Breath in.. Breath out..
Most clinicians break out in a cold sweat when it comes to masking. “Am I doing this correctly, is it effective?”. Take a deep breath and look no further.
To understand it, we must first understand the science and reasoning behind masking in audiology. We must also cover some of the related concepts such as interaural attenuation (IA) and the occlusion effect (OE). You may already know these terms, but let’s see if we can shed a little light on how they all work together.
What is masking in audiology and why is it necessary?
During testing, it is possible for the non-test ear to hear a tone presented in the test ear. We call this crossover.
Crossover happens when an air conducted signal is intense enough to cause the skull to vibrate. Sound is transmitted through bone conduction and allows the patient to hear the tone in the non-test ear. Because the skull is fused, these vibrations activate fluid motion within both the test ear and non-test ear cochleas. Tones produced by insert earphones can only cause vibrations in the deeper part of the ear canal. Therefore, tones played through inserts need greater intensity to generate crossover. While Supra-aural or circum-aural earphones, which make contact with the head, can more easily cause vibrations and affect testing as well as subsequent hearing aid fitment. In short, supra/circum-aural earphones generate crossover at lower intensity than inserts.
If crossover is present, the clinician will need to isolate the test ear from the non-test ear. To do this, we use masking.
Masking in audiology is the act of playing white noise into the non-test ear to prevent it from hearing the tones that crossover from the test ear. It helps to obtain the true threshold of the test ear, and ensures that the non-test ear is not helping out.
The amount of masking noise required depends on the amount of signal that may crossover.
To a certain degree, interaural attenuation(IA) reduces crossover.. IA is the amount of sound intensity that is reduced/attenuated when crossing from the test ear to the non-test ear. Known average IA values are available for insert and supra/circum-aural earphones.
Study results from Chaiklin (1967), Killion, Wilber & Gundmundson (1985) and Sklare & Denenberg (1987) on supra or circumaural earphones, found average IA values across all frequencies for air conduction to be around 60 dB. However, clinically, a more conservative IA of 40 dB is used. 40 dB is used clinically as it was found to be the lowest IA level.
For inserts, results from Killion et al. (1985), Konig (1962) and Sklare & Denenberg (1987) found average IA values of around 80 dB with minimum IA levels of 70 dB. Therefore, a clinician can use 70 dB during masking. However, most clinicians use a much more conservative IA of 60 dB for inserts.
Adult skulls are fused at the cranial sutures. If you vibrate any part of the head bone (i.e. Mastoid), equal vibrations will transmit to the the entire head bone and both cochleas. This means that there is almost no (>10dB) IA in BC testing. The most conservative assumption is that there is no interaural attenuation. This assumes that the ‘entire’ intensity will go to the non-test ear too.
Most patients will not experience crossover at the above levels. Yet, one cannot assume the patients attenuation value, without calculating each patients individual IA values. We therefore, use masking based on the generally validated IA values when crossover is possible .
The Occlusion Effect
The occlusion effect is the enhancement of bone-conducted sound signal caused by blocked or occluded ear canals. An occluded ear canal traps the BC signal energy. This causes an increase in sound pressure levels delivered to the tympanic membrane and inner ear. This can improve BC thresholds and deliver false test results.
Therefore, bone conduction masking requires a correction factor for the occlusion effect (OE) caused by the earphone in the non-test ear that is being masked. The correction factor increases the normal masking level to offset the added sound pressure transmitted to the cochlea and applies to 250, 500 and 1000 Hz frequencies.
Occlusion Effect Masking
Occlusion effect masking values will depend on what is occluding the ears (insert or supra-aural earphones). Various studies recommend different OE correction values. When using inserts, Dean & Martin (2000) recommend OE correctional values of 9 dB for 250 Hz, 7 dB for 500 Hz and 0 dB at 1000 Hz. The OE values when using deep inserted earphones are low. Logically, the deeper the earphone is in the ear canal, the smaller the remaining resonating area. This is why a deep insert is crucial to success with inserts using the above mentioned OE values.
When using supra-aural earphones, Goldstein & Newman (1994) recommend correctional values of 15 dB for both 250 Hz and 500 Hz, and 10 dB at 1000 Hz. Various studies show that there is very little OE difference between forehead or mastoid BC placement but is significantly less for insert earphones.
Below is a method that you can use to calculate each patient’s specific OE values, whether using insert or supra-aural earphones. Warning: This takes up valuable clinical time.
Calculating Occlusion Effect Values For Masking:
- Step 1. Place the bone vibrator on the preferred site (Mastoid or Forehead)
- Step 2 Measure the patient’s BC thresholds at 250, 500 and 1000 Hz without occluding the ears
- Step 3. Measure the patients BC thresholds with occluded ears (or occlude the non-test ear) using insert or supra-aural earphones
- Step 4. Subtract results of BC occluded thresholds from the BC unoccluded thresholds. This is the patient’s OE.
- Step 5. Now add the OE of 250, 500 and 1000 Hz to the total masking value of the noise in the non-test ear
When Should I Mask And How Much?
