Measurement of Hearing - Midterm

What is masking?

-The process by which the detection threshold of one sound (the signal) is raised by the presence of another sound (the masker)

• masking may occur unintentionally

  • environmental noise, air conditioning, other people talking…

  • or intentionally: clinical masking is intentional

masking setup for AC thresholds

-In clinical masking, a signal is presented to one ear and a masker is presented in the opposite ear

-the masker is always presented via air conduction to the non-test ear

• you can use insert earphones or supra-aural headphones

masking setup for BC thresholds

-In clinical masking, a signal is presented to one ear and a masker is presented in the opposite ear

-the masker is always presented via air conduction to the non-test ear

• you can use insert earphones or supra-aural headphones

-Present the masker via AC even when the signal is presented via BC

• headphone placement can be tricky here

Why use masking

-masking prevents cross hearing due to crossover from confounding your test results

-if cross hearing occurs, without masking, you may not be testing the ear you think you are

crossover

-when a signal to one ear travels through the skull via bone conduction (BC) and stimulates the opposite cochlea

• it could also travel around the skull via air conduction (AC)

How much crossover (in dB) occurs?

-amount of crossover via BC depends on the interaural attenuation of the signal:

• this varies with skull density, transducer, and the stimulus

What is IA? (interaural attenuation)

-The amount of sound energy in dB lost as an AC or BC signal presented to one ear travels through the skull, via bone-conduction, to the contralateral ear

-IA differs for AC and BC transducers

• and for AC transducer type

• and with freq and stimulus type

Variables affecting IA

-the person

• skull size, thickness, bone density, etc. affect IA

  • Range of IA’s of ~30 dB across subjects

-The freq of test signal

• less IA for lower freq with supra-aural phones but more with insert phones

-The type of earphone/transducer (can be big factor)

Minimum IA for inserts

~50 dB (shallow) ~55 dB (deep)

Minimum IA for supra-aural

~40 dB

Minimum IA for bone oscillator

~0 dB

when do you get max IA?

-with deeply seated inserts

• extra 5-40 dB

IA for mastoid BC

-Avg. BC IA varies (0-10 dB) with frequency

• <500 Hz IA

  • ~3-5 dB

• 500-1800 Hz

  • ~0 dB

• 3000-5000 Hz

  • ~10 dB

• 5000 Hz

  • ~4 dB

-But 20-30 dB range!

minimum IA assumptions

-Conservative estimates, it is likely that most people’s IA is substantially greater!

-Knowing an individual’s IA can help you make masking decisions

• particularly helpful when it is > the assumed minimum!

How would we know someone’s IA is > minimum IA?

-Compare unmasked threshold differences to assumed minimum IA

The minimum IA is assumed to be the larger of the following:

1. The assumed minimum IA for the transducer

   1. 40 or 50 dB for headphones and inserts

2. The difference between the AC (TE) threshold and the AC (NTE) threshold

3. The difference between the AC (TE) threshold and the BC (TE) threshold -i.e. ABG in the TE

4. The difference between the AC (TE) threshold and the BC (NTE) threshold

When do you need to mask AC thresholds conceptually?

-Whenever there is a chance that the signal to the test ear (TE) is actually being detected by the non-test ear (NTE)

-The likelihood of this happening depends on the

• air conduction thresholds (signal level) in the TE

• Interaural attenuation of the AC transducer

• BONE conduction thresholds in the NTE

When do you need to mask AC thresholds quantitatively?

-Mask if the difference between the AC threshold (signal level) in the TE and the BC threshold in the NTE > the minimum IA of the transducer

• Mask if: AC (TE) - BC (NTE) > min IA

-If we don’t know the BC threshold in the NTE, no masking until unmasked AC and BC thresholds are obtained. Alternatively, clinicians may assume initially that the BC (NTE) threshold = AC (NTE) threshold

-

When do you need to mask BC thresholds conceptually?

