A Guide to Masking

When it comes to audiological testing, everyone is a little more than anxious. You might say to yourself, “I’m a speech therapist, I evaluate and treat disorders of speech and language, not hearing.” While this statement has some merit (granted our caseload has more clients than hours in the day), as experts in communication disorders, it is essential for us to know not only basic audiological screening techniques, but also the tangential procedures that allow us to conduct a comprehensive evaluation. It is important to be cognizant about the role that auditory impairments play in disorders of speech and language, so that we can have a thorough understanding of the etiology.

If you’ve taken an audiology course in graduate school, surely you’ve heard about the masking procedure. Simply stated, masking is a procedure we use while acoustically testing two ears, separately. The process is very much like when you get an eye exam, and you don’t want to test both eyes at the same time. You separate them by covering one then testing the other to determine if it is normal or impaired. Similary, noise is used as a masker during an audiological evaluation, hence the term masking. Noise is introduced to one ear while the other ear is tested with a tone (or speech signal). To indicate that the hearing thresholds were obtained using masking, masked threshold symbols are used on the audiogram.

Here, I will explain the masking procedure in detail. Implications for when to mask, how to mask, and how much to mask will also be discussed.

Factors

Interaural Attenuation: The reduction of sound between two ears (from TE to NTE) when presented via air or bone conduction. This is the number of decibels (dB) lost in cross-over.

Shadow Audiogram:  When the thresholds of the left ear mimic the bone conduction thresholds of the right ear. This is a tell-tale sign of cross-over.

Occlusion Effect: The enhancement of low frequency bone conduction thresholds when the outer ear is occluded.

When to Mask

A/C: if there is a difference between the 2 ears of 40 dB or greater, you need to mask.

B/C: if there is an air-bone gap greater than 10 dB you need to mask.

SRT: SRTte > SRTnte by 45 dB or SRTte > BCnte in speech range by 45 dB or more.

SRS: PLte > SRTnte by 35 dB or PLte > BCnte in speech range by 35 dB or more.

How Much to Mask

Undermasking: Not enough masking in the non-test ear to mask the test stimulus.

Overmasking: Excessive level of masking crosses over and elevates thresholds.

Effective Masking: Minimum amount of noise required to just mask out the signal keeps signal from crossing-over to non-test ear.

Formulas

AC: ACnte + SF = M

BC: ACnte + SF + OE = M

SRT: (PLte – 35 dB) + PTAnte

WDS: (PLte – 25 dB) + PTAnte

SF = 10 dB

OE = 20 dB @ 250 Hz, 15 dB @ 500 Hz, 5 dB @ 1000 Hz

Remember 35 dB for SRT, 25 dB for WDS

Procedure

A/C: An earphone is placed over the test ear and masking is applied to the non-test ear through another earphone.

B/C: With mastoid placement, the bone conductor is placed on the mastoid of the test ear and masking is applied to the non-test ear through an earphone, which is placed in front of the pinna of the non-test ear.

Hood Plateau Method: 3 consecutive positives or 3 consecutive 5 dB masking level increases at the same pure tone intensity level. Width of plateau is affected by the amount of interaural attenuation, the A/C threshold of the non-test ear, and the B/C threshold of the test ear.

Central Masking: A threshold shift in the test ear resulting from the introduction of a masking signal into the non-test ear that is not due to cross-over. Average is 5 dB.

Masking Dilemma: Ineffective masking level achieved due to bilateral moderate conductive hearing loss.

What do we do when there’s a masking dilemma, you ask?

Mask anyway

But consider using the Weber Test (with asymmetrical conductive hearing loss).

Use insert earphones (insert earphones increase the interaural attenuation levels from an average of 55-65 dB to an average of 80 dB).

Also, know the immittance measures, which validate behavioral audiometric results.

Electroacoustic Principles

Impedance: reflected energy

Admittance: absorbed energy

Immittance: measured through absorbed and reflected energy

Equipment

Air pump: +/- pressure

Microphone: input (analysis system)

Loudspeaker: output (probe system)

Probe tip with cuff (hermetic seal). Goes into ear canal and delivers pressure and signal

Measurements of Immittance Measures

Static Compliance: measurement of the mobility of the tympanic membrane

Tympanometry: measurement of the pressure-compliance of the tympanic membrane

Tymp Types

Type A: represents normal middle ear function. Peak occurs between 50 to -100.