In air-conduction (AC) testing, the main concern is that loud pure tone air conduction sound may generate bone-vibration sound and heard in the non-test ear. Each patient’s physiology is different so not everyone will experience crossover at the same intensity level
Some of you might have already figured it out by now, if not, do not despair.
When to mask
A general rule is to compare the air conduction thresholds of the test ear with the bone conduction thresholds of the non-test ear.
When using supra-aural earphones, if the difference between the non-test ear BC and test ear AC thresholds is 40 dB or more, then you should mask.
For insert earphones, the difference should be 60 dB or more.
Formula to determine the need for masking:
Mask IF, Air Conduction (test ear) - IA (40 dB or 60 dB) ≥ Bone Conduction (non-test ear).
Generally, audiologists determine 'when to mask' by comparing the AC thresholds of the test ear and that of the non-test ear. This is because many audiologists often obtain AC thresholds prior to measuring and obtaining BC thresholds. A preliminary decision can be made about the need to mask by comparing the AC of the two ears (Katz, 2014).
However, it is important to remember that crossover hearing occurs primarily through the mechanism of BC. As a result, it may be necessary to re-evaluate unmasked BC thresholds in order to determine the need for masking the non-test ear during AC testing. Clinically, one may not see the need to mask by comparing AC thresholds from the patient's two ears. However, once unmasked BC thresholds are obtained, contralateral masking may be required based on the 'when to mask' formula stated above.
If you are using inserts for AC it is less likely that you will need to mask.
How much should I mask?
For air-conduction, to mask the non-test ear, add 10 dB HL to the AC threshold in the non-test ear (AC non-test ear + 10 dB HL). That will be the level you can start at for effective masking.
You may have already concluded that if the IA for BC is ~0 dB, then one should always mask. Most audiologists agree and will always mask during BC. Even ANSI (2004) assumes that masking was or is always used during BC testing. The British society of Audiology (2012) further advocates for always masking during bone conduction by stating that ‘’Without masking [during BC], it is not possible to determine which ear is responding to bone conduction… When testing without masking, thresholds may appear more acute by about 5 dB due to binaural stimulation.’’
When masking the non-test ear the listening task becomes slightly difficult and results in elevated threshold of the test ear compared to what would’ve occurred without masking the non-test ear.
Known as central masking - this is a side effect of non-test ear masking noise being near-threshold-levels which often creates a 5 dB threshold elevation. If you don’t mask, the threshold will likely be 5 dB better and present minor air-bone gaps.
So, when should you mask for BC? Short answer, Always.
Since interaural attenuation during bone-conduction is ~0 dB, the intensity at each cochlea is equal. The better hearing cochlea will detect the sound. There is no need for masking if bone-conduction test results rule out a conductive component (ie. no air-bone gap). If there are air-bone gaps, we should then mask to determine if they are ‘real’ air-bone gaps and whether they are unilateral or bilateral.
If there is a air-bone gap of more than 10 dB in the test ear, masking is needed.
Formula to determine the need for masking:
Mask IF, Air-Bone gap in test ear ≥ 10 dB.
How much masking is needed?
For bone-conduction, to mask the non-test ear, add 10 dB HL to the Air Conduction threshold in the non-test ear and add the occlusion effect (Air Conduction non-test ear + 10 dB HL + occlusion effect).
That is the level you can start masking at.
There are several masking techniques or methods around but the most popular is the Plateau Method. In this method, the above formulas for BC and AC determine the starting masking level. The starting decibel level for the test ear will be the unmasked test ear threshold you have recorded.
- Step 1 - Determine the starting levels
- Step 2 - Present the masking noise into the non-test ear while simultaneously presenting the unmasked threshold (signal) into the test ear
- Step 3 - Increase the masking noise in the non-test ear in increments of 3 by 5 dB steps (+5dB, +5dB, +5dB)
- Step 4 - If the tone is still heard after increasing the masking level three times, then that is your threshold
- Step 5 - Obtain three consecutive responses to three increments
A common issue for masking in audiology is when you cannot find the masking plateau. Known as the masking dilemma, this occurs when there is a moderate to severe bilateral conductive hearing loss. In this case, masking noise intensity presented in the non-test ear crosses over to the test ear and elevates the thresholds. This means that you will keep masking without reaching the plateau, hence the dilemma.
A simple work around for the masking dilemma in audiology is using insert earphones. Inserts provide superior inter-aural attenuation than their counterparts and reduce the need to mask in the first place.
If you still can't get a true threshold as a result of the masking dilemma, it is best to record the unmasked threshold on the audiogram as ‘’could not mask’’.
KUDUwave™ - For simple and efficient masking
The KUDUwave™ masking strategy saves the clinician valuable time in determining accurate masked thresholds. It masks both manually and automatically for BC with the occlusion effect automatically accounted for. AC masking is manual which allows you to use any masking strategy/ procedure that you believe yields the best results (we recommend the plateau masking method stated above). This is especially useful when considering tele-audiology as without it, trusting your test results when testing is performed remotely is paramount.
The combination of insert earphones, easy to understand and quick to use masking strategy is just another feature that sets the KUDUwave apart and provides improved patient comfort, infection control, ambient noise attenuation, and more.