Real Ear and Artificial Mastoid Bone Conduction Calibration

Loudness Balance Technique (Beranek 1949)

Bone and air conducted stimuli are alternated in presentation.  The listener is required to adjust the magnitude of one stimulus until it is perceived as equally loud as the other.  When averaged over a number of individuals, an approximate calibration of the bone conductor can be determined at each test frequency (Barry et al, 1981; Hedgecock, 1961).

Real Ear Procedures

This method assumes that air and bone conduction thresholds are equivalent for the test subject.  If many subjects (6 or more) are testing by air and bone conduction using an audiometer that is properly calibrated for air conduction, corrections for bone conduction can be applied by finding the difference between the two methods of testing.  The underlying principle here is that BC thresholds should be equal to AC thresholds in the absence of a conductive impairment (Hood, 1979).  Although this is true for a large population of subjects, it cannot be expected to be true for any individual or for very small groups (Studebaker, 1967; Wilber et al, 1967). 

Roach and Carhart (1956) suggest using subjects with a purely sensorineural hearing loss to avoid difficulties associated with ambient nose in the clinical setting, as well as lack of audiometric sensitivity, thus increasing the probability of obtaining true thresholds.  

Subjective Phase Cancellation

In this method, simultaneous air and bone conducted signals are presented to the subject.  The theory behind this procedure is that the subject could adjust the phase and magnitude of the stimulus of one signal (either air or bone) in such a way as to achieve cancellation of the other stimulus, thus resulting in no signal being heard.  When this state is reached, an auditory null is said to be achieved whereby the BC stimulus is equal in magnitude to the AC tone (at the cochlea) (Dempsey et al. 1990; Kapteyn, Boeze, & Snel, 1983; Kapteyn, Snel, & Vis, 1980; Levitt, 1987).  Once this is achieved, a calibration for the bone conductor can be determined at this frequency.

Artificial Mastoid Procedure

The most common and clinically preferred procedure for calibrating the bone vibrator is one involving an artificial mastoid.  In the past, no commercially available artificial mastoid met the mechanical impedance requirements of the ANSI (S3.13-1972), or IEC (IEC 373-1971) standards.  Because of this, these standards were revised to conform to the specification of an artificial mastoid that was available.  The new standard was ANSI S3.13-1987.  Audiologists may find it interesting to know that the International Standards Organization (ISO) has developed a draft standard for bone conduction thresholds (ISO / DIS 7566 -1987) that gives one set of values that are to be used for all bone-vibrators having the circular tip described in the ANSI and IEC standards.  Both the ANSI and ISO values are based on unoccluded ears using contralateral masking.  Studies have shown (Shipton et al. 1980) that air borne radiation from the bone vibrator can affect measurement variability when testing under an unoccluded ear.  For further information regarding the implications of using an unoccluded ear please see the air-borne radiation article.

The recommended RMS force values (in dB re 1 uN) for 0 dB HL for the Radioear B-71 bone vibrator is as follows:

To take a look at a sample bone conduction calibration worksheet, click here

When calibrating the bone vibrator, it is necessary to remove the vibratory mechanism from the head band before placing it onto the artificial mastoid.  Below is a diagram of the equipment set up for a bone vibrator calibration.

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