For initial orientation, hearing tests are performed using a tuning fork. Pure tone audiometry
is then used to plot hearing sensitivity on an audiogram and determine the auditory threshold, indicating the frequencies that are affected and the degree of hearing loss. Each frequency is
measured individually. It is thus possible to differentiate between conductive hearing loss, sensorineural hearing loss and a combination of both. Conductive hearing loss indicates an abnormality in the middle ear, while sensorineural hearing loss is caused by an abnormality in the inner ear (cochlear hearing loss) or in the structures located behind the inner ear (i.e. retrocochlear hearing loss, in which case the auditory nerve or auditory cortex is affected). Whether sensorineural hearing loss is cochlear or retrocochlear in origin must be investigated further with other tests, called suprathreshold audiometry.
All tests mentioned above involve examinations of how the sound is transmitted from outside the body to the brain. Whether or not the central processing of sound is impaired is tested by means of speech audiometry. All these tests require the patient to be willing and able to cooperate. The patient is asked to either press a button or else repeat words or sentences aloud and in this way give active feedback to the examiner, which is why these tests are collectively called subjective audiometry.
A pure tone audiogram takes advantage of the fact that there are two ways for sound to reach the inner ear. Air conduction, the more important of the two, involves the sound travelling through the air from the outer ear via the middle ear to the inner ear. By contrast, bone conduction is the conduction of sound directly from outside through the skull bone to the inner ear, thus bypassing the middle ear. Bone conduction is hardly noticeable in everyday life. One effect is that your own voice sounds different inside your head than outside. To create a pure tone audiogram, both types of conduction are tested, and air and bone conduction thresholds measured. For this purpose, the loudness levels are determined at which a frequency-specific pure tone is just about audible for a given individual. If both thresholds are the same, this means that the middle ear is fully functional and that the cause of the hearing loss is located behind the middle ear. If there is an air-bone gap, the middle ear is not functioning correctly. The bone conduction threshold gives information about the condition of the inner ear (‘cochlear hearing loss’) and the structures located behind it (‘retrocochlear hearing loss’ – referring to disorders of the auditory nerve and the auditory cortex).
‘Suprathreshold‘ audiometry derives its name from the loudness levels of those pure tones which are clearly audible for the individual undergoing the test. Hearing loss is measured indirectly, taking advantage of the following mechanism: A normal-functioning inner ear is able to amplify quiet sounds and decrease the volume of loud sounds, i. e. the loudness is adjusted accordingly. This reduces the dynamic range (the ratio between the softest and the loudest sound), resulting in what is called dynamic range compression. Put differently, the human brain is able to process only a relatively small dynamic range. The inner ear uses this compression to tailor the sounds to its specific needs so that the individual is able to hear considerably softer sounds and tolerate louder sounds. If this mechanism no longer works properly, or indeed at all, the individual is not able to perceive soft sounds any more, or almost immediately finds loud sounds too loud. This effect is called ‘recruitment’ and typically occurs in cases of sensorineural hearing loss. If the hearing loss is of ‘retrocochlear’ origin, however, the intensity of all sounds is reduced equally, and there is no recruitment. Suprathreshold audiometric tests verify recruitment and indirectly show whether the type of hearing loss is cochlear or retrocochlear. Outdated tests of this kind include the SISI, Langenbeck, Fowler and Lüschler tests, while a new and very reliable test is called categorical loudness scaling. These tests are also frequency-specific.
Speech operates on different frequencies. Vowels are predominantly associated with low frequencies and consonants with high frequencies. With a little experience, the analysis of a pure tone audiogram allows conclusions to be drawn on how well an individual should be able to hear certain words – if they don’t perform as well as expected, a central processing disorder seems a likely diagnosis. Various tests are used in speech audiometry. The Freiburg Speech Test, the most widely used test in Germany, examines the word recognition of monosyllabic words (in German Knie, Beil, Lauf etc.), which are mainly recognised by their consonants, thus testing high frequencies. Numbers (in German Achtzehn, Vierundzwanzig, etc.) are mainly recognised by their vowels, making them a good test of low frequencies. Speech understanding is increasingly tested in noise, mainly in the form of sentence tests (such as the Oldenburg Sentence Test and the Goettingen Sentence Test) as these tests simulate everyday situations more realistically than tests in quiet. This provides a more accurate picture of how an individual’s hearing loss affects them in day-to-day life. Such tests are also ideal for purposes such as checking the settings of any kind of assistive listening device.