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Audiometric Tests

Pure-Tone Audiometry
  1. Pure tone Audiometry- Quantitative test with Qualitative components
  2. An audiometer is an electronic device which produces pure tones, the intensity of which can be increased or decreased in 5 dB steps.
  3. This is most commonly used method of measuring hearing acuity.
  4. Usually air conduction thresholds are measured for tone of 125, 250, 500, 1000 and 2000 and 4000 and 8000 Hz and bone conduction thresholds for 250, 500, 1000 and 2000 and 4000 Hz.
  5. Most audiometers used today are calibrated to the international (ISO) standard level.
  6. The best frequency to start with is 1000 Hz.
  7. The signal is then increased in 5-dB steps until half of the tone pips are consistently heard.
  8. The amount of intensity that has to be raised above the normal level is a measure of the degree of hearing impairment at that frequency. The sound emitted by this is transmitted by the bones of the skull to the cochlea, thus by-passing the external and middle ears and giving a measure of inner-ear function. The results are charted as audiograms
  9. The threshold of bone conduction is a measure of cochlear function. The difference in the thresholds of air and bone conduction (A-B gap) is a measure of the degree of conductive deafness.
  10. It may be noted that audiometer is so calibrated that the hearing of a normal person, both for air and bone conduction, is at zero dB and there is no A-B gap, while turning fork tests normally show AC>BC.
  11. When difference between the two ears is 40 dB or above in air conduction thresholds, the better ear is masked to avoid getting a shadow curve from the non-test better ear. Similarly, masking is essential in all bone conduction studies. Masking is done by employing narrow-band noise to the non-test ear.
    1. Masking must be applied to the better ear when testing the deafer ear if the difference in threshold is found to be 40 dB or more.
Fig.: (A) Audiogram of right ear showing conductive hearing loss with A-B gap (B) Symbol used in audiogram charting

Fig: Early case of noise-induced hearing loss. Note dip at 4000 Hz.

Uses of pure tone audiogram
  1. It is a measure of threshold of hearing by air and bone conduction and thus the degree and type of hearing loss.
  2. A record can be kept for future reference.
  3. Audiogram is essential for prescription of hearing aid.
  4. Helps to find degree of handicap for medicolegal purposes.
  5. Helps to predict speech reception threshold.

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