6.6 BONE CONDUCTION AUDIOMETRY WITH MASKING/INSERT PHONE
As stated above, one gets a certain bone conduction effect from a normal
receiver caused by vibration. This is especially true for the lower
frequencies. In cases of a big difference of hearing loss in the two ears where
one has to use high masking intensities, one can get false masking.
As mentioned previously, the receiver will produce a bone conduction effect
with an intensity of approximately 40-60 dB below the air conduction effect.
If, for example, one has a left ear with air conduction loss of 80 dB and a right
ear with air conduction threshold at 20 dB at a specific frequency, and one
wants to test the bone conduction threshold on the right ear, the masking
intensity needed in the left ear would be 80 plus 15, equals 95 dB. This
intensity will produce a bone conduction signal of approximately 35 dB in the
right ear, and therefore a threshold measurement for bone conduction is
impossible.
In cases like this it is necessary to use insert masking. The insert receiver
produces less bone conduction than a normal TDH 39 receiver. The bone
conduction from an insert receiver is about 90-100 dB below the air
conduction level. In the described case, the insert will only produce about 0
dB in the right ear, and one will therefore be able to make a valid bone
conduction test.
Insert Masking
6.7 FREE-FIELD PURE TONE AUDIOMETRY
From the Select Function Menu, select: Tone, FF, Left, FF.
The following will be displayed:
Freq.
F.F.
L:Tone
F.F.
R:NBN
1000 Hz
C/P/W
20 dB On/Off
20 dB
When free-field output is selected in pure tone audiometry, the tone mode is
automatically set to warble tone (
$
5% frequency modulation). This reduces
the effects of standing waves in the test chamber.
Default Warble
Tone
Alternatively, narrow-band noise may be used as a stimulus source by
turning on the masking to the secondary channel (press Softkey 4 to activate
this channel). However, when using narrow-band noise as stimulus,
remember that this signal is calibrated to effective masking level, i.e.
normally 3-6 dB higher than threshold.
Softkey 4
Both the distance from the loudspeaker to the patient and the absorption of
the sound room are very critical.
Free-field pure tone audiometry is essentially used when testing children.
NOTE:
Contralateral, Ipsilateral and Insert are all available masking
sources in Free-Field tests.
MIDIMATE 622 OPERATION MANUAL
6. Pure Tone Audiometry
40
