defining & stimulating residual hearing ability

DEFINING & STIMULATING RESIDUAL HEARING ABILITY

The limiting historical concept of hearing instrument fitting has been that all we really needed to do was to replace the intensity at frequencies damaged or destroyed by pathology in each ear.


Clearly, if that was all it took to return a person to “normal” hearing, we would have been making perfect fittings for the past 40 years.

Today, though it is not that simple, it is possible!


Many fitting formulae have been theorized and researched in an attempt to “correct” hearing loss as defined audiometrically in decibels relative to hearing level (dB HL).


Most hearing instrument fitting formulae modify the audiometric results into an “appropriate correction” for the patient/client.

This concept of “appropriate correction” as a HI fitting “target” should be considered as a starting point--not a prescriptive requirement.


Rather than applying a conceptual target for “appropriate correction”, what we really need to do is define and then fully stimulate the patient’s residual auditory function.

The correct approach is to maximize the patient’s residual hearing!


We must identify the parameters of the patient/client’s residual auditory area.

Specifically, pure tone thresholds and loudness discomfort levels (LDLs) at discreet frequencies.

By defining the residual auditory area parameters, the hearing instrument will provide the most benefit to the patient/client.


Figure 1 depicts the auditory area of a normal hearing person. The shadowed area represents average conversational speech.

The range between just audible and uncomfortably loud is called the auditory area or the dynamic range of hearing.


Figure #1


As seen in Figure 1, speech is audible and comfortable across a wide frequency range for a normal hearing person.

Because of the large range of hearing available to them, normal hearing people can experience all the dynamics of speech; that is, its peaks and valleys and soft, average and loud levels.


Most prescriptive methods utilize insertion gain targets.

The gain suggested by a prescriptive fitting method only gets you to audibility.

Prescriptive gain targets do not always allow patient/clients to enjoy the full benefits of their residual auditory area and the dynamics of conversational speech.


Hearing instrument fitting methods must be described, so that we may achieve hearing instrument electroacoustic performance which may be adapted to the psychoacoustic parameters of each patient/client’s residual auditory ability/capacity.

Fitting methods should include not only audiometric pure tone thresholds but LDLs by frequency, as well.


After all, our goal is to ensure audibility of important sounds, especially speech, while limiting loud sounds to below the patient’s UCL.

Today’s digital electroacoustic technology enables us to do just that!


Figure #2


Figure 2, is a display of unaided results as a function of frequency.

The dotted line at the lower part of the figure represents normal hearing defined in dB SPL measured near the eardrum as a function of frequency (in Hz).

The pink circles joined by a line represent the individual’s hearing threshold levels, in dB SPL, as a function of frequency.


In figure 2, the asterisks represent average UCL values for this individual’s hearing threshold levels.

The green line represents average unamplified conversational speech generally associated with average conversational speech (representing peaks and valleys of speech). Speech sounds associated with average unamplified conversational speech lying BELOW the pink circles, will not be audible.


Fitting wide dynamic range compression (WDRC) instruments to meet gain targets helps patients hear better.

Most current fitting formulae generate targets based on frequency-specific insertion gain.

These frequency specific data are easily obtained utilizing probe microphone real-ear measurements.


Figure #3


In figure 3, aided test results are displayed as a function of frequency.

The circles joined by lines indicate hearing threshold levels (eardrum dB SPL) as a function of frequency (in Hz).

The asterisks indicate UCL.


The orange line at the top of the graph running close to, but not exceeding the asterisks, is the probe microphone measured real-ear maximum output of the hearing instrument fitted for the individual (the real-ear saturation response [RESR]—usually measured at 85dB input).


The bars show the dynamics of an amplified average conversational speech signal relative to the targets for aided average conversational speech.

If no compression were occurring for a speech-like signal, then the bars would extend about 30dB (similar to the dynamic range of speech displayed within Figure 1).


A patient/client’s residual auditory area is defined as the difference between their air conduction thresholds and their loudness discomfort levels as measured at each octave and half-octave frequencies from 250 to 6k Hz.


The goal for fitting amplification is to adjust the parameters available on the hearing instrument to amplify soft, average and loud speech within the reduced auditory area across the broadest relevant frequency range while ensuring that loud sounds do not exceed the patient/client’s LDLs.


Loudness discomfort levels should be especially evaluated at 1k, 2k, 3k and 4k Hz, because most of today’s amplification systems possess their peak output responses at those frequencies.

Also, many hearing loss pathologies exhibit significant recruitment at those very same frequencies.


Figure #4


In figure 4, the pink circles joined by a line represent the individual’s threshold levels (dB SPL eardrum) as a function of frequency.

The asterisks represent average UCL values for this individual’s hearing threshold levels.

The blue shaded region outlines the individual’s residual auditory area.


The goal for fitting amplification to this individual would be to adjust the parameters available on the hearing instrument to fit amplified soft, average and loud speech within the reduced auditory area across the broadest relevant frequency range; and to ensure that loud sounds do not exceed the recommended loudness discomfort levels.


In 1988, Dr. Skinner provided a table of estimates for most comfortable loudness (MCL) and UCL data that were derived from air conduction threshold results by frequency. This table was obtained from the study by Kamm, Dirks and Mickey (1978).

Further research found this estimated “normative data” to have a disparity among individuals by as much as seventeen decibels!


Despite the slight variability in frequency specific LDL measurement results, today’s hearing instrument technology demands that each patient/client’s LDLs be known for each ear.

These results define the residual auditory area, and are most crucial to comfortable and successful auditory stimulation.


Today’s digital hearing instrument technology provides a variety of adjustable response bands and compression ratios so that our custom fitting efforts are no longer compromised with reduced gain or excessive output.


The major and most exciting electroacoustic difference between analog and digital hearing instrument technology is that the hearing instrument gain of digital instruments is no longer directly correlated to its output.

Old custom electroacoustic fitting methods often compromised the required gain relative to the output for patient comfort.


We will learn more tomorrow regarding: 1)residual hearing ability, 2)patient/client counseling and 3)patient/client realistic expectations for hearing instrument use.

defining & stimulating residual hearing ability

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