Advanced Verification Notes

Advanced Verification - Week 6

Learning Outcomes

Explain how different signal types can influence hearing aid output.
Justify why we use the ISTS (International Speech Test Signal).
Explain the pros and cons of visible speech mapping.
Briefly touch on how visible speech mapping can be used for verifying frequency lowering (though frequency lowering itself will be discussed in detail later).
- Frequency lowering redirects high-frequency information to healthier regions of the cochlea when cochlear dead regions exist.
Discuss how verification differs with open-fit hearing aids and in cases of unusual ear canal conditions (mastoid cavity, chronic perforation affecting natural ear canal resonance).

Quick Revision - Real Ear Insertion Gain (REIG)

Gain: The difference between input and output.
- Zero gain: input = output.
- Below zero: attenuation (less output than input).
$Real\ Ear\ Insertion\ Gain = Real\ Ear\ Aided\ Gain - Real\ Ear\ Unaided\ Gain$
- Real ear unaided gain: natural ear canal resonance measured without anything in the ear.

Response vs. Gain Views

Response View: Displays output in dB (e.g., 60, 70, 80 dB).
Gain View: Displays gain (difference between input and output).
REIG protocol is typically used in the clinic, but aided response can also be selected to change the axes to output in dB.
REIG incorporates ear canal resonance.

Why Verify Hearing Aids?

To ensure appropriate and safe amplification, effective for amplifying speech signals.
Australia excels in using insertion gain for verification due to Hearing Services Program requirements.
Verification against published insertion gain targets (NAL-NL1/NL2/NL3, DSL, NAL-RP) is crucial.
Coupler measures are also used globally, involving pre-programming hearing aids in a coupler.
Coupler measures will be covered in a practical workshop next semester.

Coupler Setup

Affinity Compacts have a speaker/sound-absorbing space and a microphone inside a 2CC coupler.
Adapters are available for BTE, RIC, and ITE hearing aids.
A reference microphone sits near the hearing aid microphones to reference sound output.
The lid contains the speaker and reference microphone.
Sockets exist for battery testing (battery current drain) and induction coil testing (telecoil response), although battery drain is typically handled by manufacturers now.

Verification Methods: Probe Tubes, Couplers, Aided Thresholds

*Aided Thresholds:
* Can be useful to know how well the hearing aid will work for soft inputs.
* Occluding the ear and then testing with amplification
* Not a precise indicator for general day-to-day communication.
* Helpful only for assessing soft inputs due to hearing aid compression characteristics.

Aided thresholds are necessary for certain certifications (aviation licenses, commercial driver's licenses) and are used with Victoria Police.
Cochlear implant candidates: Real ear measurements are preferred to ensure proper hearing aid setup before considering implantation.
Aided audiograms are sometimes used in pediatric testing to confirm sufficient aided hearing levels for soft speech components.
Aided cortical tests are performed to ensure cortical response to amplified signals.

Why Use the International Speech Test Signal (ISTS)?

Essential to have a standardized signal which represents what the hearing aid is doing in normal listening situations.
Hearing aids are designed to maximize speech intelligibility by manipulating the input signal.
Early verification methods utilized swept wobble tones, pink noise, or white noise, which do not accurately represent speech fluctuations.

Signal Types

Consider the appearance, energy distribution, and appropriateness of different sounds for hearing aid verification.

Narrowband Signal (Swept Warble Tone)

Sweep Tone: Sweeping from low frequency up to high frequency (25 Hz - 8000 Hz). It covers the frequencies most important for speech, from the fundamental frequency of a male voice (125 Hz) to the upper harmonics of a child's voice(8000 Hz).

Broadband Noise (Pink Noise)

Delivers a range of frequencies from low to high.
Spectrogram shows energy bands present in different frequencies over time.
Swept noise starts low and sweeps up to 6,000 Hz.

Swept Pure Tone

In a $60\ dB\ SPL$ swept pure tone, the output at any given point is $60\ dB\ SPL$ .
The waveform is sinusoidal.
A line spectrum shows only one frequency present.
The spectral envelope shows the same frequency at output as it sweeps through.
Used for couplers measurement.

Coupler Measures with Swept Pure Tones

Input level: $60\ dB$ . Frequency range: $560\ Hz$ to above $5,000\ Hz$ .
Continuous swept pure tone recorded at every twelfth of an octave.
Frequency response shows the hearing aid's output to a $60\ dB$ input.

