Why Am I Getting a False Air-Bone Gap? Explained
“My bone oscillator’s calibration is off” is one of the most common calls our service department receives. Yet after investigation, calibration typically falls within the acceptable range defined by the ANSI S3.6 standard for audiometers.
So, if calibration isn’t the issue, what’s actually causing those unexpected air-bone gaps during bone conduction testing? In most cases, it comes down to a combination of test technique, transducer behavior, patient variability, and the natural distribution of hearing thresholds.
This guide walks through the most common contributors to a false air-bone gap and how to identify them in your clinic.
Understanding the Air-Bone Gap
An air-bone gap is the difference between a patient’s air conduction and bone conduction thresholds at the same frequency. It’s a key diagnostic indicator used to distinguish between conductive and sensorineural hearing loss on the audiogram.
In general:
- Conductive hearing loss shows poorer air conduction thresholds with normal bone conduction thresholds, producing a true air-bone gap
- Sensorineural hearing loss shows equally reduced air and bone conduction thresholds with no significant gap
- Mixed hearing loss shows reduced bone thresholds plus an additional air-bone gap
When an air-bone gap appears unexpectedly, especially in patients with otherwise normal middle ear function, it’s often a false gap caused by testing variables rather than true pathology.
Common Causes of a False Air-Bone Gap
1. Bone Oscillator Positioning
Bone oscillator placement can significantly influence the accuracy of bone conduction threshold measurements. During bone conduction audiometry, make sure to:
- Position the oscillator on the firm surface of the mastoid bone, avoiding contact with the pinna and hair
- Keep the oscillator steady throughout testing, since any motion can reduce bone conduction sensitivity
Even small shifts in placement can produce measurable differences in bone conduction thresholds, leading to an artificial air-bone gap.
2. Mastoid vs. Forehead Placement
More than 90% of bone conduction tests use mastoid placement, even though research shows forehead placement offers better test-retest reliability and fewer middle ear contributions to bone conduction thresholds.
The trade-off with forehead placement is reduced maximum output, typically about 20 dB lower at certain frequencies. However, the newer B81 bone oscillator offsets this limitation, providing forehead output equivalent to the B71 on the mastoid.
This makes forehead placement a more viable option for clinicians seeking improved reliability without sacrificing output.
3. Bone Oscillator Application Force
The tension of the bone oscillator headband determines how low bone conduction thresholds can be measured. The ANSI standard recommends a headband tension of 400 grams (about 14.11 oz).
Over time, headbands lose tension and may need to be replaced. This is typically inspected during annual calibration, but clinicians should remain aware that aging headbands can subtly affect threshold accuracy between calibrations.
4. The Occlusion Effect
The occlusion effect artificially improves bone conduction thresholds in the 250 Hz to 1 kHz range and is a well-documented phenomenon. The extent of the effect largely depends on the air conduction transducer used.
- Supra-aural headphones produce a stronger occlusion effect
- Insert earphones reduce the effect, especially when placed deeper than the cartilaginous portion of the external auditory canal
- Circumaural headphones that cover the full audiometric frequency range offer comfort, better ambient noise attenuation, and no occlusion effect
Choosing the right transducer for your patient and test condition can meaningfully reduce false air-bone gaps in the low frequencies.
5. Air Conduction Testing Factors
Air conduction testing variables can also introduce false air-bone gaps. Common contributors include:
- Transducer placement, particularly misalignment of supra-aural headphones
- Earphone cushion condition, where stiff or cracked cushions reduce low-frequency attenuation of background noise
- Insert earphone fit, where shallow insertion or incorrect foam tip size allows low-frequency tones to leak from the ear canal
Routine inspection of transducers and cushions can prevent many of these issues before they affect thresholds.
6. Pure Tone Audiometry Subject Variables
Audiometric zero (0 dB HL) represents the average hearing threshold of young, normal-hearing adults. As with any normal distribution, some individuals naturally fall at the extremes.
Both air and bone conduction thresholds follow a normal distribution, which means:
- Some normal-hearing individuals may show small, unexplained air-bone gaps
- Occasionally, bone thresholds may even test slightly worse than air at the same frequency
These results should be recorded accurately without adjustment to match clinical expectations. Additionally, age-related changes, such as eardrum flaccidity and ossicular chain looseness, can contribute to air-bone gaps, particularly at 4 kHz.
7. Audiometric Calibration Variables
Accurate transducer calibration is critical and should be performed annually by a trained technician using the correct equipment and test couplers. But because calibration happens only once a year, audiologists often ask:
How can I verify equipment accuracy between annual calibrations?
One reliable approach is to perform routine biological calibration checks by measuring air and bone conduction thresholds on a colleague with normal hearing or confirmed sensorineural hearing loss.
This establishes a baseline for ongoing comparison. As long as thresholds remain within plus or minus 5 dB of the reference, the equipment is performing within expected limits. Consistent deviations may indicate a transducer issue and warrant a service call.
Why Air-Bone Gaps Often Appear at 4 kHz
The 4 kHz air-bone gap is one of the most commonly observed, and most misunderstood, findings in clinical audiometry. It’s typically the result of several overlapping factors:
- Age-related middle ear changes
- Inaccurate bone oscillator placement
- Collapsed ear canals when using supra-aural cushions
- ANSI calibration offset at 4 kHz
While debate continues over the accuracy of the ANSI offset, adhering to the standard is essential for consistent measurement.
Key research findings
In a study by Dr. Robert Margolis at the University of Minnesota, researchers identified clear patterns in 4 kHz air-bone gaps:
- Normal-hearing participants showed minor air-bone gaps at 0.5, 1.0, and 2.0 kHz (-1.7 to 0.3 dB) and a larger gap at 4 kHz (10.6 dB)
- Participants with sensorineural hearing loss showed minor gaps at 0.5, 1.0, and 2.0 kHz (-0.7 to 1.7 dB) and a larger gap at 4 kHz (14.1 dB)
- The 4 kHz air-bone gap increased with air conduction threshold, from 10.1 dB at 5 to 10 dB HL up to 21.1 dB at thresholds greater than 60 dB
- Adjusting the 4 kHz RETFL by -14.1 dB could potentially eliminate 4 kHz air-bone gaps in patients with SNHL
This research reinforces that 4 kHz gaps are often a normal artifact of measurement variables, not a true conductive component.
How to Reduce False Air-Bone Gaps in Your Clinic
Calibration issues are rare, but technique and equipment condition can significantly affect results. To maintain accurate bone conduction testing:
- Perform periodic biological calibrations on normal-hearing staff
- Monitor 4 kHz bone thresholds for unusual changes over time
- Inspect headbands, cushions, and transducers regularly
- Choose the right transducer for the patient and test goal
- Document unexpected results accurately rather than adjusting them
These small habits help ensure your audiometric results reflect true patient hearing rather than testing artifacts.
When to Call for Support
If you consistently see unexplained air-bone gaps or suspect transducer calibration issues, your local e3 Diagnostics team is here to help ensure your equipment meets testing standards.
