LESSON 61

How to Perform Bone-Conduction Audiometry

Name: How to Perform Bone-Conduction Audiometry
Uploaded: 2026-11-04T03:15:00+03:00
Duration: 11 min 57 s
Description: Bone conduction thresholds tell us whether a hearing loss is conductive or sensorineural. In this lesson, we use a sample audiogram to look at mastoid and forehead placement of the bone vibrator, test frequencies, interaural attenuation, vibrotactile responses, acoustic radiation and the occlusion effect.

Year 1
Spring semester
Audiological Assessment Methods I
Clinical basics and pure tone audiometry
Duration: 11:57

Bone conduction thresholds tell us whether a hearing loss is conductive or sensorineural. In this lesson, we use a sample audiogram to look at mastoid and forehead placement of the bone vibrator, test frequencies, interaural attenuation, vibrotactile responses, acoustic radiation and the occlusion effect.

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Hello, and welcome to the AIHEAR Audiology Academy.
In this lesson, we learn how bone conduction audiometry is performed.
Step by step, we will look at vibrator placement, test frequencies and the pitfalls that can mislead the results.
In air conduction testing, sound passes through the outer and middle ear to reach the cochlea.
In bone conduction testing, vibration applied to the skull largely bypasses the outer and middle ear and stimulates the cochlea directly.
So bone conduction thresholds largely reflect the sensitivity of the inner ear and auditory nerve, the sensorineural system.
By comparing the two thresholds, we can tell whether a loss is conductive, sensorineural or mixed.
Recall that the difference between the air and bone conduction thresholds is called the air-bone gap.
We covered the mechanisms of bone conduction hearing in earlier lessons; here we focus on the measurement itself.
Bone conduction is usually tested when air conduction thresholds are outside normal limits, poorer than about twenty to twenty five dB HL.
Many clinics, however, test bone conduction routinely in all patients.
The test frequencies are usually the octave frequencies from two hundred fifty to four thousand hertz.
Eight thousand hertz is not tested routinely, because vibrator output is limited there and acoustic radiation increases.
When testing unmasked bone conduction, the earphones are removed and both ears are left uncovered.
In bone conduction testing, sound is delivered by a small transducer called a bone vibrator.
The vibrator turns the electrical signal of the audiometer into mechanical vibration that is passed to the skull.
The vibrator is held by a headband with a static force of about five point four newtons, as defined in the standards.
Models in common clinical use include the B-71 and the B-81.
Each vibrator must be set up on the audiometer with its own calibration values.
The vibrator can be placed in two locations: on the mastoid bone or on the midline of the forehead.
Mastoid placement is more common in the clinic, because it gives a stronger sensation for the same electrical power.
The vibrator is placed on the flattest part of the mastoid process, without touching the auricle.
Forehead placement is less sensitive to differences in placement and has better test-retest reliability.
But more power is needed at the forehead for the same sensation, so the maximum testable level is lower.
For both placements, the audiometer must be used with the calibration values for that placement.
An important feature of bone conduction is that the vibration sets the whole skull vibrating.
Whichever side the vibrator is placed on, both cochleae are stimulated at almost the same level.
That is why interaural attenuation for bone conduction is usually taken to be about zero decibels.
So an unmasked bone conduction threshold actually shows the threshold of the better of the two cochleae.
To confirm which ear the threshold belongs to, the opposite ear is masked; we will see this in the clinical masking lessons.
First, hair over the mastoid is parted, and objects that may touch the vibrator, such as glasses arms and earrings, are removed.
The vibrator is placed, and the headband is secured so that it will not slip.
As in air conduction, the threshold search uses the modified Hughson-Westlake method: down ten decibels, up five decibels.
The instructions are the same, but tell the patient that feeling vibration does not count; they should respond when they hear the sound.
The frequency order is usually as for air conduction: start at one thousand hertz, go up to the highs, then move to the lows.
The patient should not move the vibrator or touch the headband until the end of the test.
The maximum output of bone vibrators is much lower than that of air conduction earphones.
The limit varies by device; it is usually about forty to fifty dB HL at two hundred fifty hertz and sixty to seventy in the mid range.
If there is no response at this level either, bone conduction is recorded at the device limit with a no-response symbol.
So in severe losses, it is not always possible to confirm the type of loss with bone conduction.
In this sample audiogram, the air conduction thresholds of the right ear are around forty dB HL.
