Download Clinical masking

Masking Model
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card fld "Read This"
The purpose of this model is to demonstrate
phenomena related to masking: cross hearing,
the occlusion effect, and effective masking.
The following abbreviations are used:
Tac: Threshold by air conduction
Tbc: Threshold by bone conduction
ABG: Air-bone gap
S:
The signal level
N:
The noise level
MMN: Minimum masking for normals
HC: Headphone coupling factor
MR: Meatal resonance
IA: Interaural attenuation
OE: The occlusion effect
The Interaural Attenuation (IA) & Occlusion Effect (OE)
The abbreviation HC stands for headphone coupling factor, and MR stands for meatal resonance. HC represents the
difference between the level of the air conduction (AC) signal delivered by the headphone, and the bone conducted (BC)
signal also delivered (to the skull) by the headphone. So,
HC = AC - BC
HC is a measure of the efficiency of the earphone cushion as a bone conduction oscillator (i.e., a larger value for HC
implies a less efficient coupling). MR is related to the difference (occluded verus unoccluded ear canal) in dB SPL of the
sound radiated into the ear canal (from the osseous walls of the meatus) as a result of a bone conducted signal delivered
to the skull. Together with the audiometric air-bone gap (ABG), they define the IA and OE according to the following
equations:
MR = SPLoccluded - SPLunoccluded,
OE = MR - ABG,
OE ≥ 0
and
IA = HC - OE,
IA ≤ HC
The OE is the meatal resonance reduced by the ABG. However, the OE is never less than 0 dB. The IA is the
headphone coupling factor reduced by the OE, but the IA is never greater than the HC since OE cannot be less than 0 dB.
It is assumed (for purposes of the simulation) that HC and MR are dependent upon properties of the skull, and are the
same for each ear. However, since ABG varies for each ear, the IA and OE are ear dependent.
For example: if HC=60 dB and MR=20 dB, and there is no ABG, the effective interaural attenuation is 40 dB, the value
conventionally given to the IA. If there is a 5 dB ABG, then the OE will be 15 dB, and IA will be 45 dB.
To enter Tac or Tbc just click on the box that contains these names. The default values for MMN (10 dB), MR
(20 dB) and HC (60 dB) can be changed by clicking in the boxes that contain these names.
The tone or noise are presented only when their buttons are hilited. Click the "Continuous" radio button to turn either
the tone or noise on permanently.
Masking Model
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--The End--
bkgnd fld "bkgnd field id 1"
The Masking Model
This is an attempt to show how air and bone conducted sounds (pure tones and noise) "flow" through the skull and
middle ear on their way to the cochlea.
Use the audiometer to select the ears you want the tones and noise delivered to. Present the tone and watch what
happens.
Hope you enjoy it!
Questions? Call or write:
Robert de Jonge, PhD
Department of Communication Disorders
Central Missouri State University
Warrensburg, MO 64093
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•What's masking?
•Interaural attenuation
•Occlusion effect
•Central masking
•Masking efficiency
Masking Model
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card fld "Definitions…"
What's masking?
•one sound makes it more difficult to hear another
•the elevation in threshold to one sound produced by another
•the amount of masking is measured in dB, as:
dB = masked threshold - unmasked threshold
•test signal is the one being masked
•the one doing the masking is the masker
•test ear
•masked ear
•classical masking studies used to investigate properties of the ear
-forward, backward masking
-spread of masking
-critical band theory
Clinical masking
•clinical masking used during audiometry
•test ear, masked ear are different ears
•masking needed to isolate the two ears
•a goal of audiometry is to get an audiogram that reflects acuity of each ear
•the two ears are not completely isolated from one another
•without masking thresholds for the left ear may reflect hearing for the right ear (and vice-versa)
Consequences…
•inappropriate amplification
•inappropriate medical intervention from interpreting a sensorineural loss as a conductive
•see Mr. Unilateral… no hearing in left ear
•the tone presented to the left ear "crossed over" and was heard in the right
•the tone presented to the left ear was heard by transcranial hearing, cross hearing
•the audiogram for the left ear was a "shadow curve"
Interaural attenuation
•interaural attenutation (IA): the separation between ears, in dB
For example…
Assume one ear (say, the RE) is entirely normal (no middle ear pathology, cerumen, no SN loss). The other ear has no
hearing. Headphones are placed over the RE and LE. The AC threshold is 5 dB when the tone is delivered to the right
headphone. When the tone is delivered to the left headphone, the AC threshold is 65 dB. IA is 65 - 5= 60 dB.
