Добавил:
Sekretar
kiopkiopkiop18@yandex.ru
t.me/Prokururor I Вовсе не секретарь, но почту проверяю
Опубликованный материал нарушает ваши авторские права? Сообщите нам.
Вуз:
Предмет:
Файл:Ординатура / Хирургия / Библиотека им академика М.И. Перельмана / Книга_4612_Библиотеки_им_академика_М_И_Перельмана
.pdf
Audiology Review: Preparing for the Praxis and Comprehensive Examinations
https://t.me/medicina_free
422
TABLE 9–1. Differences Between Input Compression (AGC-I) and Output
Compression (AGC-O)
VARIABLE AGC-O AGC-I
AGC occurrence in
relation to amplification
Subject of compression Output signal Input signal
Role of volume control Volume control can alter
After volume control Before volume control
Change in volume control
knee point
will not change knee point
audibility for low-level consonant sounds present after higher-intensity vowel signals, but this may
result in temporal envelope distortion of the speech signal. Slow ATs and RTs will not provide as
much audibility to low-level sounds following compression (reduced gain) but results in less temporal
envelope distortion. Temporal envelope cues are important for listeners with poor frequency selectivity associated with increasing degrees of hearing loss. The choice of fast- or slow-acting compression
should be made based on the individual patient. For more information including the advantages and
disadvantages of compression speed, please review Moore (2008).
Frequency-specific manipulation of gain is made possible due to an increased number of compression channels and frequency bands.
n
Compression channels are frequency regions that can be manipulated independently of each
other across the frequency range. This allows for varying levels of compression characteristics
at different frequencies.
n
Frequency bands are regions where gain can be independently adjusted. An adjustment to one
frequency band impacts gain for all levels of input within that frequency band.
n
The ability to manipulate multiple compression channels and/or frequency bands enables the
audiologist to program HAs for a wide array of audiometric configurations.
n
The technology of multichannel compression allows the HA to more accurately respond to
acoustic input comprising one frequency region, without impacting the entire frequency
range. However, research also demonstrates that a higher number of compression channels
may lead to distortion of temporal and spectral cues (Souza, 2002), and as such, care should
be taken when adjusting compression parameters.
n
Modern HAs also feature improved noise management because of multichannel compression.
An individual with hearing loss typically experiences reduced hearing sensitivity to soft sounds
(due to OHC dysfunction and loss of the active mechanism of the cochlea). Meanwhile, the impact
on audibility of moderate to loud sounds is more limited. The audiologist must place a vast number of
environmental sounds that vary in intensity into the reduced dynamic range of the patient.
n
Wide dynamic range compression (WDRC) enables modern HAs to be programmed in such
a way that the user perceives soft sounds as soft yet audible, moderate sounds as comfortable,
and loud sounds as loud but comfortable by adjusting the amount of gain for each input level
(see Figure 9–1). See Table 9–2 for more information on WDRC. While WDRC is often the
preferred choice for HA programming, potential drawbacks include the following:
CHAPTER 9 Treatment: Topics in Amplification
https://t.me/medicina_free
423
Background noise may be amplified in situations where the background noise is softer than
the speech signal because of increased gain for lower-level inputs.
The gain may not be adequate for someone with a severe-to-profound hearing loss who
requires more gain for higher input level signals, sometimes near maximum power output
(MPO).
Patients may be bothered by soft sounds (e.g., refrigerator running, clocks ticking) that they
are not used to hearing due to increased gain for low-level inputs (which may be reduced by
expansion, which will be described).
In utilizing WDRC to make soft sounds audible, potentially bothersome, low-level sounds may
also become noticeable. This includes machine noise from household appliances or even internal noises
from the HA (referred to as equivalent input noise or EIN). Expansion is a strategy for low-level sound
reduction that helps to ensure perceived quiet when the patient is in a quiet environment.
n
Expansion provides different amounts of gain depending on the incoming signal, similar to
compression.
n
Unlike compression, expansion acts on inputs with intensity levels below the expansion knee
point and
applied). This can be visualized in Figure 9–1.
n
While the reduced amplification of low-level sounds may increase comfort for some, it also has
the potential to decrease audibility of low-level speech sounds when the expansion threshold is
set at a high intensity (above approximately 40 dB SPL). Therefore, it is especially important
to consider the impact of expansion on HA programming for pediatric patients who require
audibility of soft speech sounds for speech and language development.
n
Another potential benefit of expansion is reducing EIN (commonly referred to as circuit
noise) when listeners possess normal low-frequency hearing thresholds (typically below
10–15dB HL).
