Low-frequency hearing loss is seen in Ménière's disease, and a “U”-shape audiogram is typical for some hereditary hearing loss. High-frequency hearing loss is the most frequent type of sensorineural hearing loss, since it is typical for both age-related and noise-induced hearing damage. The shape of the audiogram can differ depending on the disease. In sensorineural hearing loss both air conduction and bone conduction curves worsen, and no air–bone gap is present ( Fig.
The difference between air conduction curve and bone conduction curve is called an air–bone gap, and indicates the possible improvement in hearing if the cause of impaired sound transmission was removed. In conductive hearing loss, air conduction thresholds worsen, so the air conduction curve is shifted down, by up to 50 dB HL, while bone conduction thresholds remain unchanged ( Fig. In normal-hearing ears both curves are placed on the audiogram plot within the range of values that do not exceed 20 dB HL ( Fig. 6 Consequently, some conductive disorders do cause minor, but significant alterations in bone conduction sensitivity because of changes in the contribution of the middle ear to the bone-conducted signal reaching the cochlea. It has been demonstrated, however, that the external ear and middle ear do provide minor, but important contributions to the bone conduction threshold in the normal auditory system. The diagnostic utility of the difference between air and bone conduction sensitivity is based primarily on two assumptions: (1) that the air conduction threshold is a measure of the function of the total auditory system, both conductive and sensorineural components, and (2) that the threshold for bone conduction is primarily a measure of the integrity of the sensorineural auditory system and is not significantly influenced by the functional status of the external or middle ear. The procedure for measuring bone conduction thresholds is similar to that for measuring air conduction thresholds, except that a vibrotactile stimulator transduces the signal, usually coupled to the mastoid of the ear being tested. The primary audiologic tests used to distinguish conductive from sensorineural hearing loss are the comparative measures of air and bone conduction thresholds.
Other hearing tests have been developed for the purpose of distinguishing among the various sites of auditory dysfunction. However, little differential diagnostic information can be obtained from this description of audiometric configuration because auditory system dysfunction at various anatomic sites may result in similar patterns of loss of sensitivity. The measure provides an indication of the magnitude and configuration of the hearing loss as a function of frequency. In summary, pure-tone air conduction testing is the initial and critical measurement for subjective hearing loss. There is increasing interest in assessing hearing between 8000 and 16,000 Hz, but testing in the ultra-audiometric range (10 to 20 kHz) is not routine. To determine hearing loss, hearing sensitivity is assessed at octave frequencies between 2 Hz. To assess hearing loss by air conduction, the examiner determines the magnitude (in decibels) by which the patient's hearing deviates from the 0-dB hearing level (i.e., normal hearing). The decibel levels used in audiometers for the normal threshold for air conduction can be found in other publications. The artificial ear simulates the impedance characteristics of the average human ear at the plane of the tympanic membrane. The output sound pressure level for standard circumaural or inserted earphones, or both, is specified when measured in a standard coupler, referred to as an artificial ear. In the United States, the American National Standards Institute (ANSI) has established standards for the calibration of clinical audiometers. Pure-tone threshold hearing sensitivity is the subjective procedure by which auditory sensitivity is determined. Richard Winn MD, in Youmans and Winn Neurological Surgery, 2017 Pure-Tone Audiometry Air Conduction.