Masking in Audiology

MASKING

Masking Definition

  • Definition:
      - "The process by which the threshold of hearing for one sound is raised by the presence of another sound (masker)."
      - "The amount by which the threshold of hearing for one sound is raised by the presence of another (masking) sound expressed in decibels."

Terminology

  • TE = Test Ear
      - The ear that is being evaluated (tested) at the time.
      - This ear has no masking in it, only the tone/word.

  • NTE = Non-Test Ear
      - The ear that is NOT being evaluated at the time.
      - This is the BETTER EAR.
      - This ear has the masking noise in it to keep it from hearing the other ear.

Everyday Masking vs. Clinical Masking

  • Everyday Masking:
      - In everyday listening, masking is a negative concept; it is unwanted background noise in most situations.
      - Viewed as interference, it is an undesirable factor and unwanted acoustic energy that prevents clear listening.

  • Clinical Masking:
      - When applied properly during clinical audiometric testing, the masking signal is intentionally delivered to the NTE.

What is the Masking Signal?

  • Masking Signals by Testing Type:
      - For AC testing: NARROW BAND NOISE.
      - For BC testing: NARROW BAND NOISE.
      - For Speech testing: SPEECH NOISE.

Clinical Masking

  • The appropriate use of masking prevents inadvertent acoustic stimulation of the NTE from a signal presented to the TE.

  • Crossover:
      - Inadvertent stimulation of the NTE occurs in patients with unilateral loss or significant asymmetrical loss.

Crossover

  • Mechanisms Leading to Crossover:
      - Sound leaking out and around the earphone cushion to travel AROUND THE SKULL and be heard by the NTE on the other side.
      - Sound traveling THROUGH THE SKULL to be perceived by the NTE via bone conduction or vibration of the skull.

Important Concept to Grasp!

  • Audiologists use masking to PREVENT crossover and apply knowledge of interaural attenuation to mask appropriately.

  • Aims: Avoid UNDERMASKING or OVERMASKING as both lead to INCORRECT TEST RESULTS.

Why Do We Mask?

  • Ensure that we are testing the ear we intend to evaluate.

  • Keep the “good” ear occupied while testing the “poorer” ear.

  • Ensure results are accurate and represent the TRUE hearing capabilities of the test ear.

Interaural Attenuation (IA)

  • Factors Influencing IA:
      - Subject Variability:
        - Head size, thickness of bone in the skull, hearing sensitivity, test-retest variability between tests in one subject.
      - Frequency Spectrum of the Test Signal:
        - Values for AC with supra-aural earphones range from 45 – 70 dB at octave frequencies from 250 to 8000 Hz.
      - Earphone Transducer Type:
        - Types include Supra-aural, Insert, and Bone Oscillator.

Interaural Attenuation Recommendations

  • Recommended Minimal Interaural Attenuation Values for AC Testing:
      - With supra-aural earphones (air conduction), interaural attenuation is approximately 40 dB.
      - With insert earphones (AC), IA is approximately 60 dB (some textbooks say 55 dB, but most adhere to 60 dB).
      - With a bone conduction vibrator, interaural attenuation is approximately 0 dB.

Air Conduction Concern

  • Concern arises when the better ear can hear tones while:
      - The BC of the better ear is 40 dB or more better than the unmasked AC of the worse ear.
      - Always compare the AC threshold of the TE to the BC threshold of the NTE.

  • A problem is identified only when there's a large difference between the AC of the worse ear and the BC of the non-test ear.

  • Audiologists must be mindful of asymmetrical differences and be aware of the BC thresholds.

Use of Masking During Testing

  • Audiologists may perform only AC testing at times where significant differences are noted, prompting the use of masking.

  • Symbols on the Audiogram:
      - AC masked symbols will be recorded using different representations for right and left ears.

Undermasking

  • Definition:
      - A minimum or sufficient amount of masking in the NTE that prevents crossover in the TE.
      - Any intensity LESS than the minimum necessary masking level constitutes undermasking.

  • Consequences of Undermasking:
      - Danger that the test signal may still be perceived in the NTE due to insufficient masking.
      - Potential reasons can include equipment calibration errors or calculation errors made by the audiologist.

Overmasking

  • Definition:
      - Not a desirable outcome in testing; occurs when the masking noise in the NTE is intense enough to influence thresholds in the test ear.

  • Threshold Impact:
      - Results in inaccurate thresholds.

  • Conditions for Overmasking:
      - Occurs when the masking noise level in the NTE equals or exceeds the IA plus the BC threshold of the TE.

The Plateau

  • Definition:
      - The intensity range between the minimum necessary masking level and the maximum permissible masking level.

  • Characteristics of the Masking Plateau:
      - Effective masking of the NTE can be achieved while obtaining valid thresholds in the TE.   - Refer to Figure 5.4 on page 145 in the text for a visual representation of the plateau and examples of undermasking and overmasking.

Plotting Masking on the Audiogram

  • Masked Symbols:
      - AC: Triangle for right, square for left.
      - BC: Bracket opening right for right, bracket opening left for left.

  • Recording Guidelines:
      - Don’t record an unmasked symbol for a frequency where masking is used; only the masked symbol should appear on the audiogram.

  • Real Thresholds:
      - The masked threshold is the real threshold; it is the ACCURATE threshold.

Thoughts on Masking

  • The masking noise is delivered to the NON-TEST ear while the threshold for the TEST ear is recorded as masked.

  • Remember, masking is merely a test protocol and does not need to be elaborated on in reporting.

  • The symbols in audiograms signify that masking was employed, and we can trust the test results are reliable and accurate due to proper testing methodologies.

What Tests Use Masking?

  • Masking is employed in:
      - Pure-tone air- and bone-conduction thresholds.
      - Speech reception thresholds (SRTs).
      - Word recognition scores.

  • Accurate percentage of word recognition abilities for each ear separately is crucial.

Shadow Curves

  • Audiologists should be vigilant for asymmetrical losses that necessitate masking.

  • Watch for “shadow” curves that reflect thresholds of the better ear.

  • Example:
      - BH - a hearing conservation client who displayed a shadow curve that the previous audiologist failed to detect.

Additional Notes

  • Focus on Effective Masking and Central Masking concepts (refer to pages 152 or 155 in the text).

  • No need to memorize any formulae for masking.