Pure Tone audiometry
Audiogram, Assessment of hearing
Pure tone audiometry is used to measure the auditory threshold of an individual. The instrument used in the measurement of auditory threshold is known as the audiometer.
An audiometer has been described by the International Electrochemical Commission in 1976 as an instrument used for the measurement of acuity of hearing, and threshold of audibility. There are two types of audiometers widely used. They are:
1. Those that require a subjective response on the part of the patient and
2. Those that require no subjective response from the patient.
1. Pure tone audiometer is the classic example of the first type
2. Impedence audiometer / BERA are examples of the second variety
Components of a Puretone audiometer:
Oscillator: The role of the oscullator in a puretone audiometer is to generate electronically standardized frequencies within +/- 3% of their nominal value. The frequencies generated are 125, 250, 500,750, 1000, 1500, 2000, 3000, 4000, 6000 and 8000 Hz.
Interrupter switch: The tones presented to the patient should be switched on and off. This feature is important because a continuous tone undergoes decay during a period of time. This switch gives the option of providing the tone in a continuous or an interrupted manner.
Equalisation circuit: This circuit contains resistors which helps in equalisation of the tones generated. This is because the threshold of human hearing is not uniform, the human ear is most sensitive at frequencies around 2 kHz. It is also insensitive at low or very high frequencies.
Output power amplifier: The signals produced by the oscillator needs to be amplified. The most important characteristic of the amplifier is that it produces very little distortion, and has a good signal to noise ratio. In most audiometers the power amplifier is run at constant high signal output levels.
Hearing level attenuator: The attenuator controls the level of the signal from the audiometer within the range of 110 - 120 dB. The attenuator can be varied in steps of 5 dB. The basic reference point is marked as 'O'. This indicates -5 to -10 dB hearing threshold levels. The attenuator steps should be accurate.
Output transducers: Is of three different types.
1. Ear phones
2. Bone vibrator
3. Loud speaker
The ear phones for audiometers are very special. They cannot be replaced or changed without calibrating the whole equipment. The pre requisites of a good ear phones are:
1. It should have a good long term stability
2. It should have a flat frequency response
3. It should be able to deliver high output sounds.
Bone vibrators: In contrast to ear phones bone vibrators have a limited dynamic frequency range. At low frequencies these vibrators show distortions.
Loud speakers: Are used in testing paediatric patients. It is used in free field audiometry.
Calibration of the audiometer involves calibration of the audiometer proper, calibration of ear phones, and calibration of bone vibrators. The basic aim of these calibration procedures is to define the audiometric zero for the chosen earphone. This can be performed using human volunteers or an artificial ear. The calibration of bone vibrators is the same as for earphones except for the measuring device which is different.
Pure tone air conduction testing:
This is a measurement of air conduction thresholds of audibility. Air conduction Threshold is in fact defined as the faintest tone a subject is able to hear via air conduction. In clinical setting pure tone audiogram is performed for two main purposes:
1. To assist in the diagnosis of ear pathology
2. To decide on the appropriate rehabilitation device which can be used to minimize the hearing disability.
Pure tone air conduction threshold is tested using head phones:
Technique of measurement: Some audiologists assess the threshold of air conduction by going from an inaudible to an audible stimulus intensity. This method is known as ascending method of estimation of threshold of hearing, while others assess the threshold of air conduction by going from an audible to an inaudible stimulus intensity. This is known as descending method of threshold estimation.
Instructions to the patient: The patient is instructed to raise the index finger if the sound is heard. The patient should respond even if the sound is faintly heard.
The head phones should be properly seated over the external auditory canal. This step should be performed with care because the patient's pinna comes in various shapes and sizes. Improper placement of head phones will cause threshold variations of even 15 - 20 dB.
The audiometer should be properly checked before performing the test. After the audiometer has had a warm up period, the tester should first place the ear phones on his own ears and listen to various frequencies and intensities of test tones. He should also listen to the masking noise, check for any audible clicks of the interrupter switch. Before placing the ear phones on the patient, the patient's ear should be examined for the presence of wax. If wax is present it should first be cleaned before the test could be performed. If the ear canal is small or tends to collapse when pinna is pressed, the test could not be valid. If the ear canal tends to collapse when pressure is applied to the pinna, plastic tubes can be inserted into the external canal to prevent such collapses.