Masking Model
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Now, assume you determine a BC threshold of 10 dB with the vibrator placed on the right mastoid. If you place the
vibrator on the left mastoid and obtain a threshold of 5 dB, the IA is 5 - 10 = -5 dB.
IA is…
•conveniently measured when one ear is dead
•varies for air conduction vs. bone conduction
•varies with frequency
•varies with individual
•is somewhat smaller for children
•is influenced by status of middle ear (amount of the air-bone gap, ABG), the occlusion effect
Three explanations for cross hearing:
1. Bow of earphone headset (disproved by Békésy)
2. Sound leakage from headphone across head, diffraction via air conduction (Békésy favored this)
3. Through bones in the skull, by bone conduction. Feldman (1963) gave evidence to support this when he found that:
-unilaterally deaf subjects heard the tone presented to their bad ear better when their good ear was covered
-IA was smaller with an occluded ear
•see Chaiklin study
About the IA…
•IA for bone is 0 dB, effectively
•IA occurs for AC since the earphone cushion acts as a (fairly inefficient) BC oscillator
•Whenever you present an AC signal, you simultaneously present a BC signal
BC = AC - IA**
**(Actually, BC = AC - HC, where HC is the headphone coupling factor. For higher frequencies, IA = HC. For lower
frequencies, IA is influenced by the OE, occlusion effect.)
•maximum air-bone gap is equal to IA
•IA for air is about 60 dB for higher frequencies
•IA is smaller for the lower frequencies
-why is this?
The occlusion effect (OE)
•OE is: the threshold for a bone conducted signal improves when an ear is covered
•since IA= 0 dB for bone, either ear can be covered for OE to occur
•OE occurs all the time for AC testing, since both ears are covered
•OE occurs for bone only during masking, when one ear is covered
•OE related to the Bing test
-low frequency tone delivered to skull via tuning fork or bone oscillator
Masking Model
-ear is then occluded
-Bing is positive if tone sounds louder
-Bing positive when normal, sensorineural loss
-Bing negative with conductive
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Theories for OE…
1. Resonance phenomena. When ear occluded the resonant frequency of the ear is changed.
•this would predict a certain band of frequencies to be heard better
•inconsistent with all lower frequencies heard better
2. Mach's outflow theory. Sound radiated out of canal by eardrum. Sound directed back in if ear occluded.
•Tonndorf removed TM of cat, found no change in OE
3. Békésy explanation. Lower jaw moves out of phase with TMJ, generating sound normally radiated out of canal.
•Guess what Tonndorf did?
4. Tonndorf's explanation…
•during bone conduction stimulation, walls of the osseus meatus vibrate
-this BC stimulation occurs for AC also
•vibrational energy is transmitted to air in canal
•low frequencies find ear canal a low impedance path, and radiate out
•when occluded, sound is redirected in, like any normal AC signal
How would you eliminate OE?
1. Not cover the ear. Not an option if you have to mask.
2. Insert a plug deep into meatus. Damp vibrations of the canal wall.
-Mead Killion's tubephone (ER-3A) based on this principle (or, the effect can occur if the plug is inserted deeply), and
-E-A-R material is soft, damps vibrations, rather that transmitting them to skull, and
-smaller contact area.