-same as AC testing- mask whenever NTE may be responsible for signal detection

-Importantly- mask whenever you need to confirm/refute the presence of a conductive component in the test ear

BC testing

-Always assume unmasked thresholds reflect the better ear-not the mastoid placement ear!

-Start BC testing on the poorer hearing ear

• i.e. masker goes into the ear with the better AC thresholds as it is easier to effectively mask the better hearing ear

-Assume min IA for BC testing = 0 dB

• but remember mean BC IA varies with freq- 0 to 10 dB

  • up to 30-40 dB in individuals

When do you need to mask BC thresholds quantitatively?

-Mask if difference between AC and BC thresholds in TE is > 15 dB

• mask if: AC (TE) - BC (TE) > 15 dB (mask if ABG (TE) > 15 dB

  • compare test ear AC & BC thresholds b/c need to confirm/refute the presence of a conductive component

• ASHA (2005) rx to mask if ABG>10 dB

  • conservative but fine approach

• Always mask if ABG (TE) is systematically > 10 dB

  • systematic differences are not random variability

What masker levels should you use?

-enough the “effectively mask” the NTE

• I.e. enough to raise threshold in NTE so signal from the TE is not detected

EM & EMLs

-masking noises are calibrated in EML (dB SPL) but audiometer dial values reflect EM in dB

• the amount of EM is = to the expected shift in pure tone threshold from 0 dB HL if the masking noise was presented in the same ear as the tone

  • e.g. a 60 dB HL pure tone would be just audible at threshold when presented in a 60 dB EM noise (60 dB masker dial value on the audiometer)

-So to mask the signal (make it inaudible in the NTE) you must add at least 5 dB more EM

• e.g. at least 65 dB EM to effectively mask a 60 dB HL tone

-For a properly calibrated audiometer, ON AVERAGE, masker level in dB HL = dB EM

Under masking

-The masker level causes a shift in threshold but is insufficient to prevent cross hearing of a signal presented at “true threshold”

• Results in a linear increase in threshold with increases in masker level

Over masking

-The masker level is so high it crosses over and masks thr signal at the TE

• Also results in a linear increase in threshold with increases in masker level

• in some cases your starting masker level may cause over masking

plateau region

-the range of masker levels that mask the NTE without under - or over- masking

• this masker level lets you confirm that responses to the signal are not due to detection by the NTE

• when you measure a masked threshold your final masker level should be in this range

-Unfortunately we typically don’t know the masker levels that make up any individual’s “plateau region”

Factors affecting plateau width

-individual’s IA

• larger IA = larger plateau

-TE BC threshold

• better thresholds = narrower plateau

-NTE AC threshold

• poorer thresholds = narrower plateau

Initial masker level - AC threshold testing

-Your starting masker level

• your starting masker presentation level (PL)

  • this may vary depending on your masking procedure

• Must be high enough to initially mask the non-test ear given the current signal level

-IML = AC threshold in NTE +10 dB

IML - BC

-same as AC testing however, we must also account for any “occlusion effect” (OE)

-recall an OE is the increase (in dB HL) of the BC signal at the NTE (or TE) when the EAC is occluded

• occurs primarily at freq <1000 Hz

-Is reduced or absent with ME conductive pathology

• i.e. when ABG due to ME pathology is about >20 dB

-IML = AC threshold in NTE + OE + 10 dB (safety factor)

What if your threshold shifts when you present a masker at your initial masking level?

-it wasn’t really at the threshold in the test ear (i.e. NTE was detecting the signal) or central masking

Central masking

-A shift in the TE threshold when a masking noise is added to the NTE in the absence of cross hearing

• can lead to a 5-7 dB shift in threshold

  • or more at high masker levels — up to 10-15 dB

  • similar for AC, BC, and speech

• May be due to efferent inhibition which reduces the sensitivity of the TE

Central masking - which threshold to record?

