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Calibration Considerations
The information provided in this section is an overview of issues related to the calibration of audiological equipment. For a comprehensive review of topics related to calibration, please refer to Champlin
and Letowski (2014) for details on air conduction, Margolis and Popelka (2014) for details on bone
conduction, and Frost and Levitt (2014) for information on calibrating the speech signal. A significant
portion of the information below is found in the abovementioned articles.
Calibration Equipment
Calibration procedures are accomplished using acoustic couplers to mimic the human ear and head.
These procedures are designed with specific shapes and volumes to allow for sound measurements using
a calibrated microphone. A primary function is to provide a standardized and reproducible recording
mechanism that mimics the impedance of the auditory system to the best degree possible. Couplers
vary in their volume depending on the transducer and its location relative to the tympanic membrane.
n
6-cc couplers (e.g., IEC 318; NBS 9–A) are used with supra-aural headphones to calibrate
frequencies from 125 to 8000 Hz. The 6-cc volume simulates the average volume of the adult
ear canal with a supra-aural headphone place on the ear. A 500-gram weight is used to mimic
the pressure induced by the supra-aural headband. To calibrate frequencies above 8000 Hz, an
adapter plate is used with circumaural headphones such as the HDA 200 or HDA 300.
n
2-cc couplers (e.g., IEC 126; IEC 711; Zwislocki occluded ear simulator) are commonly used
to calibrate insert earphones due to the reduced volume of air in the ear canal associated with
placement of the insert earphones (versus the 6-cc associated with supra-aural headphones).
HA-1 and HA-2 couplers commonly used for electroacoustic analysis of hearing aids are
variants of the previously noted couplers.
n
Artificial mastoid couplers (e.g., Bruel and Kjaer 4390; Larson Davis AMC493B) are used
to calibrate bone-conduction oscillators (Margolis & Popelka, 2014). These devices convert
mechanical energy from the oscillator to an electrical signal that is recorded by the sound-level
meter. An oscillator is coupled to the artificial mastoid with 5.4 newtons (± .5 newtons) as
specified in ANSI standard S3.6 (2010).
Sound-level meters (SLMs) are used to measure the output level of earphones, inserts, and loudspeakers. These devices are calibrated using a pistonphone set to a fixed frequency and fixed level of
either 94 or 114 dB SPL. Pressure microphones are used for calibrating earphones while random
incidence microphones are used for calibrating soundfield speakers.
n
Type 1 SLMs provide the most precise measurements and are required for exhaustive
electroacoustic calibration. SLMs can be set to “fast” or “slow” recording times. Fast recording
times analyze the incoming signal every 0.125 ms (or eight times per second). Slow weighting
is recommended for calibration purposes.
n
Various weightings are available depending on the purpose of the measurements. Figure3–18
provides example dBA, dBB, and dBC weightings. dBA weightings are used to mimic human
auditory sensitivity by frequency at low intensities where low and high frequencies are
attenuated (inverse of the MAF curve). This scale is commonly used in industrial settings for
hearing conservation purposes.
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FIGURE 3–18. Sound-level meter weightings.
123
n
SLMs also have filter characteristics that allow for the precise analysis of frequency and
reduction in ambient noise from frequencies neighboring the test frequency. These filters are
typically one octave or one-third octave wide. One-third octave filters are more common in
sophisticated (and more expensive) SLMs.
KNOWLEDGE CHECKPOINT
SLMs have numerous options from which to choose when analyzing sound levels.
One option relates to how the equipment weights the incoming frequencies
contained within a signal. The dBA weighting is commonly used in industrial
audiology when evaluating noise levels and exposure for employees because the
response mimics the human auditory system and provides the best estimate of
what the employee is experiencing. The dBC weighting should be used when a
more equal weighting of frequencies is desired. Additional details on sound-level
recording and its associated equipment are discussed in Chapter 8.
n
Multimeters are used to measure the linearity of the attenuator or the change in output level of
the audiometer with changes in the HL dial. These devices measure three electrical properties,
including resistance, current, and voltage, with voltage being of interest for determining
output level.
n
Frequency counters are used to evaluate the integrity of the tone generator by connecting
directly to the audiometer.
n
Oscilloscopes are used to visualize the waveform (output over time) of tonal signals.
