Learn more at the links below
Impact on dog's quality of life
https://www.veterinary-practice.com/article/presbycusis-impact-quality-of-life-dogs
Efficacy of hearing aids on pets
https://www.akc.org/expert-advice/health/dog-hearing-aids/
Types of hearing loss
https://www.msdvetmanual.com/ear-disorders/deafness/deafness-in-animals