-There is a debate over which threshold to record

• for example, unmasked threshold is 50 dB HL

• masked threshold is 55 dB HL

• the 5 dB shift could be central masking

-Most suggest you report your final masked threshold

What if your threshold shifts when you present a masker at your initial masking level?

-It could be due to central masking or it could be that the signal was crossing over and being detected by the NTE and you successfully masked the NTE

• this could reflect under masking or masking in the plateau region

-Or you could be “over masking”

• i.e. the signal was being detected by the test ear, but your masker level was so high it crossed over and masked the signal in the test ear!

Over masking

When masker at NTE crosses over and masks the signal in the TE (as well as the NTE)

How much masking is too much?

-mathematically you may be over masking when

• masker level in the NTE > BC (TE) + IA

• Masker level in the NTE - BC (TE) > IA

Maximum masker level (MML)

-The highest masker level (AC & BC) you can use without over masking

• MML = BC (TE) + IA

-i.e. if BC (TE) = 30 dB HL and your IA = 40 dB (TDH phones)

• you may be over masking if the masker is > 70 dB

What if your threshold shift is more than 5 dB (and you aren’t overmasking)?

-Then it is not central masking!

-so you need to find their “true” threshold

Plateau method - Hood

-Demonstrate “shadowing” and a “change over point”

-advantages:

• effective, reliable method for ensuring NTE is not affecting TE measures

• Gives clinician confidence in their threshold

-Disadvantage:

• time consuming

Plateau method - Yacullo

-AC initial masker level: AC (NTE) + 10 dB (may see 15 dB)

• BC: AC (NTE) + 10 (or 15) dB + OE

-If no (or only 5 dB) threshold shift (i.e. they respond) -

• increase masker level 5-10 dB and present tone

• continue to increase masker level for 2 (10 dB) or 3 (5 dB) steps to achieve a 15-20 dB plateau

-If threshold shifts > 5 dB - re-establish threshold then increase masker 5-10 dB steps as above until plateau is achieved

-In both cases you are looking for stable thresholds over 2 (10 dB) or 3 (5 dB) masker level increases. i.e. a 15-20 dB plateau

Plateau method variations

-get your 15-20 dB plateau various ways

• use 5 dB steps and look for 3 step 15 dB plateau

• use 10 dB steps and look for 2 steps 20 dB

• use 15-20 dB step but only 1 time

-Use smaller step size whenever concerned about a narrow plateau width (over-masking)

plateau width (PW)

-varies and is sometimes smaller than 15-20 dB

• PW(AC) = 2IA-ABG(TE)-ABG(NTE)

• PW(BC) = *IA-ABG(NTE)

  • *IA for AC transducer

-PW increases as IA increases and ABG decreases

• asymmetric losses- have larger plateau so use 10 dB steps

  • or more, especially for SNHL

  • SNHL has larger plateau than conductive loss

• Symmetric losses use smaller step size and 15 dB plateau

• Bilateral (esp. symmetric) conductive- biggest problem! may be small or no plateau, always use 5 dB steps

Formula method

-initially developed in the 1950’s along with development of calibrated masking noises

• modifications continue

-advantage

• fast (if done correctly)!

• one level and done!

• 
  

Disadvantages:

• learning the formulas and the time to calculate them

• sometimes we don’t have all the information needed to complete the formulas

The liden method

-pick a masker lever that falls between minimum and maximum. effective masking levels

-Air conduction (AC):

• AC (MIN) = PL (TE) - 40 (IA) + ABG

• AC/BC (MAX) = BC (TE) +40 (IA) dB

-Bone conduction (BC) =

• BC (MIN) = PL (TE) + ABG (NTE) (plus OE)

• AC/BC (MAX) = BC (TE) + 40 dB

-If no threshold shift occurs (or only 5 dB), then NTE was not involved, and you are done!

