Operation Manual MA 28
8506915 Rev. 5
24
05/05/2021
5.8 Performing Tone Audiometric Tests
5.8.1 Air Conduction Testing
5.8.1.1 Pretest Set-up and Instructions
Hearing threshold levels can be determined by presenting test signals to the test subject
with the included headphones (air conduction
– AC). The purpose of AC audiometry is to
establish the hearing sensitivity at various frequencies. The test can specify the AC loss
but cannot distinguish between conductive versus a sensorineural abnormality.
5.8.1.2 Threshold Determination
A threshold test is seeking the lowest level a tone is heard at least 50% of the time. The
test normally starts at 1000 Hz on the patient's better ear. Select
Right
/
Left
(
F2
key). A
procedure of
“down 10 dB, up 5 dB
” is typically utilized to establish a threshold at each
frequency. Vary the length of the tone and intervals between tone presentations to ensure
the patient is responding to the tone not just repeating the behavior.
5.8.2 Bone Conduction Testing
Place the bone conduction oscillator
on the patient’s head so that the flat, circular side
of the transducer is placed on the mastoid, at the noticeable ledge of the cranial bone
behind, but not touching the pinna. The other side of the headband is placed in front
of the opposite ear. Set the
F1
key to
Bone
and select the test ear.
Perform the test utilizing the same method as air conduction testing.
5.8.3 Masking
Masking is required if there is a notable threshold difference between the left and right
ears. It is possible for sound to be transmitted to both ears via bone conduction while
testing the poore
r ear. This is called “
crossover
”.
Crossover occurs often while testing bone conduction, but it can also occur during air
conduction testing. Relevant to crossover is the sound level received by the opposite
ear. The difference between the original test signal in the test ear and the received
signal in the opposite ear is called “
interaural attenuation
”.
For bone conduction measurements the interaural attenuation is 0 dB to 15 dB. Bone
conduction crossover is therefore possible even with a slight difference in hearing loss
between ears.
To ensure that the patient will not experience crossover, mask the opposite ear.
Masking may increase the hearing threshold of the test ear. For bone conduction the
masking signal is automatically routed to the opposite output of the phones or inserts,
based on the test ear selected.
The masking is turned on by rotating the
Masking Level dB
level control dial
to the
right. The masking sound should be continuously presented for effective masking. The
masking is done with a noise signal which is transmitted by the headphone. For pure
tone audiometry a narrowband noise is used. This noise changes its center frequency
according to the frequency of the test signal.
Adjust the level of the masking noise for the appropriate level to be presented.