TABLE 9–2. Wide Dynamic Range Compression
CHARACTERISTIC WDRC REASONING
Compression Type Multichannel Provides hearing aid programming
flexibility needed for a variety of
hearing loss configurations
Compression Knee Point Low, typically 50 dB SPL
or lower
Input or Output Control AGC-I Better to achieve goal of audibility
Attack Time and Release
Time
Compression Ratio 4:1 or less Ratios greater than 4:1 may cause
AT: <20 ms
RT: 10–100 ms (short),
>500 ms (long)
Makes soft sounds audible
HA users with lower cognitive status
may be negatively affected by fast
release times, dependent on speech
stimulus (Cox & Xu, 2010)
excessive distortion of the signal
Audiology Review: Preparing for the Praxis and Comprehensive Examinations
https://t.me/medicina_free
424
AUDIOLOGY NUGGET
The advent of power DSP HAs (circa 2003–2004) presented an opportunity to
introduce listeners with severe-to-profound sensorineural hearing loss (SNHL)
to digital HAs. Anecdotally (according to the authors of the text, JD and KF,
who are of a certain age and remember fitting these devices on patients), many
of these patients reported low-level sounds to be too intense but conversational
speech as unclear, mumbled, or garbled. These complaints were contrary to
what was expected of this high-quality DSP aid, and most patients preferred the
analog processing of their previous HAs.
So, why were these complaints likely occurring? If WDRC processing was
implemented, the complaints were likely due to the fact that having a low TK
increased gain for low-level input, to which they were not accustomed, and
compression (gain reduction) of signals at conversation speech intensity (65–75
dB SPL), thus creating the situation where low-level sounds were too intense
and conversational speech was being altered in a negative way through unwanted
reductions in gain.
Noise Reduction
Digital noise reduction (DNR) is an additional tool available in modern HAs. Noise in this case is
defined as undesirable background sounds and may include mechanical noise, multitalker babble, wind
noise, and other common environmental sounds.
n
The goal of DNR is to limit the amplification of noise compared to the speech signal, that is,
improve the signal-to-noise ratio (SNR). DNR is accomplished primarily through gain reduction
at affected bands but may also implement adaptive beam forming via directional microphones.
n
The HA utilizes an environmental classification algorithm that continuously assesses temporal,
spectral, and intensity of sound input. The HA will then determine if DNR is needed, how
much is needed, and how fast to implement changes.
n
DNR does not improve speech intelligibility for HA users, but it may improve comfort of
listening in noise and reduce listening effort.
DNR in pediatric HA programming may limit chances for incidental language learning and is not
always recommended but depending on the age of the child may be warranted. Several studies with
school-aged children found that DNR did not impact word recognition and improved novel word
learning in children ages 11 to 12 years, and therefore these technologies may be appropriate for older
children who can effectively manage their use. Special consideration should be made for children with
a greater degree of hearing loss. More severe hearing loss may result in poorer audibility in background
noise and different outcomes with DNR (McCreery et al., 2012). For more information on DNR,
please review Bentler and Chiou (2006).
Directional Microphones
Microphones can be directional or omnidirectional and have specific applications for various listening
environments.
CHAPTER 9 Treatment: Topics in Amplification
https://t.me/medicina_free
n
A directional microphone is more sensitive to sounds arriving to the front of the listener and
less sensitive to sounds arriving to the sides and back.
n
An omnidirectional microphone does not include directional sensitivity and is equally sensitive
to sounds arriving from all directions.
n
Figure 9–2 includes the polar plots of various microphone sensitivity options. The polar plot is
a 360° representation of microphone sensitivity to sounds arriving from different locations.
n
Directionality is quantified using the Directivity Index (DI), which is a single-number
indicator of directional performance (higher values providing more directionality). Typical
first-order directional microphones (two microphone systems) provide for a DI value between
approximately 3 and 4, with second-order microphones (three or more microphones) resulting
in increased directionality. For a more detailed review of directional microphones, please
review foundational information from Ricketts (2005) as well as a recent review on novel
directional microphone technology by Zhang (2020).
n
HA manufacturers are consistently updating the functionality of directional microphone
systems and techniques used to improve speech recognition in noisy listening conditions,
making it important to continually update one’s knowledge of these systems.
425
Acoustic Feedback
Feedback reduction aims to suppress the signal components of HA output that escape from the receiver
and are picked up by the microphone and reamplified (acoustic feedback loop). Two common methods
of feedback reduction include adaptive gain reduction and feedback path cancellation (often referred
to as phase cancellation).