-This method is overly complex and so not used much in this form. Modifications of this method, however, are used

The Martin method

-First to use individual IAs to select masking levels

• minimum masker levels:

  • AC (min): AC (NTE) +10 (20) dB (safety factor)

  • BC (min): AC (NTE) + 10 (20) dB + OE (individual)

• maximum masker level:

  • AC/BC (max) = BC (TE) + individual IA

-If threshold is stable (or shifts only 5 dB) you are done

• otherwise, must plateau

“optimized” method AC (Turner)

-For asymmetric losses: when unmasked AC thresholds differ by >25 dB

• i.e. when plateau width is large

-Starting masker level for AC testing:

• 10 dB below Test Ear AC threshold (75 dB-10 dB = 65 dB)

-present signal

-if threshold is stable (or 5 dB shift)- you are done!

-If no response- re-establish threshold

• then increase masker level by the amount of threshold shift

“optimized” method BC (Turner)

-Starting masker level- BC testing:

• Larger of AC (TE) - 10 OR

• 30 dB above unmasked BC (TE) threshold

-if stable threshold or only a 5 dB shift - you are done

• if not, then follow protocol for AC testing

-This approach compensates for up to 20 dB of increased signal level due to any occlusion effect

• no need to add in an occlusion effect

-If NR- re-establish threshold

• to 50 dB here

-Then increase masker level by the amount of threshold shift (25 dB)

-If stable you are done!

-If NR re-establish threshold

The masking dilemma

-in some cases, our minimum/starting masker level may exceed our maximum masker level leading to a “masking dilemma”

-typically only a concern in cases with a significant bilateral conductive component

• when conductive component exceeds IA for the individual

• be concerned whenever you have a bilateral ABG>35 dB

Recording your masking results

-if masking is used, only mark masked thresholds

-Some audiograms/clinics may want you to record your final masker level

• later cross check to ensure masking was appropriate

-Masking dilemma:

• only record unmasked symbols

• report it on the audiogram e.g. “thresholds may be poorer than shown. unable to mask effectively”

• some rx to report inability to plateau completely

White noise

-Equal energy across a wide frequency range

• 250-5000 Hz

-Freq response is limited by the transducer

-effective but not efficient, masker

narrow band noise

-band passed filtered noise

• 1/3rd to ½ octave wide

-geometrically centered at audiometric freq

-effective and more efficient than white noise

• equal masking but at lower levels

Masking noise and the critical bandwidth

-Only masker energy which falls within a “critical band” (CB) of freq will contribute to masking

• energy outside the CB will add to loudness but not to masking

-audiometers NBN’s are a bit wider than the CB

audiometer speech masking noise

-flat freq response from 100-1000 Hz, then falls at 12 dB/octave

-More effective than a white noise or NBN for masking speech

• SRT and/or word recognition

costs of masking

-time consuming

• lengthy instructions/re-instructions

• additional equipment setup

• additional threshold search measures

-Potential effects on patients

• fatiguing

• possible temporary threshold shifts (TTS) of NTE

-Possible contamination of threshold

• confusing to patient

-so only mask when needed!

• stop masking if concerns it may contaminate results or decrease likelihood of accurate results

Hearing via Bone conduction

-Recall sound occurs when a vibratory pattern is transmitted through a medium

• bone is just another medium

-Detecting an air conducted signal via BC (eg. through inserts or sound field) requires very high levels

• 40-60 dB+ above BC thresholds

-We are most sensitive to a vibrating source when it has a more direct connection to our skull (e.g. a bone oscillator)

Utility of bone conduction

-BC thresholds can provide evidence for site of lesion and can aid in differential diagnosis

BC caveats

-(Superior) semicircular canal dehiscence (SCD/(S)SCD)

• can cause an apparent conductive pathology.

• May artificially “enhance” BC thresholds

• >1 month post-op BC thresholds worsen but AC thresholds remain stable.

