What is interaural attenuation
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Here is a thorough explanation of interaural attenuation, drawn from authoritative otolaryngology textbooks:
Interaural Attenuation
Interaural attenuation (also called transcranial transmission loss) is the reduction in sound intensity that occurs as sound delivered to one ear travels through the skull and reaches the opposite (non-test) cochlea.
- Cummings Otolaryngology Head and Neck Surgery: "Interaural attenuation refers to the reduction in sound when it crosses from one ear to the other."
- Scott-Brown's Otorhinolaryngology: It is "the attenuation of sound delivered by a transducer to one ear as it reaches the opposite ear."
Why It Matters: Crossover and Shadow Curves
When a sufficiently loud signal is presented to the test ear, it can cross the skull and be perceived by the non-test ear - this is called crossover (or cross-hearing). If crossover occurs undetected, the responses recorded do not represent the test ear at all - they represent the non-test ear's thresholds elevated by the interaural attenuation value. These false readings are called "shadow responses" or a shadow curve.
Interaural Attenuation Values by Transducer Type
| Transducer | Interaural Attenuation |
|---|---|
| Supra-aural headphones (standard) | ~40-65 dB (typically cited as 40 dB) |
| Insert earphones (e.g., ER3A, ER5) | 60-90+ dB (75 dB at <1000 Hz; ~50 dB at >1000 Hz) |
| Bone conduction | ~0 dB (essentially no attenuation) |
The value varies between individuals due to differences in skull characteristics. Insert earphones provide substantially higher interaural attenuation because they have less contact with the lateral temporal bone, so less sound energy is transmitted contralaterally.
Clinical Significance: When to Mask
Because bone conduction has essentially 0 dB of interaural attenuation, there is no barrier to the signal reaching either cochlea - masking should therefore be used routinely with bone conduction testing whenever thresholds between ears are asymmetric.
For air conduction, masking is required when the air-conduction threshold of the test ear and the air-conduction or bone-conduction threshold of the non-test ear differ by the interaural attenuation factor for the frequency being tested.
For speech testing, masking is indicated when the SRT of the test ear differs from the SRT, air-conduction PTA, or bone-conduction PTA of the non-test ear by 45 dB or more.
The Masking Dilemma
In some cases, masking is necessary but impossible - this is the masking dilemma. It can arise in bilateral conductive or mixed hearing loss when bone-conduction thresholds are normal but air-conduction thresholds equal or exceed the interaural attenuation. Attempting to mask may cause overmasking, where masking noise crosses over and elevates the test ear's thresholds, making results unreliable in either direction.
Clinical Tip: Why Insert Earphones Are Preferred
The high interaural attenuation of insert earphones (often 70-90 dB) frequently eliminates the need for masking altogether in asymmetric hearing loss cases, reduces the risk of shadow responses, and avoids the masking dilemma in many patients.
- K.J. Lee's Essential Otolaryngology: "Insert earphones allow much higher interaural attenuation, and thus there is much less chance of crossover. Interaural attenuation may be as high as 70 to 90 dB, which often eliminates [the need for masking]."
Sources: Cummings Otolaryngology Head and Neck Surgery (Masking section) | Scott-Brown's Otorhinolaryngology Head & Neck Surgery Vol. 2 | K.J. Lee's Essential Otolaryngology
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