Using the Real Ear equipment, the audiologist places a probe microphone into the ear
canal and presents a known auditory signal to the patient. The information from the
microphone when the stimulus is present yields a Real Ear unaided response (REUR).
This response reveals the resonating characteristics of the ear canal without the
aid in place and can assist in formulating the best 2-cm3 coupler response for a
patient at the time a hearing aid is ordered.
A variety of prescriptive techniques for fitting hearing aids use information
from Real Ear measures. These techniques include the half-gain rule and the
prescription of gain and output (POGO).
One of the most popular prescriptive techniques is the procedure developed
by the National Acoustics Laboratory (NAL) in Australia for selecting gain and
frequency response of a hearing aid. The NAL algorithm is used to calculate
the most appropriate Real Ear gain.
From the algorithm, a hearing aid is selected with the required frequency
response and gain characteristics, and comparisons are made between the
predicted gain and the Real Ear measurements obtained from the hearing-impaired client.
This can be obtained by measuring the real-ear aided response (REAR). The
REAR is taken with the hearing aid and the probe microphone in the ear, and
the aid's gain is turned to match the calculated Real Ear gain. The REAR is
the gain in decibels relative to the stimulus level presented to the patient.
The real-ear insertion gain (REIG) is the difference between the REAR and the
REUR and is used to verify that the predetermined target insertion gain has
been achieved.
Within the past 10 years, hearing aids using digital signal processing (DSP)
have been introduced into the market. These aids, when compared to standard
analog hearing aids, allow for a more precise control over a broader range of
parameters. In 2005, more than 90% of all hearing aids dispensed in the
United States were digital.
Some analog hearing aids can be digitally programmed; the digital programmer
can adjust the gain, frequency response, and output of the analog circuit.
Some analog hearing aids also may have multiple channels (frequency bands)
that can be digitally programmed.
The difference between a DSP hearing instrument and an analog aid is that
the analog signals from the microphone are converted into a digital form by
an analog-to-digital converter. Once in the digital form, the signals are
manipulated by sophisticated processing algorithms and then converted back
to analog form by digital-to-analog conversion.
The digitally controlled hearing aids usually employ an external programming
unit that the dispenser uses to adjust the gain, output, and frequency response
of the unit. Many of these aids have multiple channels that allow the dispenser
to program individual gain, output, and compression for each frequency channel.
Most of the digital hearing aids and some of the digitally programmed analog
hearing aids use a common computer platform database called NOAH. This database
can carry the audiometric information and office-based information on each patient.
Software from each manufacturer can be installed on the platform. The aids are
connected to a common interface called HI-PRO that allows the software from the
manufacturer to interface with the hearing aid. The fitting paradigms
vary with each manufacturer.
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