-HF conductive loss could point to collapsing canal or a partial ossicular discontinuity

Conductive pathology and transducer type with BC

-With open PE tubes, inserts can “artificially” elevate AC thresholds, compared to supra-aural phones

• Creating false/exaggerated ABGs

• More common at LF’s (250-500 Hz) and when EAC volume is large (esp. in kids)

-Transducer-related differences in EAC volume affects SPL of the signal! Which affects thresholds with each transducer

• Large changes in volume with and without PE tubes when using inserts, but not supra-aural phones

BC hearing technology

-Rehabilitation:

• Baha, Ponto, Bonebridge

• Soundbite

-Listening enhancement

• headphones

-Employment

• Atlantic signal dominator

• Molar mic

  • military

  • police/fire

Mechanisms of Bone conduction

-All mechanisms start with vibration of the skull in response to BC stimulus presentation

• Pattern of vibration varies with frequency

-This activates the cochlea via several mechanisms

• Inner ear mechanism

• middle ear mechanism

• outer ear mechanism

BC Inner ear mechanism I - Distortion

-Skull vibrations cause compression of the cochlear walls resulting in BM displacement

• Largest effect for freq of >4000 Hz

BC Inner ear mechanism II - Inertia

-Vibrational inertia will set cochlear fluids in motion

• IF a “release” mechanism is present

-Possible mechanisms:

• compliance difference between oval and round windows

• A “third” window - secondary fluid paths between the cochlear and cranial fluids (cerebrospinal fluid)

  • Vestibular and cochlear aqueducts, nerve fibers, veins, and micro-channels entering the cochlea

-Very important for BC hearing <1kHz

BC middle ear mechanism - Ossicular inertia

-Creates stapes motion (similar to AC) at higher freq. due to

• Mass loading TM can improve BC thresholds (increases inertia)

-Largest effect in mid freq. (1-3kHz)

• But effect is small—i.e., removing ossicles doesn’t worsen BC thresholds much

-Only occurs for mastoid placement

BC outer ear mechanism

-Vibrations of the cartilaginous portion of EAC create sound waves in the EAC

• These can be transmitted to the cochlea via AC route

-Larger effects for mastoid (vs. forehead) bone oscillator placement

-Largest effect when EAC is occluded (occlusion effect)

• and for low freq.

Occlusion effect

-The increase in SPL at the TM with EAC closed (vs. open) when presenting BC stimuli

-Can cause a “false” improvement in BC thresholds

-Largest effect in LFs

• <1000 Hz

  • up to 20 dB at 250 Hz

-Perceived only by NH or SNHL (or a small ABG)

• Absent with large ABG

Bone conductions testing

-Always assume both cochlea are stimulated

• responses reflect hearing sensitivity from best cochlea

• must rule out non-test ear (NTE) participation

  • masking noise to NTE limits detection of signal

-Threshold testing process: same as AC

• ASHA- Test 1,2,3,4 - retest 1, 500, and 250

• In general, we do not cover the test ear (we do cover the NTE)

  • So ambient noise must not exceed MPNLs- ears not covered

Factors affecting BC thresholds

-Oscillator placement

• needs to be secure and have firm bone support

  • instruct patient to tell you if equipment moves

• not touching pinna

• clear hair, hair bands, glasses

-Adjust oscillator placement if needed to maximize loudness for audible tone in region of good hearing

-RETFL calibration “error” at 4000 Hz

• 5-15 B “false” air bone gap

-Vibrotactile responses

• VT response refers to feeling, rather than hearing, a BC stimulus and can lead to false ABGs

  • A concern whenever measuring low frequency (<1000) BC thresholds

    • suspect VT response whenever LF BC thresholds are >20-35 dB HL

    • False ABG at 250 Hz may reflect VT response

-Occlusion effect: increase in EAC SPL during BC testing when either ear is covered

• OE can be estimated by measuring threshold with and without the NTE covered

• OE varies with freq and across individuals and with transducer

Mastoid placement

-less force needed for threshold resulting in a greater “dynamic range” of testing