-increases IA in low frequencies by reducing OE
3. Create an ABG in occluded ear, ABG ≥ OE. Before reaching cochlea, AC signal is attenuated by ABG.
-IA is greater (lower frequencies) for people with conductive loss
-you don't know this before testing
-see Goldstein & Hayes article
-see definitions for headphone coupling factor (HC) and meatal resonance (MR) at beginning of the stack in field, "Read
This."
Central masking
•change in tonal threshold produced by presence of noise in contralateral ear
-provided no peripheral masking occurs
Masking Model
•Why is central masking important? Hood plateau procedure identifies true threshold as…
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-the threshold that remains constant as noise level in contralateral ear increases
-if threshold changes with noise in contralateral ear, there is no plateau
•most masking occurs at the periphery (peripheral vs. central masking)
-tone and noise interact at the level of the cochlea, tone is lost in the traveling wave
•central masking is masking within the CANS
-tone enters cochlear nucleus on one side, noise enters cochlear nucleus on the other
•noise interferes with audibility of tone via interaction between crossed and uncrossed pathways
-lowest level would be SOC, but many other opportunities in ascending pathways
•noise could interfere via activation of efferent pathways
-direct efferent suppression of contralateral cochlear output via crossed olivocochlear bundle
•see article by Dirks & Malmquist
•central masking does occur, but the magnitude is small, less than 10 dB, in most cases less than 5 dB
-generally, ignore it
Masking efficiency
•A normal listener has a threshold, in the right ear, of 5 dB for a pure tone. Consider two noises, A and B. Each noise has
a SPL of 57 dB. Threshold is determined for the tone when each noise is delivered to the right ear.
-when noise A is presented, the threshold shifts to 20 dB
-when noise B is presented, threshold shifts to 35 dB
•noise B is the more efficient masker
•Disadvantages of an inefficient masker…
-noise may become uncomfortably loud for listener before threshold is determined
-audiometer limits for noise may be reached before threshold is determined
•see article by Sanders & Rintelmann
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card fld "Caption"
Mr. Unilateral
•no hearing in left ear (dead ear)
•typical interaural attenuation values for air &
bone
Masking Model
•attempt surgery to improve hearing?
•otoscopic appearance similar to otosclerotic ear
•Shambaugh… improper masking cause of diagnostic error for otosclerosis
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card fld "Caption…"
Interaural attenuation and cross hearing in air
conduction audiometry. Chaiklin, 1967.
•Five normal hearing college students, except
for dead ear (4 mumps, other etiology
unknown).
•tested conventionally, and with an ear plug
inserted deep into the auditory meatus of the
better ear.
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card fld "Caption…"
Interaural attenuation and cross hearing in air
conduction audiometry. Chaiklin, 1967.
•mean values for interaural attenuation,
occlusion effect
•comparison to Killion's ER-3A (tubephone)
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card fld "Caption…"
The occlusion effect in bone conduction
hearing. Goldstein & Hayes, 1965.
•Mean values for the occlusion effect
(actually, the Meatal Resonance) for 28
subjects, mastoid placement of bone vibrator
•Standard deviations
•maximum values (95th percentile)
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Masking Model
•Main point: You don't know what the OE (or MR) will be for a given patient.
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card fld "Caption…"
The occlusion effect in bone conduction
hearing. Goldstein & Hayes, 1965.
•relationship between ear canal SPL
(measured via probe tube) and change in
threshold produced by a TDH-39 earphone in
MX41/AR cushion
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card fld "Caption…"
Changes in bone conduction thresholds
produced by masking in the non-test ear
Dirks & Malmquist, 1964.
•Central masking measured with 10 subjects
•air conduction and bone conduction
threshold shift to narrow band noise
presented to the contralateral ear
•Central masking produced by efferent
suppression via contralateral olivo-cochlear
bundle?