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AUDIOLOGY NUGGET
Extensive calibration of audiological equipment is required to be conducted
annually, but it is good practice to do daily calibration checks of the audiometer.
These include cycling through the different frequencies for each transducer type,
listening to the changes in intensities presented, listening for intermittency in
the signal, and checking for any distortion in the signal. These daily checks alert
the audiologist to any potential problems with equipment prior to testing their
first patient that may skew the test findings. Individuals with normal hearing are
those who routinely run the biologic checks. Records of these checks may prove
useful if the validity of the results obtained from a clinic are questioned in legal
proceedings.
General Calibration Characteristics
Calibrating audiological equipment is a technical process requiring attention to detail. The following
information provides some general topic areas involved in the calibration of hearing testing equipment.
n
Output levels are measured for transducers, including earphones, inserts, bone oscillators,
and soundfield speakers, using the appropriate coupler and a SLM. Calibration of these
transducers is designed to match the normal pattern of human auditory sensitivity by
frequency. Reference equivalent threshold sound pressure levels (RETSPLs) and reference
equivalent threshold force levels (RETFLs) are used to calibrate air-conduction and boneconduction signals, respectively, and represent the signal levels required to reach absolute
thresholds of hearing among otologically normal listeners (0 dB HL or audiometric zero).
Over the years, RETFLs were remeasured and converted to RETSPLs (Margolis &
Popelka, 2014). For a comprehensive review of RETSPLs for air- and bone-conduction
transducers, please review Table 2 in both Champlin and Letowski (2014) and Margolis
and Popelka (2014). Note specific RETSPL values for mastoid vs. forehead placement in
the latter.
RETSPLs for signals presented via soundfield speakers require a reference point at least
1meter away from the speaker at which all signals are measured. The signals can include
FM tones (commonly referred to as warble tones) or one-third octave band noise. The angle
of incidence also plays a role in RETSPL values, with specific values contained in Table4
of Champlin and Letowski (2014). Specific angles specified are 0°, 45°, and 90°. Note
the lowest RETSPL levels between 3 and 4 kHz for all angles due to the natural resonant
properties and sensitivity to frequency of the human auditory system that coincides to the
lowest point on the MAF curve.
n
Attenuator linearity refers to the predictable (and equal) change in audiometer output
with changes in dB HL (a decrease from 70 to 65 dB HL should result in a commensurate
reduction in output on the multimeter). This should be checked across output levels to ensure
proper function at low, mid, and high output levels. A change of 5 dB requires the output to
be with ±1.5 dB of this decibel change.
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n
Frequency integrity should be assessed using a frequency counter to ensure proper function of
the pure-tone generator. Frequency values must be within ±1% of the test frequency (e.g., if
1000 Hz is the intended frequency, the output must be within 990 and 1010 Hz).
n
Harmonic distortion (often referred to as total harmonic distortion or THD) is defined as
distortion occurring at integer multiples (harmonics) of a test frequency (e.g., distortion at
2, 3, and 4 kHz with a 1 kHz test frequency). This is measured using a SLM set to one-third
octave filters and tested at the highest audiometer level setting. THD of greater than 2.5% is
considered out of calibration.
n
Signal presentation via the interrupter switch is evaluated in three ways, including the on-off
ratio, crosstalk, and rise-fall time.
On-off ratio refers to the output level of the signal with the tone on (interrupter button
pressed) and the tone off (interrupter button not pressed). This ratio is considered out of
compliance if the value is less than 70 dB.
Crosstalk refers to the situation where signal information from one channel is present and
can be heard in the other channel. The output is measured in one earphone (test) and
compared to the output of the other earphone (nontest). Significant crosstalk is present if
the difference between these two measurements is less than 70 dB.