• a big deal in some cases

-Some interaural attenuation, which can help with masking

Forehead placement

-more stable, less variability and better test-retest reliability

-better estimate of “pure cochlear” function

RETFLs

-Reference equivalent threshold force levels

-RETFLs vary with frequency and oscillator placement

-at threshold, need ~40 dB less force at high (4 kHz) vs low (250 Hz) frequencies

• lets us present at higher levels (dB HL) in HFs

-8-14 dB more force needed at threshold for forehead placement

• separate calibration factors required

-Maximum audiometer outputs (type 1) varies with frequency

occlusions effect with transducers

-OE with shallow inserts is similar to supraurals

-OE with deep inserts is a greater difference. Lot less than supraural

-OE magnitude is also reduced with circumaural, compared to supraural headphones

Recommended OE values for when masking at VBMC clinics

-250 Hz —→ 15 dB OE

-500 Hz —→ 15 dB OE

-1000 Hz —→ 10 dB OE

-2000 Hz+ —→ 0 dB OE

Tuning fork tests

-still used by physicians

• differentiates NH, conductive and (some) SNHL losses

-Most useful for unilateral or very asymmetric “pure” SNHL or conductive losses

• not as useful for symmetric losses or mixed losses

-Common tuning fork tests

• weber

• rinne

• bing (lesser extent)

• some of these can also be done using an audiometer

Weber test

-Is a unilateral or asymmetric loss, conductive or sensorineural?

• use tuning fork or audiometer - 512 (500) Hz

-Tuning fork/oscillator placed midline/forehead

-Ask “where do you hear the tone?”

Where do normal/ symmetric SNHL patients hear tone in Weber test?

hears it midline

Where do conductive loss patients hear tone in Weber test?

Hears it in “poorer” ear (or ear with larger ABG)

Where do SNHL patients hear tone in Weber test?

Hears it in “better ear”

Why does the tone lateralize to a giver ear in Weber test? - SNHL

-SNHL: “Stenger” effect causes sound to be hear only in the better hearing ear

• Stenger test is a common test for identifying false/exaggerated HL

  • AKA- pseudohypacusis or non-organic hearing loss

Why does the tone lateralize to a giver ear in Weber test? - conductive loss

-Reason varies with pathology

• Occlusion effect- an occluded ear canal (e.g. impacted cerumen) causes the tone to be louder in the poorer ear

  • causing a “stenger effect”

• lower resonant frequency of middle ear system - “mass load” system (e.g. otitis media with effusion) so that ear is more sensitive to the LF BC signal

  • making it louder and causing a “stenger effect”

• Phase change- fixing, or disrupting the ossicles may alter phase relation between ossicles and temporal bone such that sound arrives “sooner” to the poorer ear

  • We lateralize towards the “earlier” sound

Rinne test

-Is a loss conductive or sensorineural?

• tuning fork moved from mastoid to pinna (~1”)

• Ask “is the tone louder/longer via AC or BC?”

  • use tuning fork 256, 512, 1024 Hz

Positive Rinne

-sounds louder listening via AC

• NH or SNHL

• false positive Rinne-

  • mild conductive losses

Negative Rinne

-Sounds louder listening via BC

• conductive loss

• False negative- asymmetric SNHL. better ear may detect tone

Bing test

-Is a loss conductive or sensorineural?

• “Is the tone louder with the EAC open or closed?”

-Tuning fork/oscillator (512 (500) Hz) on mastoid

• alternate EAC open/closed

-looking for presence/absence of an occlusion effect (OE)

Positive bing

-Louder when closed

• OE is present - NH or SNHL

negative bing

-No change closed

• No OE present- conductive loss

What is “normal” hearing?

-Normal depends, among other things, on:

• How you do your testing to determine “normal”

  • i.e. your protocol

• Who you test to determine “normal”

  • screened or unscreened sample?

