Maico MA 42, Руководство по эксплуатации

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Operating Instructions MA 42
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5.1.1.1 Masking
When measuring a pure tone audiogram you assume that the measured
hearing threshold is correct. But if you recognize that sound is also transmitted
through bone conduction over the entire skull it is probable that the opposite
ear has also received sound. This is called “crossover”.
Crossover can also occur when measuring air conduction because a small
amount of air conducted sound is received by the skull and transmitted by
bone. Whether the crossover signal can be heard by the opposite ear depends
on its inner ear function.
Relevant to crossover is the sound level which is received by the opposite ear.
The difference between the original test signal at the test ear and the received
signal at the opposite ear is called “interaural attenuation”.
For bone conduction measurement the interaural attenuation is 0 to 15 dB.
Bone conduction crossover is therefore possible even with a slight difference in
hearing loss between ears.
Important: Please advise the patient to tell you in which ear test signal
is heard. It is thereby easier to detect crossover.
To ensure that the patient will not experience crossover you must mask the
opposite ear. Masking increases the hearing threshold of the opposite ear.
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.
Manual masking
For the opposite ear set the receiver selector to Phone or Insert and the signal
selector to NOISE.
Adjust the required masking level with the level control dial.
Notice that the masking sound should be continuously presented for effective
masking. This can be done by pressing the corresponding STIM MODE key to
keep the masking noise on.
To mask when performing bone conduction testing, place the headphone on
the opposite ear so that the receiver is at the correct height (the sound output
grid exactly faces the ear canal). Adjust the headband of the headphones. The
receiver on the side where the bone conductor is placed should sit directly on
the cheek bone.