Speaker Placement
Most enclosures have some amount of acoustical re
fl
ection or interference during open
fi
eld
stimulus presentation. The effects will vary depending on the size of the enclosure, the angle at
which the speaker is positioned, and the materials used or added for sound dampening. To
minimize the effects of distortion or interference, position the stimulus speaker on the same
plane as the subject's ear and set at an angle from the sides of the enclosure. See
for more information on speaker placement.
Vibrations
Vibration from nearby large equipment is another common source of noise. If this is a problem
in your lab, the cage and subject can be positioned on an air table or anti-vibration table.
Heating Pads
When the subject is sedated for testing, it cannot generate enough body heat to maintain core
temperature. When the subject loses body heat, its hearing system is not as responsive which
can arti
fi
cially raise hearing thresholds. A heating pad is typically used to maintain the
subject's temperature. Heating pads that require AC power inside the chamber are not suitable
for ABR recordings.
Below are two alternative types of heating pads used for this purpose:
An Isothermal Pad
contains a phase change material that changes state near the subject's
body temperature. It can be heated in a microwave and can typically hold a constant
temperature for an hour or more.
Example:
http://www.braintreesci.com/images/DP.pdf
A Warm Water Recirculator
moves warm
fl
uid through a small subject heating pad to keep the
subject's body temperatures stable. Because this method uses an external heating element
and pump to transfer water to the pad via non-ferrous tubes, the recordings will not be
affected.
Examples:
http://www.braintreesci.com/prodinfo.asp?number=HTP-1500
and
www.braintreesci.com/prodinfo.asp?number=TP-700
Your Test Area | 12