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card fld "Caption…"
•from Sanders and Rintelmann, 1964
•the experiment compared masking efficiency
of three types of noises
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Masking Model
-complex noise, saw-tooth noise. Fundamental frequency of 78 Hz, all even and odd harmonics
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-broad band noise. White noise with a bandwidth limited by frequency response of earphone (TDH-39)
-narrow band noise. White noise filtered so that center frequency of each band was (more or less) at an audiometric
frequency
•note: curves representing noises are not
drawn precisely to scale
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card fld "Caption…"
Masking Model
•filters shape the spectrum by attenuating some frequencies, while allowing others to pass through
•fo is the center frequency of the filter
•f1 is the low frequency cut-off
•f2 is the high frequency cut-off
•f1 and f2 measured at the "3 dB down" point
•BW= filter bandwidth= f2 - f1. Also called the "pass band"
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Masking Model
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card fld "Caption…"
How threshold shifts with increase in noise
level
•When the EML > 0 dB, masking occurs
(threshold is shifted from 0 dB HL)
•The dB SPL corresponding to 0 dB EML is
different for each noise
-varies with spectrum, bandwidth
•before 0 dB EML, no masking occurs
•beyond 0 dB EML, every dB increase in noise
level increases threshold by 1 dB
•Critical band theory predicts, very accurately, what dB SPL corresponds to 0 dB EML
•threshold for a tone is mostly related to the amount of noise energy in a narrow band surrounding the tone
•noise energy outside the critical band contributes to…
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Masking Model
-the overall SPL of noise
-the loudness of the noise
-masking inefficiency of the noise
-but not to shifting the threshold of the noise
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card fld "Overall SPL"
When SPL increases, loudness typically
increases. If the BW of the noise increases, the
extra frequencies increase the SPL of the noise.
In this experiment, the spectrum level (overall
SPL of each single frequency component of the
noise) was reduced as the BW increased. This
kept the overall SPL of the noise constant.
This experiment suggests that:
•Loudness remains the same if the energy
within the CB remains constant.
•Loudness increases if the BW of the noise exceeds the CB — so energy spreads to other CBs.
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card fld "Critical Bands"
Results of the critical band experiments:
•The auditory system responds to sound as if
it is listening through a narrow filter (the CB).
•The auditory system can focus effectively on
a small region of the spectrum, ignoring the
irrelevent.
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Masking Model
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•For the noise, only a narrow frequency region surrounding the test frequency is effective in masking the tone.
•Noise outside the critical band contributes to overall SPL and loudness, but not masking.
•The width of the band increases with frequency. CBs are about equal to 1/3 octave bands.
•Each band probably occupies a constant distance along the basilar membrane.
•The width of the critical band for loudness is about 2.5 times the band for masking, referred to as the Critical Ratio.
•Threshold in noise is obtained when the SPL of the noise within the Critical Band (actually, Critical Ratio) equals the SPL
of the tone.
For any noise, its overall level (OLn):
OLn = LPC + 10*log(BW)
Since CR = BW and threshold in noise (THn) equals the SPL of the tone at threshold:
THn = LPC + 10*log(CR)
(See next card)
•Or, alternatively, threshold in noise is obtained when the SPL of the noise within the Critical Band (not Critical Ratio) is
4 dB higher than the SPL of the tone (S/N= -4 dB).
--------The End--------
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card fld "Info"
Spectrum level, overall level, and bandwidth
==================================
===========================
Variables:
f1,
f2 = the low and high frequency cut-offs
(corner frequencies)
of the narrow band of noise pictured on
the graph. Change
these values by dragging them along the
frequency scale.
fo = the center frequency of the narrow band
of noise. It is the
geometric mean of the corner frequencies.
BW= the bandwidth (f2 - f1) of the narrow band of noise
LPC= the dB level per cycle (or spectrum level) of each of the
frequency components of the narrow band of noise. You
change it just like you change the corner frequencies, by
dragging it up and down the ordinate.
OLN= the overall level of the noise in dB SPL
CB= the critical band in Hz
CR= the critical ratio in Hz
THN= the threshold (in dB SPL) of a tone whose frequency equals the
center frequency of the noise band. It is assumed that
the listener's threshold in quiet is better than the threshold
in noise.
Masking Model
S/N= the signal-to-noise ratio at threshold (tone to overall noise
level).