125
Rise-fall time refers to the time it takes for the signal to go from off to on (rise time)
and from on to off (fall time). To measure this, the signal is monitored using a digital
oscilloscope to monitor the rise and fall time. Rise-fall times generally cannot be less than
20 ms or greater than 200 ms for pure-tone signals.
n
Masking noises commonly used in audiological testing include speech noise, narrowband
noise, and sometimes pink or white noise. For narrowband noise maskers, the frequency range
(band limits in Hz) and band reference levels (levels in dB) are specified and evaluated using a
spectrum analyzer. For broadband maskers (white noise), the frequency distribution from 250
to 5000 Hz must be within 5 dB of the level at 1000 Hz.
n
Ambient noise levels may have a dramatic effect on the validity of audiological testing. ANSI
S1.1-1999 (R2003) specifies maximum permissible ambient noise levels (MPANLs) for
audiometric test suites. These values are found in Table 3–4 and are specified for ears covered
(earphones or inserts) and ears uncovered (soundfield speakers). As can be seen, lower ambient
levels are required for ears uncovered due to a lack of passive attenuation provided when using
a transducer on or in the ear. These levels have been determined to produce negligible masking
of equal to or less than 2 dB when testing tonal signals at 0 dB HL (or RETSPL values)
specified by ANSI S3.6 (2010).
Immittance Equipment
A tympanometer (sometimes referred to as an immittance bridge) is a piece of equipment used for the
measurement of acoustic impedance within the human external ear canal and is helpful in the evaluation, identification, documentation, and diagnosis of external and middle ear disorders. Examples
of clinically available tympanometers include the GSI Tympstar Pro™, the Interacoustics Titan™,
and the Maico TouchTymp™. Many tympanometers analyze middle ear function using tests such as
tympanometry, acoustic/middle ear muscle reflex threshold, acoustic reflex decay, and Eustachian tube
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TABLE 3–4. ANSI Maximum Permissible Ambient Levels for Audiometric Test
Rooms (Comparison of Ears Covered and Ears Uncovered)
OCTAVE BAND CENTER
FREQUENCY (Hz)
125 39 35
250 25 21
500 21 16
1000 26 13
2000 34 14
4000 37 11
8000 37 14
Source: American National Standards Institute. (2003). Maximum permissible ambient noise
levels for audiometric test rooms
MAX dB SPL WITH
EARS COVERED
(Rev. ed.) (ANSI S3.1-1999).
MAX dB SPL WITH EAR
UNCOVERED
function testing. A probe tone frequency of 226 Hz is regularly used for patients older than 6 months
of age (due to a stiffness-dominated middle ear system), but 678 Hz and 1000 Hz are also available
stimulus frequencies (which evaluates for mass-related conditions). Wideband tympanometry is an
available measurement that uses a wideband click as a stimulus. Additional details on tympanometry
are provided in Chapters 5 and 6 of this text.
n
A test cavity is typically included to verify the calibration of the tympanometer and is
recommended to be part of a daily calibration routine. A biological calibration check is
also recommended for tympanometry and reflex tests by performing a daily check on an
otologically normal ear. Extensive calibration is recommended under ASNI S3.39-1987
(R2020) Standard Specifications for Instruments to Measure Aural Acoustic Impedance and
Admittance (Aural Acoustic Immittance).
Hearing Aid Verification Equipment
A hearing instrument analyzer is used in the process of fitting and verifying the electroacoustic performance of hearing instruments connected to a standard acoustic coupler (test box measures) or while
worn on the ear by the HA user (on-ear or probe microphone measures). Examples of this manufactured equipment are called Audioscan™ Verifit 1 and 2, Natus Aurical™ Hearing Aid Fitting System,
MedRX™ Avant Arc, and Rem + Real Ear Measures and Live Speech Mapping Systems.
n
On-ear measures provide information regarding HA function while the HA is being used by
an individual. Examples of these measures include the real ear unaided response (REUR), real
ear unaided gain (REUG), real ear aided response (REAR), real ear insertion gain (REIG), real
ear saturations response (RESR), and real ear to coupler difference (RECD), all of which are
commonly used.
