before, by two inductive-capacitive sections. On the other hand, the waterfilled section of the canal is represented as an inductance only--the capacitance being negligible due to .the very small compliance of this medium. Finally, when water completely fills the ear canal, the equivalent circuit reduces to an inductance plus an end correction factor. Apparently, Bauer believes that our 1969 data are

not consistent

with

this

model.

Bauer's arguments also are based on the assumption However,

this relationship may not hold when the ear is sub-

merged. For example, if it can be assumed that placement of an earphone in close proximity to the skull

(underwater)causesit to act as a boneconductor,it might be expected that the middle ear is not necessary

for the excitation of the cochlea. Nevertheless, it must 1488

against other portions of the head. A significantly louder sound was heard with the earphone held against the ear than for any other locations." Even though this observation may have resulted from the earphone characteristics rather than the functioning of the auditory _

system, the argument is a cogent one. We do point out, however, that the mechanical coupling from earphone to skull is more direct in water than in air. Hence, the phenomenon Bauer is alluding to could be due to the loudness variation often reported by subjects when a bone conduction

unit is moved to various

sites on the

skull (Isele, Berger, Lippy, andRotolo,1068). Bauer and Torick also contend that "...

when the ear-

phoneis held at arm's length, all sense of direction

ceased," and "... if receptionoccurredthroughnormal auditory channels, then directional perception via the

earphones wouldbepossible,whereas,if it tookplace through the bony portions of the skull, then earphones

could not be used for directional perception." (They did, in fact, find directional perception with the ear-

phones.) On the other hand, in light of the overwhelming amount of evidence that humans can localized

under-

water sound(AndersonandChristensen,1060; Feinstein, 1066, 1971, 1072; Hollien, 1060, 1072; Hollien, Lauer, and Paul, 1070; Ide, 1044; Leggiere, MeAniff, Sehenek, and van Ryzin, 1070; Norman, Phelps, and Wightman, 1071), it is difficult to accountfor the inability of their subject to locate a sound source at arm's

length. In any ease, the criticisms by Bauer and his

that placing the sound source in front of the ears is

equivalent to excitation of the middle ear.

"... the earphoneswere placed againstthe ears and

J. Acoust.Soc.Am., Vol. 57, No. 6, PartII, June1975

associates add justification to our further pursuit of information concerning diver auditory sensitivity. Another approach to the problem is to test auditory sensitivity with the meatus either occluded or unoccluded and with the attenuation

of the bone conduction

mode un-

der varied experimental control. This condition is easily achieved by requiring the subjects to wear a neoprene

hood. One can expect, on theoretical as well as empirical grounds, that the foam neoprene will yield a signillCopyright¸ 1975by the Acoustical Societyof America

1488

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1489

HollienandFeinstein: Externalauditowmeatus andunderwater sensitivity

ELECTRONIC

RECORDING

ATTEN

SWITCH

TRANSI AM•

ATTENUATOR

FORMER

I

I

IOSCILLATOR CRO

CONTROL BOX

ß

GRAPHIC

LEVEL

1489

FIG. 1. Block diagram of stimulus generating equipment and response sys-

DIVIDER

RECORDER

tem.

AMP

WATER

i--[q

J9

PRO JECTOR

F36

HYDROPHONE

CONTROL SWITCH

cant frequency dependent attenuation across most of the

audiorange (see Montagueand Strickland, 1961, for example). Thus, if a small channelthroughthe hood (and into the meatus) is provided--and if the middle ear is functional--greater

auditory sensitivity would be ex-

A second system was used for calibration purposes

(in SPL re O. 0002 gbar).

It consisted of a calibrated

F-36 hydrophone fixed to DICORS at the level of the

diver's ears, an amplifier (Ithaca, model250), and a graphic level recorder (GeneralRadio, type 1521-B)

pected than for the case where no channel is provided

coupled mechanically to a beat frequency oscillator;

through the hood.

a CRO was used to visually monitor the stimuli. Standard procedures for measurement of the output of an

I. METHOD

underwatersoundprojector were employed(Bobber,

A.

•9•).

Research facility and apparatus Since the general procedure used in this experiment

was describedin an earlier report (BrandtandHollien, 1967), only a brief review follows. The field research facility of the Naval Research Laboratory's Underwater

SoundReference Division, Leesburg, Florida, was the site of the present study.

Located directly over a deep

fresh-water spring (temperature 22 øC) is a large floating barge with two laboratory

rooms situated one on

either side of a well through which DICORS (Diver Communication Research System: Hollien and Thompson, 1967) was lowered to the proper ear depth. DICORS is essentially an open framework diving cage constructed

of polyvinylchloride (PVC) tubing used to support subjects

and equipment employed in diver communication

research.

The stimulus-generating equipment and response units which are depicted in Fig. i consisted of equipment common to audibility threshold experiments. Sinusoidal test stimuli were generated by a beat frequency oscil-

lator (GeneralRadio, type 1304-B) coupledto an electronic switch (Grason-Stadler, model829D). The output of the switch was fed through a fixed T-pad attenuator serving as an impedance-matching device to a re-

cordingattenuator(Grason-Stadler, modelR362A), an impedance-matching transformer (United Transformer

Company, LS-33), a Navy developedpower amplifier, and a J9 transducer

mounted

on the frame

of DICORS

The Minimum Audible Field (MAF) underwater was determined by means of a modified Bekesy technique. Sinusoidal stimuli at 250, 500, 1000, 4000, and 8000 Hz were gated ON and OFF with a period of 500 msec and a 25-msec rise and fall time. The stimulus duty cycle was 1 sec, and the attenuation rate of the record-

ing attenuator was 8 dB/sec. B.

Procedure

The research was carried out on seven subjects (four males and three females) who were competent divers and experienced in taking hearing tests in air and in water. DICORS was lowered to an ear depth of 30 ft.

The diver, wearing open circuit SCUBA equipment and a full wet suit, descended to DICORS and firmly fixed himself at exactly 10 ft from the J9 transducer by means

of (1) a chest bar and (2) a lead-weighted belt. When the diver had equalized the air pressure in the middle ear to the pressure in the external meatus and was ready to

begin the threshold test, he signaled an experimenter on the surface. The test stimulus was initially presented

at a high enoughlevel to be clearly audible. The diver/ listener varied the stimulus SPL around the audibility threshold by activating a water- and pressure-proofed hand switch connected through a control box to the recording attenuator.

ducerwas usedas a soundprojector (loudspeaker);it

As in all diver auditory acuity experiments, the threshold measures were taken while the diver/listener held his breath, thus reducing the noise level to a minimum.

has a frequency range of 40 to 20 000 Hz.

Considerable bubble noise is generated when air is ex-

i m in front of the diver/listener's

ears.

The J9 trans-

J. Acoust. Soc. Am., Vol. 57, No. 6, Part II, June 1975

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Hollienand Feinstein:Externalauditow meatusandunderwatersensitivity

1490

TABLE I. Mean threshold SPL (decibels ze 0.0002 #bar) in water as a function of head covering for seven listeners. Standard deviations are in parentheses. All values (except probabilities) are in decibels.

Frequency (Hz) 250

Bare head

Hood with holes

Hood withoui holes

74.71

70.85

73.14

(7.25)

(7.90)

500

56.43

60.57

1000

58.28

4000

74.42

(6.55)

(9.18) P