Cardioid Hypercardioid
0
330
300
270
0
-20
-40
240
210
30
60
90
120
150
270
Bidirectional Supercardioid
0
330
300
240
0
-10
-20
-30
-40
30
60
90
120
210
FIGURE 9–2. Polar plots for hearing aid microphone configurations. Microphone directionality as seen
from a 360° graphic representation. Source: From Essentials of Modern Hearing Aids: Selection, Fitting, and
Verification (pp. 1–888) by Ricketts, T. A., Bentler, R., & Mueller, H. G. Copyright © 2019 Plural Publishing,
Inc. All rights reserved.
180
150
180
Audiology Review: Preparing for the Praxis and Comprehensive Examinations
https://t.me/medicina_free
426
n
Through adaptive gain reduction, the HA works to automatically reduce gain at a specific
channel when feedback is detected.
n
Feedback path cancellation (phase cancellation) creates an internal signal that is opposite in
phase, thus reducing/eliminating the feedback. This method does not require significant gain
reduction.
n
Additional methods to prevent feedback include creating a well-fitting earmold, modifying
venting parameters, and ensuring proper placement of the receiver or earmold in the ear canal.
n
Limiting gain is typically considered a last resort for dealing with acoustic feedback.
Frequency Lowering
Although the use of modern HA features discussed above allows the audiologist to adequately fit a
range of hearing loss configurations with expanded frequency ranges, high-frequency amplification
continues to be variable above approximately 6000 Hz. For patients with high-frequency hearing loss,
this often means certain high-frequency consonant sounds (e.g., /s/, /sh/, /th/) are left underamplified and inaudible. Frequency-lowering (FL) technology is implemented to deal with the limitations
of high-frequency amplification for certain and more severe degrees of hearing loss. Four major FL
techniques are utilized across different manufacturers, including linear frequency compression, linear
frequency transposition, nonlinear frequency compression (NLFC), and spectral envelope warping
(Alexander, 2013).
n
Linear frequency compression uses a spectral balance detector to analyze the frequency
components of incoming signals. If the energy of the signal is greater in the frequency region
above 2500 Hz, compared to the region below 2500 Hz, frequency lowering will activate. If
not, frequency lowering will not be implemented.
n
Linear frequency transposition identifies a spectral peak in a designated frequency region.
Aband of input around the spectral peak is resynthesized into the target frequency region.
This process is continuous when activated, but the spectral content of the input will determine
what signals are impacted.
n
NLFC uses a start frequency that may be manipulated by the HA programmer, which
typically ranges between 1500 Hz and 6000 Hz. When activated, this FL method will
continuously impact input frequencies above the starting frequency but not those below the
starting frequency.
n
Spectral envelope warping occurs as the HA looks for characteristics of high-frequency
information that may be indicative of speech. This information is then inserted into a lowerfrequency input while preserving harmonic structure and high-frequency information of the
original signal.
For detailed information on these techniques, including illustrated depictions, please see Alexander
(2013). As these techniques and how manufacturers implement them may evolve over time, the authors
recommend reviewing specific (and current) details provided by each manufacturer.
When activating FL in HA programming, it is important to verify that the high-frequency sounds
are both audible and acoustically distinct from one another. For example, consider the fricative speech
sounds /s/ and /sh/. FL may place those sounds into an audible range for the patient, but they may
not be distinguishable from one another and should be verified. Figures 9–3, 9–4, and 9–5 provide
techniques by which FL is commonly verified.
CHAPTER 9 Treatment: Topics in Amplification
https://t.me/medicina_free
FIGURE 9–3. Difference in HA acoustic output for /s/ and /sh/ with frequency lowering off vs. on.
427
n
Figure 9–3 shows the acoustic output produced by a HA for the /sh/ and /s/ sound with
frequency lowering off and on. Note the expected shift in peak energy to a lower frequency for
both speech sounds.
n
Figure 9–4 shows Speechmapping results for a steeply sloping audiogram with frequency
lowering off and on. With frequency lowering off (the full bandwidth option), the highfrequency targets (4000 Hz and above) are not met. This is often the case with steeply sloping
audiograms and highlights the difficulties amplifying significant high-frequency hearing loss
FIGURE 9–4. Speechmapping results with frequency lowering off
vs. on.
