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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 logarithmic 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. Therefore, during hearing testing we record loudness in decibels (dB).
The ear requires much more sound pressure level (SPL) for a sound to be detected in the bass region, compared with a mid-pitch tone. As a result, scientists have developed a corrected hearing threshold level (HTL) measurement scale. This plots 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 threshold can influence accuracy. As a result, a particular procedure is used during hearing screening 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 everyday lives is a complex amalgam of various frequencies. For example, a simple vowel sound actually comprises frequencies across a broad spectrum. However, to understand how the cochlear is performing, the hearing clinic must ensure that it does 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 for the hearing centre to accurately measure hearing threshold, the headphones must be calibrated against an international standard, and the test performed in acoustical conditions that preclude the influence of extraneous noise upon that test.
A specialised device, known as a bone conductor, that is 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 indicate 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, as it would produce inaccurate hearing test results.
Restoring a patient’s ability to understand speech is the primary focus of an audiologist’s rehabilitative effort. Via free hearing tests, 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 informs 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 makes each word list equally difficult 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.