The unmasked bone conduction thresholds measured from the right mastoid are within normal limits.
In this example, the air conduction thresholds of the left ear are also normal.
There appears to be an air-bone gap of about thirty decibels in the right ear.
But we do not yet know whether this unmasked bone conduction belongs to the right or the left cochlea.
If there is a gap of fifteen decibels or more in the test ear, bone conduction is usually retested with the opposite ear masked.
We find the type of loss by reading the bone conduction result together with air conduction.
In conductive loss, air conduction is poor, bone conduction is normal, and there is a clear gap between them.
In sensorineural loss, air and bone conduction worsen together, and there is no significant gap between them.
In mixed loss, bone conduction is also poor, but air conduction is even worse.
Differences of up to ten decibels are usually treated as measurement variability, not as a gap.
In this sample audiogram, the air conduction thresholds of the right ear are between fifty five and seventy five dB HL.
The bone conduction thresholds measured with masking are also outside normal limits, between thirty and forty five dB HL.
At one thousand hertz, there is a gap of about thirty decibels between the two thresholds.
The reduced bone conduction shows a sensorineural component, and the gap shows a conductive component.
The picture in this example is consistent with a mixed hearing loss.
At high levels, especially at two hundred fifty and five hundred hertz, the bone vibrator produces vibration that can be felt on the skin.
The patient may mistake this vibration for sound and respond; this is called a vibrotactile response.
Such a response is not a true hearing threshold, and it can create a false air-bone gap.
This is especially likely in severe sensorineural losses, where it can look like a low-frequency conductive component.
If you suspect it, ask the patient whether they heard the sound or felt the vibration, and note this on the audiogram.
As the bone vibrator vibrates, it also radiates some sound into the surrounding air; this is called acoustic radiation.
This radiation is most noticeable at high frequencies, in the two thousand to four thousand hertz region.
Because the ear canal is open, the radiated sound can be heard by air conduction, and the bone threshold looks better than it is.
As a result, a false air-bone gap may appear, especially at four thousand hertz.
If in doubt, the test ear can be closed with a foam plug and four thousand hertz retested.
Because the occlusion effect is negligible at high frequencies, this check does not artificially improve the threshold.
When the ear canal is closed, low-pitched sounds heard by bone conduction seem louder; this is called the occlusion effect.
The effect is greatest between two hundred fifty and one thousand hertz, and it is clear in normal or sensorineural ears.
In ears with conductive loss, the occlusion effect is usually small or absent.
That is why unmasked bone conduction is measured with the ears uncovered.
When an earphone is placed on the opposite ear for masking, this effect must be taken into account.
Deeply inserted insert earphones markedly reduce the occlusion effect.
Bone conduction results are very sensitive to placement; small errors can change thresholds by several decibels.
If the vibrator touches the auricle, the auricle vibrates too, and the results become unreliable.
A loose headband reduces the contact force and makes thresholds look worse than they are.
Hair or a glasses arm in between also weakens the contact.
If the vibrator slips during the test, reposition it and repeat the most recently tested frequencies.
A tip from practice: normally, the bone conduction threshold cannot be clearly worse than the air conduction threshold.
This is because sound delivered by air conduction also ends up at the same cochlea.
Differences of up to ten decibels are accepted as measurement variability.
If bone conduction is more than ten decibels worse than air conduction, first check the vibrator placement and the headband.
If the problem persists, the bone conduction calibration may need to be reviewed.
Bone conduction largely bypasses the outer and middle ear and stimulates the cochlea directly.
The vibrator is placed on the mastoid or the forehead; the mastoid is more common, and the forehead is more consistent.
Interaural attenuation for bone conduction is about zero, so an unmasked threshold shows the better cochlea.
Vibrotactile responses, acoustic radiation and the occlusion effect are three key pitfalls that can mislead bone conduction results.
Test yourself: In bone conduction testing, what is the approximate interaural attenuation in decibels?
The correct answer is A: about zero decibels.
Vibration spreads through the skull and stimulates both cochleae almost equally, so unmasked bone conduction cannot show which ear heard it.
In the next lesson, we will look in detail at audiogram symbols, masked recordings and the no-response symbol.
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How to Perform Bone-Conduction Audiometry

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