Pure Tone Audiometry

An audiometer is an electronic device which produces
pure tones, the intensity of which can be increased or
decreased in 5 dB steps (Fig. 4.2). Usually air conduction
thresholds are measured for tones of 125,250,500,1000,
2000 and 4000 and 8000 Hz and bone conduction
thresholds for 250, 500, 1000 and 2000 and 4000 Hz.
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. It is charted in the form of a
graph called audiogram. 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. 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
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 nontest
better ear. Similarly, masking is essential in all bone
conduction studies. Masking is done by employing
narrow-band noise to the non-test ear.

Uses of pure tone audiogram

(i) It is a measure of threshold of hearing by air and
bone conduction and thus the degree and type of
hearing loss.
(it) A record can be kept for future reference.
(iii) Audiogram is essential for prescription of hearing
aid.
(iv) Helps to find degree of handicap for medicolegal
purposes.
(v) Helps to predict speech reception threshold.