Your Test Area
The small size (1 uV - 2.5 uV @ 90 dB stimulation) of typical ABR response signals makes it especially important to minimize electrical noise in the test area and to ensure all conditions are carefully controlled during testing. Be sure to consider the elements detailed below in setting up your testing area.
Grounding
Use a designated and grounded electrical outlet.
Enclosure Considerations
Many labs are full of cables, metal structures and other items that generate noise as well as electrical wires that serve as antennas for electrical noise. We recommend using a sound attenuating chamber with a built in Faraday cage for ABR testing. It is equally important to use good practices when using the selected enclosure. A mains power cord that extends from outside the enclosure to inside the enclosure undermines its integrity.
To maintain the integrity of the enclosure:
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Keep AC electrically powered devices (wall/mains powered) outside.
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Ensure the preamplifier is completely inside your enclosure.
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Ensure the preamp charger is unplugged and outside the enclosure.
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Ensure electrode wires are completely inside the enclosure.
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Verify the enclosure door shuts securely.
Speaker Placement
Most enclosures have some amount of acoustical reflection or interference during open field 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 Sound Chamber Fundamentals 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 artificially 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.
A Warm Water Recirculator moves warm fluid 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: https://www.braintreesci.com/temperature-measurement-control/heat-therapy-pumps-pads/adroit-heat-therapy-pump/ and https://www.braintreesci.com/temperature-measurement-control/heat-therapy-pumps-pads/gaymar-heating-cooling-t-pump-back-in-stock/
Important
It is critical that you place the heat pump and all associated electrical connections outside the Faraday cage when using this type of pad.