Audiology Review: Preparing for the Praxis and Comprehensive Examinations
https://t.me/medicina_free
428
AUDIOLOGY NUGGET
Frequency lowering is especially important to the pediatric population. Infants
and young children are learning speech and language and require access to all
speech sounds (Stelmachowicz et al., 2004). HAs can be limited in their ability
to provide access to soft high-frequency sounds (e.g., /s/, /z/, /sh/) through
traditional gain. The use of FL technology in pediatric patients has proven to
be beneficial to high-frequency speech detection and recognition (Glista et
al., 2009). FL should be considered in HA fittings when the hearing loss and
capabilities of HA output prevent the HA from meeting prescriptive targets in
frequency regions that encompass high-frequency speech sounds. Note, however,
that adults who have not previously experienced FL may have complaints about
the sound quality of the aids: Sounds may be “slushy” or “mechanical” to them.
with traditional techniques. With FL on, the shift in peak energy and reduction of highfrequency output is seen. With programming adjustments, it may have been possible to better
approximate the high-frequency targets in this case, but this example is used to illustrate
concepts related to FL and its applications. Additionally, in the FL on condition, gain
adjustments to better approximate targets at frequencies 1 to 4 kHz are needed.
FIGURE 9–5. Example of Speechmapping verification of distinction between /s/ and /sh/
with frequency lowering on.
/sh/
/s/
CHAPTER 9 Treatment: Topics in Amplification
https://t.me/medicina_free
n
Another means of verifying FL is to evaluate whether the processed /s/ and /sh/ sounds are
429
acoustically distinct, meaning the /s/ sound produces a HA output that is distinctly different
from the /sh/ sound. This is shown in Figure 9–5, where the two sounds (both recorded
with FL on) produce a distinctive acoustic output for the listener. This technique must be
interpreted with the caveat that while the acoustic output of the HA produces two distinct
tracings, it should not be assumed that this difference is perceptible to the listener and should
be verified with behavioral discrimination.
Connectivity
Many HAs are equipped with short-range communication abilities. Depending on the manufacturer,
this may connect via near-field magnetic induction (NFMI), 2.4 GHz (commonly referred to as Bluetooth technology), or 900 MHz communication. This allows for communication between a pair of
HAs and communication to manufacturer-proprietary accessories.
n
Ear-to-ear communication between HAs allows for optimized signal processing such as
speech-in-noise and advanced directionality features.
NFMI is often used for this type of communication due to the short-range capabilities and
relatively low battery drain.
n
Accessory devices such as basic remote microphones, television audio streaming devices, and
remote controls also use short-range communication.
2.4 to 2.48 GHz or 900 MHz are often used for these functions due to the longer-range
capabilities compared to NFMI.
An additional feature within HAs is the telecoil. The telecoil (T-coil) is a small copper wire within
the HA casing that acts as a receiver. T-coils can be found in landline telephones or looped systems
around venues. With the advent of Bluetooth compatibility, T-coil use has declined in the United
States. Venues such as public theaters, community meeting rooms, and places of worship may be
equipped with a looped system that patients may utilize.
n
The advent of Bluetooth compatibility for HAs allows users to wirelessly stream audio to their
HAs from connected devices such as smartphones and tablets. This can include phone calls,
music, video audio, and more. The specific Bluetooth capabilities of the device are dependent
on the type of Bluetooth technology incorporated into the HA and varies by manufacturer.
Additional uses for Bluetooth include:
Wireless programming
Identifying last known location of a lost HA
Mobile app use for easily accessible program and/or volume adjustment
In addition to traditional HAs, remote microphone hearing assistive technology (RM-HAT) is
commonly used. These include frequency modulation (FM) systems and digital modulation (DM) systems.
n
Both FM and DM systems can be utilized to improve the signal-to-noise ratio (SNR) with the
primary differences being the transmission method.
n
RM-HAT systems consist of a remote microphone placed near the desired speech signal. That
signal is transmitted to the users’ HAs via a receiver, an ear-level nonamplifying device, or a
speaker system.
Audiology Review: Preparing for the Praxis and Comprehensive Examinations
https://t.me/medicina_free
430
n
The benefits of RM-HAT are pervasive in the literature in support of using ear-level RM-HAT
systems in children with hearing loss to improve SNR in the classroom (Anderson &
Goldstein, 2004; Anderson et al., 2005).
n
RM-HAT systems have also proven beneficial in normal-hearing patients with a central
auditory processing disorder (Schafer et al., 2020).
n
The recent advent of integrated RM-HAT receivers into HAs without an additional attached
receiver component has increased ease and accessibility of system use.
