"My bone oscillator's calibration is off" is a frequent call to our service department. After investigating, the calibration usually falls within the acceptable range, as stated in the ANSI S3.6 standard for audiometers.

So, what leads to these confusing air-bone gaps? This blog provides answers.

Factors Behind False Air-Bone Gaps in Bone Conduction Testing

Bone Oscillator Positioning

Bone oscillator placement can significantly influence the precision of bone conduction threshold measurements. During bone conduction testing, make sure you:

1. Position the oscillator on the firm surface of the mastoid bone, avoiding contact with the pinna and hair.
2. Make sure the bone oscillator remains steady, as any motion during testing can decrease bone conduction hearing sensitivity.
   

Mastoid versus Forehead Placement

More than 90% of bone conduction tests utilize mastoid placement despite studies indicating that forehead placement offers improved test/retest reliability with fewer middle ear contributions to bone conduction thresholds.

However, putting the bone oscillator on the forehead has a downside. It lowers the maximum sound output by about 20 dB at certain frequencies.

While there is a reduction in the overall output at all frequencies, the new B81 oscillator overcomes the 20 dB loss or output. The overall output now on the forehead is equivalent to the B71 on the mastoid.

Bone Oscillator Application Force

The degree of tension in the oscillator's headband indicates how low bone conduction thresholds can be measured. The ANSI Standard for audiometers recommends a headband tension of 400 grams (about 14.11 oz).

With time, bone conduction headbands inevitably lose tension and may require replacement. Inspection of the headband takes place during annual calibration.

Occlusion Effect

The occlusion effect artificially reduces bone conduction thresholds in the 250Hz-1KHz range and is a well-known phenomenon. The extent of this effect depends on the choice of air conduction transducer.

Supra-aural headphones induce a more significant occlusion effect than insert phones, especially when inserted beyond the ear canal's cartilaginous portion.

Circumaural headphones that can test the full audiometric frequency test range are now available. These offer several advantages regarding comfort and ambient noise attenuation and do not create any occlusion effect.

Air Conduction Testing Factors

Several air conduction testing factors can contribute to air-bone gaps, including:

Incorrect placement of transducers, especially supra-aural headphones, can affect air conduction thresholds.

The condition of the earphone cushions can contribute to inaccurate air conduction thresholds, especially in the low-frequency range. If the pads are stiff and/or have cracks, they diminish the attenuation of background noise at low pitches.

Transducer placement & size. When using insert phones, inserting them deeply and using the right size is essential. If foam tips don't fit properly, low-frequency pure tones can leak out of the ear.

Pure Tone Audiometry Subject Variables

Audiometric zero (0dBHL) represents the average hearing threshold for a typical group of young, normal-hearing adults. However, like in most "normal" populations, there is variability around this average level.

Both air conduction and bone conduction threshold levels follow a normal distribution. Because of this distribution, some individuals classified as having normal hearing will fall at the extremes of this distribution.

This variance can lead to unexplained air-bone gaps and, in some cases, bone thresholds that test worse than air at the same frequency. It's crucial to accurately record these results without manipulation to align with one's expectations.

Additionally, age-related changes in the middle ear, such as increased eardrum flaccidity and ossicular chain looseness, can contribute to air-bone gaps, especially at 4KHz.

Audiometric Calibration Variables

Accurate audiometer transducer calibration is crucial and should be done by a trained technician using the right equipment and test couplers. Since audiometer calibrations take place annually, the question is:

How can I verify if my equipment meets acceptable standards between yearly calibrations?

One approach is to perform routine biological checks by measuring air and bone conduction thresholds on a colleague or an individual with normal hearing or confirmed sensorineural hearing loss.

This test sets a reference for future comparisons, ensuring the thresholds stay within an acceptable +/- 5dB range. If this range is exceeded or if you notice a consistent issue during patient testing, it might suggest a problem with a particular transducer. In such cases, contacting e3 Diagnostics immediately for calibration is a good idea.

Gaps at 4kHz Explained

The 4kHz air-bone gap is likely due to a combination of factors, including aging, inaccurate bone oscillator placement, collapsed canals with supra-aural cushions, and the ANSI calibration offset at 4kHz.

Debate over the accuracy of this offset continues. However, adhering to the ANSI standard for audiometers is essential to maintain measurement precision.

Various published articles explore the theories surrounding this issue, encompassing elements like chance, radiation, middle ear changes, and reference-equivalent threshold force levels at 4kHz. It's essential to remember that air-bone gaps arise from comparing two variables with normal distributions, occasionally leading to unexplained gaps.

In a study by Dr. Robert Margolis at the University of Minnesota, several key findings emerged:

1. Participants with normal hearing displayed minor air-bone gaps at 0.5, 1.0, and 2.0 kHz (-1.7 to 0.3 dB) and a more significant air-bone gap at 4 kHz (10.6 dB).
2. Participants with sensorineural hearing loss (SNHL) showed minor air-bone gaps at 0.5, 1.0, and 2.0 kHz (-0.7 to 1.7 dB) and a more substantial air-bone gap at 4 kHz (14.1 dB).
3. The 4-kHz air-bone gap increased with the air-conduction threshold, ranging from 10.1 dB for air-conduction thresholds of 5-10 dB HL to 21.1 dB HL for thresholds greater than 60 dB.
4. Adjusting the 4-kHz RETFL by -14.1 dB could potentially eliminate the 4-kHz air-bone gaps in individuals with SNHL.
   

Calibration issues are rare unless you regularly measure air-bone gaps in all patients, including those with normal or sensorineural hearing loss.

To ensure accuracy, periodically perform biological calibrations on a couple of your normal-hearing office staff members and monitor their 4KHz bone thresholds for unusual changes.

We are committed to ensuring your equipment meets testing standards, and if you suspect transducer calibration issues, contact your local e3 office for support.

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