THE SCIENCE BEHIND A HEARING TEST
Although at first glance the concept of a hearing test might appear
simple, there exists a considerable volume of science behind the
process. An Audiologist is trained specifically to perform and
interpret the results of your diagnostic hearing assessment.
The Threshold of Hearing
Threshold is the softest sound that can be heard by an individual at the frequency (pitch) being measured.
The loudness of the sound at a particular frequency is represented
on a logarthmic scale. This is because the range of intensity from the
very softest to the uncomfortably loud is too vast to fit on a standard
graph. Hence, we record loundness in decibels (dB).
The ear requires much more sound pressure level (SPL) for a sound to
be just detected in the bass region compared with a mid pitch tone. As
a consequence scientists have developed a “corrected” hearing threshold
level (HTL) measurement scale. This plotts the threshold of hearing for
the average normal ear at “zero” at all frequencies for ease of
reference.
It is recognised that threshold is not a black or white phenomena.
Shades of grey exist. Research has shown that the way we measure
threshould can influence accuracy. As a consequence a particular
procedure is used that takes the average of responses near threshold to
be used as the clients hearing threshold level.
Pure Tones and the Audiogram
Sound in our everday lives is a complex amalgum of various
frequencies. For example, a simple vowel sound actually comprises
frequencies across a broad spectrum. However, to understand how the
cochlear is performing, we must ensure that we do so with “pure tones”
that contain only one frequency.
The hearing test, then requires the repetition of threshold
measurement with pure tones of various frequencies to construct the
hearing chart known as the audiogram.
The audiogram is thus a chart of threshold levels, measured in decibels, against frequency.
Headphones and Bone Conductors
In order to accurately measure hearing threshold the headphones must
be calibrated against an international standard and the test performed
in acoustical conditions the preclude the influence of extraneous noise
upon the test.
A specialised device, known as a bone conductor, that is also
calibrated against international standards is used, so thresholds
recorded with this transducer can be compared with those obtained via
the headphones. Discrepancies between the two are indicative of the
nature of the impairment and point to the anatomical location of the
problem.
It is therefore impossible to accurately identify the degree of a
hearing impairment or its nature over the telephone or internet thus
having the potential to significantly misinform the consumer. The
disclaimer should therefore be read with great care!
Speech Discrimination
Restoring a patients ability to understand speech is the primary
focus of an Audiologists rehabilitive efforts. The audiogram provides
considerable information about the degree and nature of the hearing
impairment but further detail is required to understand how an
individual with a given loss might perform in real life. A test of
speech discrimination is therefore insightful in understanding the
degree of distortion that is taking place in the hearing pathway. This
in turn can inform your Audiologist as to the most appropriate
rehabilitation strategy to undertake.
A speech discrimination test takes the form of word lists balanced
for their relative frequency of appearance in the english language.
This selection process thus makes each list of words approximately
equal in their level of difficulty to understand. The Audiologist then
administers the word list under headphones at varying intensities to
assess how well the ear can process complex sound and correlates this
with the findings of the pure tone test.
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