REUR — SPL, as a function of frequency, at a specified measurement point in the ear canal
for a specified auditory signal with the unoccluded ear canal
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REUG — difference, in dB as a function of frequency, between the SPL at a specified
measurement point in the ear canal and the SPL at a field reference point, for a specified
auditory signal with the ear unoccluded; amount of natural gain provided by the resonance
properties of the external auditory system
REAR — SPL at a specified measurement point in the ear canal for a specified sound field,
with the HA in place and turned on
REIG — difference between aided and unaided ear canal SPL; amount of gain provided by
an amplification device found using the formula: REAR – REUR.
RESR — value obtained using a narrowband signal at a high level to saturate the HA
(usually 85–90 dB SPL)
RECD — difference between the SPL in an occluded ear and the SPL in the HA-1 (2 cc
coupler) produced by the same sound source with the same acoustic coupling
Most systems come with the equipment needed to perform daily/weekly calibration checks
for both on-ear and test box measures. For instance, the Verifit 2 has specific ANSI couplers
(blue) to be used for test box calibration.
n
Test box measurements provide information about HA performance such as measures of gain,
distortion, input/output, compression, directionality, and digital noise reduction measured in
a coupler (not in situ or in the ear).
127
n
Speechmap fitting environment is also commonly used as it provides various stimuli, including
speech and noise, for fitting HAs and testing their performance. These stimuli are available
in the test box or on-ear test environment. Verification software can also calculate a Speech
Intelligibility Index (SII), valued in percentages, using unaided or aided thresholds to estimate
the audibility of speech. Please review Amlani et al. (2002) for a comprehensive review of the
SII and its uses. Comparing unaided and aided SII results and meeting prescriptive targets at
various input levels is commonly used to counsel patients on HA performance.
Figure 3–19 provides an example of a probe-microphone (on-ear) analysis. The figure is
labeled and notes important information such as UCLs, prescriptive targets, SII values (for
55, 65, and 80 dB input), and audiometric (threshold) information. To the top right are the
characteristics of the fitting such as HA style, coupling (occluding in this case), real ear to
coupler difference (RECD), binaural or monaural fitting, and patient age, among others. It
is important to enter this information as each parameter influences prescriptive targets for
the fitting.
In interpreting Figure 3–19, one should note a few items. For most frequencies, prescriptive
targets are not being met (except for approximately 2 kHz), which is the reason for lower
obtained SII values (39 for 55 dB SPL input and 43 for 65 dB SPL input). This translates
to 39% of the speech signal at 55 dB SPL being audible to the listener and 43% audibility
at 65 dB SPL (average conversational intensity level). These values can be used to estimate
speech recognition performance of various stimulus types (e.g., digits, words, sentences) by
referring to page 55 of the Verifit 2 manual (which is freely available at https://docs
.audioscan.com/userguides/vf2manual.pdf)
●
Rounding to an SII of 45 (from 43), it can be estimated from the data that 100% of
digits, 94% of IEEE sentences, and 63% of NU-6 words would be correctly identified at
65 dB SPL. As demonstrated in this example, SII values alone do not provide a percent
correct value for recognizing the speech signal but rather a measure of audibility that is
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UCLs
Targets
Thresholds
FIGURE 3–19. Audioscan Verifit2 printout.
SII values
then used to estimate speech recognition performance. It is important to note that these
speech recognition values are estimates and are influenced by a host of factors, including
degree of hearing loss, auditory processing capabilities, environmental acoustics (noise
and reverberation), and cognitive abilities of the listener.
The readers are encouraged to review the Audioscan Verifit 2 manual (as one example)
for detailed information regarding testing protocols, technical details related to available
signals, and interpretation of findings.
n
On-ear calibration compares the probe microphone response with the flat calibrated response
of the on-ear reference microphone. It then compensates all probe-tube measurements for
the difference noted. This is done by positioning the probe tube in front of the reference
microphone and holding the probe dock 15 to 90 cm (depending upon the equipment) away
from and directly in front of the speaker. The microphone to be calibrated faces the speaker.