  • Screening using self report or records review to identify those with risk factors for hearing loss

Carefully controlled laboratory testing

-very precise, but a slow process. So generally small N’s (# of subjects)

-Careful subject screening/selection, instructions and practice

-Threshold often measured multiple times

• subject selection criteria may be bias threshold estimates

Mass/survey testing

-Test a lot of people (large Ns) in a short time

-To do this you generally have:

• A less controlled setting (e.g. state fair; clinic)

• Less instruction/practice prior to testing

• Threshold only measured once (or twice)

• a less strict screening/selection process.

Screened population

-Screened excludes (based on self report) anyone with risk factors for hearing loss such as

• occupational or leisure noise exposure

• previous ear infections

• head injury

• known use of ototoxic medications

• known hearing loss

• self report of perceived hearing difficulties

“normal” hearing thresholds

-Today, normal hearing is defined by the RETSPL values (AKA audiometric zero) - the “reference” thresholds (in dB SPL) for normal hearing

• RETSPL: Reference Equivalent Threshold in dB SPL

-Where do RETSPL values come from?

• recall thresholds (the “T” is RETSPL) depend on

  • how you do the testing (carefully controlled vs mass survey)

  • And who you test (screened vs unscreened)

RETSPL

“Reference equivalent threshold sound pressure level”

-The mean/modal SPL at threshold (at a specified frequency, measured in a specified coupler), based on data from a large number of “otologically normal” adults (18-25/30 years old)

-Our American standard RETSPLs now mirror European International Standards Organization (ISO) standard RETSPLs

• ISO 389-1 for supra-aural earphones

• ISO 389-2 for insert earphones

Problem with thresholds in dB SPL

Large array of “normal” thresholds because “normal” values vary with the transducer/coupler combination

What measurement unit do we use on audiograms?

dB HL (dB hearing level)

dB hearing level (dB HL)

-The difference between the hearing threshold of an individual (in dB HL) and the RETSPL, also in dB SPL

• i.e. individual threshold minus the mean/modal threshold of a large group of “otologically normal hearing” young adults

-0 dB HL is the softest sound that can elicit a response in a series of ascending trials at least 50% of the time (i.e. threshold) in “otologically normal” individuals

• This is true regardless of the transducer or frequency

important points to remember about dB HL and SPL

-0 dB HL does not equal 0 dB SPL

-In dB SPL “normal” varies with frequency and transducer

• we’re most sensitive in the 1-4 kHz range

-In dB HL “normal” is constant across freq and transducer

• 0dB

How to calculate dB HL

-Start with individuals “equivalent threshold in dB SPL” (ETSPL)

-subtract RETSPL from this value

-Difference is threshold in dB HL

Sensation level in dB (dB SPL)

-The level (in dB) above threshold for that sound

• SL can be used to describe levels for simple sounds like pure tones

Hearing threshold definition

-Hearing threshold of an individual is “the lowest decibel hearing level (dB HL) at which responses occur in at least one half of a series of ascending trials.”

• minimum of two, or three, responses at a single level

The audiogram

-Many different kinds but all have:

• freq on the x-axis (octave scale)

• Hearing level in dB on the y-axis (dB HL)

• Spacing: 20 dB ~1 octave

-Additional space for

• speech audiometry

• acoustic immittance

-y-axis should also report source for “reference threshold”

Assessing the auditory system

-Behavioral pure tone thresholds

• a primary component of the audiologic test battery

• important for many purposes

-For diagnostic (site of lesion) purposes we measure and compare two thresholds obtained via the

• air conduction pathway and bone conduction pathway

-Dysfunction at any point along these pathways results in HL

Air conduction pathway

-Air conduction (AC)

• outer ear —> middle ear —> inner ear and beyond

-Elevated AC thresholds mean dysfunction somewhere along the pathway

• but you dont know where!

-Bone conduction testing can help