To plot the recordings, red color ink is used to plot values of right ear, and blue color ink is used to plot values of left ear.
"Up 5-down 10" method of threshold estimation: This technique is based on Hughson - Westlake ascending technique.
Tones of short duration is used for threshold estimation. This method of threshold estimation involves the following steps:
Step I : The better ear is tested first in order to determine the need for masking.
Step II : Start with a 1000 Hz tone at a level above the threshold to allow easy identification of the tone. This tone is selected because it is an important speech frequency, and the patient is less apt to mistake the frequency. If the patient is suspected to be having a profound hearing loss then the testing should be started with 250Hz frequency. This is because of the fact that the individuals with profound hearing loss often have testable hearing only in the low frequency range.
Step III : The patient's understanding of the listening task should be checked by using both short and long duration test tones. The patient should be instructed to raise the index finger as soon as the sound is heard.
Step IV : During testing, the examiner should vary the interval between tone presentations to avoid telegraphing the stimulus. Tone should not be presented while the attenuator dial is being rotated, because switching artifacts may contaminate the results. As the threshold levels are being reached, a check should be made for the existence of abnormal tone decay. This is done by sustaining the tone for several seconds longer than usual. If the index finger drops before the tone is discontinued, abnormal tone decay should be suspected.
Step V : The starting intensity of the test tone is reduced in 10 dB steps following each positive response, until a hearing threshold level is reached at which the subject fails to respond. Then, the tone is raised by 5 dB, if the subject hears this increment, the tone is reduced by 10 dB; if the tone is not heard then ti is raised by another 5 dB increment. This 5 dB increment is always used if the preceding tone is not heard, and a 10 dB decrement is always used when the sound is heard. The threshold is defined as the faintest tone that can be heard 50% or more of the time, and is established after several threshold crossings.
If there is no response at the maximum output of the audiometer, an arrow pointing downward should be attached to the symbol designating the test ear and placed on the audiogram at the hearing threshold level coinciding with the maximum output for the test frequency. If the tone is heard at the minimum level, the audiogram should be marked in similar fashion but should have the arrow pointing upward.
Step VII : Testing of the second ear should begin with the last frequency used to test the first ear. There is no need to start again with a 1000 Hz tone because if one side of the heard has learned the listening task, the other side knows it as well. The test is terminated after all desired frequencies have been examined.
With air conduction readings alone one cannot reliably come to a diagnosis. Even a basic distinction between conductive and sensorineural hearing loss cannot be made with air conduction hearing tests. When there is a hearing loss in air conduction audiogram, it represents a cumulative deficits from the outer, middle, inner ear and retro cochlear system of a subject. Bone conduction audiometry is a must before a classification of the deafness could be made.
Scale of hearing threshold estimation in air conduction audiometry:
|- 10 to 15||Normal|
|16-19||Mild hearing loss|
|30-44||Moderate Hearing loss|
|45-59||Moderately severe hearing loss|
|60-79||Severe hearing loss|
|80 and above||Profound hearing loss|
Bone conduction audiometry:
This is an important measurement of hearing threshold using a bone vibrator. This helps to differentiate conductive from sensori neural hearing loss. The equipment necessary is just a bone vibrator connected to the audiometer. The bone vibrator is placed over the mastoid process of the side to be tested. The auditory threshold is assessed as described for air conduction assessment. The only difference is that the better hearing ear should be masked using a masking tone delivered via a head phone. If the values of bone conduction audiometry is better than that of air conduction, then the patient is said to have conductive deafness. In sensori neural hearing loss both the air and bone conduction curves show a dip. This dip is frequently seen for higher frequencies.
In bone conduction audiometry high frequencies cannot be used for testing. Frequencies above 4000 Hz cannot be used because they are beyond the vibrating capabilities of the bone vibrator.