This is the most common technique used for hearing assessment. Pure tones are delivered to the ear through headphones for air conduction and by bone vibrator for bone conduction.
Plotting: Hearing level in decibels (dB) above the normal threshold is plotted against frequency.
Range: Frequency tested usually ranges from 250 to 8000 Hz.
Symbols for Plotting
The right ear curve is recorded in Red colour and left ear curve with Blue colour.
| Test Mode | Right Ear (Red) | Left Ear (Blue) |
|---|---|---|
| Air Conduction (Earphones) |
O (Unmasked) △ (Masked) |
X (Unmasked) □ (Masked) |
| Bone Conduction (Mastoid) |
< (Unmasked) [ (Masked) |
> (Unmasked) ] (Masked) |
Masking
Masking is done to prevent the hearing from the normal ear while testing the deaf ear. It occurs due to the crossover of sound from testing ear to the normal ear.
- Ideal Method: Hood's plateau method.
- Alternative: Barany noise box.
- OPD Method: Rough masking effect can be produced by the rustling sound of paper (Tragus rubbing), but it is not very accurate.
Interpretation & Grading
Pure Tone Average (PTA): The average of the hearing threshold levels at 500, 1000, and 2000 Hz only.
Grading of Deafness
| Hearing Level (dB) | Category |
|---|---|
| -10 to 15 dB | Normal hearing |
| 16 to 25 dB | Minimal hearing loss |
| 26 to 40 dB | Mild deafness |
| 41 to 55 dB | Moderate deafness |
| 56 to 70 dB | Moderately Severe deafness |
| 71 to 90 dB | Severe deafness |
| Above 90 dB | Profound deafness |
Types of Deafness
Distinguished by comparing Air Conduction (AC) and Bone Conduction (BC).
- Conductive Deafness:
Bone conduction is Normal.
Air-Bone gap is 20 dB or more. - Sensorineural Deafness (SNHL):
Bone conduction is > 20 dB (abnormal).
Air-Bone gap is 10 dB or less. - Mixed Deafness:
Bone conduction is > 20 dB (abnormal).
Air-Bone gap is 10 dB or more.
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