Interaural attenuation values for clinical masking
What interaural attenuation values should you assume when deciding whether to mask?
Written by Erica Guberman, Au.D. · Updated September 9, 2026
Use the minimum of the reported range so cross-hearing is never missed: 40 dB for supra-aural earphones, 55 dB for insert earphones, and 0 dB for bone conduction. Insert earphones buy the most headroom because they contact far less of the skull than a supra-aural cushion, which is why they resolve so many masking impasses.
Why it matters
Every masking decision comes down to one comparison: could the signal you presented to the test ear have crossed the skull and been heard by the non-test ear instead? Interaural attenuation (IA) is how much level a signal loses crossing from one side of the head to the other, and it is what stands between a real threshold and one that actually belongs to the other ear.
Clinicians assume the minimum of the reported range rather than the average or the maximum. Reported values vary by transducer, frequency, and skull shape, and using anything higher than the minimum risks missing cross-hearing in exactly the patients whose skulls transmit sound most efficiently.
The values to assume
Supra-aural earphones sit on the pinna and couple to the skull fairly well, so their minimum interaural attenuation is the lowest of the three. Insert earphones couple through a foam or silicone tip deep in the canal, touching far less of the skull, which pushes their minimum well above supra-aurals, especially at low frequencies. A bone-conduction vibrator on one mastoid sets the entire skull vibrating, so for practical purposes it reaches both cochleae at nearly the same level.
- Reported supra-aural values span roughly 40 to 85 dB depending on frequency and skull; 40 dB is the floor you assume.
- Insert values can exceed 70 dB below 1000 Hz, but 55 dB is the conservative figure used across frequencies.
- For speech, interaural attenuation is conventionally taken as somewhat higher than the pure-tone value, roughly 45 to 50 dB supra-aural and 60 dB insert. Sources disagree on the exact number, so state which one you're using.
| Transducer | Minimum interaural attenuation assumed |
|---|---|
| Supra-aural earphones | 40 dB |
| Insert earphones | 55 dB |
| Bone conduction (mastoid or forehead) | 0 dB |
When to mask air conduction
Mask the non-test ear during air conduction testing when the presentation level to the test ear, minus interaural attenuation, equals or exceeds the non-test ear's bone-conduction threshold at that frequency. That comparison is always against bone conduction, never air conduction, because cross-hearing travels through the skull by bone conduction no matter which transducer produced the original signal.
With supra-aural earphones, subtract 40 dB. With insert earphones, subtract 55 dB instead, and the larger number is exactly why switching to inserts so often removes the need to mask at all.
When to mask bone conduction
Because interaural attenuation for bone conduction is assumed to be 0 dB, any air-bone gap of 10 dB or more in the test ear obligates masked bone conduction at that frequency; a 10 dB threshold is used because a 5 dB gap falls within the normal variability of the two measurement systems. This is also why a mixed loss can never be diagnosed from unmasked bone-conduction data: without masking, you cannot tell which cochlea actually produced the response.
Bone-conduction masking has one extra wrinkle air conduction doesn't: the occlusion effect. Covering the non-test ear's canal with an earphone or insert tip during masking artificially improves that ear's own bone-conduction sensitivity at low frequencies, which means more masking noise is needed to actually reach the cochlea than the raw numbers suggest. Add the occlusion effect to the initial masking level whenever the non-test ear is normal or sensorineural: a conductive ear is already effectively occluded and shows no occlusion effect of its own. Deeply inserted foam tips reduce the correction substantially by seating past the cartilaginous canal.
Common traps
A few patterns account for most masking questions that trip students up.
- Comparing against the wrong ear's air conduction instead of the non-test ear's bone conduction. The comparison is always to bone conduction, because that's how cross-hearing actually travels.
- Averaging the interaural attenuation range instead of using the minimum. The minimum is the conservative choice that never lets cross-hearing go undetected.
- Forgetting the occlusion effect correction when masking bone conduction on an ear with normal or sensorineural hearing.
- Mistaking central masking for overmasking. A masked threshold that shifts about 5 dB worse than the unmasked one, and then stays flat as masker level keeps rising, is central masking, a real but small effect of noise in the opposite ear, not evidence of overmasking chasing the plateau.
- Not recognizing the masking dilemma: when the air-bone gap is large in both ears, no masking level exists that's high enough to mask the non-test ear without also crossing over and masking the test ear itself. There's no clean fix; it has to be named and worked around, not masked away with more noise.
Frequently asked
Why use the minimum interaural attenuation instead of an average or typical value?
Interaural attenuation varies by frequency, transducer, and individual skull, and the reported ranges are wide. Using the minimum means you'll never fail to mask a patient whose skull happens to transmit sound unusually well. Using an average would let cross-hearing go undetected in exactly those patients.
Why is bone conduction's interaural attenuation assumed to be 0 dB?
A bone-conduction vibrator set on one mastoid puts the entire skull into motion, so it drives both cochleae at nearly the same level. There's no meaningful attenuation to rely on, so masking decisions for bone conduction treat it as none at all.
Do insert earphones ever eliminate the need to mask?
Often, yes. Because inserts carry a much higher assumed interaural attenuation (55 dB versus 40 dB for supra-aurals), the same air-conduction threshold and bone-conduction comparison that would require masking with supra-aural earphones frequently doesn't cross the threshold for masking once you switch to inserts.
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