Audiometer - The Testing Instrument
The instrument used for performing Audiometry is called an Audiometer, a device capable of generating pure tone signals that can be adjusted in both frequency and intensity, independently. An audiometer can be manually operated and some of them can be set to an automatic mode. The manual audiometry is done by an Audiologist, who adjusts the frequency and the loudness levels and records the lowest sound responded by the subject at each frequency.
An automatic audiometer -
• Presents the tones of different frequencies and loudness levels
• Records the responses of the subject
• Adjusts the level of the tone
• Determines the threshold at each frequency as the lowest sound level to which the subject responds fifty percent of the time.
The accessories of an audiometer include standard audiometric headphones, insert earphones, a patient response switch and external power supply.
The Procedure of Audiometry
The subject is made to sit in a sound treated room for testing. Standard headphones or the insert earphones are fixed. The subject is required to give a response on hearing a tone, like lifting hand or finger or pressing a button. The procedure can be carried out automatically or manually. The lowest loudness level heard by the subject in each frequency is recorded and is called Threshold. Thresholds are determined for tones with frequencies starting at about 125 Hz and increasing in frequency by octaves or half-octaves to about 8000 Hz.
Hearing tests of the right and left ear are done independently. Air conduction testing and bone conduction testing are carried out to determine the type of hearing loss. Bone conduction testing is done with the help of the oscillator fixed to a headband, positioned behind the ear.
Audiogram - The Graph of Frequency vs Intensity
Audiogram is the graph of frequency vs intensity, where the thresholds of each ear are marked in different colors and shapes and the graph is plotted. The bone conduction results are also marked in the same graph. The frequencies used most often during testing are 250Hz, 500 Hz, 1000 Hz, 2000 Hz, 4000 Hz and 8000 Hz.
The intensity or loudness of the tone is measured in decibels or dB. Each mark on the audiogram represents the lowest loudness level the subject has heard for that particular frequency, in the ear tested. The range of intensity depicted is from 0 dB to 120 dB.
Pure tone average is calculated as the average of thresholds (softest sound heard) at the 500 Hz, 1000 Hz and 2000 Hz for each ear and this average is used to determine the type and severity of hearing loss in the ear for the particular ear.
The bone conduction testing helps determine the type of hearing loss. While sound from the earphone travels along the outer and the middle ear to the inner ear, the sound from the oscillator in bone conduction testing is received directly by the inner ear. Thus, the difference between the two test results can help locate whether the defect is in the outer or middle ear (conductive hearing loss) or in the innermost part of the ear (sensorineural hearing loss). People with conductive hearing loss may hear sounds, but less loud than normal. People with sensorineural hearing loss usually hear louder sounds, and also have difficulty in understanding speech.
The hearing loss can be monaural (loss in one ear), binaural (loss in both ears), symmetrical (relatively even in both ears) or asymmetrical (loss in one ear significantly worse than the other ear).
The severity of hearing loss depends on the pure tone average and is categorized as below-
|Severity of hearing loss||Pure tone average||Hearing levels|
|Normal hearing||0 to 20 dB||0 dB is not the absence of sound|
|Mild hearing loss||21 to 40 dB||Difficulty in following speech especially in noisy situations|
|Moderate hearing loss||41 to 55 dB||Difficulty in following speech and other quiet sounds|
|Moderately severe hearing loss||56 to 70 dB||Unable to hear speech even in quiet surroundings|
|Severe hearing loss||71 to 90 dB||Only very loud sounds like traffic noises can be heard|
|Profound hearing loss||Above 91 dB||Unable to hear sounds unless extraordinarily loud|
What can an Audiologist predict from the Audiogram?
Depending on the thresholds at various frequencies in each ear and also the bone conduction test results, the audiologist can predict the type of hearing loss and where the damage is located. The graph can be typical for certain types of hearing loss and the various causal factors of the hearing loss in the individual like problems in the middle ear, otosclerosis (fixation of the stapes bone), age-related hearing loss (presbycusis), noise-induced hearing loss, Menieres disease and others can be interpreted.
Other Types of Audiometry
Behavioural Audiometry: This procedure is used for children who cannot understand instructions or do not know how to respond. The child is allowed to play inside the sound treated room and can be accompanied by a caretaker. The caretaker is instructed not to show any response to the sounds heard. Pure tones of different loudness levels are presented through loudspeakers in the room and the response of the child is recorded. This is a subjective procedure and can only be used for screening purposes.
Speech Audiometry: The procedure of Speech Audiometry is almost the same as in pure tone audiometry, except that this uses human speech instead of pure tones for testing.
Tympanometry: This procedure measures the responses of the eardrum to sound inputs and pressure variations in the ear canal. It is also called Impedance Audiometry.
Immitance Audiometry: It measures the response or resistance of the structures of the ear to the incoming sounds using a special audiometer called Electro-Acoustic Immitance Bridge. The instrument measures the sounds produced in the inner ear, called Otoacoustic emissions or OAE, in response to external sound stimuli.
Brain Stem Evoked Response Audiometry (BERA): This is mostly used for young children as they are unable to respond or follow instructions. It is a painless, non-invasive procedure in which the changes in the nerve activity in response to sound are recorded using electrodes.