It is recommended to be performed daily or weekly.
n
Ambient-level check measures can also be done to measure the ambient noise level in the
room where the real ear measurement is performed. Similarly, the test box also establishes
a correction curve for an uncalibrated test box reference microphone. This is done by
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positioning the reference microphones about 1 to 2 mm from the coupler microphone. It
is recommended to be performed daily or weekly. ANSI standards exist for many of the
specific tests.
Signals Used in Verification
n
Narrowband signals
Sinusoidal (pure tones): used in ANSI HA tests, frequency response tests, insertion gain
tests (e.g., MPO stimulus contains a series of 128-ms tone bursts with 128-ms gaps at
an SPL of 90 dB SPL in the test box and 85 dB in the sound field, uses 1/12 octave
frequencies or 1/3 octave frequencies for MPO)
n
Broadband signals
Pink noise signal: pseudo-random signal composed of 1,024 simultaneous tones summed
to provide a crest factor of 12 dB; spectrum is controlled by the reference microphone
with a digital feedback loop. Spectrum of pink noise decreases at approximately 3 dB per
octave.
Real-speech signals: in Speechmap for on-ear and test box measurements, using four speech
passages, the International Speech Test Signal (ISTS), and the single-talker International
Collegium of Rehabilitative Audiology (ICRA) noise
●
Examples: ISTS consists of 500-ms segments from recording of six female talkers
reading the same passage in English, Arabic, Chinese, French, German, and Spanish.
Segments are spliced together to maintain appropriate pauses and filtered to be
representative of the long-term average spectrum reported in Byrne et al. (1994). ICRA
noise is a recording of an English-speaking talker that has been digitally modified
to make the speech largely unintelligible while preserving the temporal and spectral
properties of the signal.
129
For additional information, please review the Audioscan Verifit 2 manual (https://docs.audioscan
.com/userguides/vf2manual.pdf) for one verification tool example currently utilized in clinical practice.
Verification equipment from other manufacturers is available; however, the authors are most familiar
with the Audioscan Verfit and its mention in this chapter is not an endorsement of the product.
Recommended Readings
Akeroyd, M. A. (2006). The psychoacoustics of binau-
ral hearing. International Journal of Audiology, 45,
S25–S33.
Lentz, J. L. (2020). Psychoacoustics: Perception of normal
and impaired hearing with audiology applications.
Plural Publishing.
Moore, B. C. J. (2022). Listening to music through
hearing aids: Potential lessons for cochlear implants.
Trends in Hearing, 26, 1–13.
Musiek, F. E., & Baran, J. A. (2020). Cochlear physi-
ology I: Mostly mechanics. In The auditory system
anatomy, physiology, and clinical correlates (2nd ed.,
pp. 117–132). Plural Publishing.
Oxenham, A., & Bacon, S. (2003). Cochlear compres-
sion: Perceptual measures and implications for normal and impaired hearing. Ear & Hearing, 24(5),
352–366.
Audiology Review: Preparing for the Praxis and Comprehensive Examinations
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Practice Questions
1. Otoscopy outcomes for pediatric patients are displayed in the tables using “effusion” or “no
effusion” criteria to identify possible otitis media with effusion. With the population of 200,
what is the calculated specificity of otoscopy for detecting “no effusion” in cases of suspected
otitis media with effusion?
Predicted
No
4%
a.
b. 22%
c. 78%
Effusion
Effusion 96 22
Actual
No
Effusion
4 78
Effusion
131
d. 22%
Explanation: Remember that specificity is how false alarms are related to true negatives. In most
matrices, this is completed by using the calculation d/(c + d) where box d is the true negative value
(here, 78) and box c is the false-positive value (here, 22). 78/(78 + 22) = 78/100 = 0.78 or 78%.
Therefore, the answer is c, 78%.
2. From the visual below, what is the phase difference between the two tonal signals?
a. 90°
b. 180°
c. 270°
d. 360°
Explanation: In the figure, when one signal is at its maximum positive displacement, the other is at
its maximum negative displacement. This represents a phase difference of 180°. Therefore, the correct
answer is b.
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