AUDIOLOGY NUGGET
Children require more favorable SNRs compared to adults because children are
less able to utilize contextual cues and are still developing language and auditory
processing skills. The ANSI Standards for classroom acoustics includes a SNR of
at least +15 dB at the child’s ears. Despite this, typical classrooms are regularly
impacted by high reverberation times and background noise sources such heating
and cooling systems. Rabelo et al. (2014) found average classroom SNRs at 54
to 74 dBA. As such, implementing RM-HAT systems can positively impact the
SNR for children with hearing loss.
Additional Features
HA programming software allows the audiologist to create multiple programs for the patient to
manually adjust situational HA function. Although modern HAs are able to make real-time changes in
response to the environment, there are certain situations where the patient may benefit from manually
switching to different predetermined settings. Examples of programs include:
n
Noise program: a HA patient may consistently experience increased difficulty hearing in noise
when entering a particular space. For example, a college student may have difficulty hearing
friends when in the cafeteria. A noise program may be created by increasing directionality of
the microphones and increasing noise reduction.
n
Music program: for patients who enjoy listening to live music or playing an instrument, a
music program is often recommended. Programming changes to this may include disabling
features such as noise reduction, feedback management, and frequency lowering.
n
Tinnitus program: patients with tinnitus may utilize customizable tinnitus maskers and
tinnitus management activities within manufacturer apps aimed to distract the HA user from
their tinnitus.
Datalogging is a feature in modern HAs that provides the audiologist with a numerical representation of HA wear time between programming sessions.
n
Typically, this value is the number of hours worn divided by the number of days since the
HA was last connected to the manufacturer software. This is provided as an average daily
wear time.
n
Programming software may also provide insight into how often the patient spends in each of
their HA programs and how often they spend in various types of listening environments (e.g.,
quiet, noise, speech-in-noise).
CHAPTER 9 Treatment: Topics in Amplification
https://t.me/medicina_free
n
HA wear time can be a useful indicator of how well the patient is complying with HA use.
n
Datalogging is especially useful when working with pediatric patients. If a pediatric patient is
seen for a HA check and datalogging reveals an average daily wear time of 0.5 hours per day,
additional counseling and goal setting with the patient and their family is warranted.
Q & A
Question: What is the current status of over-the-counter (OTC) amplification
devices and when is an OTC appropriate to recommend to a patient?
431
Answer:
A relatively new factor to consider when assessing HA candidacy and
counseling patients on audiologic intervention is OTC hearing devices. In an
effort to increase accessibility of amplification to Americans with hearing loss, the
United States Food and Drug Administration (FDA) released a final ruling on
OTC HAs effective October 17, 2022. The ruling states that OTC HAs will be
available for adults ages 18 years or older with a mild-to-moderate hearing loss.
The devices may come ready to wear out of the box or may utilize technology
for consumer self-fitting. Additionally, consumers interested in OTC HAs are
not required to obtain an audiologic evaluation or prescription for the devices,
nor will they be required to pursue any follow-up care from a licensed hearing
healthcare professional. At this time, the exact features that will be available in
OTC devices are not known.
Hearing Aid Types and Styles
HAs are available in a variety of styles offering a range of comfort, cosmetic appeal, power, and performance. Appendix 9–A displays a list of traditional HA styles and their characteristics.
n
One additional HA style is the invisible in-the-canal (IIC) HA, which is smaller than a CIC. The
HA manufacturer Phonak has created an IIC known as the Lyric™. This device is inserted into
the external auditory canal by professionals who have received specific training with the device.
The Lyric remains in the ear canal at all times and is not removed until the battery needs to
be replaced. Phonak states that the Lyric™ can last a maximum of 120 days before it needs
replacing.
Other manufacturers create IICs that are removable by the patient.
Other HA options exist beyond the traditional HA types listed above. This includes the contralateral routing of signals (CROS) and bilateral contralateral routing of signals (BICROS).
n
CROS, named for its function of contralateral routing of signal, is an amplification option
recommended for patients with unilateral hearing loss when the poorer-hearing ear cannot
benefit from a traditional HA due to the severity of the hearing loss and/or poor word
recognition scores and the better-hearing ear has normal hearing.
A patient with a CROS wears an ear-level device on each ear (a transmitter on the poorer
ear and receiver on the normal-hearing ear). Recently, these are receiver-in-the-canal (RIC)
devices with an open fitting.
Соседние файлы в папке Библиотека им академика М.И. Перельмана