Type C: retracted TM. Static compliance is within normal limits but peak is off. More negative than -200. Can indicate beginning or resolution of Otitis media; beginning of Eustachian tube malfunction.

Type As: Very shallow. Represents abnormal stiffness. Compliance measures are abnormally low. Could be due to scarring on the TM or the beginning stages of Otitis media.

Type Ad: Represents excessively flaccid. Peak is abnormally high – appears as if it doesn’t come together at the top. Could be due to scarring on the TM or disarticulation of the ossicles.

Type B: Flat. Represents restricted mobility, a great deal of reflection is occurring. Compliance measures are abnormally low. Could be due to active Otitis media or significant fixation of the ossicles.

C1= volume of ear canal from the tip of probe all the way to tm

C2= how much mobility how much compliance you have with the TM

TM Perforation: C1 value will be very high. Air is going right through to the Eustachian tube.

Effusion: Value of C1 has to be > .27

Canal Wall: if the probe is up against the canal wall C1 will read 0 or .1

Types of Units

Relative: data on a set scale of 0-10

Absolute: data in exact units of measure (daPA)

C1 – C2 = SC/SA (C1 = ear canal volume, C2 = measurement at peak)

Normal =     0.3 – 1.4 mmho/cc3/m

Limited/Restricted =    < 0.3 mmho/cc3/m

Excessive/High =        > 1.4 mmho/cc3/m

Acoustic Reflexes: stapedial reflex of middle ear muscle contracting and causing a change in TM position. A good way to confirm behavioral results.

Ipsilateral AR Pathway

Outer Ear

Middle Ear

Inner Ear (cochlea)

Auditory Nerve (CN VIII)

Cochlear Nucleus (ventral)

Superior Olivary Complex (medial)

Contralateral AR Pathway

Facial Nerve

Superior Olivary Complex

Facial Nerve

Middle Ear

Present vs. Absent AR

Acoustic reflex occurs: 70 – 90 dB (normal). Absent = hearing loss (< 50 dB = hearing loss with recruitment)

Absent acoustic reflex due to: hearing loss (severe to moderate), neurological involvement, middle ear pathology

Special Tests

Eustachian Tube Function: Valsalva maneuver (pinch and blow), Toynbe maneuver (pinch and swallow).

Reflex decay: Hold signal for 10 seconds. Normal will show reflex for 10 seconds, pathologic system will not.

Evoked Potentials

Electrocochleography: Monitors the function of the inner ear. Often used prior to cochlear implantation to make sure auditory nerve is intact. It’s also to screen for Meniere’s Disease. You’ll need a sound delivery tube, electrode wire, foam ear plug, and electrode tip.

Auditory Brainstem Evoked Responses: Assess the function of the auditory pathway up to and including the brainstem.

Middle Latency Auditory EP: Relating responses to generators. Middle latency responses are at VI (medial geniculate in the thalamus) and VII (auditory radiations, thalamo-cortical).

Late Cortical Auditory EP: At the cortex

Otoacoustic Emissions

Otoacoustic Emissions are acoustic energy produced by the healthy cochlea and recorded in the external auditory canal. These are acoustic signals, not electrical potentials.

Evoked: from stimulating the cochlea – getting pre-neural activity. This is an efferent response.

Non-evoked: spontaneous. Nothing stimulates it. The ear acts as a sound generator – 20 dB SPL. Found in 60% of normal ears. Females tend to have stronger spontaneous OAEs. Right ears tend to have stronger spontaneous OAEs. Usually measurable in the 1K-2K Hz range. The same person can have multiple emissions.

Distortion Product (DP OAEs)

Tonal responses located at precise frequencies determined by 2 simultaneously presented pure tones. Choose which part of the cochlea you want to stimulate.

Two discrete pure tone frequencies, F1 and F2 are introduced into EAM.

Each tone has specific amplitude L1 or L2 (50-75 dB SPL).

Input frequencies are related by a pre-determined ratio called a “fratio” (1:1 – 1:3). If F1 = 1K Hz, then F2 = 1.1 – 1.3K Hz.

Transient (TEOAEs)

With a transient signal (click, gated tone pip). Children have more robust TEOAEs. Amplitude decreases with age. There is a correlation between stimuli and response amplitude: intense signal yields intense response to a point.

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