Broadband Signals

Have energy over a wide range of frequencies.
White noise has equal energy in each frequency band.
Pink noise has more low-frequency energy than high-frequency energy.
Pressure Spectrum Level: The energy present in a one hertz-wide band.

Pressure Spectrum Level

If white noise is measured at $100\ dB\ SPL$ overall, and bandpass-filtered between $10,000\ Hz$ , we need to determine the energy in a one-hertz wideband.
$L_{PS} = dB(broadband\ signal) - 10\log(bandwidth)$
Where:
- $L_{PS}$ is the pressure spectrum level.
- Bandwidth is the width of the band (e.g., $10,000\ Hz$ ).
If the overall $dB$ is $100$ and bandwidth is $10,000\ Hz$ :
$L_{PS} = 100 - 10\log(10,000)$
$L_{PS} = 100 - 10\log(10^4)$
Apply log rules to simplify:
$L_{PS} = 100 - 10 * 4 * \log(10)$
$L_{PS} = 100 - 40 * 1$
$L_{PS} = 60\ dB$
For a flat spectrum of white noise, every “fence paling” (one hertz wideband) has the same amount of paint (energy) on it.
$L_{PS} = 60\ dB$

Narrowband Signal at 1,000 Hz

Hotspot centered around 1,000 Hz.
Waveform is somewhat random with harmonics.
Frequency response shows a broad peak with skirts of the filter.
Energy is more concentrated into a narrower range, causing less adjacent activation.

Pure Tone

Theoretically, energy is only at 1,000 Hz (though short durations have spectral spread).
Appropriate for measuring the maximum output level of hearing aids.

OSPL 90 and MOSPL

May differ depending on whether run in a coupler or real ear.

Signals Recommendation

Pure tones are appropriate to use when specific maximum level of hearing aids need to be measured because it can measure the maximum power output of the hearing aid.
Broadband signals are more appropriate for general hearing aid characteristics.
Hence, pink noise or white noise was used for verifying old hearing aids.

Manufacturer Specifications

Hearing aids come with manufacturer specs, including maximum output in a coupler (ear simulator or 2CC coupler).
Variations exist for low-powered vs. medium-power receivers.
Frequency responses show how the hearing aid should perform in a coupler.
Coupler measures can be compared to published specifications to identify faulty hearing aids.

Pure Tone Limitations

Not recommended for real ear measurement due to not being speech-like.
Standing waves cause inconsistency.
Interacts with noise reduction features, potentially classifying tones as feedback or music.
Compression characteristics need consideration (compression thresholds and ratios).
Swept pure tone vs. broadband: Swept pure tones in couplers can cause “blooming,” where low-frequency amplification is exaggerated due to isolation from high frequencies.

Broadband Information

More relevant to environmental sounds, including speech.
Broadband noise can also be problematic due to hearing aid processing features and noise reduction algorithms.
Hearing aids process sounds differently based on the presence of speech.

Speech-Shaped Test Signal

Modulated broadband signal that mimics speech.
Has temporal fluctuations and higher low-frequency levels.
The ideal signal for verifying nonlinear hearing aids in a realistic manner.
Examples: ISTS, ICRA signals.
- ISTS is used primarily because it is very well validated and very well adopted around the world.
- The ISTS signal has modulation, harmonics and fundamental frequency.

ISTS (International Speech Test Signal)

*Has modulation, harmonics, and the fundamental frequency of speech.
*Shaped according to the long-term average speech spectrum (female voice is weighted between male and child).
*Internationally recognized for hearing aid verification.

Speech shaped signal, it has the speech modulation, it's got harmonics, it's got the fundamental frequency that comes through.
Created by combining recordings of the North Wind and the Sun passage in multiple languages (Spanish, Arabic, German, English, Mandarin, French).
Very dynamic signal with gaps, pauses, and formant transitions.
Periods of silence are critical for allowing hearing aid compressors to activate and release.

Signal Intensity

Soft: ~ 50-55dB; Ensure awareness of Background noise.
Normal Speech: ~65dB.
Loud:~75-80dB.

Other Considerations for Signals

Signal Duration
Manufacturer curves: Families of Gain and Output

Verifying Open-Fit Hearing Aids

Open-fit devices can cause gain leakage, interfering with the reference microphone.
Reference microphone monitors noise level to maintain 65 dB SPL.
Reference mics are that far apart so they're quite close. * Leakage causes the reference mic to reduce input, altering compression characteristics and potentially creating a feedback loop.
Solution: Perform open-fitting verification with the reference microphone switched off using the substitution method or modified pressure method.