By changing each of the variables, you can illustrate how:
•fo and BW change as a function of f1 and f2
•how the OLN changes as a function of both bandwidth and LPC of
the noise
•how threshold varies as a function of spectrum level, center
frequency, and critical ratio.
---The End---
bkgnd fld "Critical band"
Center Frequency, Critical Band Width
350,100
450,110
570,120
700,140
840,150
1000,160
1170,190
1370,210
1600,240
1850,280
2150,320
2500,380
2900,450
3400,550
4000,700
4800,900
5800,1100
7000,1300
8500,1800
10500,2500
13500,3500
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card fld "Rule…"
Threshold in noise is determined by either the
noise or the patient's sensitivity:
Floor = DialReadingNoise - MMN
If Tquiet > Floor then
Tnoise = Tquiet
else
Tnoise = Floor
end if
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Masking Model
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card fld "Terms"
Effective Masking Level (EML)
The level in dB of a pure-tone at threshold in
the ear that contains the masking noise.
•for example, if a person is listening to 80 dB
SPL of NBN, 100 dB of BBN, 120 dB of
sawtooth noise
-if the pure-tone threshold is 70 dB HL, the
EML is 70.
•the definition presumes that the noise is
intense enough to shift the threshold from
what it would be in quiet
•the audiometer dial (controlling the NBN) can be calibrated in units of EML
-for example, if the dial controlling the noise is set to 30 dB HL, and the pure-tone threshold is 30 dB, then the dial is
calibrated in units of EML
-also, MMN (see below) would be 0
•EML does not change with the degree of hearing loss
-40 dB EML is still 40 dB EML, even if the person has a profound hearing loss
-just like 40 dB HL is 40 dB HL, regardless of whether the person can hear the tone
•Note: In the formulae used to calculate EML (like MinEML), the value calculated is the dial reading needed to reach the
EML.
Minimum Masking for Normals (MMN)
The difference between X and Y (X minus Y), where
X is the dB value of the dial controlling the noise
Y is the dB value of the dial controlling the tone
and,
Masking Model
the tone is at threshold in the noise.
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•the noise and tone are delivered to the same ear
•normal hearing people are used to measure MMN (a mean is taken of 6 to 8 people, usually)
•the level of the noise is usually set to a fairly high level (70 dB)
-for example, if X= 70 dB and Y= 50 dB, MMN=20
Minimum Effective Masking Level (MinEML)
The minimum level of noise just needed to mask out a tone heard at threshold in quiet in the ear that the noise is
presented to
Important!
•MinEML is NOT dependent upon how the tone is heard. A tone at threshold by AC, BC, crossing over and heard by
BC… all are masked by the same noise level
Maximum Effective Masking Level (MaxEML)
The highest level of noise that can be presented to the NTE that will not cross over and mask out a tone at threshold in the
TE
•MaxEML is primarily related to Tbc in TE and the IA
•the true Tbc in TE is often not known… it is estimated from the unmasked Tbc (which could be for either ear)
•IA for noise is the same as for tones
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card fld "Answers"
•All thresholds for the left ear are valid, all
thresholds for the right ear are questionable
and need to be masked
250
500
1000
2000
MaxEML
50
55
50
60
MinEML
10
15
15
25
Plateau
Bone
Air
50
40
60
40
50
25
50
25
Masking Model
4000
75
8000
75
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35
40
50
35
40
Note:
•MinEML for AC equals MinEML for BC, provided BC tone is at threshold with occluded ear
•To calculate beginning of plateau, MinEML and MaxEML go to card "Calculating Plateau"
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card fld "Note"
Note, for the formulae:
OE(NTE) = MR(NTE) - ABG(NTE)
•The OE is assumed to be in effect only for
lower frequencies
MaxEML = Tbc(TE) + MMN + IA
Tbc(TE) is the unmasked BC threshold for
TE.
For simplicity, set IA to 40 dB.
