Pure Tone Audiometry
Definition: An electronic device which produces pure tones. The intensity can be increased or decreased in 5 dB steps.
Parameters & Calibration
1. Frequencies Tested
| Type | Frequencies (Hz) |
|---|---|
| Air Conduction (AC) | 125, 250, 500, 1000, 2000, 4000, 8000 Hz |
| Bone Conduction (BC) | 250, 500, 1000, 2000, 4000 Hz |
2. Interpretation of Values
- Hearing Impairment: The amount intensity is raised above normal level at a specific frequency.
- Audiogram: The graph where these thresholds are charted.
- Normal Calibration:
- Normal person (AC & BC) = 0 dB.
- No A-B Gap.
- (Contrast: Tuning fork tests normally show AC > BC).
Clinical Significance
- BC Threshold: Measure of Cochlear Function (Sensorineural reserve).
- A-B Gap (Difference in AC & BC): Measure of the degree of Conductive Deafness.
Masking
Method: Employment of narrow-band noise to the nontest ear.
Indications for Masking
| Conduction Type | Rule | Reason |
|---|---|---|
| Air Conduction | Difference between two ears is 40 dB or above. | To avoid getting a shadow curve from the better (nontest) ear. |
| Bone Conduction | Essential in ALL studies. | To ensure true bone threshold is measured. |
Uses of Pure Tone Audiogram
Diagnostic & Rehabilitative Tool
- Measures threshold of hearing (Air & Bone) → determines degree and type of hearing loss.
- Essential for Prescription of Hearing Aid.
- Helps predict Speech Reception Threshold.
- Helps find degree of handicap for medicolegal purposes.
- Record keeping for future reference.
📚 Source: ENT Dhingra
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