Methods

Substitution method: Calibrate probe microphone with the reference mic off. Requires the client to remain still.
Modified pressure method (concurrent equalization): Keep reference microphone on.
Modified pressure method (stored equalization): calibrate with the client in the room, then switch the microphone off to store the output. Recommended for open-fit.

Affinity Setup

Calibrate for open fit in Affinity to mitigate the impact of the hearing aid on ear canal acoustics.
Use a modified pressure method with stored equalization.
Perform REUG with hearing aid muted, calibrate for open fit, then unmute and proceed with REIG.
Supervise patient carefully to evade movements or noises.

Research and Clinical Practice

Only calibrate for open fit if hearing aids are providing more than 20 dB of gain.
High-frequency gain with an open-fit device may demand calibration for open fit; if amplified signal leaking back in the room, switch to pressure method.

Visible Speech Mapping (VSM):

VSM is just another hearing aid fitting technique as the REIG
Utilizes Speech Signals (ISTS).
Output in dB SPL.
Provides live data of sound through the hearing aids inside the canal.
Is speech mappign same as Real Ear Insertion Gain.
Verifies hearing aids with same rigor as VSM if using ISTS.

Details

*It merges real ear verification and counseling into one Screen.
*There are speech mapping targets using NL NL $2$ , our DSL or we can verify the hearing aid.
*You can use Speech Mapping to verify the hearing aid.

Visible Speech Mapping Setup (Affinity)

Select audiogram and enter UCLs.
Calibrate probe tube.
Select VSM protocol in the REM module.
Measure probe tube length (5 mm beyond hearing aid).
Use otoscope to confirm proximity to eardrum.
Insert hearing aid.
Perform visual speech map using ISTS.
Adjust the hearing aid to meet targets.

Live Voice for Visible Speech Mapping

Live voice cannot be used for verification(HCP), but is useful for validation.
Get partner of clint to be the stimulus in Clinic.
Configure addiotnal program to shap and booost partener of cilent voice.

What can Live Speech Mapping do?

Check directional Microphones so they cna determine where the source sounds.
Directional Mics can cut out those partner voices.
Speech mapping done with them palying their instrument in the room.

Steps for Setting Up

Standard Steps.
Rainbow Passage calibrated and found in clinic.
If not the calbirated one, any calibrated passage helps.

Benefits

Uses speech signals, which is critical for live speech.
Real-time visual representation of the audibility during the fitting.
Takes into Account client dynamic range.
Aids will use hearing aid Features, compression and feedback, noise reduction.
Can make a Very Useful counseling Tool, family Participation and reduce post fitting appt.

Disadvantages

Takes time.
More espensive but it get client use the equipment.
Scancity based evidence based research.

The Phonak Patient View

Phonak Patient view can show client what there hearng aid. What thier hearing aid with their channels. With real time gain.
Shows there are particular speech.

Surgically Altered and Atypical Ears

What an Atypical ear can cause.

What happeneds to average ear Canal Resonance. That effects what the hearing aids are done to them need to be modified so there incorporating those changes.

Surgical/Atypical Ears (REUG)

*We know that most people have a peak around $2,3$$ k hertz that indicates the ear canal resonance canal.
*The greater space (a mastoid Cavity) it going to resonate lower frequencies.

Mastoid Cavity

You’ll see angular self, The Facial Ridge.
*Resonant at 1500 instead of 1500 hertz so its larger which means Lower Freq.
*Real ear insertion gain needs to be there to then turn things and turn other stuffs down..
*When u measure aided gain you needs to have that extra gain.
*Use aided Response instead of the REIG.
Use average REUG, rather than measure do measured (measured is what we bamg bout).
Measured is what to use, unless not a typical you can use predicted.

Aided Targets for Surgically Atypical Ears

*Aided targets are response for where have different axes.
*If y ou had uer REUG than ur seeing the difference from aided or unaided.

In summary:

*When verifying your fits, Make sure the hearing aids match with SPL
*Ececeise to the cause , do appropiate signal.

Use prober for speech, use breadth band.
*The high pitched tends to drop a lil it out so dont stop.
*Consider the REFF MIC dont take messurments out on the STREET.
*Aied gain or adied response s probably the best to do.