•The true MaxEML would be based upon the actual BC threshold for the TE, the masked Tbc(TE). Using the unmasked
Tbc(TE) represents a conservative estimate of MaxEML; the real MaxEML is probably higher. Also, the masked Tbc(TE)
is not known at the beginning of the masking procedure.
•Setting IA to 40 is a simplification. The actual IA is calculated as:
IA = HC - OE(TE)
Masking Model
OE(TE) = MR - ABG(TE)
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The OE for the TE is 0 dB if masking for BC, since the TE is not occluded. If masking for AC, remember that the OE
operates only in the lower frequencies. Also, the ABG(TE) may not be known if masking has not been completed for the
TE.
Also note for the formulae:
MinEML = Tac(NTE) + MMN
•This formula assumes that threshold has been re-established, if masking for BC, since the OE may improve hearing.
The variable, Tunmasked(TE)
•If masking for bone conduction, Tunmasked(TE) is the threshold obtained with the ear unoccluded — not the reestablished threshold. The re-established threshold (Tunmasked(TE) - OE) could be used in estimating the threshold
shift, but then the BC Plateau would be
BC Plateau = MinEML + Tshift
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card fld "Terms"
Masking dilemmas are situations when, even
though you have done everything correctly,
you still do not (can not) obtain a valid
masked threshold.
Masking dilemmas usually occur for either
one of two reasons:
•there is a great amount of hearing loss in the
NTE
-So, you run out of noise before the plateau
can be established
-conditions #1 and #2 below
•there is a large air-bone gap in the NTE, one that approaches the IA
-So, the masking noise crosses over, and masks
the tone in the TE
-the same noise level reaches both cochleae so
that you cannot separate the ears
-condition #3 below
Specific conditions
1. MinEML + Plateau > Limits
•Plateau: is usually taken to be 15 dB, minimum
•Limits: is limits of the audiometer for the noise
2. MinEML + Plateau + Threshold Shift > Limits
•Shift: is masked minus unmasked threshold
Masking Model
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•In the process of establishing the plateau, before the true masked threshold is reached, you run out of noise.
3. MinEML + Plateau > MaxEML
•Before you validate that you have reached the true masked threshold, you overmask (mask the tone being heard in the
test ear).
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card fld "Terms"
Minimum Masking for Normals (MMN) for
Speech
The difference between X and Y (X minus Y),
where
X is the dB value of the dial controlling the
speech noise
Y is the dB value of the dial controlling the
speech
and,
the speech is at threshold in the noise.
•the noise and speech are delivered to the same ear
•normal hearing people are used to measure MMN (a mean is taken of 6 to 8 people, usually)
•the level of the noise is usually set to a fairly high level (70 dB)
-for example, if X= 70 dB and Y= 50 dB, MMN=20
Minimum Effective Masking Level (MinEML)
The minimum level of noise just needed to mask out speech heard at threshold in quiet in the ear that the noise is
presented to
for determining the SRT:
MinEML = SRT(NTE) + MMN
for speech recognition testing
MinEML= SRT(NTE) + MMN + (PL - IA - Tbest)
PL= presentation level of speech (on the dial)
PL - IA = bone conduction level of speech
PL - IA - Tbest= bone conduction sensation level
of the speech
Another formula:
MinEML= SRT(NTE) + MMN + Shift + SL
Shift= Masked SRT(TE) - Unmasked SRT(TE)
SL= the sensation level of speech presented to
the test ear (usually 30, or 40 dB)
Maximum Effective Masking Level (MaxEML)
Masking Model
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The highest level of noise that can be presented to the NTE that will not cross over and mask out speech at threshold in
the TE
MaxEML= SRTbc(TE) + MMN + IA
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card fld "Example"
Enter a mild, gradually falling, conductive loss
for the left ear. Assume that you need to mask
this ear while testing word recognition for the
right ear. The SRT for the RE is 35 dB HL.
Assume MMN = 10 dB, IA = 40 dB.
(You need 60 dB HL of noise in the LE.)