jillio is right. A hearing/service dog aids in alerting a deaf or hard of hearing person to sounds in their environment. It does not take the place of a terp.
Here is more information about the ADA and how it applies to service dogs:
Guide Dogs for the Blind: An Overview of the Americans with Disabilities Act
An Overview of the Americans with Disabilities Act
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Section 36.302(c) of the Americans with Disabilities Act requires public accommodations generally to modify policies, practices, and procedures to accommodate the use of service animals in places of public accommodation.
Service animal means any guide dog, signal dog, or other animal individually trained to do work or perform tasks for the benefit of an individual with a disability, including, but not limited to, guiding individuals with impaired vision, alerting individuals with impaired hearing to intruders or sounds, providing minimal protection or rescue work, pulling a wheelchair, or fetching dropped items.
Places of public accommodation mean facilities, operated by a private entity, whose operations affect commerce and fall within at least one of the following categories:
Places of lodging
Establishments serving food or drink
Places of exhibition or entertainment
Places of public gathering
Sales or rental establishments
Service establishments
Stations used for specified public transportation
Places of public display or collection
Places of recreation
Places of education
Social service center establishments
Places of exercise or recreation
For more information, call the ADA Hotline at 800-514-0301.